# GASP XXXIV: Unfolding the thermal side of ram pressure stripping in the
jellyfish galaxy JO201

M. Giulia Campitiello Dipartimento di Fisica e Astronomia, Università di
Bologna, via Piero Gobetti 93/2, 40129 Bologna, Italy  INAF, Osservatorio
Astronomico di Bologna, via Piero Gobetti, 93/3, 40129 Bologna, Italy
Alessandro Ignesti INAF-Padova Astronomical Observatory, Vicolo
dell’Osservatorio 5, I-35122 Padova, Italy  Dipartimento di Fisica e
Astronomia, Università di Bologna, via Piero Gobetti 93/2, 40129 Bologna,
Italy  Myriam Gitti Dipartimento di Fisica e Astronomia, Università di
Bologna, via Piero Gobetti 93/2, 40129 Bologna, Italy  INAF, Istituto di
Radioastronomia di Bologna, via Piero Gobetti 101, 40129 Bologna, Italy
Fabrizio Brighenti Dipartimento di Fisica e Astronomia, Università di Bologna,
via Piero Gobetti 93/2, 40129 Bologna, Italy  Department of Astronomy and
Astrophysics, University of California, 1156 High Street, Santa Cruz, CA
95064, USA  Mario Radovich INAF-Padova Astronomical Observatory, Vicolo
dell’Osservatorio 5, I-35122 Padova, Italy  Anna Wolter INAF-Osservatorio
Astronomico di Brera, via Brera 28, I-20121 Milano, Italy  Neven Tomičić INAF-
Padova Astronomical Observatory, Vicolo dell’Osservatorio 5, I-35122 Padova,
Italy  Callum Bellhouse INAF-Padova Astronomical Observatory, Vicolo
dell’Osservatorio 5, I-35122 Padova, Italy  University of Birmingham School of
Physics and Astronomy, Edgbaston, Birmingham B15 2TT, England  Bianca M.
Poggianti INAF-Padova Astronomical Observatory, Vicolo dell’Osservatorio 5,
I-35122 Padova, Italy  Alessia Moretti INAF-Padova Astronomical Observatory,
Vicolo dell’Osservatorio 5, I-35122 Padova, Italy  Benedetta Vulcani INAF-
Padova Astronomical Observatory, Vicolo dell’Osservatorio 5, I-35122 Padova,
Italy  Yara L. Jaffé Instituto de Física y Astronomía, Universidad de
Valparaíso, Avda. Gran Bretaña 1111 Valparaíso, Chile  Rosita Paladino INAF,
Istituto di Radioastronomia di Bologna, via Piero Gobetti 101, 40129 Bologna,
Italy  Ancla Müller Ruhr University Bochum, Faculty of Physics and Astronomy,
Astronomical Institute, Universitätsst 150, 44801 Bochum, Germany  Jacopo
Fritz Instituto de Radioastronomía y Astrofísica, UNAM, Campus Morelia, A.P.
3-72, C.P. 58089, Mexico  Ana C. C. Lourenço Instituto de Física y Astronomía,
Universidad de Valparaíso, Avda. Gran Bretaña 1111 Valparaíso, Chile  Marco
Gullieuszik INAF-Padova Astronomical Observatory, Vicolo dell’Osservatorio 5,
I-35122 Padova, Italy

(Received 01.02.2021; Revised 01.03.2021; Accepted 05.03.2021)

###### Abstract

X-ray studies of jellyfish galaxies play a crucial role in understanding the
interactions between the interstellar medium (ISM) and the intracluster medium
(ICM). In this paper, we focused on the jellyfish galaxy JO201. By combining
archival Chandra observations, MUSE H$\alpha$ cubes, and maps of the emission
fraction of the diffuse ionised gas, we investigated both its high energy
spectral properties and the spatial correlation between its X-ray and optical
emissions. The X-ray emission of JO201 is provided by both the Compton thick
AGN (L${}_{\text{X}}^{0.5-10\text{keV}}$=2.7$\cdot$1041 erg s-1, not corrected
for intrinsic absorption) and an extended component
(L${}_{\text{X}}^{0.5-10\,\text{keV}}\approx$ 1.9-4.5$\cdot$1041 erg s-1)
produced by a warm plasma (kT$\approx$1 keV), whose luminosity is higher than
expected from the observed star formation (L${}_{\text{X}}\sim$3.8$\cdot
10^{40}$ erg s-1). The spectral analysis showed that the X-ray emission is
consistent with the thermal cooling of hot plasma. These properties are
similar to the ones found in other jellyfish galaxies showing extended X-ray
emission. A point-to-point analysis revealed that this X-ray emission closely
follows the ISM distribution, whereas CLOUDY simulations proved that the
ionisation triggered by this warm plasma would be able to reproduce the
[OI]/H$\alpha$ excess observed in JO201. We conclude that the galactic X-ray
emitting plasma is originated on the surface of the ISM as a result of the
ICM-ISM interplay. This process would entail the cooling and accretion of the
ICM onto the galaxy, which could additionally fuel the star formation, and the
emergence of [OI]/H$\alpha$ excess in the optical spectrum.

galaxies: evolution – galaxies: clusters: general

††journal: ApJ

##  1 Introduction

The evolution of galaxies is strongly influenced by all those phenomena that
can alter their gas content. Due to internal processes, like feedback from
supernovae (SN) and active galactic nuclei (AGN) (e.g., Ho et al., 2014), or
external processes, like harassment (e.g., Moore et al., 1998), strangulation
(e.g., Larson et al., 1980), and tidal interactions (e.g., Springel, 2000),
galaxies can either acquire gas, increasing their star formation (SF), or,
loose their gas, turning into passive systems (e.g., van Gorkom, 2004, and
references therein). One of the main processes involved in removing gas from
galaxies is ram pressure stripping (RPS). This mechanism was proposed for the
first time by Gunn & Gott (1972) to explain the lack of gas-rich galaxies in
clusters. Anytime a galaxy falls into the intracluster medium (ICM), it
experiences a force in the opposite direction of its relative motion. If this
force overcomes the gravitational one, the gas component is stripped away,
causing a transformation that can turn the galaxy into a quenched system. The
condition for gas loss is given by $\rho_{\text{ICM}}v^{2}_{\text{in}}>2\pi
G\Sigma_{\text{s}}\Sigma_{\text{g}}$, that is when the ram pressure is higher
than the gravitational pressure. Here $\Sigma_{\text{s}}$ and
$\Sigma_{\text{g}}$ are, respectively, the surface density of stars and gas,
$\rho_{\text{ICM}}$ is the ICM mass density and $v_{\text{in}}$ the in-fall
velocity of the galaxy. Starting from this relation, it is possible to
identify regions of the phase-space diagram in which the stripping process is
heavily favoured; they are characterised by small cluster radii (where the ICM
density is higher) and high galaxy velocity (Yoon et al., 2017; Jaffé et al.,
2015, 2016). In these regions in particular, the presence of peculiar galaxies
was observed: these objects show clear signs of the ongoing stripping process,
i. e. tails or ”tentacles” of diffuse gas extended in the opposite direction
of the galaxy motion (Jaffé et al., 2018; Gullieuszik et al., 2020). The most
extreme examples of galaxies undergoing strong ram pressure are the so called
jellyfish galaxies (Fumagalli et al., 2014; Smith et al., 2010; Ebeling et
al., 2014; Poggianti et al., 2017a), objects that show extra-planar,
unilateral debris visible in the optical/UV light and striking tails of
H$\alpha$ ionised gas.

Jellyfish galaxies represent the transitional phase between infalling star-
forming spirals and quenched cluster early-type galaxies and provide a unique
opportunity to understand the impact of gas removal processes on both the star
formation and the AGN activity. One of the most recent research projects in
this field is GAs Stripping Phenomena (GASP), an European Southern Observatory
(ESO) Large Program carried out with the Multi Unit Spectroscopic Explorer
(MUSE) at the Very Large Telescope (VLT), to observe 94 stripping candidates
at z=0.04-0.07 (Poggianti et al., 2017a). These galaxies were selected by
visual inspection based on the presence of ram-pressure stripping signatures,
such as displaced tails and asymmetric morphologies, from the WIdefield Nearby
Galaxy-cluster Survey (WINGS Fasano et al., 2006; Moretti et al., 2014), its
extension OmegaWINGS (Gullieuszik et al., 2015; Moretti et al., 2017) and the
Padova-Millennium Galaxy and Group Catalogue (Calvi et al., 2011).

