# Spatially-resolved properties of early-type group-dominant galaxies with
MUSE: gas content, ionisation mechanisms and metallicity gradients

P. Lagos,1,2 S. I. Loubser,2,3 T. C. Scott,1 E. O’Sullivan,4 K. Kolokythas,2
A. Babul,5 A. Nigoche-Netro,6 V. Olivares, 7,8 and C. Sengupta9
1Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP,
Rua das Estrelas, 4150-762 Porto, Portugal
2Centre for Space Research, North-West University, Potchefstroom 2520, South
Africa
3National Institute for Theoretical and Computational Sciences (NITheCS),
Potchefstroom 2520, South Africa
4Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
5Department of Physics and Astronomy, University of Victoria, Victoria, BC,
V8W 2Y2, Canada
6Instituto de Astronomía y Meteorología, Av, Vallarta 2602. Col. Arcos
Vallarta. Guadalajara, Jalisco. C.P. 44130 México
7LERMA, Observatoire de Paris, PSL Research Univ., CNRS, Sorbonne Univ., 75014
Paris, France
8Department of Physics and Astronomy, University of Kentucky, 505 Rose Street,
Lexington, KY 40506, USA
9Purple Mountain Observatory, No. 8 Yuanhua Road, Qixia District, Nanjing
210034, China E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

With the goal of a thorough investigation of the ionised gas and its origin in
early-type group-dominant galaxies, we present archival MUSE data for 18
galaxies from the Complete Local-Volume Groups Sample (CLoGS). This data
allowed us to study the spatially-resolved warm gas properties, including the
morphology of the ionised gas, EW(H$\alpha$) and kinematics as well as the
gas-phase metallicity (12 + log(O/H)) of these systems. In order to
distinguish between different ionisation mechanisms, we used the emission-line
ratios [O iii]/H$\beta$ and [N ii]/H$\alpha$ in the BPT diagrams and
EW(H$\alpha$). We find that the ionisation sources in our sample have variable
impacts at different radii, central regions are more influenced by low-
luminosity AGN, while extended regions of LINER-like emission are ionised by
other mechanisms with pAGBs photoionisation likely contributing significantly.
We classified our sample into three H$\alpha$+[N ii] emission morphology
types. We calculate the gas-phase metallicity assuming several methods and
ionisation sources. In general, 12 + log(O/H) decreases with radius from the
centre for all galaxies, independently of nebular morphology type, indicating
a metallicity gradient in the abundance profiles. Interestingly, the more
extended filamentary structures and all extranuclear star-forming regions
present shallow metallicity gradients. Within the uncertainties these extended
structures can be considered chemically homogeneous. We suggest that group-
dominant galaxies in our sample likely acquired their cold gas in the past as
a consequence of one or more mechanisms, e.g. gas-clouds or satellite
mergers/accretion and/or cooling flows that contribute to the growth of the
ionised gas structures.

###### keywords:

galaxies: abundances – galaxies: groups: general – galaxies: ISM – galaxies:
elliptical and lenticular, cD, S0 – galaxies: nuclei

††pubyear: 2015††pagerange: Spatially-resolved properties of early-type group-
dominant galaxies with MUSE: gas content, ionisation mechanisms and
metallicity gradients–G

##  1 Introduction

Most galaxies are found in groups and clusters. The distinction between them
is made by the number of their members, clusters have $\gtrsim$50 member
galaxies and are more massive than groups. Bright galaxies at the centres of
galaxy groups (hereafter BGGs) or group-dominant galaxies typically have
stellar masses of M∗ $\gtrsim$1010-13 M⊙ (e.g., Gozaliasl et al., 2016;
O’Sullivan et al., 2018; Kolokythas et al., 2022), with some BGGs also
displaying X-ray emission (e.g., O’Sullivan et al., 2017) and containing cold
H i and/or molecular gas in the range $\sim$108-10 M⊙ (e.g., O’Sullivan et
al., 2015, 2018). A significant fraction ($\sim$25%; Gozaliasl et al., 2016)
of BGGs lie on the main sequence of the star-forming (late-type) galaxies at z
$\lesssim$ 0.4, and this fraction increases with redshift. However, most BGGs
are early-type galaxies (ETGs), i.e., elliptical and/or lenticular (S0)
galaxies. Early-type BGGs (e.g., Kolokythas et al., 2022), and many other ETGs
in the literature (e.g., Shapiro et al., 2010; Fang et al., 2012; Gomes et
al., 2016), contain warm ionised gas and ongoing low-level star-formation (SF)
(SFR${}_{FUV}\sim$0.01–0.4 M⊙ yr-1; Kolokythas et al., 2022), while in late-
type BGGs the SF can reach $\sim$10 M⊙ yr-1 (Oliva-Altamirano et al., 2014;
Gozaliasl et al., 2016). This same activity is also found in a significant
fraction of Brightest Cluster Galaxies (BCGs; e.g., Bildfell et al., 2008;
Pipino et al., 2009; McDonald et al., 2010; Donahue et al., 2011; McDonald,
Veilleux & Mushotzky, 2011; Werner et al., 2014; Loubser et al., 2016).

ETGs (e.g., Phillips et al., 1986; Trinchieri & di Serego Alighieri, 1991;
Annibali et al., 2010), including some early-type BGGs111See the list of
galaxies in this paper. also display low-ionisation nuclear emission-line
regions (LINERs, Heckman, 1980). The spectra of LINERs exhibit strong low-
ionisation optical emission lines such as [O iii]$\lambda$5007, [O
ii]$\lambda\lambda$3726/29, [N ii]$\lambda$6584, [S
ii]$\lambda\lambda$6717,6731 and hydrogen Balmer lines (H$\alpha$, H$\beta$,
etc), commonly with [N ii]$\lambda$6584 > H$\alpha$ emission. Several
explanations for LINER emission, in galaxies, have been proposed including
shock-ionisation (e.g., Heckman, 1980; Dopita & Sutherland, 1995, 1996; Allen
et al., 2008), photoionisation by ongoing low-level SF (e.g., Schawinski et
al., 2007; Fogarty et al., 2015; Shapiro et al., 2010), hot evolved post-
asymptotic giant branch (pAGB) stars (e.g., Binette et al., 1994; Stasińska et
al., 2008; Annibali et al., 2010; Sarzi et al., 2010; Cid Fernandes et al.,
2010, 2011), photoionisation by the "cooling" X–ray–emitting gas (Voit &
Donahue, 1990; Donahue & Voit, 1991; Ferland, Fabian & Johnstone, 1994; Voit,
Donahue & Slavin, 1994; Polles et al., 2021), thermal conduction from the hot
gas (Sparks, Macchetto & Golombek, 1989) and ionisation by low-luminosity
active galactic nuclei (AGN) (e.g., Ferland & Netzer, 1983; Barth et al.,
1998; Ho, 1999). Observational evidence suggests that more than one mechanism
are likely to be at work in LINERs (e.g, Edwards et al., 2007; Fogarty et al.,
2015, in BCGs). For instance, the contribution of AGN to the photoionisation
of LINERs may be restricted to the nuclear region (e.g., Sarzi et al., 2010),
while pAGB stars are likely responsible for the extended gas emission (e.g.,
Binette et al., 1994; Sarzi et al., 2010; Gomes et al., 2016). Extended low-
ionisation emission line regions (LIERs; Belfiore et al., 2016) or LINER-like
extranuclear diffuse ionised gas (DIG) emission in ETGs has been detected and
studied (e.g., Sarzi et al., 2006; Gomes et al., 2016) as part of the SAURON
(Bacon et al., 2001) and Calar Alto Legacy Integral Field Area (CALIFA,
Sánchez et al., 2012) surveys. These studies suggests low-luminosity AGN are
likely contributing to LINER emission in a less than dominant way (Yan &
Blanton, 2012).

Extended and DIG filaments are observed in BGGs/BCGs (e.g., Heckman et al.,
1989; Crawford et al., 1999; O’Sullivan et al., 2012; Tremblay et al., 2018;
Olivares et al., 2019). These ionised gas filaments seem to be co-spatial with
large quantities of H i and molecular gas, traced by CO, and soft X–ray
emission features (Werner et al., 2014). McDonald et al. (2010) found that the
extended warm gas in cluster galaxies, in general, is spatially coincident
with cooling intracluster medium (ICM) flows. McDonald, Veilleux & Mushotzky
(2011); McDonald, Veilleux & Rupke (2012) rule out collisional ionisation by
cosmic rays, thermal conduction, photoionisation by ICM X-rays and AGN as
strong contributors to the ionisation of the warm gas, in both the nuclei and
filaments of cool core clusters. They argue that the data are adequately
described by a composite model of slow shocks and SF. According to McDonald,
Veilleux & Mushotzky (2011), the H$\alpha$ filaments in BGGs/BCGs are more
strongly correlated with the cooling properties of the ICM than with the radio
properties of the BCGs. The molecular gas in the filaments is located along
old radio jets, lobes and/or below X-ray cavities, suggesting that AGN
activity supplies a regular input of energy in those systems (Russell et al.,
2016; O’Sullivan et al., 2021). Generally, the dominant ionisation mechanism,
as well as their locations (i.e., nuclear and/or extended regions), is not
clear and different processes may be at work during the evolution of each type
of system.

Hot (107 K) gas-phase metallicity is another key property for studying the
effect of galactic environments and AGN/SF feedback in groups and clusters
(for a review see Gastaldello et al., 2021). The X-ray emitting intra-group
medium (IGrM) shows several emission lines typical of elements synthesised by
stars/supernovae (SNe) and deposited in the IGrM/ICM via galactic winds from
the group/cluster members (e.g., Liang et al., 2016), ram-pressure
stripping/tidal interactions (Taylor & Babul, 2001; McCarthy et al., 2008) and
others (for a review see Schindler & Diaferio, 2008). Several studies (e.g.,
Mernier et al., 2017; Gastaldello et al., 2021, and references therein) show
that the average abundance profiles of Fe and others elements (e.g., O, Si,
Ar) in the ICM/IGrM increase towards the core of cool-core clusters/groups up
to values of $\sim$1.0 Z⊙, and decrease at large radii. This peaked
distribution is associated with the release of metals from the galaxy members
via mechanical feedback from AGN/SF activity, gas turbulence (Rennehan et al.,
2019) or infalling galaxies through ram-pressure stripping across the
evolution of the systems. Whereas some studies (e.g., Werner et al., 2006)
report a rather flat O and Mg profiles, non-cool-core clusters and groups do
not exhibit clear Fe abundance gradient in their cores. Mechanisms such as
merger–induced sloshing motions (O’Sullivan, David & Vrtilek, 2014, and
references therein) can also transport the released metals from the galaxies
into the IGrM/ICM. On the other hand, the spatial distribution of abundances
of the warm (104 K) gas-phase in the interstellar medium (ISM) of the galaxies
is also important for understanding the formation history of these systems.
Studies of local star-forming galaxies have shown a negative metallicity
gradient (the metallicity decreases radially) with increasing galactocentric
radius (e.g., Martin & Roy, 1992; van Zee et al., 1998, among many others).
However, this radial distribution can present deviations from this simple
negative gradient, namely a steep decrease of oxygen abundance (12 + log(O/H))
in the nuclear regions and a flattening of the gradient in the outer parts
(see Sánchez-Menguiano et al., 2018; Sánchez, 2020, and references therein).
Several mechanisms have been proposed to explain these features such as radial
migration, infall of gas or satellite accretion. In early-type group-dominant
galaxies, the presence of these features and the origin of their gas are
poorly understood (e.g., Annibali et al., 2010). The cold gas in group and
cluster ETGs potentially originates from two main sources: internal production
(a large fraction of stellar mass is believed to be released in the form of
cold gas, Davis et al., 2011) and external accretion, i.e., mergers, stripping
gas from satellites, gas from the IGrM/ICM (e.g., Jung et al., 2022). A
kinematical decoupling between gas and stars as evidence for external gas
origin was found in many ETGs in the SAURON (Sarzi et al., 2006, 2010) and
CALIFA (Gomes et al., 2016) surveys.

CLoGS (O’Sullivan et al., 2017) is an optically-selected sample of 53 groups
at distances of less than 80 Mpc and is the first statistically complete
sample of nearby galaxy groups studied with an extensive multi-wavelength
dataset, including X-ray (Chandra and XMM-Newton; O’Sullivan et al., 2017),
radio (GMRT and VLA; Kolokythas et al., 2018, 2019) and sub-mm (IRAM-30m and
APEX; O’Sullivan et al., 2018) observations. In addition, we have analysed
spatially-resolved long-slit spectroscopy from the 10m Hobby-Ebberly Telescope
(van den Bosch et al., 2015) for a sub-sample of 23 of the CLoGS dominant
central galaxies (see Loubser et al., 2018). Archival GALEX FUV and WISE
photometry for the group-dominant galaxies are presented in Kolokythas et al.
(2022). In this paper, we use archival optical ESO Very Large Telescope (VLT)
Integral Field Unit (IFU) Multi-Unit Spectroscopic Explorer (MUSE)
observations of a sample of 18 early-type group-dominant CLoGS galaxies. We
aim to investigate the relation between the properties (i.e., emission line
ratios, chemical abundances, etc) and structure of each galaxy’s ISM in order
to constrain the ionisation processes, the origin of their gas and its
chemical abundance distribution. In related papers, using the same MUSE
dataset, Olivares et al. (2022) presents a detailed analysis of the ionised
gas kinematics, while in Loubser et al. (2022) we study the stellar kinematics
of the fast and slow rotators in this sample. The paper is organized as
follows: Section 2 contains the technical details regarding the observations,
data reduction, spectral fitting and measurement of line fluxes; Section 3
describes our results on the structure and physical properties of the ionised
gas; Section 4 discusses the results. Finally, in Section 5 we summarise our
conclusions. In the following analysis we adopt as solar abundances Z⊙ =
0.0142, X⊙ = 0.7154, and [12 + log(O/H)]⊙ = 8.69 from Asplund et al. (2009).

##  2 The data and emission line measurements

###  2.1 Observations and data reduction

Our sample consists of 18 group-dominant galaxies selected from the CLoGS
nearby groups sample (O’Sullivan et al., 2017). Each galaxy has archival ESO
VLT/MUSE (Bacon et al., 2010) data. Four of our targets were previously
observed in the CALIFA IFU survey (NGC 677, NGC 924, NGC 1060 and NGC 7619).
The observations (program ID 097.A-0366(A)) were made with the MUSE
spectrograph, which used the Nasmyth B focus on Yepun the 8.2m VLT UT4. The
observations were made in the wide field mode over a 1${}^{\prime}\times$1′
Field of View (FoV) sampled by a system of 24 spectrographs with an spectral
coverage between $\sim$4800 and 9300 $\rm\AA$, a spectral resolution of
$\sim$2.6 $\rm\AA$, spectral sampling of 1.25 $\rm\AA$ and spatial sampling of
0.2′′ per spaxel, leading to a total of 90,000 spectra per exposure. For each
galaxy three exposures were taken. These observations were dithered with a few
arcsecond dither pattern in order to cover a large portion of each galaxy’s
ISM. Offset sky observations were used for sky subtraction. In Table 1 we show
the general parameters for the sample members and the observation log.

The data were reduced using the MUSE pipeline (v2.6.2, Weilbacher et al.,
2020), with steps including: bias correction, wavelength calibration,
construction of datacubes from the individual spectra from the detectors,
correction of the spectra to the heliocentric frame, skysubtraction, and
merging of the individual exposures to form a combined datacube with a total
of $\sim$200,000 spectra per each galaxy. Finally, the data cubes were
corrected for redshift and galactic extinction using the reddening function
given by Cardelli, Clayton & Mathis (1989). In Figure 1 we show the nuclear
spectra of each member of our sample. More details in Section 2.2.

Table 1: General parameters of our sample and observing log. Col. (1) gives the galaxy name, Col. (2) the morphology, Cols. (3) and (4) give their coordinates, Col. (5) presents the galactic extinction AV (mag) (Schlafly & Finkbeiner, 2011) and Cols. (6)-(7) gives the distance and redshift, respectively. D (Local Group) and z from NED assuming Ho = 73.0 km/sec/Mpc. In Col. (8) we give X-ray morphology (extent of the gas halo) and core type as cool-core/non-cool-core (CC/NCC) based on their temperature profiles (O’Sullivan et al., 2017) for systems where thermal emission was detected: group-like (GRP, extent >65 kpc), galaxy-like (gal, extent $\sim$10-65 kpc) and point-like (pnt, unresolved, extent smaller than the XMM-Newton PSF). Radio morphology (Kolokythas et al., 2018): point-like (pnt, radio source with sizes $\lesssim$11 kpc), diffuse emission (diffuse, with no clear jet or lobe structure), small-scale jets (jet, <20 kpc jets confined within the stellar body of the host galaxy) and large-scale jets (JET, >20 kpc jets extending beyond the host galaxy and into the IGrM. Finally, Col. (9)-(11) shows the observation date, exposure time and mean airmass during the observations. Name | Morph. | RA | DEC | AV | D | z | X–ray / Radio | Date | Exp. time | Mean |
---|---|---|---|---|---|---|---|---|---|---|---
|  | (J2000) | (J2000) | (mag) | (Mpc) |  | morphology | of observation | (s) | airmass |
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11) |
NGC 193 | E | 00h39m18.6s | +03d19m52s | 0.062 | 62.7 | 0.014723 | GRP (CC) / JET | 2016-06-08 | 3$\times$900 | 1.524 |
NGC 410 | E | 01h10m58.9s | +33d09m07s | 0.161 | 75.8 | 0.017659 | GRP (CC) / pnt | 2016-08-16 | 3$\times$900 | 1.937 |
NGC 584 | E | 01h31m20.7s | -06d52m05s | 0.116 | 25.9 | 0.006011 | …/ pnt | 2016-07-01 | 3$\times$900 | 1.243 |
NGC 677 | E | 01h49m14.0s | +13d03m19s | 0.243 | 71.9 | 0.017012 | GRP (CC) / diffuse | 2016-07-20 | 3$\times$900 | 1.344 |
NGC 777 | E | 02h00m14.9s | +31d25m46s | 0.128 | 71.4 | 0.016728 | GRP (NCC) / pnt | 2016-08-16 | 3$\times$900 | 1.823 |
NGC 924 | S0 | 02h26m46.8s | +20d29m51s | 0.467 | 63.1 | 0.014880 | …/ pnt | 2016-08-14 | 3$\times$900 | 1.651 |
NGC 940 | S0 | 02h29m27.5s | +31d38m27s | 0.246 | 72.6 | 0.017075 | pnt / pnt | 2016-08-21 | 3$\times$900 | 1.814 |
NGC 978 | E/S0 | 02h34m47.6s | +32d50m37s | 0.253 | 67.3 | 0.015794 | gal (NCC) / pnt | 2016-08-16 | 3$\times$900 | 1.866 |
NGC 1060 | E/S0 | 02h43m15.0s | +32d25m30s | 0.532 | 73.4 | 0.017312 | GRP (CC) / jet | 2016-08-22 | 3$\times$900 | 1.845 |
NGC 1453 | E | 03h46m27.2s | -03d58m08s | 0.289 | 52.9 | 0.012962 | GRP (NCC) / pnt | 2016-07-22 | 3$\times$900 | 1.358 |
NGC 1587 | E | 04h30m39.9s | +00d39m42s | 0.197 | 50.0 | 0.012322 | GRP (NCC) / diffuse | 2016-08-18 | 3$\times$900 | 1.360 |
NGC 4008 | E | 11h58m17.0s | +28d11m33s | 0.064 | 49.0 | 0.012075 | gal (NCC) / pnt | 2016-05-13 | 3$\times$900 | 1.668 |
NGC 4169 | S0 | 12h12m18.8s | +29d10m46s | 0.058 | 51.4 | 0.012622 | pnt / pnt | 2016-05-19 | 3$\times$900 | 1.728 |
NGC 4261 | E | 12h19m23.2s | +05d49m31s | 0.049 | 28.4 | 0.007378 | GRP (CC) / JET | 2016-04-17 | 3$\times$880 | 1.311 |
ESO0507-G025 | E/S0 | 12h51m31.8s | -26d27m07s | 0.245 | 41.1 | 0.010788 | …/ diffuse | 2016-04-12 | 3$\times$900 | 1.107 |
NGC 5846 | E | 15h06m29.3s | +01d36m20s | 0.153 | 23.1 | 0.005711 | GRP (CC) / jet | 2016-05-16 | 3$\times$880 | 1.579 |
NGC 6658 | S0 | 18h33m55.6s | +22d53m18s | 0.339 | 61.6 | 0.014243 | pnt / … | 2016-04-23 | 3$\times$870 | 1.557 |
NGC 7619 | E | 23h20m14.5s | +08d12m22s | 0.224 | 54.6 | 0.012549 | GRP (CC) / pnt | 2016-05-23 | 3$\times$870 | 1.444 |

###  2.2 Spectral fitting and emission-line measurement

We fitted and removed the stellar component within each spectrum (bin or
spaxel), in order to obtain pure emission line spectra using the spectral
synthesis code Fitting Analysis using Differential evolution Optimization
(FADO, Gomes & Papaderos, 2017) version V1.B. Simple stellar population (SSP)
templates from the Bruzual & Charlot (2003) libraries were used with
metallicities between Z=0.004 and Z=0.05 and stellar ages between 1 Myr and 15
Gyr, for a Chabrier initial mass function (IMF, Chabrier, 2003). FADO self-
consistently reproduces the nebular characteristics of a galaxy, including the
hydrogen Balmer-line luminosities, equivalent widths and nebular continuum. An
important advantage of FADO is that its convergence scheme employs genetic
differential evolution optimization. This results in improvements with respect
to the uniqueness of spectral fits and the overall efficiency of the
convergence schemes. Artificial intelligence is used to eliminate redundancies
in the SSP base libraries which increases the computational efficiency. The
fit was performed in the 4800-7000 $\AA$ spectral range assuming the Cardelli,
Clayton & Mathis (1989) extinction law. The data cubes were tessellated using
the Voronoi binning method (Cappellari & Copin, 2003) to achieve an adequate
signal to noise (S/N; we choose a S/N$\sim$30-50) per bin for the continuum
between 6000 to 6200 $\AA$. The Voronoi binning method lead to the dilution of
the emission lines within the bins, so the shape of extended and diffuse
emission lines was lost (see Erroz-Ferrer et al., 2019, for a similar
approach). Then, the best fit stellar continuum or synthetic Spectral Energy
Distribution (SED) was subtracted from the observed spaxels, with S/N $>$ 3 in
the continuum, to obtain pure emission-line data cubes. FADO provides as
results, among others, mass contributions of individual stellar populations,
mass- and luminosity-weighted stellar ages and metallicities, emission-line
fluxes, equivalent widths (EWs), FWHMs and estimates of their uncertainties.
In Figure 2 we show an example to illustrate the spectral modelling results
with FADO from a nuclear 3" aperture for the galaxy ESO0507-G025. In order to
test the accuracy of these results, we carried out a single-Gaussian fit to
each emission line. Our fit results were in good agreement with those from
FADO, consequently for the analysis in this study we create the following
emission line maps using FADO: H$\alpha$, H$\beta$, [O iii]$\lambda$5007, [N
ii]$\lambda$6584 and [S ii] $\lambda\lambda$6717, 6731. We note that, when
deriving the maps we only use spaxels with emission fluxes $>$3$\sigma$ above
the background. We did not detect significant broad asymmetric emission-line
profiles, so multiple-component decomposition of the emission lines was not
necessary, because each emission line is well described by a single Gaussian
profile. More details about the morphologies of these maps is given in Section
3.1.

Figure 1: Observed MUSE spectra for our sample of group-dominant galaxies
extracted from a nuclear 3" aperture and covering the entire MUSE wavelength
range.

Following a similar procedure to Parkash et al. (2019) we extracted for each
galaxy a spectrum from a circular region at the galaxy continuum centre with a
diameter of 3" in order to mimic the SDSS spectral fibre aperture. From here
on we refer to this circular region as the nuclear region and the spectra from
this region as the nuclear spectrum (see Figure 1). The nuclear spectra of the
galaxies are characterized by the presence of strong [N ii]$\lambda$6584
emission with [N ii]$\lambda$6584/H$\alpha>$ 1, which is consistent with low-
ionisation nuclear emission-line regions. Additionally, we extracted an
integrated spectrum for each galaxy within an aperture which encompasses the
extended emission in Figure 3 (see Section 3.1.1) with [N ii]$\lambda$6584
emission > 3$\sigma$ per pixel. In summary, we find 15/18 objects with
intensive nuclear ionised-gas (LINERs) with [N ii]$\lambda$6584 $>$ H$\alpha$.
Pagotto et al. (2021) found the strongest and most spatially extended
emission, in their sample of massive ETGs in densest clusters of galaxies,
came from the [N ii]$\lambda$6584 lines. We find the same result for our
sample. In Table 2 we present for each galaxy the 3" nuclear and integrated
observed F(H$\alpha$) flux, F(H$\alpha$)/F(H$\beta$) multiplied by a factor of
100, F([N ii]]$\lambda$6584) flux and EW(H$\alpha$), respectively. We note
that using the spectral synthesis code FADO (Gomes & Papaderos, 2017) and the
Bruzual & Charlot (2003) stellar spectral library, we resolved weak [O
iii]$\lambda$5007, H$\beta$, H$\alpha$ and [N ii]$\lambda$6584 emission in the
nuclear region of NGC 6658. However, Olivares et al. (2022) using pPXF
(Cappellari, 2017) and the Indo-U.S. Coudé Feed stellar library (Valdes et
al., 2004) find no emission lines in the spectrum of this galaxy using the
same MUSE dataset. The results for this galaxy are therefore uncertain
especially given that no clear emission line peaks are observed in the
integrated spectrum.

Figure 2: Example of the stellar continuum fit modelling with FADO for the galaxy ESO0507-G025 from the nuclear 3" aperture. The black and red upper lines correspond to the observed and modeled stellar spectrum, respectively. By subtracting the latter from the former we obtain the pure nebular emission line spectrum (lower black line). We have labelled the emission lines used in our analysis. Table 2: H$\alpha$+[N ii] morphology, observed emission line fluxes and EW(H$\alpha$) for our sample galaxies from a simulated 3" diameter and an integrated (int) aperture. The radial size of the integrated aperture is shown in parentheses to the right of F(H$\alpha$). | H$\alpha$+[N ii] | F(H$\alpha$) | (F(H$\alpha$)/F(H$\beta$))$\times$100 | F([N ii]$\lambda$6584) | EW(H$\alpha$)
---|---|---|---|---|---
| morphology | ($\times$10-14 erg cm-2 s-1) |  | ($\times$10-14 erg cm-2 s-1) | ($\AA$)
|  | 3" | int | 3" | int | 3" | int | 3" | int
NGC 193 | i | 0.43$\pm$0.02 | 1.64$\pm$0.11 (6.0$"$) | 245.16$\pm$0.31 | 134.39$\pm$0.25 | 0.59$\pm$0.06 | 1.70$\pm$0.22 | 2.71 | 1.65
NGC 410 | i0 | 0.52$\pm$0.01 | 0.83$\pm$0.02 (4.5$"$) | 146.81$\pm$0.11 | 112.64$\pm$0.14 | 0.52$\pm$0.04 | 0.87$\pm$0.08 | 1.63 | 0.69
NGC 584 | i | 0.66$\pm$0.07 | 5.83$\pm$0.67 (21.0$"$) | 205.26$\pm$0.69 | 131.12$\pm$0.88 | 0.90$\pm$0.08 | 4.1$\pm$0.7 | 0.77 | 0.53
NGC 677 | i | 0.63$\pm$0.03 | 4.51$\pm$0.45 (7.5$"$) | 322.66$\pm$0.36 | 313.84$\pm$0.60 | 0.62$\pm$0.07 | 3.87$\pm$0.60 | 3.43 | 3.13
NGC 777 | i0 | 0.10$\pm$0.01 | … | 25.55$\pm$0.09 | … | 0.19$\pm$0.01 | … | 0.28 | …
NGC 924 | ii | 0.83$\pm$0.06 | 3.83$\pm$0.30 (24$"$) | 257.83$\pm$0.66 | 246.32$\pm$1.24 | 0.96$\pm$0.06 | 3.48$\pm$0.25 | 2.24 | 1.60
NGC 940 | ii | 0.52$\pm$0.04 | 10.01$\pm$0.24 (13.5$"$) | 436.02$\pm$1.12 | 287.04$\pm$0.31 | 1.14$\pm$0.10 | 6.49$\pm$0.77 | 2.22 | 5.06
NGC 978 | i0 | 0.33$\pm$0.02 | 0.81$\pm$0.08 (4.5$"$) | 148.83$\pm$0.19 | 104.83$\pm$0.21 | 0.28$\pm$0.02 | 0.70$\pm$0.08 | 1.23 | 0.81
NGC 1060 | i | 0.37$\pm$0.05 | 1.37$\pm$0.23 (6.0$"$) | 202.02$\pm$1.96 | 129.84$\pm$0.73 | 0.46$\pm$0.04 | 1.18$\pm$0.19 | 0.86 | 0.63
NGC 1453 | i | 1.35$\pm$0.06 | 8.14$\pm$0.79 (13.5$"$) | 188.92$\pm$0.21 | 91.13$\pm$0.19 | 2.07$\pm$0.08 | 9.43$\pm$0.82 | 2.60 | 2.00
NGC 1587 | i | 0.63$\pm$0.04 | 5.14$\pm$0.18 (15$"$) | 227.91$\pm$0.40 | 161.05$\pm$0.33 | 0.75$\pm$0.04 | 4.13$\pm$0.22 | 1.50 | 1.41
NGC 4008 | i0 | 0.16$\pm$0.02 | … | … | … | 0.14$\pm$0.02 | … | 0.46 | …
NGC 4169 | ii | 0.99$\pm$0.05 | 4.67$\pm$0.28 (10.5$"$) | 325.06$\pm$0.57 | 235.69$\pm$0.76 | 1.71$\pm$0.06 | 6.63$\pm$0.36 | 2.24 | 1.35
NGC 4261 | i0 | 4.31$\pm$0.58 | … | 450.88$\pm$0.78 | … | 9.44$\pm$0.64 | … | 2.61 | …
ESO0507-G025 | ii | 2.59$\pm$0.14 | 20.37$\pm$1.34 (27.0$"$) | 363.09$\pm$0.32 | 291.93$\pm$0.29 | 2.9$\pm$0.18 | 13.68$\pm$1.95 | 5.93 | 3.65
NGC 5846 | i | 0.74$\pm$0.07 | 11.33$\pm$1.27(18$"$) | 189.27$\pm$0.35 | 139.11$\pm$0.30 | 0.98$\pm$0.09 | 11.46$\pm$1.44 | 2.10 | 1.49
NGC 6658 | i0 | 0.15$\pm$0.02 | … | 242.79$\pm$1.89 | … | 0.23$\pm$0.02 | … | 0.47 | …
NGC 7619 | i0 | 0.19$\pm$0.05 | … | 36.63$\pm$0.13 | … | 0.47$\pm$0.04 | … | 0.31 | …