To investigate the effects and the features of such a complex process as the
ram pressure stripping, multi-wavelength studies are required. With this
approach it is in fact possible, to observe the different phases of the
stripped gas (e.g. Poggianti et al., 2019a), to reconstruct the star formation
history of the galaxy (visible in the optical band, e.g., Bellhouse et al.,
2019) and to understand the impact of the magnetic fields on the stripping
process (visible in the radio band, e.g. Müller et al., 2021). However, the
high-energy side of jellyfish galaxies is still deeply unexplored, with only
few objects studied in details: ESO 137-001 and ESO 137-002 in Abell 3627 (Sun
et al., 2010; Zhang et al., 2013), the GASP galaxy JW100 in Abell 2626
(Poggianti et al., 2019a) and also NGC 6872 in the galaxy group Pavo (Machacek
et al., 2005), NGC 4569 in Virgo (Tschöke et al., 2001) and UGC 6697 in A1367
(Sun & Vikhlinin, 2005), although for these three latter galaxies a further
contribution arising from tidal interactions cannot be completely excluded.
These studies found that the galactic extended X-ray emission is produced by a
warm plasma with temperature in the range kT $\sim$ 0.7 - 1 keV, likely
originated by the interplay between the interstellar medium (ISM) and the ICM
on the surface of the stripped tails. From the analysis of JW100, it was
observed that the star formation rate (SFR) is not able to explain the
observed X-ray luminosity of the galaxy and, thus, an additional emission
mechanism is required. Moreover, a spatial correlation between X-ray and
H$\alpha$ surface brightness was found and was interpreted as evidence of the
strict connection between the X-ray emitting plasma and the ISM. Based on
these results and on previous studies about the phase transformation of the
ram pressure stripped gas through shocks, heat conduction, magneto-
hydrodynamic waves and turbulence (e.g., Cowie & Songaila, 1977; Nulsen, 1982;
Gavazzi et al., 2001; Sun et al., 2006, 2007, 2010; Fossati et al., 2016;
Boselli et al., 2016), it was argued that the origin of the extended X-ray
emission observed in jellyfish galaxies could be related to the ongoing
stripping process, with the X-ray emitting plasma being the results of the
complex ICM-ISM interaction triggered by the stripping, which causes either
the heating of the ISM through shocks and conduction, or the cooling of the
ICM onto the galaxy or the ICM-ISM mixing.

With the aim to expand the sample of jellyfish galaxies with detailed X-ray
analysis, and with the purpose to address the open questions triggered by the
JW100 study, we carried out an X-ray investigation of another jellyfish galaxy
of the GASP sample, JO201. This galaxy has been extensively studied at many
wavelengths (e.g., Bellhouse et al., 2017, 2019, 2021; George et al., 2018;
Moretti et al., 2020; Ramatsoku et al., 2020) and disposes of deep, archival
Chandra observations, necessary for a detailed X-ray analysis. This paper is
structured as follows. In Section 2 we present the main properties of JO201
and its host cluster Abell 85; in Section 3 we describe the procedures adopted
for the Chandra data reduction and extraction of spectra. In Section 4 we
present the results of our detailed spectral analysis, and discuss them in
Section 5. Conclusions are presented in Section 6. Throughout this paper, we
assume H0= 70 km s-1 Mpc-1, $\Omega_{M}=0.3$ and $\Omega_{\Lambda}$= 0.7 (1
arcsec = 1.085 kpc at z = 0.05586) and errors are given at the 1$\sigma$
level.

##  2 JO201 and the cluster Abell 85

As mentioned in the Introduction, to fully understand the ongoing stripping
process in JO201, it is necessary to use a multi-wavelength approach and to
take into account the properties of the environment in which the galaxy is
located. For this reason, we briefly present in the following subsection the
results of previous studies of both JO201 and its host cluster Abell 85 (A85).

###  2.1 The galaxy cluster Abell 85

Abell 85 (z=0.05586 and M200=1.58 $\cdot 10^{15}$$M_{\odot}$, Moretti et al.,
2017) is one of the brightest galaxy clusters in the X-ray sky (Edge et al.,
1990) and hosts the largest brightest central galaxy (BCG) ever observed in
the optical band (López-Cruz et al., 2014), namely Holm 15A. Despite being
classified as a cool-core cluster, A85 is undergoing a merger with two sub-
clusters: one from the south (S) and the other one from the south-west (SW).
In order to investigate the peculiar dynamical state of A85, Ichinohe et al.
(2015) analysed its X-ray emission by means of a deep Chandra exposure. This
analysis highlighted the presence of an apparent brightness excess spiral,
starting north of the core and extending counter-clockwise outward from the
core out to $\sim$600 kpc. This feature is attributable to the sloshing of the
ICM in the gravitational potential of the cluster triggered by previous merger
events. Furthermore, it was observed that the S sub-cluster core is almost
entirely stripped of the low-entropy gas, providing a case of efficient
destruction of a cool core during a merger. Starting from the X-ray spectral
analysis, radial profiles of the thermal properties of the cluster were
derived. According to them, in the annular region between r${}_{\text{1}}$=200
kpc and r${}_{\text{2}}$=400 kpc, where JO201 is located, a temperature of
kT$\simeq 6.7$ keV, a density of n${}_{\text{e}}\simeq$ 10-3cm-3 and an ICM
pressure of p$\simeq$10-2 keV cm-3 were measured.

Figure 1: Exposure-corrected mosaic of the cluster Abell 85, Gaussian-smoothed
with $\sigma$=2.48 arcsec, obtained by combining the five multi-chip Chandra
observations and by filtering in the 0.5-2.0 keV band (see §3.1 for details).
In the zoomed-in image is shown the galaxy JO201: white contours represent the
H$\alpha$ emission with S/N$>$3 (Bellhouse et al., 2019), while the gold
contour represents the stellar disk region (Gullieuszik et al., 2020).

###  2.2 JO201: the current state of the art

JO201 is a Seyfert 2 galaxy (RA 00:41:30.325, Dec -09:15:45.96, Bellhouse et
al., 2017) located at a distance of 360 kpc north west (NW) from the BCG (RA
00:41:50.54, Dec -09:18:13.07, Edwards et al., 2016); it is characterised by a
stellar mass of $M_{s}=3.55^{+1.24}_{-0.23}\cdot 10^{10}$ M⊙ and a line of
sight velocity of v$=3363.7$ km $\text{s}^{-1}$ with respect to the mean
velocity of the cluster (Bellhouse et al., 2017; Vulcani et al., 2018). Its
proximity to the centre of A85, along with its very high velocity, make the
stripping process extremely efficient. Referring to the main questions that
the GASP project aims to answer (Poggianti et al., 2017a), in this subsection
we present both the results obtained from previous optical and radio studies
of JO201 and the open questions.

  1. 1.

For how long, where and why does gas removal occur? The motion of JO201
develops almost exclusively along the line of sight in the direction of the
observer, providing a unique perspective on the stripping process. The
orientation of the galaxy is indeed face-on and the interaction with the ICM
occurs frontally. This allows to recognise a particular pattern in the
distribution of the H$\alpha$ emission attributable to the unwinding of the
spiral arms (Bellhouse et al., 2021). Observations revealed that the
efficiency of the stripping is such that gas was removed even from the
innermost part of the stellar disk. From N-body simulations it was estimated
that the process has been ongoing for $\sim$ 0.6-1.2 Gyr and thus a long
extension of the gas tail is expected (Bellhouse et al., 2019). H$\alpha$
observations highlight the presence of gas up to a distance of 50 kpc from the
disk, but this value could be underestimated due to the particular orientation
of JO201. For this reason, Bellhouse et al. (2019) resorted to simulations to
investigate the motion and dynamics of a cloud of gas that is accelerated away
from the galaxy, after being stripped by ram pressure, finding that the real
length of the tail is probably $\sim$ 94 kpc;

  2. 2.

What is the impact of the RPS process on star formation? Previous studies
found that JO201 has a total integrated SFR of $6\pm 1$ M⊙yr-1, which is
0.4–0.5 dex above the main sequence of non-stripped disk galaxies (Vulcani et
al., 2018), and shows a molecular gas content 4-5 times higher than normal
galaxies of similar mass (Moretti et al., 2020). In particular, the neutral
gas mass (HI) is 14 times lower than the molecular gas mass ($H_{\text{2}}$),
which is in contrast with the behaviour of normal spiral galaxies. The
hypothesis suggested until now to interpret these features, is that ram
pressure could be able to compress the gas, increasing the SF and converting
HI into H${}_{\text{2}}$ effectively (Ramatsoku et al., 2020);

  3. 3.