Finally, we explored the data cubes of our target BGGs (see appendix A) in
order to find additional emission line galaxies in the FoV. We found two
additional objects in the FoV of NGC 677 and one in the FoVs of NGC 777, NGC
924 and NGC 1453. These detections are not physically associated (0.1
$\lesssim$ z $\lesssim$ 0.5) with the main galaxies. In Figure 14 we show the
position of these objects in the FoV of the galaxies, while in Table 8 we
summarize their main properties.

##  3 Results

###  3.1 Ionised gas structure: H$\alpha$ and [N ii]$\lambda$6584 emission
line maps and extinction correction

####  3.1.1 Emission line morphology

To assist understanding of the physical conditions and detection of the
extended and diffuse line emitting gas we created continuum-subtracted
H$\alpha$+[N ii]$\lambda\lambda$6548,6584 (H$\alpha$+[N ii]) emission line
images by subtracting the continuum SED models, as explained in Section 2.2,
and then summing the flux in the data cubes between 6528 $\rm\AA$ and 6604
$\rm\AA$. In Figures 3 and 15 (Appendix B) we show the maps resulting from
this procedure for each galaxy. We also show, in these figures, the continuum-
subtracted MUSE spectra from the nuclear 3" aperture covering the wavelength
range from 6400 $\AA$ to 6800 $\AA$ as an inset panel within each image. The
DIG and extended filamentary line emitting H$\alpha$+[N ii] gas reaches values
of $\sim$10-19 (e.g., NGC 4008) to $\sim$10-17 (e.g., NGC 5846) erg cm-2 s-1
per pixel.

We used the continuum-subtracted H$\alpha$+[N ii] images to classify the
galaxies in our sample. To identify the morphology of the line emitting
regions and determine how much these lines correlate with the overall
morphology and properties of our sample galaxies, we divided the galaxies into
three morphological groups or types: type i0 \- strong or diffuse nuclear
emission (i.e. within a $\sim$1.5" radius from the galaxy centre) with (or
without) unextended ($\lesssim$1 kpc) filamentary structures connected to the
nuclear region, type i \- strong or diffuse nuclear emission with extended
(several kpc) filamentary structures beyond the nuclear region and type ii \-
i0 or i plus extranuclear H ii regions (well-defined or in distorted ring-like
structures). Using this simple classification scheme we find that 7/18 (NGC
410, NGC 777, NGC 978, NGC 4008, NGC 4261, NGC 6658 and NGC 7619) are type i0,
another 7/18 are type i (NGC 193, NGC 584, NGC 677, NGC 1060, NGC 1453, NGC
1587 and NGC 5846) and 4/18 galaxies are type ii (NGC 924, NGC 940, NGC 4169
and ESO0507-G025).

Figure 3: Example of H$\alpha$+[N ii]$\lambda\lambda$6548,6584 emission line
maps from our sample. We smoothed the emission line maps using a 3$\times$3
box filter. MUSE continuum-subtracted spectra from a nuclear 3" aperture
covering the wavelength range from 6400 $\AA$ to 6800 $\AA$ are shown in the
inset panels. The vertical lines in the inset panels indicate the wavelengths
of the [N ii]$\lambda$6548, H$\alpha$, [N ii]$\lambda$6584, [S
ii]$\lambda$6717 and [S ii]$\lambda$6731 emission lines. Fluxes in units of
$\times$10-15 (erg s-1 cm-2 $\AA^{-1}$). In appendix B we show the emission
line maps of all galaxies in our sample. The companion galaxy of NGC 978 is
indicated in the figure. North is to the top and East to the left.

####  3.1.2 Intrinsic reddening

We computed the extinction corrections with the colour excess E(B-V) from the
gas using the Balmer decrement H$\alpha$/H$\beta$=2.86 assuming case B
recombination (Osterbrock & Ferland, 2006, ne=100 cm-3 and Te=104 K). The
E(B-V)gas is calculated as follows

$E\left(B-V\right)_{gas}$=$\frac{2.5}{\kappa\left(H\beta\right)-\kappa\left(H\alpha\right)}\log\left(\frac{\left(H\alpha\diagup
H\beta\right)_{o}}{2.86}\right),$ (1)

where $\kappa$(H$\alpha$) and $\kappa$(H$\beta$) are the extinction values
from the Cardelli, Clayton & Mathis (1989) curve with RV = 3.1.
(H$\alpha$/H$\beta$)o is the observed ratio between H$\alpha$ and H$\beta$.
Therefore, we corrected the emission lines for extinction using Iλ=Fλ,o
10${}^{0.4E\left(B-V\right)_{gas}\kappa\left(\lambda\right)}$. The reddening
parameters were set to 0.0 for unrealistic values of (H$\alpha$/H$\beta$)o <
2.86 and when H$\beta$ was not detected. The assumption of
(H$\alpha$/H$\beta$)o = 2.86 instead of 3.1 in spectra with AGN(-like)
features does not significantly affect the results. Galaxies with
(H$\alpha$/H$\beta$)o > 2.86 have low-reddening E(B-V)gas values from 0.13 to
0.46 mag. This result is not surprising as low-balmer decrements are also
found in other ETGs (e.g., Annibali et al., 2010) and in the nuclei and
nebular filaments of cool-core clusters (McDonald, Veilleux & Rupke, 2012). On
the other hand, $\sim$72% (13/18) and $\sim$83% (15/18) of our galaxies have
unrealistic nuclear and integrated (H$\alpha$/H$\beta$)o values, respectively.
This indicates that the derived H$\beta$ fluxes are larger for their
corresponding H$\alpha$ fluxes. For galaxies with (H$\alpha$/H$\beta$)o > 2.86
the E(B-V)nebular are similar or slightly lower than the E(B-V)stellar, while
for galaxies with (H$\alpha$/H$\beta$)o < 2.86 the E(B-V)stellar is higher.
This might produce an under subtraction of the observed stellar populations.
It is worth noting that unrealistic extinctions are also obtained with other
SSP codes (e.g., Annibali et al., 2010; Herpich et al., 2018) in ETGs.
However, uncertainties in the reddening correction produced by the stellar
fitting will not significantly affect the emission line ratios nor the
properties obtained from them in our case. Therefore, large errors in the
H$\alpha$/H$\beta$ are due to the uncertainty in the H$\beta$ emission.

###  3.2 [O iii]$\lambda$5007/H$\beta$, [S
ii]$\lambda\lambda$6717,6731/H$\alpha$ and [N ii]$\lambda$6584/H$\alpha$
emission line ratio maps and BPT diagrams

Using the information derived in the previous sections it is possible to
distinguish between different ionisation mechanisms using BPT (Baldwin,
Phillips & Terlevich, 1981) diagrams. To do this, we used the following
emission line ratios: [O iii] $\lambda$5007/H$\beta$ ([O iii]/H$\beta$) and [N
ii] $\lambda$6584/H$\alpha$ ([N ii]/H$\alpha$). The [O iii]
$\lambda$5007/H$\beta$ emission line ratio is an excitation indicator and
provides information about the available fraction of hard ionising photons. On
the other hand, [N ii] $\lambda$6584 is a low-ionisation emission line tracer,
which is usually weak in pure H ii regions. Therefore, the emission line
ratios [N ii]/H$\alpha$ and [O iii]/H$\beta$ can effectively discriminate
between photoionisation under physical conditions that are typical for H ii
regions and other excitation mechanisms (e.g., AGN or shocks) which are likely
present in our sample. In Figure 4 we show an example of the emission line
ratio maps log([N ii]/H$\alpha$) and log([O iii]/H$\beta$, the log([N
ii]/H$\alpha$) vs log([O iii]/H$\beta$ BPT diagram and the 2D BPT map for the
galaxy ESO0507-G025. In this figure, we separate star-forming and AGN-ionised
regions (i.e., Seyferts and LINERs) with blue demarcation lines from models by
Kauffmann et al. (2003) (dotted line), Kewley et al. (2001) (solid line) and
Schawinski et al. (2007) (dashed line). We colour coded the data points
located in the different areas of the BPT diagram, i.e, AGN/LINERs in red,
composite in orange and the star-forming H ii dominated area in green. In
Appendix C we show the emission line ratio maps and the BPT diagrams/maps for
our entire sample. The BPT maps show that most of our data points/spaxels in
the nuclear regions (see Figure 4) are dominated by AGN/LINERs while the SF
becomes more important in the extended regions. We note that, in most cases,
we obtain a very small or nonexistent number of data points ($\sim$17% of the
spaxels in ESO0507-G025) in the H ii area of the BPT diagrams (see figures in
Appendix C). Most spaxels lie in the AGN/LINERs region of the diagrams
($\sim$71% of the spaxels on average), while only in two cases (NGC 978 and
ESO0507-G025) are most spaxels in the composite region with 62% and 55% of the
spaxels, respectively.

Figure 4: Example of our emission line ratio maps log([N ii]/H$\alpha$) and
log([O iii]/H$\beta$ (top row), and BPT diagrams (bottom row) for the galaxy
ESO0507-G025. In the BPT diagrams we include the Kewley et al. (2001; solid
line), Kauffmann et al. (2003; dotted line) and Schawinski et al. (2007;
dashed line) model boundary lines which divide regions dominated by star-
forming H ii (green), composite (orange data points) and AGN/LINERs (red data
points). The filled black data point corresponds to value in the 3" aperture.
In Appendix C we show the emission line ratio maps and the BPT diagrams/maps
for our entire sample. North is to the top and East to the left.

In Table 3 we present for each galaxy the emission line ratios log([O
iii]/H$\beta$), log([N ii]/H$\alpha$) and log([S ii]/H$\alpha$) from the 3"
and integrated apertures. Figure 5 shows the BPT diagram [O iii]/H$\beta$
versus [N ii]/H$\alpha$ of the nuclear regions (small squares) for galaxies
where [O iii] $\lambda$5007 and H$\beta$ are detected. In the figure, we
compare these with values obtained from the integrated apertures (circles). We
use the same colour code to identify the galaxies in this diagram throughout
the paper. In most cases, the nuclear values lie in the LINER region of the
BPT diagram, while the integrated ones are in the composite region. The log([O
iii]/H$\beta$) for most galaxies decreases significantly when we consider the
larger integrated aperture, which changes their positions in the BPT diagram.
In the case of ESO0507-G025, NGC 924 and NGC 5846 the log([O iii]/H$\beta$)
increases and the log([N ii]/H$\alpha$) ratio decreases. Figure 5 clearly
shows that the ionisation structure of our sample galaxies can not be
correctly assessed from small fixed apertures (e.g., SDSS aperture) and the
ionisation sources and structure of the warm gas may be different in different
parts of the galaxies (see Appendix C). In Section 4.1 and 4.2 we explore the
effect of using different apertures on the observed properties of the ISM and
the ionisation mechanisms likely present in our sample galaxies.

Figure 5: [O iii]/H$\beta$ vs [N ii]/H$\alpha$ diagnostic diagram for the nuclear spectra using equivalent 3" apertures to SDSS spectra (squares) and integrated emission (circles). The Kewley et al. (2001; solid line), Kauffmann et al. (2003; dotted line) and Schawinski et al. (2007; dashed line) model boundary lines in blue discriminate between areas of the diagram dominated by SF (H ii) and AGN/LINERs emission. The mean errors are shown in the lower of the figure. Table 3: Main properties of our sample galaxies from the simulated SDSS spectral fibre diameter (3") and integrated (int) apertures. SFR(H$\alpha$) calculated from the L(H$\alpha$) in the largest aperture int/3". | log ([O iii]/H$\beta$) | log ([N ii]/H$\alpha$) | log ([S ii]/H$\alpha$) | L(H$\alpha$) | SFR(H$\alpha$)
---|---|---|---|---|---
|  |  |  |  |  |  | ($\times$1039 erg cm-1) | (M⊙ yr-1)
| 3" | int | 3" | int | 3" | int | 3" | int |
NGC 193 | -0.27$\pm$0.22 | -0.49$\pm$0.40 | 0.14$\pm$0.16 | 0.02$\pm$0.20 | -0.02$\pm$0.15 | -0.14$\pm$0.22 | 2.01$\pm$0.12 | 7.70$\pm$0.52 | 0.0354$\pm$0.0024
NGC 410 | … | … | 0.01$\pm$0.11 | 0.02$\pm$0.12 | -0.33$\pm$0.15 | -0.13$\pm$0.17 | 3.55$\pm$0.08 | 5.72$\pm$0.13 | 0.0263$\pm$0.0006
NGC 584 | -0.04$\pm$0.40 | -0.17$\pm$1.03 | 0.13$\pm$0.19 | -0.15$\pm$0.29 | -0.16$\pm$0.34 | -0.44$\pm$0.75 | 0.53$\pm$0.06 | 4.68$\pm$0.54 | 0.0215$\pm$0.0025
NGC 677 | -0.06$\pm$0.22 | -0.15$\pm$0.30 | -0.01$\pm$0.16 | -0.07$\pm$0.23 | -0.03$\pm$0.08 | -0.09$\pm$0.15 | 5.22$\pm$0.24 | 34.71$\pm$3.50 | 0.1597$\pm$0.0161
NGC 777 | … | … | 0.28$\pm$0.22 | … | 0.30$\pm$0.28 | … | 0.61$\pm$0.09 | … | 0.0028$\pm$0.0004
NGC 924 | 0.05$\pm$0.37 | 0.14$\pm$0.60 | 0.06$\pm$0.14 | -0.04$\pm$0.15 | -0.10$\pm$0.20 | -0.12$\pm$0.27 | 3.97$\pm$0.28 | 18.24$\pm$1.41 | 0.0839$\pm$0.0065
NGC 940 | 0.19$\pm$0.43 | -0.55$\pm$0.36 | 0.34$\pm$0.17 | -0.19$\pm$0.14 | 0.02$\pm$0.25 | -0.45$\pm$0.20 | 8.84$\pm$0.75 | 63.67$\pm$1.50 | 0.2929$\pm$0.0069
NGC 978 | -0.09$\pm$0.17 | -0.15$\pm$0.22 | -0.08$\pm$0.14 | -0.07$\pm$0.22 | -0.18$\pm$0.21 | -0.11$\pm$0.42 | 1.81$\pm$0.21 | 4.40$\pm$0.49 | 0.0203$\pm$0.0023
NGC 1060 | … | … | 0.10$\pm$0.23 | -0.06$\pm$0.33 | 0.09$\pm$0.44 | 0.18$\pm$0.57 | 2.38$\pm$0.33 | 8.80$\pm$1.51 | 0.0405$\pm$0.0069
NGC 1453 | -0.27$\pm$0.26 | -0.52$\pm$0.37 | 0.19$\pm$0.08 | 0.06$\pm$0.18 | 0.09$\pm$0.08 | -0.12$\pm$0.17 | 4.52$\pm$0.19 | 27.23$\pm$2.64 | 0.1253$\pm$0.0121
NGC 1587 | 0.27$\pm$0.16 | -0.05$\pm$0.34 | 0.07$\pm$0.11 | -0.09$\pm$0.09 | 0.03$\pm$0.16 | -0.11$\pm$0.23 | 1.88$\pm$0.11 | 15.37$\pm$0.52 | 0.0707$\pm$0.0024
NGC 4008 | … | … | -0.07$\pm$0.25 | … | 0.09$\pm$0.41 | … | 0.47$\pm$0.06 | … | 0.0022$\pm$0.0003
NGC 4169 | -0.05$\pm$0.32 | -0.16$\pm$0.51 | 0.24$\pm$0.08 | 0.15$\pm$0.12 | 0.10$\pm$0.16 | 0.10$\pm$0.15 | 4.24$\pm$0.19 | 14.75$\pm$0.90 | 0.0679$\pm$0.0041
NGC 4261 | 0.25$\pm$0.48 | … | 0.34$\pm$0.20 | … | -0.07$\pm$0.29 | … | 12.08$\pm$1.62 | … | 0.0556$\pm$0.0074
ESO0507-G025 | -0.08$\pm$0.35 | 0.02$\pm$0.34 | 0.05$\pm$0.12 | -0.17$\pm$0.21 | 0.07$\pm$0.15 | -0.16$\pm$0.26 | 9.17$\pm$0.52 | 42.56$\pm$2.96 | 0.1958$\pm$0.0136
NGC 5846 | -0.40$\pm$0.19 | -0.24$\pm$0.41 | 0.12$\pm$0.18 | 0.01$\pm$0.24 | -0.12$\pm$0.21 | -0.26$\pm$0.43 | 0.47$\pm$0.04 | 7.23$\pm$0.81 | 0.0333$\pm$0.0037
NGC 6658 | 0.54$\pm$0.79 | … | 0.19$\pm$0.19 | … | … | … | 0.65$\pm$0.08 | … | 0.0030$\pm$0.0004
NGC 7619 | … | … | 0.40$\pm$0.36 | … | … | … | 0.66$\pm$0.17 | … | 0.0030$\pm$0.0008

In summary, we find that all galaxies in our sample, with [O iii]
$\lambda$5007 and H$\beta$ detections, have a dominant AGN/LINER nuclear
region. Extended LINER-like regions are observed in most galaxies with
filamentary structures (type i galaxies) and ring-like structures and/or
extranuclear H ii regions (type ii galaxies). Most spaxels in these regions
fall in the BPT area of mixed contribution (composite) from SF and/or AGN.

###  3.3 Velocity field maps

We obtained the radial velocity V([N ii]) and velocity dispersion $\sigma$([N
ii]) maps by fitting a single Gaussian to the [N ii]$\lambda$6584 emission
line profiles. The velocity dispersion $\sigma$ (or FWHM=2.35$\sigma$) was
obtained as $\sigma^{2}$ = $\sigma^{2}_{obs}$ \- $\sigma^{2}_{inst}$ where
$\sigma_{inst}$ is the instrumental dispersion, see Section 2.1. The detailed
kinematical analysis of the emitting gas is beyond the scope of this paper,
however in Figure 6 and Appendix D we show the velocity field maps for the
type ii galaxies ESO0507-G025 and NGC 924, NGC 940 and NGC 4169, respectively.
Radial velocity and velocity dispersion maps for our full sample are shown in
Olivares et al. (2022). In order to include the kinematical information from
the extended and filamentary structures we considered spaxels with S/N
$\gtrsim$ 2\. In Figure 6 we see an example of these velocity fields for the
galaxy ESO0507-G025. Clearly, this galaxy has two independent velocity
structures or rotating discs, while the velocity dispersion in the nuclear
region shows a bicone-shaped structure which increases in velocity dispersion
with distance from the centre. The gas in these bicone structures is located
in the AGN/LINERs region of the BPT diagram (see Figures 4 and 5) suggesting
an outflow perpendicular to the observer.

Figure 6: Radial velocity V([N ii]) (left panel) and velocity dispersion
$\sigma$([N ii]) (right panel) maps for the galaxy ESO0507-G025. The position
of the continuum maximum is indicated by a X symbol. A zoom-in of the central
region of the velocity dispersion map is shown in the inset panel. North is up
and east is to the left.

Our velocity fields (V and $\sigma$) are in agreement with those obtained
independently by Olivares et al. (2022). In general, the radial velocity
fields in our sample galaxies (see Appendix D and Olivares et al. 2022) show
rotation or gradients but with relatively small velocity ranges of around
$\pm$200-300 km s-1. These variations are similar in both, the compact and
extended gas emission regions. We see the highest $\sigma$([N ii]) values, in
most cases, at positions close to the continuum flux maximum with values of
$\sim$150 to $\sim$300 km s-1, while extended regions or filaments show lower
values. In general $\sigma$([N ii]) values in the filaments vary little across
the galaxies. This trend was also reported by McDonald, Veilleux & Rupke
(2012) for a sample of galaxies in galaxy clusters. They found that the most
extended optical filaments in their sample, likely originated from ICM cooling
and were experiencing only minor turbulence.

###  3.4 [S ii]$\lambda$6716 / [S ii]$\lambda$6731 ratio

The [S ii]$\lambda$6716 / [S ii]$\lambda$6731 intensity ratio was used to
determine the electron density ne([S ii]). We computed the values of ne(S ii)
using the IRAF STS temden package assuming te([O iii])=10000 K. We set
unrealistic (saturated) values of [S ii]$\lambda$6716 / [S ii]$\lambda$6731 >
1.43 to ne([S ii]) $\sim$1 cm-3 and for [S ii]$\lambda$6716 / [S
ii]$\lambda$6731 < 0.46 to $\sim$52452 cm-3. In Table 4 we show the electron
density for the nuclear (3" aperture) regions for our sample. We note that the
[S ii]$\lambda$6716 / [S ii]$\lambda$6731 ratio is greater than 1 for 12/18
galaxies, which indicates a predominantly low density regime ($\sim$100 cm-3).
Only 6/18 galaxies show values $\gtrsim$1000 cm-3 in the nuclear regions. In
Figure 32 (appendix E) we show the [S ii]$\lambda$6716 / [S ii]$\lambda$6731
ratio maps for our sample galaxies.

Table 4: Electron density for the nuclear (3" aperture) regions of our sample of group-dominant galaxies. We show the unrealistic (saturated) values of [S ii]$\lambda$6716 / [S ii]$\lambda$6731 by $\checkmark$ symbols. Name | [S ii]$\lambda$6716 / [S ii]$\lambda$6731 | Saturated | ne([S ii])
---|---|---|---
|  | values | (cm-3)
NGC 193 | 0.92$\pm$0.16 |  | 839.3$\pm$397.4
NGC 410 | 1.85$\pm$0.23 | $\checkmark$ |  $\lesssim$1.0
NGC 584 | 1.46$\pm$0.70 | $\checkmark$ |  $\lesssim$1.0
NGC 677 | 1.29$\pm$0.09 |  | 131.3$\pm$98.0
NGC 777 | 1.11$\pm$0.30 |  | 377.5$\pm$653.8
NGC 924 | 1.38$\pm$0.36 |  | 43.7$\pm$257.3
NGC 940 | 1.06$\pm$0.34 |  | 471.4$\pm$992.9
NGC 978 | 1.23$\pm$0.37 |  | 200.8$\pm$436.0
NGC 1060 | 0.66$\pm$0.37 |  | 2788.0$\pm$1126.3
NGC 1453 | 1.29$\pm$0.10 |  | 131.3$\pm$109.2
NGC 1587 | 1.21$\pm$0.24 |  | 226.4$\pm$229.5
NGC 4008 | 0.69$\pm$0.39 |  | 2352.9$\pm$960.4
NGC 4169 | 1.07$\pm$0.25 |  | 451.4$\pm$584.7
NGC 4261 | 1.12$\pm$0.36 |  | 360.4$\pm$649.3
ESO0507-G025 | 0.46$\pm$0.09 |  | 48893.1$\pm$21200.4
NGC 5846 | 1.06$\pm$0.24 |  | 471.4$\pm$574.6
NGC 6658 | 0.29$\pm$0.12 | $\checkmark$ |  $\gtrsim$52452.0
NGC 7619 | 0.74$\pm$0.11 |  | 1823.7$\pm$1122.5

Interestingly, the ionisation cones observed in ESO0507-G025 (see the emission
line ratio maps [O iii]/H$\beta$ and [N ii]/H$\alpha$ in Figure 4 and velocity
fields in Figure 6) together with the high electron density and the shape of
the spatially resolved [S ii]$\lambda$6716 / [S ii]$\lambda$6731 ratio map
indicate that the central region of this galaxy is almost certainly ionised by
an AGN (e.g., Kakkad et al., 2018). This clear pattern is not observed in our
other galaxies. In general, the presence of high electron density in the
nuclear regions and extended enhanced regions with densities $\gtrsim$1000
cm-3 could indicate outflowing ISM likely driven by expanding hot gas heated
by SNe, SF stellar winds, AGN activity and/or the collision/interaction of
galaxies (Westmoquette et al., 2012). We will discuss in detail in Section 4
the results obtained in this section and their implications for the
evolutionary stages of the gas in our sample galaxies.

###  3.5 Star-formation rate

Following a common practice, the current star-formation rates (SFRs) were
estimated from the integrated H$\alpha$ luminosity L(H$\alpha$), assuming the
Kennicutt (1998) formula, for a solar metallicity, after correction for a
Chabrier (2003) initial mass function, i.e., SFR(H$\alpha$) (M⊙ yr-1) = 4.6
$\times$ 10-42 $\times$ L(H$\alpha$) (erg s-1) (Parkash et al., 2019).
However, it is unlikely in our case that all H$\alpha$ emission results from
SF, since the BPT diagnostics indicate a LINER or composite nature for most
spaxels in our emission line maps. Therefore, the derived SFRs are likely to
be an overestimates and are included here for comparison with other studies.
Table 3 shows the resultant SFRs, obtained from the integrated apertures of
the galaxies.

We note that, our average SFRs is $\sim$16% lower than that found by
Kolokythas et al. (2022) using GALEX FUV fluxes as SF indicator, while FIR
SFRs (O’Sullivan et al., 2015) for some of our galaxies (NGC 777, NGC 940, NGC
1060 and NGC 5846) are one or two orders of magnitude higher than our
SFR(H$\alpha$). Contamination of the FIR luminosity as a SF tracer can not be
discarded. Most galaxies in both samples are AGN-dominated at FIR wavelengths
and their derived FIR SFRs are greater than that expected from SF. However, in
the case of NGC 940, they do not confirm the presence of a radio AGN
(O’Sullivan et al., 2015). In summary, we find that in this and the
aforementioned studies, the SFR in type i and type ii group-dominant galaxies,
on average, are higher than in type i0 galaxies.

##  4 Discussion

###  4.1 Aperture effects

As shown in Section 3.2, aperture selection and 2D mapping can impact
conclusions about the dominant mechanism ionising the gas in our sample. This
was demostrated in Figure 5 where we show the [O iii]/H$\beta$ vs. [N
ii]/H$\alpha$ diagnostic diagram for the nuclear and integrated apertures. The
displacement of the data points in this diagram when the aperture is
increased, indicates how our interpretation about the ionising sources can
change depending on aperture size. The limitations of using SDSS spectra for
nearby galaxies has also been highlighted recently using IFU spectroscopy (see
Gomes et al., 2016, and references therein). In our case, we can see that in
the inner most part of the galaxies, the gas is likely dominated by shocks
and/or AGN activity given that the nuclear gas emission line ratios lie in the
LINER region of the BPT diagram, however, at large apertures photoionisation
by OB stars likely becomes increasingly more important.