What is the impact of the AGN activity on the RPS process? JO201 hosts an AGN
whose activity influenced the central region, both ionising the surrounding
gas (Poggianti et al., 2017a; Bellhouse et al., 2019; Radovich et al., 2019)
and quenching the star formation (George et al., 2019). However, a detailed
analysis of its X-ray emission has not been carried out yet.

Therefore, it clearly appears that there are still several shortcomings in the
understanding of the ongoing stripping process in JO201, regarding the nature
of the interaction between the ISM and the ICM, and the properties of the AGN.
In this paper, we aim at investigating the origin of the extended X-ray
emission associated to JO201, by means of a dedicated re-analysis of the
archival Chandra observations (first presented by Ichinohe et al., 2015). In
particular, we aim at discerning if the X-ray emission is linked to the
intense ongoing star formation or to a further contribution caused by the RPS
process and also at analysing the AGN, in order to understand the origin of
its high-energy emission.

##  3 Data analysis

###  3.1 Chandra data processing

We analysed archival Chandra observations of the cluster A85 consisting of
five different exposures (ObsID 15173, 15174,16263, 16264, 904, see Table 1)
hosting the galaxy JO201. Each observation was obtained with the ACIS-I
instrument in the VFAINT mode and the cluster emission extends over all the
four chips. Data were downloaded from the Chandra Data
Archive111https://cda.harvard.edu/chaser/ and were reprocessed with the
software package CIAO (version 4.11) and CALDB (version 4.8.3) to apply the
latest calibration and to remove bad pixels and flares. The net exposure times
obtained after this phase of data cleaning are summarised in Table 1. We also
improved the absolute astrometry identifying the point sources with the task
WAVDETECT and cross-matching them with the optical catalogue
USNO-A2.0222http://tdc-www.harvard.edu/catalogs/ua2.html. For background
subtraction we first identified and re-project the ACIS background files
(blanksky) that matches our data; then we treated each chip of each blanksky
separately, normalising them to the count rate of the corresponding chip of
the source image in the 9-12 keV band.

For each observation we produced an image in units of counts (counts/pixel),
an exposure map (cm${}^{2}\cdot$s$\cdot$counts/ photons) and an image in units
of flux (photons/(cm${}^{2}\cdot$s$\cdot$pixel)) resulting from the division
between the image in units of counts and the exposure map. Combining the five
images in units of flux and filtering data in the 0.5-2.0 keV band, we
produced the exposure-corrected mosaic shown in Figure 1.

Table 1: Summary of the observations; the net exposure times are the values after cleaning. ObsID | Date |  | Net exposure
---
time (ks)
PI
15173 | 2013-08-14 | 40.2 | S. Allen
15174 | 2013-08-09 | 38.2 | S. Allen
16263 | 2013-08-10 | 36.3 | S. Allen
16264 | 2013-08-17 | 34.3 | S. Allen
904 | 2000-08-19 | 37.9 | C.L. Sarazin

###  3.2 X-ray morphology

From the exposure-corrected mosaic (Figure 1), it is possible to recognise the
main features of the cluster A85 presented in §2.1. In particular, in the S
and SW regions we identify the two sub-clusters that are falling towards the
centre of A85. Despite the presence of these two mergers, the gas distribution
follows a spherical symmetry elsewhere in the cluster.

To the NW of the BCG, in correspondence to the position of JO201, it is
possible to observe an extended X-ray emission. Specifically, we distinguish a
central region related to the centre of the galaxy disk including the AGN and
a southern, arcuate region resembling the unwinding of the spiral arm
(Bellhouse et al., 2021). The X-ray emission is less extended than the optical
disk, but, as in the case of JW100 (see §1), it coincides spatially with the
H$\alpha$ emission (see also Figure 6).

###  3.3 Point-to-point analysis

In order to unfold the connection between the X-ray emitting plasma and the
galactic warm gas, we carried out a point-to-point study of the spatial
correlation between the two respective emissions, i.e. X-rays and H$\alpha$.

We combined the Chandra image with MUSE H$\alpha$ cubes and emission fraction
of the diffuse ionised gas (DIG) maps (Reynolds, 1985; Haffner et al., 1999;
Reynolds et al., 1999; Tomičić et al., 2021). The H$\alpha$ observations of
JO201 were taken as part of the GASP program with the MUSE IFU instrument at
the Very Large Telescope and processed following the standard procedure for
GASP galaxies (Poggianti et al., 2017a). The observations was further
corrected for extinction by dust within the Milky Way by estimating the
contribution using the NASA/IRSA infrared science archive. In order to extract
emission line fluxes, the cube was analysed using the IDL code kubeviz
(Fossati et al., 2016) after smoothing spatially with a 5x5 kernel. kubeviz
fits a series of gaussian components to selected emission lines in each spaxel
of the IFU data. The resulting map of H$\alpha$ emission line flux was used in
the following analysis.

The maps of emission lines corrected for attenuation, were calculated assuming
the intrinsic Balmer line ratio of $\rm H\alpha/H\beta=2.86$, a ionised gas
temperature of $\rm T\approx 10^{4}$ K and case B recombination (Osterbrock &
Martel, 1992). Finally, we discriminated the ionisation source of the
H$\alpha$ emission (SF, LINER or AGN) according to the [OI] diagnostic BPT
diagram (Baldwin et al., 1981; Veilleux & Osterbrock, 1987). As reported in
Poggianti et al. (2019b), at distances of 10-50 kpc from the galaxy centre,
JO201 shows [OI]/H$\alpha$ ratios much higher than what observed in SF
regions. The other BPT line ratios ([NII]/H$\alpha$ and [SII]/H$\alpha$) are
instead closer to values measured in SF regions. Since the LINER-like emission
is extended and not concentrated in the nucleus, in this work we refer to
these regions as ”LIERs” rather than LINERs, following Belfiore et al. (2016).

Unlike the dense ionised gas in HII regions, the DIG component of the ISM has
low densities (n $\sim 10^{-1}$ cm-3), high temperatures (higher than $\sim
10^{4}$ K), and can reach large spatial distances from the HII regions (up to
1-2 kpc, Haffner et al., 2009). The DIG exhibits lower H$\alpha$ surface
density and higher $\rm[S\textsc{ii}]/H\alpha$ line ratio compared to the
dense gas emission. Fitting the anti-correlation between
$\rm[S\textsc{ii}]/H\alpha$ ratio (accounted for radial gas-phase metallicity
gradient) and the extinction corrected H$\alpha$ surface brightness,
$\rm\Sigma H\alpha_{corr}$, we derived the maps of fraction of H$\alpha$
emission due to DIG (labelled hereafter as $\rm C_{DIG}$) through JO201. The
details of the DIG fraction estimation are described in Tomičić et al. (2021).

The following point-to-point analysis was carried out with the PT-REX code
(Ignesti et al., 2020, Ignesti in preparation,) which is based on the Common
Astronomy Software Application package333https://casa.nrao.edu/ (CASA v. 6.0).
For sampling the H$\alpha$ emission, we resort to a grid composed of 5
arcsecond-large cells, corresponding to $\sim$5 kpc. This sampling-scale
allows us to reconcile the X-ray S/N of each cell (which increases with the
size of the cell) with the number of sampled points (which decreases with the
size of the cell). The resulting sampling scale is larger than the typical
size of SF-blobs ($\sim$1-2 kpc) evidenced by the H$\alpha$ emission in MUSE,
and therefore these substructures are sub-sampled in our analysis. In order to
optimise the signal of the galactic emission, we considered only the regions
of the H$\alpha$ image with S/N$>$3 and we excluded all those pixels that,
according to the BPT diagram, are characterised by the AGN-related emission.
Finally, we used the stellar disk mask (Figure 2) to discriminate the cells
within the stellar disk (i.e. those that overlap with the stellar disk mask
for $>50\%$ of the cell area) from those located over the tail.

We further refined our sampling by identifying those cells with $>50\%$ SF-
related emission (red) and $>50\,\%$ LIER-related emission (blue). However,
since it was not possible to determine a BPT classification for every spaxel,
which is only accepted if all the 4 lines invoked have S/N$>$3, we discern the
dominant process only in the brightest regions of the galaxy. For this reason,
the BPT-spatial correlation analysis does not cover the whole galaxy, but is
still able to provide insights of the general trends.