In Figure 7 we show the effects of using different apertures on the observed
properties of our sample. In panel a) we show the L(H$\alpha$) surface density
$\Sigma$(H$\alpha$) calculated as L(H$\alpha$)/area, in b) the EW(H$\alpha$)
and panel c) shows the log([N ii]/H$\alpha$) emission line ratio. In this
figure, the apertures were selected to increase in steps of 1.5" of radius.
The $\Sigma$, in panel a) of Figure 7, in all cases decreases with radius and
shows the presence of extended H$\alpha$ emission beyond the SDSS aperture.
Cid Fernandes et al. (2011) used the EW(H$\alpha$) as an alternative method
(explained below in Section 4.2) to the BPT diagrams (emission-line
classification). Using this method, Gomes et al. (2016) argue that evolved
pAGB stellar background is sufficient to photoionise the diffuse gas in ETGs
and explain the observed EW(H$\alpha$) in the range 0.5 - 2.4 $\AA\rm$. Most
EW(H$\alpha$) values in panel b) irrespective of aperture lie within the area
(grey region in the panel) which is consistent with pure pAGB photoionisation
(Cid Fernandes et al., 2011; Gomes et al., 2016). We find that one type ii
galaxy (ESO0507-G025) and three type i galaxies (NGC 193, NGC 677 and NGC
1453) show EW(H$\alpha$) $>$2.4 $\AA$ at their centres. These results indicate
that pAGBs alone do not explain the observed values in these regions. The
EW(H$\alpha$) values of remaining galaxies are consistent with pure pAGB
emission. The [N ii]/H$\alpha$ found in the nuclear regions (SDSS aperture;
see panel c) and at large radii, in most cases, is consistent with LINER
emission since log([N ii]/H$\alpha$) $\gtrsim$ 0.0. We observe that the log([N
ii]/H$\alpha$) declines with increasing apertures in almost all cases,
although the radial profiles differ significantly.

Figure 7: Observed gas properties as a function of 1.5" aperture bins. a)
Luminosity surface density $\Sigma$(H$\alpha$). b) EW(H$\alpha$), the grey
region between 0.5–2.4 $\AA$ is consistent with pure pAGB photoionisation. For
points with EW(H$\alpha$) $>$2.4 $\AA$ an additional gas excitation source is
needed to account for the observed EW(H$\alpha$) (Gomes et al., 2016). c)
log([N ii]/H$\alpha$) emission line ratio. The vertical dotted line indicates
the SDSS radial spectra fibre aperture.

###  4.2 Ionisation mechanisms

In this section we examine the mechanisms responsible for the ionisation which
produces the optical emission lines in our sample. For this, we use
photoionisation models from the literature, which incorporate the expected
physical conditions in our sample galaxies.

In Figure 8 we show the BPT diagram for the nuclear emission for our sample
galaxies. The fill colour of the data points indicates the EW(H$\alpha$) from
the colour scale and the edge colour of the circles identifies the galaxies
from the legend. Superimposed on the plot are shock models from Allen et al.
(2008) calculated with the MAPPINGS III shock and photoionisation code. These
shock models have solar metallicity, densities from n = 0.01 cm-3 to 1000
cm-3, velocities from v = 100 kms-1 to 1000 kms-1, and magnetic field B = 1
$\mu$G. The models for v = 100 kms-1 (purple), 200 kms-1 (red), 300 kms-1
(green), 500 kms-1 (orange) and 1000 kms-1 (blue) are shown as coloured lines.
The nuclear regions in our sample lie in the velocity range of > 200 kms-1 to
$\gtrsim$1000 kms-1 and density between $\sim$0.1 cm-1 to < 100 cm-1. Annibali
et al. (2010) compared the [S ii] densities obtained for a sample of
ETGs/LINERs with ionised gas with the Allen et al. (2008) models. The majority
of their galaxies had n${}_{e}\gtrsim$ 100 cm-3 for an assumed T = 10,000 K.
But as in our case (see Table 4), a significant fraction of galaxies have
saturated [S ii] ratios around 1.45 which are consistent with pre-shock
densities (n${}_{e}\approx$ 0.01 to $\lesssim$100 cm-3). These pre-shock
regions are observed in some regions of Figure 32. This could indicate
together with the increasing [S ii]$\lambda$6716 / [S ii]$\lambda$6731 ratios
that AGN/SF-driven shocks are ionising the gas mainly in the nuclear regions.
This would be consistent with the Molina et al. (2018) conclusion that the
line emission of LINERs with low-luminosity AGN are predominantly powered by
shocked gas due to jets or outflows (see also Edwards et al., 2007).

Figure 8: BPT [O iii]/H$\beta$ vs [N ii]/H$\alpha$ diagnostic diagram, similar
to Figure 4, for the nuclear spectra (circles). Superimposed the shock models
of Allen et al. (2008) at solar metallicity (more details in the text) and the
Kewley et al. (2001; solid line), Kauffmann et al. (2003; dotted line) and
Schawinski et al. (2007; dashed line) boundaries as in Figure 5. The fill
colour of the circles indicates the EW(H$\alpha$) and the edge colours
identifies the galaxies from the legend.

In Figure 9 (upper panels) we show the BPT diagrams, as in Figure 8, but
including two different photoionisation model grids from Krabbe et al. (2021).
Their models consider the main (LINER) properties observed in our sample of
group-dominant galaxies for two distinct SEDs. One model assumes pAGB stars
(upper left panel) with different Teff as the ionising source and the other
considers AGN SED with a multicomponent continuum (upper right panel). The
models were created using the CLOUDY code version 17.0 (Ferland, 2017) for
metallicities Z/Z⊙ = 0.2, 0.5, 0.75, 1.0, 2.0 and 3.0 and ionisation
parameters U222The ratio of the ionizing photon density to the particle
density. in the range of -4.0 $\leq$ log(U) $\leq$ -0.5 in steps of 0.5 dex.
Their assumed density of ne 500 cm-3 is typical for BCGs (e.g., Ciocan et al.,
2021) and it is compatible with our values from Section 3.4. In both cases,
most galaxies in our sample lie in the region between Z/Z⊙ = 0.75 (12 +
log(O/H)= 8.56) and Z/Z⊙= 1 (12 + log(O/H) = 8.69). However, NGG 4261 and NGC
940 have values compatible with models at 2-3 Z⊙ (12 + log(O/H) = 8.99 -
9.17), assuming AGN activity. While for pAGB models, the values for NGG 4261,
NGC 940, NGC 4169 and NGC 6658 are compatible with models at > 1.0 Z⊙
metallicities. These results indicate that the nuclear regions in our sample
are consistent, within the uncertainties, with metallicities slightly (above)
solar, independent of the ionising sources. In Section 4.3 we estimate the gas
phase metallicity for each nuclear region in our sample.

In Figure 9 (Bottom pannel) we show the BPT diagram overlaid with the
predicted emission line ratios from CLOUDY models obtained by Polles et al.
(2021), which include photoionisation by X-ray emission. We show, in this
figure, models for three values of metallicity Z/Z⊙ = 0.3, 0.65 and 1 and
several values of X-ray emission log(Gx) from 2.8 to 1.4 in steps of 0.2 dex
with the turbulent velocity (produced by e.g., AGN jets, turbulent mixing
between the hot and cold phases and the collisions between filaments) fixed to
10 km s-1 (dotted colored lines and dotted black lines). In the same panel, we
added three models for the aforementioned metallicities and turbulence
velocities vtue = 30, 10, 2 and 0 km s-1 (solid colored lines and small dotted
black lines). The X-ray radiation field Gx is fixed to 100. In general, these
grid of models can reproduce the observed values without an excess in X-ray
luminosity, even if the optical depth AV increases (see figures E1 and E2 in
Polles et al., 2021). In the case of NGC 5846, the observed emission line
ratios (maroon open circle) are reproduced by models at very low (or no)
turbulence velocities. Also, we find no sign of a clear rotational pattern
(disturbed velocity field) in the FoV of the galaxy (see the vr map in
Olivares et al., 2022) together with extended filaments of low velocity
dispersion. This is in agreement with a cooling flows scenario, where the cool
gas may have cooled in (or close to) the centre of the group (Temi et al.,
2018; Jung et al., 2022). In addition, in this galaxy there is a good
correlation between the CO cloud positions, detected by Temi et al. (2018),
and the warm gas emission.

Figure 9: BPT [O iii]/H$\beta$ vs [N ii]/H$\alpha$ diagnostic diagrams for
several photoionisation models. Upper panels: Colored lines represent the
photoionisation models from Krabbe et al. (2021) considering pAGB stars (left
panel) and AGN activity (right panel) for metallicities from Z/Z⊙ = 0.2 to 3.
Black dotted lines correspond to different ionisation parameters from log(U) =
-1 to 4.0. Bottom panel: Colored lines represent photoionisation models from
Polles et al. (2021) for three metallicities Z/Z⊙ = 0.3, 0.65 and 1
considering the variation of X-ray emission log(Gx) from 2.8 to 1.4 in steps
of 0.2 dex (dotted coloured lines and dotted black lines) and including
turbulence velocity vtue = 30, 10, 2 and 0 km s-1 (solid colored lines and
small dotted black lines). In these models the optical depth AV = 0.1 mag. The
fill colour of the circles indicates the EW(H$\alpha$). The position of NGC
5846 is indicated by the open maroon circle. Superimposed in blue the Kewley
et al. (2001; solid line), Kauffmann et al. (2003; dotted line) and Schawinski
et al. (2007; dashed line) boundaries as in Figure 5.

Cid Fernandes et al. (2011) introduced the WHAN diagram which uses the
EW(H$\alpha$) (or WHα) in order to discriminate low ionisation AGN from
galaxies that are ionised by evolved pAGB stars. This diagram identifies 5
classes of galaxies: 1) pure star-forming galaxies: log([N ii]/H$\alpha$)
$<$-0.4 and EW(H$\alpha$) $>$3 $\AA$, 2) strong AGN emission: log([N
ii]/H$\alpha$) $>$-0.4 and EW(H$\alpha$) $>$6 $\AA$, 3) weak AGN: log([N
ii]/H$\alpha$) $>$-0.4 and EW(H$\alpha$) between 3 and 6 $\AA$, 4) retired
galaxies: EW(H$\alpha$) $<$3 $\AA$ and 5) passive galaxies: EW(H$\alpha$) and
EW([N ii]) $<$0.5 $\AA$. According to this classification scheme most of our
sample (see Figure 10) can be classified as retired galaxies likely ionised by
pAGB stars, independent of their position on the BPT diagram (see the
EW(H$\alpha$) colour scale values in Figures 8 and 9) and H$\alpha$+[N ii]
emission morphology. Only in ESO0507-G025 and NGC 677 do we see clear
indications of AGN activity in the nuclear region from the WHAN diagram, with
the EW(H$\alpha$) $\sim$3–6 $\AA$ indicating a weak AGN). However, some
galaxies in our sample have clear signatures of AGN activity as reported in
studies at other wavelengths. For example, NGC 4261 is the brightest galaxy in
the NGC 4261 group. Hubble Space Telescope (HST) WFPC2 observations (Jaffe et
al., 1993) and our MUSE data reveals a bright nuclear optical source
surrounded by a disc of gas and dust, while radio observations identified two
jets perpendicular to the disc (Birkinshaw & Davies, 1985; Scheneider, 2006;
Kolokythas et al., 2015). Recent kinematical studies using ALMA CO (Boizelle
et al., 2021) find a dynamical mass for the central supermassive black hole
(BH) in NGC 4261 of 1.67$\times$109 M⊙. So, the observed LINER properties in
this galaxy may be explained by an obscured low-luminosity AGN (Zezas et al.,
2005). In addition, 17/18 galaxies in our sample show detected radio continuum
emission (Kolokythas et al., 2018); 4/18 show small ($\lesssim$20 kpc; NGC
1060 and NGC 5846) and large-scale ($>$20 kpc; NGC 193 and NGC 4261) jets,
3/10 diffuse (NGC 677, NGC 1587 and ESO0507-G025) and 10/18 a point-like or
unresolved point source radio continuum morphology ($\lesssim$11 kpc; NGC 410,
NGC 584, NGC 777, NGC 924, NGC 940, NGC 978, NGC 1453, NGC 4008, NGC 4169 and
NGC 7619).

Figure 10: EW(H$\alpha$) vs. log [N ii]/H$\alpha$ (WHAN) diagram. The light
and dark grey regions represents the division between gas ionised by weak AGN
and retired galaxies (ionisation by pAGBs). The colour of the data points
correspond to our H$\alpha$+[N ii] morphological groups: green are type i0,
blue are type i and red corresponds to type ii galaxies.

The results in this section indicate that EW(H$\alpha$), log [N ii]/H$\alpha$
and the BPT diagrams, alone, cannot distinguish between dominant ionising
sources producing the central and extended LINER-like emission in our sample.
However, the overall log [N ii]/H$\alpha$ (Figure 5) shows that this emission
line ratio decreases as the extent of the emission line region increases,
indicating that the ionisation sources are having different impacts at
different radii. The same decreasing value with the distance pattern is
observed for $\sigma$([N ii]), the 12 + log(O/H) abundances and in most cases
the EW(H$\alpha$). Central regions are almost certainly are influenced by low-
luminosity AGN, while the extended regions are ionised by other mechanisms (SF
and/or cooling flows shocks and pAGBs). The EW(H$\alpha$) in the outer parts
is in the range of 0.5-2.4 $\AA$ (see Figure 7), thus indicating that pAGBs
are likely contributing significantly to the ionization of these regions.
Finally, our interpretations are based on the comparison with models. Factors
like $Ly_{c}$ photon escape and dilution of nuclear EWs (Papaderos et al.,
2013) have not been considered. Consequently, the addition of these processes
may constitute an important element in understanding of ETGs with extended
optical LINER-like emission.

###  4.3 Gas-phase abundances 12 + log(O/H) and metallicity gradients

Warm gas-phase abundances in our sample are difficult to obtain at optical
wavelengths due to the uncertainty about the ionisation mechanism (see Section
1 and previous results in Section 4.2). The wavelength range covered by MUSE
does not allow the detection of [OII]$\lambda$3727 and [OIII]$\lambda$4363\.
Therefore, the 12+log(O/H) abundance in each nuclear region was derived using
linear interpolation between the models (AGN, pAGBs and X-ray emission) and
their measured emission line ratios (log([O iii]/H$\beta$ and log([N
ii]/H$\alpha$)). Additionally, we used the following calibrators: i) AGN
calibration (AGN N2 calibrator) proposed by Carvalho et al. (2020):

$12$ + $log\left(\frac{O}{H}\right)$ = $12$ +
$log\left((\frac{Z}{Z_{\odot}}\times
10^{log\left(\frac{O}{H}\right)_{\odot}}\right),$ (2)

where Z/Z⊙ = 4.01N2 \- 0.07 for 0.3 < (Z/Z⊙) < 2.0, and N2=log([N
ii]$\lambda$6584/H$\alpha$). Although this calibration was developed for
seyfert 2 nuclei, it has been used to derive the O/H abundance in narrow line
AGN regions and LINERs (e.g., Krabbe et al., 2021; do Nascimento et al.,
2022). ii) most of our extended regions lie to the right of the empirical
Kauffmann et al. (2003) boundary line on the BPT diagrams in Figures 4 and 18
(appendix C). The O3N2 ratio was suggested by Kumari et al. (2019) as a
metallicity tracer of DIG and LI(N)ERs (EW(H$\alpha$)< 3 $\rm\AA$) since these
regions are likely ionised by pAGB stars, star-forming clusters and weak AGN.
Therefore, the abundances in these regions can be estimated by:

$12$ + $log\left((\frac{O}{H}\right)$ = $7.673$ +
$0.22\times\sqrt[]{25.25-9.072\times O3N2}$ + $0.127\times O3,$ (3)

with O3N2 = log([O iii]$\lambda$5007/H$\beta\times$H$\alpha$/[N
ii]$\lambda$6584) and O3 = log([O iii]$\lambda$5007/H$\beta$.

Finally, iii) we compare those values with the ones inferred from the N2
diagnostic, calibrated by Marino et al. (2013) (star-forming H ii calibrator)
obtained using empirically calibrated direct abundance data (Te-based
measurements) from H ii regions in the CALIFA survey. The Marino et al. (2013)
calibration is defined as:

$12$ + $log\left(\frac{O}{H}\right)$ = $8.743$ + $0.462\times N2,$ (4)

We note that the emission line ratios used in any of these relations are not
highly affected by reddening.

In Table 5 we show the oxygen abundance for the nuclear 3" region of each
galaxy using the aforementioned methods and their distributions are shown in
Figure 11. In the figure and table we see that the 12 + log(O/H) derived from
the AGN model and the AGN N2 calibrator are in agreement within the
uncertainties (of $\pm$0.1 dex) with those inferred from the H ii calibration
in $\sim$62% (8/13) and $\sim$89% (16/18) of the galaxies, respectively. These
values drop to $\sim$38% (5/13) and $\sim$0% (0/13) when compared to pAGB and
X–ray emission models, respectively. However, 100% (13/13) of the nuclear
DIG/LI(N)ERs abundances are in agreement, at 1$\sigma$ level, with the H ii
values. We note that nuclear metallicities obtained from pAGB and X-ray
emission models are, in most cases, lower compared to those obtained from the
H ii-based method. Two of our galaxies are included within the Annibali et al.
(2010) sample of ETGs with ionised gas: NGC 1453 and NGC 5846. Those authors
found 12 + log(O/H) = 8.55$\pm$0.19 and 8.84$\pm$0.17 using the calibration in
Kobulnicky, Kennicutt & Pizagno (1999) for NGC 1453 and NGC 5846,
respectively. Our measurements are in reasonable agreement within the errors
with those derived by Annibali et al. (2010) considering apertures and the
intrinsic differences between the calibrations.

Figure 11: Oxygen abundance distributions for the nuclear SDSS 3" apertures in panels a) ionization by stars (H ii) using the N2 calibration by Marino et al. (2013). Panel b) shows the abundances from DIG/LI(N)ERs O3N2 calibration by Kumari et al (2019), respectively. Panel c) shows the distribution of abundances from AGN photoionization model (Krabbe et al. 2021; K21) and Carvalho et al. (2020; C20) calibration. Panel d) shows the same but considering pAGB photoioniazation (K21) and X-ray (Polles et al. 2021; P21) models. The vertical grey lines indicate the solar abundance 12 + log(O/H)⊙ = 8.69. Table 5: Oxygen abundance determinations for the nuclear region (3" aperture) assuming pure H ii regions (Marino et al. (2013; H ii), AGN models (Krabbe et al. 2021) and the AGN N2-based calibrator (Carvalho et al. 2021), the DIG/LI(N)ERs O3N2 calibrator (Kumari et al. 2019; DL), and pAGB (Krabbe et al. 2021) and X-ray (Polles et al. 2021) emission models, respectively. | 12 + log(O/H)HII | 12 + log(O/H)AGN | 12 +log(O/H)DL | 12 + log(O/H)pAGB | 12 + log(O/H)X-ray
---|---|---|---|---|---
| N2 | model | N2 | O3N2 | model | model
NGC 193 | 8.81$\pm$0.09 | 8.76$\pm$0.16 | 8.75$\pm$0.10 | 8.82$\pm$0.10 | 8.63$\pm$0.16 | 8.68$\pm$0.10
NGC 410 | 8.75$\pm$0.05 | … | 8.66$\pm$0.07 | … | … | …
NGC 584 | 8.80$\pm$0.10 | 8.71$\pm$0.16 | 8.74$\pm$0.11 | 8.81$\pm$0.16 | 8.60$\pm$0.17 | 8.67$\pm$0.15
NGC 677 | 8.74$\pm$0.08 | 8.56$\pm$0.08 | 8.65$\pm$0.10 | 8.78$\pm$0.10 | 8.55$\pm$0.05 | 8.48$\pm$0.27
NGC 777 | 8.87$\pm$0.13 | … | 8.84$\pm$0.13 | … | … | …
NGC 924 | 8.77$\pm$0.07 | 8.57$\pm$0.11 | 8.70$\pm$0.08 | 8.79$\pm$0.15 | 8.55$\pm$0.12 | 8.60$\pm$0.18
NGC 940 | 8.90$\pm$0.11 | 8.90$\pm$0.10 | 8.88$\pm$0.10 | 8.83$\pm$0.17 | 8.92$\pm$0.17 | >8.69
NGC 978 | 8.71$\pm$0.07 | 8.56$\pm$0.01 | 8.64$\pm$0.08 | 8.77$\pm$0.08 | 8.55$\pm$0.03 | 8.33$\pm$0.25
NGC 1060 | 8.79$\pm$0.12 | … | 8.72$\pm$0.14 | … | … | …
NGC 1453 | 8.83$\pm$0.06 | 8.83$\pm$0.06 | 8.78$\pm$0.05 | 8.83$\pm$0.10 | 8.74$\pm$0.14 | >8.69
NGC 1587 | 8.78$\pm$0.06 | 8.59$\pm$0.11 | 8.70$\pm$0.07 | 8.77$\pm$0.08 | 8.54$\pm$0.10 | 8.60$\pm$0.13
NGC 4008 | 8.71$\pm$0.12 | … | 8.61$\pm$0.15 | … | … | …
NGC 4169 | 8.85$\pm$0.06 | 8.85$\pm$0.04 | 8.81$\pm$0.05 | 8.83$\pm$0.12 | 8.81$\pm$0.10 | >8.69
NGC 4261 | 8.90$\pm$0.13 | 8.90$\pm$0.13 | 8.88$\pm$0.12 | 8.83$\pm$0.19 | 8.91$\pm$0.26 | >8.69
ESO0507-G025 | 8.76$\pm$0.06 | 8.57$\pm$0.08 | 8.69$\pm$0.07 | 8.79$\pm$0.14 | 8.55$\pm$0.07 | 8.59$\pm$0.16
NGC 5846 | 8.80$\pm$0.10 | 8.75$\pm$0.18 | 8.74$\pm$0.11 | 8.83$\pm$0.09 | 8.59$\pm$0.16 | 8.67$\pm$0.14
NGC 6658 | 8.83$\pm$0.11 | 8.81$\pm$0.28 | 8.78$\pm$0.11 | 8.78$\pm$0.31 | 8.81$\pm$0.32 | >8.69
NGC 7619 | 8.93$\pm$0.20 | … | 8.91$\pm$0.22 | … | … | …

It is important to bear in mind that the N2 metallicity diagnostics are known
to have a dependency on the ionisation parameter and the nitrogen-to-oxygen
ratio of the gas, given that metallicities increase with N-enrichment.
Unfortunately, we are unable to explore the extended metallicity distribution
using methods that do not have a dependence on the aforementioned parameters,
given that only the [N ii]$\lambda$6584 and H$\alpha$ emission lines were
detected in most of our sample galaxies. On the other hand, the O3N2 method
gives nuclear abundances that are in agreement, within the uncertainties, with
the ones found using the N2-based abundances. Therefore, we used the N2 and
O3N2 indicators, in galaxies with extended [N ii]$\lambda$6584 emission, as a
way of obtaining the spatially resolved morphologies of the ionised gas which
can be translated into metallicities in a relative rather than absolute way.
In Section 3.2 we showed that most spaxels in our spatially resolved BPT maps
lie in the AGN and composite areas of the diagrams. Therefore, we calculated
the pixel-by-pixel 12 + log(O/H) abundance in those regions by adopting the
Carvalho et al. (2020) (AGN N2; green dots) and Kumari et al. (2019)
(DIG/LI(N)ERs O3N2; blue dots) calibrators. While values from the N2
calibration by Marino et al. (2013) (H ii; black dots) are included for
comparison. In Figure 12 we show for each galaxy in our sample the 12 +
log(O/H) abundances as function of the radius from the galaxy’s centre. The
dots are the median values within circular bins of 1.5" radii, except for the
first bin which has a radius of 0.5". The error bars, in this figure, denote
the 1$\sigma$ distribution of the H ii 12 + log(O/H) per bin.

In Figure 12 we see that the calibrators predict comparable metallicities but
with an small offset ($\lesssim$0.1 dex in most cases) in the innermost
regions, while for the extended regions this difference can reach values of
$\sim$0.3 dex in the case of ESO0507-G025. From this figure it is clear that
there is a break in the metallicity gradient slope with a very steep gradient
in the central region which, as indicated in Section 4.2, is more influenced
by low-luminosity AGN. We calculated the metallicity gradient
($\nabla_{\text{O/H}}$) as the slope of the linear fit to the median 12 +
log(O/H) values separately for the innermost and extended regions for all the
calibrations considered. In Table 9 we present for each galaxy the results of
our linear fitting and statistics of all pixels/spaxels used to create the 12
+ log(O/H) profiles. From this table we see that the central metallicity
gradients are in all cases negative, while the extended regions show a flat
gradient.

Figure 12: 12 + log(O/H) profiles for the sample galaxies by adopting the N2
calibrations by Marino et al. (2013) (HII; black data points), Carvalho et al.
(2020) (AGN; green data points) and Kumari et al. (2019) (DIG/LI(N)ERs; blue
data points). The data points corresponds to the median 12 + log(O/H) from
bins in steps of 1.0" radii, except the central bin which is 0.5". The error
bars denote the 1$\sigma$ distribution of the y-values per each bin. The solar
abundance 12 + log(O/H)⊙ = 8.69 is indicated by the grey horizontal lines.

###  4.4 Properties of the gas and its origin

####  4.4.1 Properties of the gas and metallicity gradients

From the previous section we see that the mean gas-phase metallicity in the
nuclear and extended regions are $\langle$(O/H)${}_{nuclear}\rangle$ >
$\langle$(O/H)${}_{extended}\rangle$ with the 12 + log(O/H) abundance
generally decreasing with radius, independently of the ionisation source or
method considered, with a flattening of metallicity gradients for the
outermost regions. In Figure 13 we show the relationship between metallicity
gradients $\nabla_{\text{O/H}}$ for the central and extended regions and
H$\alpha$+[N ii] morphology of our sample. While in Table 6 we summarise the
average metallicity gradients for each region and calibrator considered. A
weak positive Spearman’s correlation $\sim$0.6 between metallicity gradients
and morphological H$\alpha$+[N ii] types of the galaxies is found in Figure
13. Despite the low number statistics in our study, these results suggest that
the nuclear $\nabla_{\text{O/H}}$ of type i group-dominant galaxies, on
average, is lightly higher than type i0 and type ii galaxies. However, the
intrinsic uncertainties associated with these values lead us to consider the
metallicity gradients may not be statistically significant. Therefore, we
argue that the similarity in the shape of the nuclear metallicity gradients in
all galaxy types and the flattening of the outer regions in our sample of
group-dominant galaxies with type i (strong nuclear emission plus extend
filamentary structures) and type ii (strong or diffuse nuclear emission plus
extranuclear H ii regions) morphologies are a common property in this group of
galaxies. Several groups (e.g., Sánchez et al., 2014; Belfiore et al., 2017,
and references therein) have studied the metallicity gradients in local galaxy
samples. In Sánchez et al. (2014) and Sánchez (2020) they show that a
significant fraction of galaxies in the CALIFA sample exhibit shallow
metallicity slopes in their innermost and/or outermost regions. In particular,
they argue that the flattening in the outer regions is a universal property of
disc galaxies, which is independent of the inclination, mass, luminosity,
morphology and the presence of bars. In this and other similar works, several
mechanisms like radial motion, inside-out growth, metal-poor/rich gas
accretion, turbulent transport and outflow of gas (e.g., Kewley et al., 2010;
Sánchez-Menguiano et al., 2018; Sánchez, 2020, see references therein) are
invoked as the sources of producing the gas metallicity profiles.

Table 6: Average and standard deviation (sd) metallicity gradients ($\nabla_{\text{O/H}}$), in units of dex/arcsec, for the central and extended regions (left and right values in each column) using the calibrators H ii N2 regions (Marino et al. 2013), AGN N2 (Carvalho et al. 2020) and O3N2 DIG/LI(N)ERs (Kumari et al. 2019). | H ii N2 | AGN N2 | DIG/LIERs O3N3
---|---|---|---
| central | extended | central | extended | central | extended
|  $\langle\nabla_{\text{(O/H)}}\rangle$/sd |  $\langle\nabla_{\text{(O/H)}}\rangle$/sd |  $\langle\nabla_{\text{(O/H)}}\rangle$/sd |  $\langle\nabla_{\text{(O/H)}}\rangle$/sd |  $\langle\nabla_{\text{(O/H)}}\rangle$/sd |  $\langle\nabla_{\text{(O/H)}}\rangle$/sd
type i0 | -0.032/0.022 | … | -0.040/0.037 | … | -0.010/0.012 | …
type i | -0.026/0.019 | 0.0011/0.0043 | -0.031/0.028 | -0.0013/0.0053 | -0.010/0.005 | -0.0012/0.0027
type ii | -0.029/0.020 | -0.0039/0.0019 | -0.038/0.027 | -0.0025/0.0017 | -0.008/0.005 | -0.0000/0.0003

The properties of the extended warm ISM in our sample of type i and type ii
galaxies suggest, within the uncertainties, nearly solar ($\pm$0.2 dex)
homogeneous chemical abundances (see Figure 12 and Table 9). This likely
requires mechanisms (e.g., radial motions, gas-clouds or satellite
accretion/interactions, AGN/SF-driven outflows) for the efficient gas
transport, mixing and radial flattening of metallicity into the outer regions
of the galaxies in a relative short time scale (e.g., Werk et al., 2011;
Bresolin, Kennicutt & Ryan-Weber, 2012; Rennehan et al., 2019; Rennehan,
2021), likely similar to other low redshift galaxy classes.