Figure 2: MUSE H$\alpha$ image with the stellar disk (magenta) and the
sampling grid (black) on top, including the SF- and LIER-dominated regions in,
respectively, red and blue.

The sampling grid obtained is reported in Figure 2 and is composed of 63
cells, of which 18 were classified as SF-dominated and 7 as LIER-dominated. We
note that, considering the resolution of the image, the large number of
sampling cells assure us that our results are not biased by the choice of the
geometry of the sampling grid (we refer to Ignesti et al., 2020, for a more
detailed discussion of this issue).

For each cell, we measured both the total H$\alpha$ surface brightness and the
DIG fraction, which we estimated as the total DIG flux density (obtained by
multiplying $\rm C_{DIG}$ and H$\alpha$ in each spaxel) of each cell divided
by the H$\alpha$ flux density, and compared them to the X-ray surface
brightness. The correlations were evaluated not only for the whole population,
but also for the SF and LIERs regions separately (Figure 4). In particular,
using the orthogonal BCES algorithm, which is able to account for the internal
scatter of the data, (Akritas & Bershady, 1996), we fitted the data with a
power-law relation $I_{H\alpha}\text{, DIG fraction}\propto I_{\text{X}}^{k}$.
Finally, in order to estimate the strength of the linear correlations, we
computed the Pearson and Spearman correlation ranks ($\rho_{\text{P}}$ and
$\rho_{\text{S}}$), which are reported in the legend of Figure 4, the p-values
($p_{\text{P}}$ and $p_{\text{S}}$), and the correspondent correlation
probabilities ($P_{\text{c,P}}$ and $P_{\text{c,S}}$). The best-fitting
indexes, correlation ranks, p-values and probabilities are reported in Table
3.

###  3.4 Spectral Analysis

####  3.4.1 Spectra extraction procedure

The spectral analysis of JO201 was carried out with the software package XSPEC
(version 12.10.1f, Arnaud, 1996). For each observation, we masked all the
point sources identified by the WAVDETECT in §3.1 except the AGN of JO201. In
order to study the nature of the interaction between the ICM and the galaxy,
we followed the approach proposed in Poggianti et al. (2019a) by considering
three regions. The first two relate to the galaxy: one (Figure 6) follows the
contours of the H$\alpha$ emission (Bellhouse et al., 2019) and resembles the
X-ray emission; the other one (Figure 3b, right panel) traces the contours of
the stellar disk and was chosen to compare the SFR estimated from the X-ray
luminosity to the SFR obtained in a previous study on the H$\alpha$ emission
in the disk (Vulcani et al., 2018; Gullieuszik et al., 2020). The third region
(Figure 3b, left panel) was used to determine the properties of the ICM at the
same clustercentric distance of JO201. Under the assumption of the spherical
symmetry of the plasma distribution in the northern region of the cluster in
which JO201 is moving, we selected an annular region excluding the southern-
west part of Abell 85, where the merger with the sub-clusters S and SW is
ongoing (see §2.1).

(a)

(b)  Figure 3: Left panel: exposure corrected mosaic of the cluster A85 in the
0.5-2.0 keV, in green is reported the region used for the ICM spectra
extraction. Right panel: zoomed-in image of JO201, in green is represented the
region used to extract spectra from the stellar disk mask, while in gold is
reported the contour of the stellar disk (Gullieuszik et al., 2020) used as
reference.

In order to generate the appropriate response matrices, spectra were extracted
separately from each observation (or from each chip of each observation in the
case of the annular region) and were fitted jointly in the 0.5-7.0 keV band
after background subtraction. In the case of the ICM analysis, background
spectra were extracted from blanksky files in the same annular region
described above. For the galaxy analysis, instead, we tested different
extraction procedures which we summarise in Table 2. On the one hand, we
considered two different backgrounds, i.e.: case 1) normalised CALDB blanksky
files (see §3.1), or case 2) local background extracted in an annular region
at the same distance and on the same chip of the galaxy. The procedure
followed in this latter case allows to subtract the contribution of the ICM
located along the line of sight, that has not been modelled in the fit. On the
other hand, we treated the AGN with two different approaches: case A)
including it in the spectrum and modelling in the fit, or case B) excluding it
during the spectrum extraction, by masking an elliptical region of the same
size of the Chandra PSF at that distance from the pointing centre.

####  3.4.2 Spectral models

Table 2: Configuration used for the spectra extraction and background/AGN components added to the component that models the galactic emission (i.e. apec, cemekl or mkcflow)  |  | AGN included
---
(case A)
| AGN excluded
---
(case B)
| blanksky
---
(case 1)
| apec+pow
---
| apec
---
| Local background
---
(case 2)
| pow
---
| -
---

Following the approach presented in Poggianti et al. (2019a), we first
investigated the properties of the environment surrounding JO201 to model the
contribution of the ICM to the galactic X-ray emission. We fitted the ICM
spectra with an absorbed thermal model, tbabs$\cdot$apec. The tbabs component
represents the galactic absorption, and its column density parameter,
nH=2.7$\cdot$1020cm-2 was fixed; the apec component instead, describes the
emission of a single temperature plasma and its temperature, metallicity and
normalisation parameter were let free to vary. The abundance tables used for
the entire spectral analysis refer to Asplund et al. (2009).

We then focused on the galaxy to investigate the origin of its extended high-
energy emission. In particular, we explore four possible scenarios, which
attribute the emission to: i) the ongoing SF, ii) the stripping of a galactic
hot halo that the galaxy may have possessed, iii) either the cooling of the
ICM onto the galaxy, or the heating of the ISM through shocks and conduction,
iv) the mixing of the ICM cooling and ISM heating.

To discern which of these hypothesis is correct, we carried out the spectral
analysis of JO201. In particular, we tested complex models made of a galactic
component chosen among the following one:

  * •

an apec model, which describes the emission of a single temperature plasma and
can represent the SF, the hot halo scenarios, and, in a first approximation,
the ICM cooling- or the ISM heating-dominated scenarios;

  * •

a cemekl model, describing the emission of a multi-phase and multi-temperature
plasma where the emission measure, EM= $\int n_{\text{e}}n_{\text{H}}dV$,
scales with temperature as $EM\propto T^{\alpha}$ and T has an upper limit,
$T_{\text{MAX}}$. This model can represents both the hot halo and the ICM-ISM
mixing and from the index $\alpha$ it is possible to derive the mass-weighted
temperature of the plasma as $T_{mw}=T_{\text{MAX}}(1+\alpha)/(2+\alpha)$ (Sun
et al., 2010);

  * •

a mkcflow model, which is usually used to describe the thermal cooling of a
gas from a maximal, T${}_{\text{MAX}}$, to a minimal temperature,
T${}_{\text{min}}$. This model returns the mass accretion rate parameter
($\dot{\text{M}}$) and, in our picture, can represent the ICM cooling scenario
(Mushotzky & Szymkowiak, 1988).

We include also an additional component that, depending on the extraction
procedure used (see §3.4.1), models either the ICM along the line of sight or
the AGN emission. In particular, we add to the galactic component:

  * •

an apec component when the blanksky was used and the AGN was excluded (case
1B). This component takes into account the ICM emission along the line of
sight and its parameter were set to the value determined from the ICM
analysis;

  * •

both an apec and a powerlaw component when the blanksky was used and the AGN
was included (case 1A). We assume in fact, that the emission of this central
source can be modelled with a power law;

  * •

a powerlaw component to model the emission of the AGN when it was included and
the local background was used (case 2A);

  * •

no component when the AGN was excluded and the local background was used (case
2B).

In each test we account for the galactic absorption by including a tbabs
component with a column density of n${}_{\text{H}}$=2.7$\cdot$1020cm-2
(Kalberla et al., 2005). In Table 2 we summarise the four configurations used
to extract spectra and the relative components added to the galaxy emission
model.

##  4 Results

###  4.1 Spatial correlation

Figure 4:  Resulting $I_{H\alpha}$ (top) and DIG fraction (bottom)
correlations obtained with the grid presented in Figure 2. In the left panels
we report the distributions obtained by using every cells, where we report in
magenta the cells within the disk and in black those in the tail. In the right
panels are shown the spatial correlations of the SF (red) and LIER (blue)
regions. We best fit indexes are reported in the legends and summarised in
Table 3.

In Figure 4 and in Table 3 we report the results of the point-to-point
analysis carried out with the grid presented in Figure 2. The majority of
regions are characterised by lower H$\alpha$ and X-ray surface brightness and
higher DIG fraction, whereas the brightest spots, which are located in the
innermost part of the disk, show a lower DIG fraction. The BPT-based sampling
highlights that the SF-dominated regions have, generally, an higher
$I_{\text{H$\alpha$}}$ and lower DIG fraction with respect to the LIER-
dominated regions.