Figure 13: Relationship between metallicity gradients $\nabla_{\text{O/H}}$
and H$\alpha$+[N ii] morphology of the galaxies. More details about the
classes of emission line morphology are given in Section 3.1. Horizontal lines
indicate the average of the metallicity gradients $\nabla_{\text{O/H}}$ for
central (stars) and extended (crossed) fit components considering three
different calibrators for H ii regions (black; Marino et al. 2013), AGN
(green; Carvalho et al. 2020) and DIG/LIERs (blue; Kumari et al. 2019). The
standard deviation for each quantity is indicated by the filled regions.

####  4.4.2 Cold gas content

H i is a sensitive tracer of external environmental mechanisms in galaxies. A
study of H i in ETGs by Serra et al. (2012) found that H i detections were
relatively uncommon near Virgo cluster centre (10%) but were common (40%) in
field with the detected H i mass inversely related to environment density. H i
morphology was also found to vary in a continuous way from regular, settled H
i discs and rings in the field to unsettled gas distributions (including tidal
or accretion tails) at the Virgo cluster centre, with the H i and CO-richest
galaxies found in the poorest environments where the SF detection rate was
also higher. This implies that galaxy group processing is involved in evolving
pre-existing ETG gas properties.

In our sample, 8/18 galaxies have the H i properties available from the
literature and 18/18 have been observed in CO (see our Table 10 for a summary;
O’Sullivan et al., 2015, 2018). In Figure 34 we show single dish H i spectra
from the literature for 7 of these galaxies. Excluding the two galaxies in
Table 10 which have a caveat about their H i properties (see Table 10 note)
the mean H i in type ii galaxies is 17.2$\times$109 M⊙ compared to the mean H
i in type i galaxies of 1.4$\times$109 M⊙, i.e., the type ii have an order of
magnitude more H i. NGC 940 was excluded from the above calculation because of
the high uncertainty about it’s H i detection, however it has a large M(H2)
mass (6.1$\times$109 M⊙). The H2 mass of NGC 940 together with the H i masses
of the other type ii galaxies confirms that the type ii galaxies are cold gas
rich. To varying degrees, all the H i in our galaxies display double horned H
i profiles which are indicative of rotating discs, with the clearest examples
being NGC 924 and NGC 940 which also have the lowest H i profile asymmetries
as measured by the Aflux parameter (Espada et al., 2011).

Its seems likely the cold and warm discs in the type ii galaxies are part of
the same kinematical gas structures, with support for this coming from the H i
profiles and the [N ii] velocity fields in Figure 6 and Appendix D. In the [N
ii] velocity fields we see that all of the type ii galaxies show clear
rotating disc patterns, with NGC 940 presenting a highly symmetric case. The
high levels of symmetry in the velocity fields, especially in NGC 940,
together with the flattened metallicity gradients argue against the gas having
been recently acquired. In particular, using Romulus hydrodynamic cosmological
simulations Jung et al. (2022) examined the re-emergence of gaseous and
stellar discs in BGGs following their destruction by mergers. They find that
ordered discs take $\sim$1 Gyrs to be established. We suggest that our gas-
rich systems obtained their cold gas at least $\sim$1 Gyr ago (Holwerda et
al., 2011; Jung et al., 2022), i.e., the H i virialization time scale after
gas-clouds or satellite mergers/accretion. However, we observe that
$\sigma_{gas,central}\gtrsim\sigma_{gas,extended}$, so the warm gas in the
central regions is unlikely to be dynamically relaxed (Olivares et al., 2022)
in the gravitational potential of the galaxy, indicating an AGN outburst
contribution. Those results are in agreement with the morphology of the warm
gas distribution and velocity fields observed in our sample (Section 3.3).

####  4.4.3 Summary of properties, chemical abundances and possible scenarios
for the origin of the gas in group-dominant galaxies

The interpretation of internal (stellar mass loss) and external mechanisms
(cooling from the IGrM or mergers/interactions) for the origin of the gas in
our sample is difficult, since different mechanisms could be acting at
different evolutionary stages. Kolokythas et al. (2022) examined the
relationships between radio power, SFR(FUV) and stellar mass for CLoGS, which
includes our analysed galaxies, and find no correlations between these
quantities. This suggests a mix of origins for the cool gas in these systems,
including stellar mass loss, cooling from the IGrM/galaxy halo, settling gas
due to mergers and tidal or ram pressure stripping (see also Loubser et al.,
2022). In systems like NGC 978 and NGC 1587 the observed ionised gas could be
associated with an interaction with the companion galaxy. Although, no cold
gas has been reported in NGC 978 (see Figure 15), but the interaction debris
gas may eventually enter the galaxy’s (hot) halo to trigger the SF and feed
the AGN activity. On the other hand, the SF in extranuclear regions (type ii
galaxies) is likely a later stage after streams gas settling from orbiting
satellites (Jung et al., 2022). Kolokythas et al. (2022) argued that S0 group-
dominant galaxies (type ii galaxies) occupy X-ray faint systems and have
point-like radio sources (Kolokythas et al., 2018), which indicates that the
cold gas is more likely to be the result of gas-rich mergers or tidal
interactions instead of cooling from a hot IGrM. While in some type i galaxies
(as in the case of NGC 5846 where the ionised gas morphology supports a
cooling flow) the cooled gas from the IGrM may be an effective mechanism
forming filaments and rotating discs in the galaxy nuclei (O’Sullivan et al.,
2015; Olivares et al., 2022, and references therein). 3/4 galaxies (NGC 193,
NGC 1060 and NGG 5846) with radio jet-like morphology present filamentary
ionised gas structures characteristic of ICM cooling. The presence of
misaligned or counter-rotating ionised gas discs with respect to the stellar
body is a strong indication of external accretion of gas. We find direct
evidence of this, Olivares et al. (2022) found that most of the galaxies in
our sample have ionised gas kinematically decoupled from the stellar
component, which suggests an external origin of the gas. Observational
evidence (e.g., Sarzi et al., 2006; Gomes et al., 2016) and simulations
(Starkenburg et al., 2019; Khim et al., 2021; Jung et al., 2022, among others)
indicate possible origins of this misalignment in galaxies regardless of the
morphology and environment, with early-type having higher misaligned
fractions. We argue that stellar mass loss is unlikely to be the dominant
source of cold gas in our sample (see Olivares et al., 2022).

Table 7: Comparison between Z (this work) and IGrM metallicities in units of Z/Z⊙. Column (1) average value between the AGN N2 and interpolated abundances from the AGN models. Column (2) abundances in the extended regions from DIG/LI(N)ERs O3N2 calibrator. Column (3) corresponds to IGrM metallicity from O’Sullivan et al. (2017). | nuclear 3" | extended | IGrM
---|---|---|---
| AGN | DIG/LI(N)ERs |
| (1) | (2) | (3)
NGC 193 | 1.15 | … | 0.68
NGC 410 | 0.94 | … | 0.42
NGC 584 | 1.08 | 1.35 | $\dots$
NGC 677 | 0.84 | … | 0.38
NGC 777 | 1.41 | $\dots$ | 0.63
NGC 924 | 0.89 | 0.85 | $\dots$
NGC 940 | 1.57 | 1.07 | 0.06
NGC 978 | 0.82 | … | >0.29
NGC 1060 | 1.08 | … | 0.28
NGC 1453 | 1.30 | 1.29 | 0.42
NGC 1587 | 0.91 | 1.05 | 0.03
NGC 4008 | 0.84 | $\dots$ | 0.32
NGC 4169 | 1.38 | 1.35 | 0.11
NGC 4261 | 1.58 | $\dots$ | 0.23
ESO0507-G025 | 0.88 | 0.98 | $\dots$
NGC 5846 | 1.12 | 1.32 | 0.27
NGC 6658 | 1.27 | $\dots$ | <0.18
NGC 7619 | 1.67 | $\dots$ | 0.54

If we assume that the metallicity of the warm gas, in our sample, is
represented by a single calibrator (AGN N2 or DIG/LI(N)ERs O3N2), on average,
the nuclear regions are more metal rich than their extended structures, i.e.,
$\langle$(O/H)${}_{nuclear}\rangle\gtrsim\langle$(O/H)${}_{extended}\rangle$.
However, in Section 4.2 we find that the ionisation sources are having
different impacts at different radii. Therefore, the abundance in the nuclear
regions is well represented by the 3" apertures and they can be obtained as
the average between the AGN N2 and the interpolated AGN abundances from the
models (Figure 9). While for the extended regions the average abundances are
from the data points in Section 4.3 (see Table 9) assuming the DIG/LI(N)ERs
O3N2 calibrator. This is a reasonable assumption given that most of the
spaxels in these regions are in the composite area of the BPT diagrams. In
Table 7 we summarize and compare the gas-phase metallicities (Z = [O/H] =
$\langle$log(O/H)$\rangle$ \- log(O/H)⊙) found in this work with those in the
IGrM by O’Sullivan et al. (2017). In the nuclear regions the metallicities
range from $\sim$0.9 to 1.7 Z⊙. The metallicity in the extended structures
often rise to values approaching the solar $\sim$1.0 Z⊙ or higher
($\lesssim$0.3 dex), while the IGrM has metallicities down to $\sim$0.1-0.7
Z⊙. In the case of NGC 940, NGC 4261 and NGC 7619 the nuclear metallicities
are $\gtrsim$0.6 dex higher than the solar value. Interestingly, in four cases
(NGC 584, NGC 1587, ESO0507-G025 and NGC 5846) we found a drop in the nuclear
metallicity with respect to the extended regions of $\lesssim$0.2 dex in NGC
584 and NGC 5846 and $\sim$0.1 dex for NGC 1587 and ESO0507-G025. This suggest
the accretion of metal-poor gas to the central AGN (e.g., do Nascimento et
al., 2022). Since metallicity in the nuclear regions represent the average
metallicity within the 3" apertures and the uncertainties on the abundances
are of the order of $\sim$0.1 dex, we find in our sample of group-dominant
galaxies that $Z_{nuclear}\gtrsim$Zextended>ZIGrM.

The mixing and dispersion of heavy elements in the ISM of galaxies, in
general, should follow the "evolutionary" stages of disc growth at different
spatial scales. It might be suggested, in our case, by a correlation between
gas-phase metallicity gradients $\nabla_{\text{O/H}}$ and H$\alpha$+[N ii]
morphology (see Figure 13), since the effect of gas flows over the lifetime of
the galaxies seems to produce the flattening of abundances out to large radii
(e.g., Kewley et al., 2010; López-Sánchez et al., 2015; Sánchez et al., 2014),
following the formation of the extended structures. However, we find no
correlation between the metallicity gradients and morphology (Section 4.4.1)
in our sample of BGGs. This is in agreement with the idea of relatively short
time scales for the radial dispersion and mixing of metals to large spatial
scales likely produced by the AGN/SF-driven outflows, gas accretion and
mergers/interactions. Furthermore, some of these metals will be transport by
these mechanisms from the galaxies into the IGrM/ICM. In particular, group-
dominant galaxies often host radio AGN that are interacting with the
surrounding gas by forming cavities and shock fronts (see Olivares et al.,
2022, for a description of these structures in our sample). As seen in Section
3.3 we observe large gas velocity dispersion in the central regions of the
galaxies, likely associated with the presence of AGN activity. Therefore,
group-dominant galaxies likely acquired their cold gas as a consequence of
several possible mechanisms, i.e., gas-clouds or satellite mergers/accretion
and cooling flows which together with the AGN/SF activity are likely
contributing to the growth of the ionized gas structures and flattening the
metallicity gradients.

##  5 Conclusions

In this paper, we present archival MUSE observations for a sample of 18 group-
dominant galaxies from the CLoGS sample (O’Sullivan et al., 2017). We derive
and removed the stellar continuum for all galaxies by fitting the stellar SEDs
using the spectral synthesis code FADO (Gomes & Papaderos, 2017). We studied
the properties (i.e., emission line ratios, chemical abundances, etc) and
structure of the warm gas, in each galaxy, in order to constrain the
ionisation processes, the origin of their gas and its chemical abundance
distribution. We summarise our main results as follows:

  * •

We used the continuum-subtracted H$\alpha$+[N ii] images (see Figure 3) to
classify the galaxies into three morphological groups or types: type i0 \-
strong or diffuse nuclear emission with (or without) unextended filamentary
($\lesssim$1 kpc) structures connected to the nuclear region, type i \- strong
or diffuse nuclear emission with extended (several kpc) filamentary structures
beyond the nuclear region and type ii \- i0 or i plus extranuclear H ii
regions (well-defined or in distorted ring-like structures). We find that 5/18
(NGC 410, NGC 978, NGC 4008, NGC 4261 and NGC 6658) are type i0, 9/18 of these
objects are type i (NGC 193, NGC 584, NGC 677, NGC 777, NGC 1060, NGC 1453,
NGC 1587, NGC 5846 and NGC 7619) and 4/18 galaxies are type ii (NGC 924, NGC
940, NGC 4169 and ESO0507-G025).

  * •

In order to distinguish between different ionisation mechanisms, in Section
3.2 we used the following emission line ratios [O iii]/H$\beta$ and [N
ii]/H$\alpha$ and the equivalent width EW(H$\alpha$). The spatially resolved
log [N ii]/H$\alpha$ ratios decreases as the extent of the emission line
region increases, indicating that the sources of the ionisation are acting at
different spatial scales. The same decreasing pattern with the distance is
observed for the velocity dispersion $\sigma$([N ii]), the 12 + log(O/H)
abundances and in most cases the EW(H$\alpha$). Using emission-line diagnostic
diagrams (or BPT diagrams) we find that all galaxies in our sample have a
dominant LINER/AGN nuclear region. Extended LINER-like regions are observed in
most galaxies with filamentary structures. In the same section, we studied the
mechanisms (pAGBs, AGN and X–ray emission) responsible for the ionisation
which produce the optical emission lines in our sample. Although, AGN, pAGBs
and X–ray emission models are able to reproduce the observational data, we
suggest that central regions are more influenced by a low-luminosity AGN,
while extended regions are ionised by other mechanisms with pAGBs
photoionisation likely contributing significantly as suggested by their
EW(H$\alpha$) values.

  * •

We calculated the gas-phase metallicity (12 + log(O/H)) using linear
interpolations between the AGN, pAGBs and X-ray emission models (Krabbe et
al., 2021) and their measured emission line ratios (log([O iii]/H$\beta$ and
log([N ii]]/H$\alpha$)). We also used the AGN N2 (Carvalho et al., 2020) and
DIG/LI(N)ERs O3N2-based (Kumari et al., 2019) calibrators. Using a single
calibrator (AGN N2 or DIG/LI(N)ERs O3N2), the 12 + log(O/H) in the nuclear and
extended regions (see Figure 12) are
$\langle$(O/H)${}_{nuclear}\rangle\gtrsim\langle$(O/H)${}_{extended}\rangle$.
We found that the metallicity gradients for the pixel-by-pixel data points
are, in most cases, negative in the innermost regions with a flat gradient for
the extended areas beyond the centre, which includes extended structures and
some star-forming regions. In this sense, the morphological H$\alpha$+[N ii]
types defined in this study indicate that group-dominant galaxies with
extended filamentary structures (type i) and S0 galaxies with extranuclear SF
regions (type ii), on average, have shallow metallicity gradients. Therefore,
extended regions and ring-like structures of ionised gas can be considered
chemically homogeneous (nearly solar) within the uncertainties. If the
ionisation sources have different impacts at different radii (as seen in
Section 3.2) we use the AGN N2 calibrator and AGN models to estimate the
nuclear (3" aperture) abundances and the DIG/LI(N)ERs O3N2 calibrator for the
extended regions. Therefore, we found in NGC 584, NGC 1587, ESO0507-G025 and
NGC 5846 a slight drop in the nuclear metallicity with respect to the extended
regions, suggesting the accretion of metal-poor gas to the central regions.
However, we find within the uncertainties that
$Z_{nuclear}\gtrsim$Zextended>ZIGrM.

  * •

We suggest that group-dominant galaxies likely acquired their cold gas in the
past as a consequence of one or more external mechanisms where gas-clouds or
satellite mergers/accretion (Section 4.4.2) and cooling flows are likely
supplying the gas for the growth of the ionised gas structures and AGN/SF
activity. Our results favor a scenario in which metals are distributed across
the ISM of the galaxies on short timescales.

## Acknowledgements

We thank the reviewer for his/her careful reading of the manuscript and
helpful comments which substantially improved the paper. PL (contract
DL57/2016/CP1364/CT0010) and TS (contract DL57/2016/CP1364/CT0009) are
supported by national funds through Fundação para a Ciência e Tecnologia (FCT)
and the Centro de Astrofísica da Universidade do Porto (CAUP). SIL and KK are
supported in part by the National Research Foundation of South Africa (NRF
Grant Numbers: 120850). Opinions, findings and conclusions or recommendations
expressed in this publication is that of the author(s), and that the NRF
accepts no liability whatsoever in this regard. EOS ackowledges support for
this work from the National Aeronautics and Space Administration through XMM-
Newton award 80NSSC19K1056. AB acknowledges support from NSERC through its
Discovery Grant Program. PL thanks Polychronis Papaderos for his very useful
comments. We thank Angela Krabbe for providing us with the CLOUDY models used
in this work.

## Data availability

The data underlying this article will be shared on reasonable request to the
corresponding author.

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##  Appendix A Other emission line galaxies in our MUSE datacubes

Other emission line galaxies in our MUSE FoVs. We found two objects in the FoV
of NGC 677 and one in the FoV of NGC 777, NGC 924 and NGC 1453. In Figure 14
we show the position of these objects in the FoV. Using the H$\alpha$ and [N
ii]$\lambda$6584 emission lines we calculated: the redshifts, H$\alpha$ SFRs
and 12 + log(O/H) abundances using the H ii calibrator. In Table 8 we
summarize their main properties.

Figure 14: Emission line galaxies found in the FoV of the galaxies NGC 677, NGC 777 NGC 924 and NGC 1453. Table 8: Properties of the emission line galaxies detected in the field of our sample galaxies. (a) z calculated using H$\beta$. FoV | z | SFR(H$\alpha$) | 12 + log(O/H)
---|---|---|---
|  | (M⊙yr-1) | N2
NGC 677 |  |  |
R1 | 0.283658 | 0.0154$\pm$0.0001 | 8.50$\pm$0.09
R2 | 0.282776 | 0.0030$\pm$0.0010 | 8.53$\pm$0.17
NGC 777 |  |  |
R1 | 0.232878 | 0.0046$\pm$0.0001 | 8.48$\pm$0.15
NGC 924(a) |  |  |
R1 | 0.491319 | … | …
NGC 1453 |  |  |
R1 | 0.118373 | 0.0003$\pm$0.0001 | 8.56$\pm$0.15

##  Appendix B H$\alpha$+[N ii]$\lambda\lambda$6548,6584 emission line maps

Figure 15: H$\alpha$+[N ii]$\lambda\lambda$6548,6584 emission line maps from
our sample. We smoothed the emission line maps using a 3$\times$3 box filter.
MUSE continuum-subtracted spectra from a nuclear 3" aperture covering the
wavelength range from 6400 $\AA$ to 6800 $\AA$ are shown in the inset panels.
The vertical lines in the inset panels indicate the wavelengths of the [N
ii]$\lambda$6548, H$\alpha$, [N ii]$\lambda$6584, [S ii]$\lambda$6717 and [S
ii]$\lambda$6731 emission lines. Fluxes in units of $\times$10-15 (erg s-1
cm-2 $\AA^{-1}$). North is to the top and East to the left.

Figure 16: continued

Figure 17: continued

##  Appendix C Emission line ratio maps and BPT diagrams of the galaxies

Figure 18: Emission line ratio maps log([N ii]/H$\alpha$), log([O
iii]/H$\beta$ and log([O iii]/H$\beta$ vs log([N ii]/H$\alpha$) BPT diagram
for emission lines with S/N $>$ 3\. In the BPT diagram we include the Kewley
et al. (2001; solid line) and Kauffmann et al. (2003; dashed line) lines to
divide between regions dominated by H ii and AGN/LINERs. Filled black data
points correspond to our measured values in the 3" aperture enclosing the
nuclear region. Figure 19: Similar to Figure 18. Figure 20: Similar to Figure
18. Figure 21: Similar to Figure 18. Figure 22: Similar to Figure 18. Figure
23: Similar to Figure 18. Figure 24: Similar to Figure 18. Figure 25: Similar
to Figure 18. Figure 26: Similar to Figure 18. Figure 27: Similar to Figure
18. Figure 28: Similar to Figure 18. Figure 29: Similar to Figure 18. Figure
30: Emission line ratio maps log([N ii]/H$\alpha$) for galaxies with no
H$\beta$ and/or [O iii]$\lambda$5007 emission detection. North is to the top
and East to the left.

##  Appendix D [N ii]$\lambda$6584 velocity fields

Figure 31: Radial velocity v([N ii]) (left panel) and velocity dispersion
$\sigma$([N ii]) (right panel) maps for our type ii galaxies NGC 924, NGC 940
and NGC 4169. See also Figure 6 for ESO0507-G025. The position of continuum
maximum is indicated by an X symbol. North is to the top and East to the left.

##  Appendix E [S ii]$\lambda$6716 / [S ii]$\lambda$6731 ratio maps

Figure 32: [S ii]$\lambda$6716 / [S ii]$\lambda$6731 ratio maps. Using the
temden IRAF STS package assuming te([O iii])=10000 K, for reference, we find
for [S ii]$\lambda$6716 / [S ii]$\lambda$6731 = 1.43, 1.32, 1.00, 0.51 and
0.46 an electron density ne(S ii) = $\sim$2, $\sim$100, $\sim$606, 10910 and
48893 cm-3, respectively. North is up and east is to the left. Figure 33:
continued

##  Appendix F Results of the linear fitting

Table 9: Results of the linear fitting of Figure 12 and statistics for all data points or spaxels using the H ii N2 (Marino et al. 2013), AGN N2 (Carvalho et al. 2020) and AGN/LI(N)ERs O3N2 (Kumari et al. 2019) methods, respectively. The slope (metallicity gradient) from the linear fitting is indicated by $\nabla_{\text{O/H}}$. | nuclear region | extended region |
---|---|---|---
Name | intercept | slope $\nabla_{\text{(O/H)}}$ | mean | sd | intercept | slope $\nabla_{\text{(O/H)}}$ | mean | sd |
|  | (dex/arcsec) |  |  |  | (dex/arcsec) |  |  |
NGC 193 | 8.87/8.83/8.83 | -0.022/-0.026/-0.009 | 8.80/8.74/8.81 | 0.09/0.11/0.04 | …/…/… | …/…/… | …/…/… | …/…/… |
NGC 410 | 8.87/8.82/… | -0.034/-0.027/… | 8.80/8.76/… | 0.10/0.12/… | …/…/… | …/…/… | …/…/… | …/…/… |
NGC 584 | 8.84/8.78/8.81 | -0.036/-0.042/-0.009 | 8.77/8.69/8.85 | 0.13/0.18/0.05 | 8.69/8.63/8.78 | 0.0030/0.0016/-0.0002 | 8.77/8.69/8.82 | 0.12/0.17/0.08 |
NGC 677 | 8.76/8.68/8.80 | -0.008/-0.008/-0.008 | 8.73/8.65/8.78 | 0.10/0.12/0.02 | …/…/… | …/…/… | …/…/… | …/…/… |
NGC 777 | 8.85/8.81/… | -0.030/-0.027/… | 8.83/8.78/… | 0.12/0.15/… | …/…/… | …/…/… | …/…/… | …/…/… |
NGC 924 | 8.80/8.74/8.79 | -0.009/-0.009/-0.002 | 8.72/8.75/8.60 | 0.17/0.24/0.04 | 8.79/8.68/8.75 | -0.0060/-0.0025/0.0001 | 8.76/8.72/8.62 | 0.16/0.12/0.03 |
NGC 940 | 8.97/8.96/8.85 | -0.055/-0.072/-0.017 | 8.60/8.51/8.70 | 0.04/0.06/0.02 | 8.63/8.53/8.82 | -0.0051/-0.0052/-0.0005 | 8.68/8.56/8.72 | 0.14/0.20/0.05 |
NGC 978 | 8.75/8.66/8.79 | -0.005/-0.005/ -0.009 | 8.74/8.65/8.78 | 0.08/0.11/0.03 | …/…/… | …/…/… | …/…/… | …/…/… |
NGC 1060 | 8.75/8.67/… | -0.014/-0.006/… | 8.79/8.72/… | 0.05/0.06/… | 8.73/8.70/… | 0.0081/0.0010/… | 8.75/8.66/… | 0.04/0.06/… |
NGC 1453 | 8.86/8.83/8.83 | -0.021/-0.028/-0.008 | 8.83/8.78/8.80 | 0.14/0.19/0.04 | 8.77/8.69/8.78 | -0.0011/0.0019/0.0013 | 8.80/8.74/8.80 | 0.13/0.18/0.04 |
NGC 1587 | 8.79/8.72/8.78 | -0.016/-0.013/-0.004 | 8.75/8.67/8.72 | 0.08/0.11/0.04 | 8.74/8.73/8.80 | -0.0047/-0.0119/-0.0058 | 8.71/8.69/8.71 | 0.13/0.07/0.05 |
NGC 4008 | 8.79/8.72/… | -0.017/-0.015/… | 8.76/8.69/… | 0.13/0.17/… | …/…/… | …/…/… | …/…/… | …/…/… |
NGC 4169 | 8.88/8.85/8.83 | -0.042/-0.059/-0.006 | 8.71/8.61/8.78 | 0.05/0.07/0.03 | 8.81/8.74/8.81 | -0.0036/-0.0004/0.0004 | 8.74/8.69/8.82 | 0.2/0.25/0.06 |
NGC 4261 | 8.90/8.88/8.82 | -0.010/-0.015/0.005 | 8.89/8.86/8.83 | 0.08/0.10/0.04 | …/…/… | …/…/… | …/…/… | …/…/… |
ESO0507-G025 | 8.81/8.74/8.82 | -0.011/-0.014/-0.007 | 8.81/8.75/8.68 | 0.09/0.13/0.02 | 8.58/8.50/8.68 | -0.0010/-0.0019/-0.0001 | 8.74/8.66/8.68 | 0.06/0.09/0.01 |
NGC 5846 | 8.91/8.89/8.85 | -0.068/-0.094/-0.020 | 8.82/8.77/8.84 | 0.04/0.05/0.08 | 8.78/8.71/8.81 | 0.0002/0.0011/-0.0002 | 8.79/8.72/8.81 | 0.08/0.11/0.05 |
NGC 6658 | 8.88/8.89/8.83 | -0.064/-0.105/-0.025 | 8.78/8.74/8.79 | 0.16/0.20/0.08 | …/…/… | …/…/… | …/…/… | …/…/… |
NGC 7619 | 8.97/8.97/… | -0.065/-0.089/… | 8.88/8.85/ | 0.12/0.17/… | …/…/… | …/…/… | …/…/… | …/…/… |

##  Appendix G Cold gas content

Figure 34: H i profiles from NED with standardised x and y-axis. In the standardised plots, the NGC 677 and the ESO spectra velocities have been binned $\times$2. Table 10: H i and H2 properties and M∗ available from the literature. Name | H$\alpha$+[N ii] | V(H i) | W20(H i) | A flux(H i)333Asymmetry in the H i profiles using the method from Espada et al. (2011). | M(H i)444From H i compilation in O’Sullivan et al. (2018), although as noted by those authors H i was not detected in NGC 940 and NGC 5846 during the ALFALFA survey as expected. | M(H2)555From O’Sullivan et al. (2015) and O’Sullivan et al. (2018) | M∗666From Kolokythas et al. (2022) | H i spectrum
---|---|---|---|---|---|---|---|---
| morphology | (km s-1) | (km s-1) |  |  $\times$109 M⊙ |  $\times$109 M⊙ |  $\times$1011 M⊙ | source
NGC 584 | i | … | … | … | 0.12 | <0.01 | 2.13 | …
NGC 677 | i | 5138$\pm$6 | 272$\pm$12 | 1.10$\pm$0.07 | 1.70 | <0.23 | 3.52 | Haynes et al (2018)
NGC 924 | ii | 4428$\pm$5 | 509$\pm$10 | 1.07$\pm$0.08 | 9.12 | 0.05 | 1.88 | Haynes et al (2018)
NGC 940 | ii | 5127$\pm$4 | 218$\pm$8 | 1.02$\pm$0.08 | 9.14 | 6.10 | 2.94 | Paturel et al. (2003)
NGC 1587 | i | 7163$\pm$1 | 302$\pm$2 | 1.29$\pm$0.04 | 2.51 | 0.23 | 3.03 | Gallagher, Knapp & Faber (1981)
NGC 4169 | ii | 3811$\pm$7 | 470$\pm$13 | 1.56$\pm$0.07 | 10.71 | 0.14 | 1.27 | Haynes et al (2018)
ESO0507-G025 | ii | 3248$\pm$8 | 450$\pm$16 | 1.26$\pm$0.51 | 31.62 | 0.42 | 2.84 | Barnes et al. (2001)
NGC 5846 | i | 1804$\pm$14 | 502$\pm$27 | 1.4$\pm$0.04 | 0.28 | 0.01 | 3.39 | Bottinelli & Gouguenheim (1979)