We observe a positive correlation between H$\alpha$ and X-ray surface
brightness (Figure 4, top-left panel), and a putative anti-correlation between
DIG fraction and X-ray surface brightness (Figure 4, bottom-left panel). The
two correlations become more evident when we limited our analysis to the SF-
dominated regions. On the contrary, LIER-dominated regions do not show any
evident correlation (Figure 4, right panels).

Table 3:  Results of the point-to-point analysis presented in Figure 4. From left to right: sampled points; Index of the best-fit power-law; Pearson rank; Pearson p-value; Pearson correlation probability; Spearman rank; Spearman p-value; Spearman correlation probability. | $k$ | $\rho_{\text{P}}$ | $p_{\text{P}}$ | $P_{\text{c,P}}$ | $\rho_{\text{S}}$ | $p_{\text{S}}$ | $P_{\text{c,S}}$
---|---|---|---|---|---|---|---
$I_{H\alpha}$ vs $I_{\text{X}}$
Total | 5.17$\pm$0.73 | 0.63 | 3.32$\times 10^{-8}$ | 0.99 | 0.58 | 7.99$\times 10^{-7}$ | 0.99
SF | 3.32$\pm$0.55 | 0.76 | 2.86$\times 10^{-4}$ | 0.99 | 0.80 | 5.82$\times 10^{-5}$ | 0.99
LIER | none | 0.02 | 0.96 | 0.04 | -0.39 | 0.38 | 0.62
DIG fraction vs $I_{\text{X}}$
Total | -2.58$\pm$0.74 | -0.37 | 2.86$\times 10^{-3}$ | 0.99 | -0.26 | 3.95$\times 10^{-2}$ | 0.96
SF | -1.29$\pm$0.30 | -0.65 | 3.25$\times 10^{-3}$ | 0.99 | -0.66 | 3.16$\times 10^{-3}$ | 0.99
LIER | none | -0.14 | 0.82 | 0.18 | 0.25 | 0.59 | 0.41

###  4.2 Results of the spectral analysis

In this subsection we report the results obtained by the spectral analysis of
both the AGN and the galactic emission.

####  4.2.1 AGN

The central source in JO201 has a shape consistent with a point source at a
distance of $\sim$5 arcmin from the aimpoint. We extract the spectrum from a
circular region of 4 arcsec enclosing the central source, and we use the
surrounding annulus as a background that includes the underlying cluster (A85)
background, the instrumental background and the galaxy diffuse background. The
resulting number of net counts is not enough to allow a thorough spectral
analysis: with around 100 net counts, the extracted spectrum is well fitted by
a power law, absorbed by the Galactic n${}_{\text{H}}$. However, the resulting
slope, $\Gamma$ = 2.7$\pm$ 0.2 is steeper than usual in AGNs. A more complex
model is therefore adjusted to the data: a pexrav with the addition of a
Gaussian line. The model describes the reflection spectrum off the walls of
the torus surrounding the AGN. The match is good with an intrinsic ”canonical”
$\Gamma$ = 1.7 and a high degree of reflection, as in a Compton Thick AGN. The
energy of the Gaussian line is consistent with a 6.4 keV K$\alpha$Fe emission
redshifted at the z=0.055 of the galaxy. The resulting luminosity is
L${}_{X}^{0.5-10keV}$=2.7 $\times 10^{41}$ erg s-1 444We note that this is
lower than reported in Poggianti et al. (2017b) (L${}_{X}^{0.3-8keV}$=7.3
$\times 10^{41}$ erg s-1) due to the different spectral model we adopted..
However, the intrinsic luminosity could be at least a factor of a hundred
higher, given the torus absorption. This is also in accordance with the
optical classification of the source as a Type 2 (Radovich et al., 2019). A
higher statistics, or observations at higher energies would be needed to
derive precise parameters for the source.

####  4.2.2 Galaxy

Our spectral analysis of the X-ray emission associated to JO201 did not reveal
significant discrepancies between the four procedures tested for the spectra
extraction (see §3.4.1). In this section we report the tests performed with
the configuration that requires the simplest model and ensures the higher
number of counts, i.e. 1B (blanksky and AGN subtraction). The spectral results
obtained with the H$\alpha$ and the stellar disk mask are consistent in terms
of both the best-fit parameters and the $\chi^{2}$ values. For this reason,
aiming to realise a comparison between the SFR estimated from the X-ray
luminosity and the SFR found in (Vulcani et al., 2018) from the H$\alpha$
emission in the disk, we choose to present the results obtained with the
stellar disk mask, whereas those obtained with the H$\alpha$ mask are
presented in the Appendix A.

Following the approach described in Section 4, we first estimated the
properties of the surrounding ICM. In agreement with previous results of
Ichinohe et al. (2015) (see § 2.1), we found a temperature of kT=$7.1\pm 0.2$
keV and a metallicity of Z=$0.21\pm 0.04$ Z⊙. Based on the temperature, we
estimated a local speed of sound $c_{s}\simeq 520\sqrt{k\text{T}/\text{(1
keV)}}\simeq 1376$ km s-1, which entails a galactic Mach number
$\mathcal{M}\simeq 2.4$ .

We then carried out the modelling of the X-ray emission from the region of the
stellar disk, following the procedure described in section 3.4.2. In
particular, we built our model by adding to the galactic component (apec,
cemekl or mkcflow) an apec component that takes into account the ICM emission
along the line of sight (1B configuration in Table 2); we report the results
of the fits in Table 4.

Our criterion to attest the validity of a model, was based not only on the
values of the $\chi^{2}$ obtained, but also on the values of the temperature,
T, and metallicity, Z revealed by the fits. Thus, we have found a temperature
of kT=0.79${}^{+0.08}_{-0.08}$ keV for the model 1) apec+apec,
kT${}_{\text{MAX}}$=1.2${}^{+0.4}_{-0.2}$ keV for the model 3) apec+mkcflow,
and kT${}_{\text{MAX}}$=1.1${}^{+0.5}_{-0.2}$ keV for the model 4)
apec+mkcflow. In all these three cases, the values obtained are lower than the
ICM temperature, sign that the emitting plasma is in an intermediate phase
between the one of the ICM and of the ISM. For the model 2) apec+cemekl,
instead, we noticed that by leaving the temperature parameter free to vary it
was not possible to obtain a good fit of the data. Since the cemekl model
describes the emission of a multi-phase and multi-temperature plasma, we
supposed that the maximal value that the temperature can assume is the value
of the ICM temperature, and for this reason we fixed $T_{\text{MAX}}=7.1$ keV.
The best-fit $\alpha=0.01^{+0.10}_{-0.01}$ would entail a $T_{mw}\simeq 3.5$
keV and an almost uniform EM of the plasma.

Regarding metallicity, we note that, when it was set free (models 1 and 4),
the Z values obtained are lower than that of the ICM. We considered these
values implausible, because the X-ray emitting plasma is either the ICM, or
the ISM or a mixing of these two components and thus its metallicity is
expected to vary between the values of the ICM metallicity
(Z${}_{\text{ICM}}$=0.21 Z⊙, see § 3.4.2) and of the ISM metallicity (typical
values are Z$\sim$1 Z⊙ ). In the other two cases (models 2 and 3) it was not
possible to obtain a good fit of the data by letting this parameter free, for
this reason we fixed it to the value of the ICM.

Under a statistical point of view, the four models are indistinguishable
because they fit equally well the observations, as it is possible to notice
from the values of the $\chi^{2}_{\text{R}}$.

Table 4: Fit results for spectra extracted from the ICM region (”Control region”, Figure 1) and from the stellar disk mask (”JO201”, Figure 3b, right panel). For this latter region, we resort to the 1B extraction configuration, which models the ICM emission with an additional apec component (see §3.4.2). The best-fit parameters shown in the third column refer only to the galactic component indicated in the second column (namely 1) apec, 2) cemekl, 3) and 4) mkcflow). | Model |  Parametersa |  |  $\chi^{2}$/dof
---
$\chi^{2}_{\text{R}}$
Control region (ICM) |  |  0)tbabs$\cdot$apec
---
|  kT=7.1$\pm 0.2$ , Z=0.21$\pm 0.04$
---
F=(8.65$\pm$0.17)$\cdot$10-13, L=(2.73 $\pm$ 0.08)$\cdot$1042
| 1694.7/1617
---
1.048
JO201 |  |  1) tbabs$\cdot$(apec+apec)
---
|  kT=0.79${}^{+0.08}_{-0.08}$ , Z=0.08${}^{+0.08}_{-0.04}$
---
F=(4.1$\pm$0.5)$\cdot$10-14, L=(1.9 $\pm$ 0.3)$\cdot$1041
| 43.18/47
---
0.9187
|  2) tbabs$\cdot$(apec+cemekl)
---
Z=0.21 and
kT${}_{\text{MAX}}=7.1$ keV (fixed)
|  $\alpha$=0.01${}^{+0.10}_{-0.01}$
---
F=(9.4$\pm$0.8)$\cdot$10-14, L=(4.5$\pm$ 0.4)$\cdot$1041
| 40.76/48
---
0.8492
|  3) tbabs$\cdot$(apec+mkcflow)
---
Z=0.21 (fixed)
|  kT${}_{\text{min}}$=0.08${}^{+0.27}_{-0.08}$ ,
kT${}_{\text{MAX}}$=1.2${}^{+0.4}_{-0.2}$
---
$\dot{M}=1.7_{-0.6}^{+0.4}$
F=(4.4$\pm$0.6)$\cdot$10-14, L=(1.9 $\pm$ 0.3)$\cdot$1041
| 40.07/47
---
0.8526
|  4) tbabs$\cdot$(apec+mkcflow)
---
|  kT${}_{\text{min}}$=0.08${}^{+0.35}_{-0.08}$ ,
kT${}_{\text{MAX}}$=1.1${}^{+0.5}_{-0.2}$
---
$\dot{M}=2.1_{-1.1}^{+3.5}$. Z=0.13${}^{+0.27}_{-0.09}$
F=(4.3$\pm$0.5)$\cdot$10-14, L=(3.3 $\pm$ 0.4)$\cdot$1041
| 39.78/46
---
0.8609
Notes: The unit measure used are: keV for temperature (kT), Z⊙ for metallicity
(Z), M⊙ yr-1 for the mass accretion rate ($\dot{\text{M}}$), erg
s${}^{\text{-1}}$ cm ${}^{\text{-2}}$ for flux (F) and erg s${}^{\text{-1}}$
for luminosity (L). The unabsorbed flux and luminosity refer only to the
galactic component and were measured in the 0.5-10.0 keV energy band.

##  5 Discussion

###  5.1 Origin of the X-ray emission

The correlation between X-ray and H$\alpha$ surface brightness (Figure 4, top
panels) indicates that the galactic X-ray emitting medium closely follows the
spatial distribution of the ISM. This result suggests that the X-ray emission
arises from local, small scale processes taking place in the close proximity
of the ISM. A similar behaviour was also observed both in the disk of isolated
galaxies (Ranalli et al., 2003; Symeonidis et al., 2011; Mineo et al., 2014)
and in previous studies of Sun et al. (2010) and Poggianti et al. (2019a) on
jellyfish galaxies. In the first case, given the lack of a hot surrounding
ICM, the correlation was interpreted as a consequence of recent star forming
processes. In the second case, since JW100 and ESO-137-001 are located in a
cluster environment and show an X-ray luminosity higher than what expected
from star formation, it was suggested that the high-energy emission arises
from the local interplay between the ICM and the ISM and that it could be
produced by the interface between the ICM and the ISM.

Starting from these two different scenarios we investigated the contribution
of the SF to the X-ray luminosity of JO201, following the approach presented
in Poggianti et al. (2019a). In presence of SF indeed, the X-ray luminosity,
of a galaxy is usually dominated by the contribution of both high mass X-ray
binaries, whose characteristic lifetime is $10^{7}$ yr, and hot ISM, which is
ionised by supernovae and massive stars (e.g., Mineo et al., 2014).Since the
sum of these contributions correlates with other SFR indicators, many
relations have been developed to link the $L_{\text{X}}$ of a galaxy to its
SFR (and vice versa).
Using the $L_{\text{X}}$-SFR calibration of Mineo et al. (2014) converted from
a Salpeter to a Chabrier IMF and from 0.5-8 keV to 0.5-10 keV assuming a
factor 1.11:

$L_{X,0.5-10.0keV}=7.6\cdot 10^{39}\,\text{SFR}\,\,\,\text{erg s}^{-1}$ (1)

where the SFR is in unit of M⊙yr-1, we estimated a $L_{\text{X}}\sim 3.8\cdot
10^{40}$ erg s-1, which is lower by one order of magnitude than the values
found by our models (see Table 4).

From this result, we conclude that the SF alone is not able to produce the
observed X-ray luminosity and that an additional contribution is necessary,
likely coming from the ongoing stripping process. This discrepancy between
predicted and observed X-ray luminosity has been observed also for JW100 (by a
factor of $4-10$ Poggianti et al., 2019a), thus we suggest that it could be a
common feature among jellyfish galaxies with extended X-ray emission. Further
studies in this direction are now necessary to test our speculations.

The point-to-point analysis also revealed that the SF regions are generally
above the LIERs ones in the H$\alpha$-X plot (Figure 4). An alternative view
on these results is that, for similar values of $I_{H\alpha}$ or DIG fraction,
the LIER regions show higher values of $I_{\text{X}}$ than SF regions. This
latter feature is similar to what was previously observed in JW100 (Poggianti
et al., 2019a). We also note that the main correlations are driven by the SF-
dominated regions, whereas the LIER regions, which are present only in the low
H$\alpha$ brightness or high-DIG fraction degree part of the plots, are less
correlated.

Concerning the connection between DIG fraction and X-ray emission, we observe
that LIER regions, which are located mostly along the stripped arm (Figure 2),
show a generally higher DIG fraction. This behaviour suggests that the DIG
regions in stripped tail may present more LIER features compared to the dense
gas in the stellar disk, which is SF-dominated. Finally, the putative anti-
correlation between the DIG fraction and $I_{\text{X}}$ could be explained by
the fact that the DIG dominated regions typically have lower H$\alpha$ flux
compared to the dense gas dominated regions (e.g., Madsen et al., 2006;
Haffner et al., 2009; Tomičić et al., 2021). Thus, following the
$I_{\text{H$\alpha$}}$-$I_{\text{X}}$ correlation, the DIG dominated regions
would show lower X-ray emission compared to the dense gas dominated regions.

Having established that the SF alone is not able to generate the X-ray
emission of JO201, we are left with the other three scenarios presented in
§3.4.2, i.e. stripping of the galactic hot halo, cooling of the ICM onto the
galaxy or heating of the ISM through shocks and conduction, mixing of the ICM
cooling and ISM heating. The stripping of the hot galactic halo that JO201 may
have possessed could produce an extra-planar X-ray emission. However, the
face-on orientation of the galaxy makes it difficult to verify this scenario.
Nevertheless, many studies (e.g. Larson et al., 1980; Bekki et al., 2002;
Zinger et al., 2018; Jaffé et al., 2018; Poggianti et al., 2019a; Gullieuszik
et al., 2020) highlighted that the majority of galaxies loose their halo in
their fall towards the cluster centre, before reaching the virial radius.
Given the proximity of JO201 to the cluster centre, well inside the virial
radius, we considered that the loss of its halo has already occurred and thus
that this scenario is implausible. Therefore, the remaining scenarios are the
heating of the ISM (through shocks, conduction or turbulent mixing resulting
from the interaction with the ICM), and the cooling of the ICM onto the galaxy
(again through conduction or mixing or a combination of the two scenarios).
From our spectral analysis we found that the models tested lead to
statistically equivalent results (see §4.2.2) and for this reason we are not
able to discern which of these hypothesis is the better one. Nevertheless, in
the following section we sift through each scenario trying to deepen our
knowledge about their physics and their implications.

###  5.2 Insights from the X-ray spectral models

In order to get further insights from the X-ray spectral analysis of JO201, we
explored in more depth the results obtained from the different spectral
models. As first step, we considered the model 1) tbabs$\cdot$(apec+apec) to
estimate the mass of the hot galactic plasma. As explained in Section 3.4.2,
the first apec represents the ICM emission along the line of sight. For this
reason, we can assume that the best-fit values reported in Table 4 and related
to the galactic component (namely to the second apec) are de-projected
quantities. Starting from this consideration, the normalisation of the apec
model is linked to the volume of the X-ray emitting plasma through the
relation:

$V=\frac{4\pi N[D_{\text{A}}(1+z)]^{2}}{n_{\text{e}}n_{\text{p}}10^{-14}}$ (2)

where $N$ is the normalisation parameter of the apec model, $z$ is the
redshift, D${}_{\text{A}}$=5.26$\cdot$1026cm is the angular distance, and
$n_{\text{p}}$ is the proton number density, (n${}_{\text{p}}$=0.8
n${}_{\text{e}}$, e.g., Gitti et al., 2012). We then followed two parallel
procedures. In the first case, we started from the assumption of pressure
equilibrium between the ICM (n${}_{e}\sim$10-3 cm-3 and kT=7.1 $\pm$ 0.2 keV)
and the galactic hot (kT $\sim$ 1 keV) plasma, finding that this latter
component is characterised by an electron density of
ne=(6.4$\pm$0.2)$\cdot$10-3cm-3. Using the relation 2 we deduced that this
plasma is included in a volume, V=(1.8$\pm$0.3)$\cdot$1069 cm3 and
consequently its mass is:

$M_{\text{gas}}=\rho_{\text{gas}}V=1.9n_{\text{e}}m_{\text{p}}\mu V=(1.1\pm
0.2)\cdot 10^{10}M_{\odot}$ (3)

where $\rho_{\text{gas}}$ is the mass density of the emitting gas, $\mu=0.6$
is the mean molecular weight and $m_{\text{p}}$ is the proton mass.

The second procedure adopted to estimate the mass assumes that the hot
galactic plasma occupies a cylindrical volume, with an height equal to the
length of the gaseous tail (94 kpc, Bellhouse et al., 2019) and an elliptical
basis with the same dimensions of the stellar disk (semi-minor axis,
$b_{\text{min}}$=$10.7\pm 0.5$ kpc, and semi-major axis,
$b_{\text{max}}$=$13.3\pm 0.5$ kpc, Gullieuszik et al., 2020), uniformly
filled by the hot plasma (i.e. with a filling factor $\phi=1$). As a result of
these assumptions, we consider our estimates to be an upper limit of the real
volume of the tail. We then estimate the n${}_{\text{e}}$ parameter by
inverting equation 2. Also in this case, we can assume that the normalisation
of the apec component associated to the galactic emission is a de-projected
quantity, because we are modelling the contribution of the ICM along the line
of sight with an additional apec component (see §3.4.2). The electron number
density measured in this case is $n_{\text{e}}=(8.0\pm 0.5)\cdot 10^{-3}$ cm-3
and consequently, using equation 3, the mass of the X-ray emitting gas is
M${}_{\text{gas}}=(9.2\pm 1.5)\cdot 10^{9}$ M⊙. We note that our estimates are
close to the predicted mass of gas with temperature $7\times 10^{5}\text{
K}<T<7\times 10^{7}\text{ K}$ reported by Tonnesen et al. (2011) for an high-
velocity jellyfish galaxy ($9.1\times 10^{9}$ M⊙).

The second model studied in detail is the tbabs$\cdot$(apec+mkcflow), which
describes the ICM cooling scenario. As reported in section §2.2, JO201 shows
in fact a high fraction of molecular gas and a high SFR, which suggest the
presence of an additional source of cold gas in the galaxy. For this reason,
we took into consideration the hypothesis that the dominant process
responsible for the extended X-ray emission could be the cooling of the ICM
onto the galaxy. In this scenario, the increase of the fraction of molecular
gas and the corresponding enhancement of the SFR would be fuelled by the
continuous accretion of cooling ICM onto the galaxy. In this treatment, we
refer to the model 3) (see Table 4) where the metallicity of the mkcflow
component is fixed to Z$=$ 0.21 Z⊙555By using the model 4), where the
metallicity was let free to vary, both a value of metallicity lower than that
of the ICM and a higher error for the mass accretion rate, $\dot{\text{M}}$,
were measured.. The spectral analysis determined a cooling rate of
$\dot{M}=1.7^{+0.4}_{-0.6}$ M⊙ yr-1, which is close to the observed excess of
SFR, $\sim 3M_{\odot}$ yr-1, with respect to the median SFR expected for a
galaxy of similar mass (Vulcani et al., 2018). This supports the idea that the
SFR could be actively fuelled by the cooling gas.

Finally, in order to constrain the limits of this pure radiative cooling
scenario, we estimated the cooling time of the X-ray emitting plasma with the
relation (e.g., Gitti et al., 2012):

$t_{cool}=\frac{5}{2}\frac{kT}{\mu X_{H}n_{e}\Lambda(T)}$ (4)

where XH=0.71 is the hydrogen mass function, and $\Lambda$(T) is the cooling
function (we have interpolated the table by Sutherland & Dopita (1993) as a
function of temperature and metallicity). By adopting the values of the
galactic hot plasma (ne=6.4$\pm$0.2 $\times$ 10-3cm-3 and kT$\sim$1 keV), the
resulting time-scale is t${}_{cool}\approx$ 3.6 Gyr, which is not consistent
with the time in which the galaxy has experienced the stripping process
(0.6-1.2 Gyr, see §2.2). This tension could be relieved if the galactic hot
plasma has a filamentary structure composed of several clumps with an higher
local density (i.e. a volume with $\phi<<1$), which would produce a patchy
thermal emission with local peaks where, due to the higher X-ray emissivity,
the ICM cools down faster. On the other hand, the discrepancy between the
average cooling time estimated above and the stripping time points out that
the cooling process could be not only radiative, but it could occur also
through conduction or mixing with the ISM (e.g., Fielding et al., 2020).

Figure 5: Comparison of line ratios in spaxels classified as LIERs (black
dots, distributed as shown in Figure 2) and CLOUDY photoionization models with
different metallicities (from left to right: $Z/Z_{\odot}=0.3,0.5,1$) and
ionization parameters ($-4\leq\log U\leq-3$), and $n_{H}$ = 10 cm-3. The upper
and lower panels shows the results obtained with column densities $\log
N_{H}=17,19.5$ respectively. The black solid and dotted curves display the
SF/LIER classification by Kewley et al. (2006).

###  5.3 Origin of [OI]/H$\alpha$ excess

A number of jellyfish galaxies show an excess of [OI]/H$\alpha$ emission in
their tails, whose origin is unclear (Fossati et al., 2016; Poggianti et al.,
2019a, b). On the basis of our results and those presented in Poggianti et al.
(2019a), we speculate that it might be related to the presence of a warm
plasma surrounding the stripping filaments, found in §4.2.2. To test this
hypothesis, we used the CLOUDY C17.02 (Ferland et al., 2017) photoionisation
code to predict the emission line ratios produced by a cold ISM cloud embedded
in a warm plasma, whose properties are constrained by our results (§ 4). To
this end, we first derived with CLOUDY the continuum emitted by a warm plasma
described as a collisionally ionised gas with $kT\simeq 1$ keV and a hydrogen
density $n_{H}\sim 10^{-2}$ cm-3: we then computed a grid of models for clouds
ionised by this continuum, varying the ionisation parameter ($U$), hydrogen
density ($n_{H}$) and column density ($N_{H}$), and metallicities ($Z$). The
ionisation parameter is defined as the ratio of the surface flux of ionizing
photons to the hydrogen density, $U=\Phi(H)/(n_{H}c)$ (Osterbrock & Ferland,
2006). We then looked for the models that provide the best fit to the line
ratios measured in the spaxels classified as LIER.

A good agreement is obtained with the following parameters: $n_{H}\leq 10$
cm${}^{-3},\log U\leq-3.5$, $Z/Z_{\odot}\sim 0.3-0.5$, $\log N_{H}\sim 19.5$.
The constraint on the column density is given by the relatively high
[OI]/H$\alpha$ ($\sim 0.1$), which requires that the clouds must be ionisation
bounded. The BPT diagram is displayed in Figure 5 and compares the observed
line ratios with those produced by photoionisation models in a region around
the best-fit parameters.

We conclude that the ionisation triggered by the warm plasma would be able to
reproduce the [OI]/Ha excess in the LIER regions, or at least significantly
contribute to it. A similar excess in [OI]/Ha was also found in the stripped
gas component of ESO137-001 (Fossati et al., 2016) and UGC 6697 (Consolandi et
al., 2017), who interpreted it to the presence of shocks. A more detailed
analysis, including data from more jellyfish galaxies, will be the subject of
a separate paper.

###  5.4 Comparison with previous works

Finally, we compare the results of our work with the previous studies of the
galaxies NGC 4569 (Tschöke et al., 2001), NGC 6872 (Machacek et al., 2005),
UGC 6697 (Sun & Vikhlinin, 2005; Consolandi et al., 2017), ESO 137-001 and
ESO110137-002 (Sun et al., 2010; Zhang et al., 2013) and JW100 (Poggianti et
al., 2019a). Despite the difference between these galaxies, in terms of
stellar mass and velocity, and the properties of the respective environments,
the X-ray analysis revealed a number of similarities that can help outlining
the origin of the X-ray emitting plasma. Here we list the common aspects:

  * •

The X-ray temperature derived for these galaxies with the single temperature
model, i.e. the average spectral temperature, is in between 0.7 and 1.0 keV.
This value is not consistent with neither the typical temperature of hot gas
in normal spirals (kT $\sim 0.3$ keV e.g., Strickland et al., 2004; Mineo et
al., 2012), nor the temperature of the surrounding environment;

  * •

The galaxies for which it was possible to reliably estimate the velocity show
similar Mach numbers $\mathcal{M}=2-2.5$;

  * •

For the only exception of NGC6872 which is part of a galaxy group, the ICM
thermal pressure surrounding these galaxies ranges between 0.8 and 4.2
$\times$ 10-11 erg cm-3. These values are consistent with threshold of $\sim
0.9\times 10^{-11}$ erg cm-3 defined in Tonnesen et al. (2011) for the
formation of bright H$\alpha$ and X-ray filaments.

For JW100 and JO201 we identify additional similarities based on the MUSE
analysis:

  * •

These galaxies show excess of [OI]/H$\alpha$ emission along their tails in
correspondence of LIER/composite regions. For JW100 the LIER emission
represent the majority of the H$\alpha$ emission outside the stellar disk,
whereas in JO201 we observe a lower fraction of LIER emission, but we can not
exclude a role of projection effects in this;

  * •

We observe in these galaxies a positive spatial correlation between H$\alpha$
and X-ray surface brightness, which suggests that in both galaxies the X-ray
emitting plasma follows closely the distribution of the stripped ISM (or vice
versa);

  * •

Although the different sampling techniques, we observe a similar behaviour
between star forming and LIER regions, where the former are generally located
to the top-left part of the $I_{\text{H$\alpha$}}$-$I_{\text{X}}$ plane with
respect to the latter (Figure 4). This could be interpreted either as an
excess of H$\alpha$ emission of the star forming region with respect to the
LIER regions, or as an excess of X-ray emission of the LIER regions respect to
the star forming blobs. However, we note that contrary to Poggianti et al.
(2019a), the LIER emission does not seem to correlate with the X-ray. This
difference could be due to projection effects, where the orientation of the
tail of the galaxy with respect to the line of sight reduces the number of
sampling cells at our disposal and strongly affects our results.

This comparison pictures a scenario where the X-ray emission arises from the
interplay between the ISM and the ICM over the stripped tails (as highlighted
by the spatial correlation) that is driven, among the other, by the galactic
Mach number and the environmental pressure. This physical framework resembles
that of a cold cloud embedded in an hot wind, which has been deeply explored
in literature by means of numerical simulations (e.g., Vollmer et al., 2001;
Tonnesen et al., 2011; Scannapieco & Brüggen, 2015; Sparre et al., 2020;
Kanjilal et al., 2021). Albeit all the works predict that the ICM-ISM
interplay would lead to the origin of a warm ”mixing layer” between the two,
the properties and the fate of this are still debated. Therefore, we suggest
that the X-ray emitting plasma we studied could correspond to the high-
temperature component of the ”mixing layer” predicted by the simulations.
Furthermore, we argue that the emergence of the [OI]/H$\alpha$ excess in the
stripped tails could be another consequence of this ICM-ISM interplay, as
consequence of ionisation of the ISM due to the proximity of a warm plasma.

The study of the polarised radio emission of jellyfish galaxies can provide
complementary insights into this scenario. Müller et al. (2021) discovered
that the magnetic field of another jellyfish galaxy of the GASP sample, JO206,
is remarkably ordered along the stripped tail. They suggest that this is
consequence of the draped accretion of the ICM onto the galaxy during its
motion, which produces a magnetised ”draping sheath” lit up by the cosmic rays
(Sparre et al., 2020). Transposed to our findings, this ICM draping process
could play a role in the origin of the X-ray emitting plasma by favouring the
ICM accretion onto the stripped tail of jellyfish galaxies. Exploring in
details this scenario requires similar radio studies for JO201 and/or deep
X-ray observations of JO206.

##  6 Summary and conclusions

In this paper we have investigated the thermal side of one of the most extreme
objects of the GASP sample, namely JO201. This jellyfish galaxy is
experiencing a strong stripping process triggered by the ICM of its host
cluster A85 and shows an extended X-ray emission. We carried out the first
detailed investigation of the X-ray emission of JO201 by means of both a
spectral analysis and a point-to point spatial correlation between the
H$\alpha$ and the X-ray emission. The dataset used was composed of five
Chandra archival observations (total exposure time t${}_{exp}\sim$ 187 ks),
MUSE H$\alpha$ cubes and emission fraction of the DIG maps.

We found that the X-ray luminosity of the galaxy is provided by two
contributions. The first one is related to the AGN
(L${}^{0.5-10\,\text{keV}}_{X}=2.7\cdot 10^{41}$erg s-1, not corrected for
intrinsic absorption), whose emission describes the reflection spectrum of the
walls of the torus surrounding the black hole. We reveal an intrinsic photon
index of $\Gamma$ =1.7 and a high degree of reflection, as expected in a
Compton-Thick AGN. The second contribution is provided by an extended source
associated to a warm plasma (kT$\approx$1 keV) whose X-ray luminosity
$L_{X}^{0.5-10\,\text{keV}}\approx$ 1.9-4.5 $\cdot$ 1041 erg s-1 is one order
of magnitude higher than the X-ray luminosity expected from the only star
formation (L${}_{X}^{0.5-10.0\,\text{keV}}$ $\sim$ 3.8 $\cdot$ 1040 erg s-1).
The correlation between the H$\alpha$ and X-ray surface brightness emerged
from the point to point analysis reveals that this galactic X-ray emitting
plasma closely follows the spatial distribution of the ISM. These behaviours
suggest that the X-ray extended emission associated to the galaxy results from
the local interplay between the ICM and the ISM and could be produced by the
interface between the two components which envelopes the stripped ISM.

In order to investigate the nature of this interplay we tested three different
spectral models. However, since they all provide statistically consistent
results, we are not able to discern if the X-ray emission is caused by the
heating of the ISM, the cooling of the ICM onto the galaxy or the mixing of
the ISM and ICM through shocks and conduction. We have therefore deepened our
study by taking into consideration the following peculiar properties of JO201
emerged from previous studies: the galaxy shows a high fraction of molecular
gas (4-5 times higher than what observed in galaxies of the same mass) and a
SFR 0.4–0.5 dex above the main sequence of non-stripped disk galaxies;
furthermore, it is characterised by an excess of [OI]/H$\alpha$ emission in
correspondence of its tail.

Starting from the high fraction of molecular gas and the high SFR, we suggest
the presence of an additional source of cold gas which could arise from the
cooling of the ICM onto the galaxy. We thus compare the cooling rate obtained
from the spectral analysis ($\dot{M}=1.7^{+0.4}_{-0.6}$ M⊙ yr-1) to the excess
of SFR ($\sim 3M_{\odot}$ yr-1), finding that the two values are very close.
However, the cooling time of the X-ray emitting plasma, t${}_{cool}\approx$3.6
Gyr, is higher than the stripping time, tstripping=1.2-0.6 Gyr. On the one
hand this discrepancy could be the result of a a filamentary structure
composed of several clumps with an higher local density where, due to the
higher X-ray emissivity, the ICM cools down faster. On the other hand, the
high cooling time could indicate that the cooling process is not only
radiative, but occurs also through conduction or mixing with the ISM.

In this work we also investigated the origing of the [OI]/H$\alpha$ excess
observed in JO201, as well as in other jellyfish galaxies. Specifically, we
tested the possibility that this emission could be originated by the presence
of the warm plasma surrounding the stripped ISM. For this reason, we simulated
with CLOUDY the behaviour of a cold cloud embedded in a warm plasma, whose
thermal properties were constrained by the spectral analysis. From the
comparison between the results of the simulations and the line rations
observed in JO201 by MUSE, we found that a good agreement is obtained assuming
sub-solar values for the metallicity of the warm plasma ($Z=0.3-0.5$). This
results is going to be analyzed in more details in a separate paper.

The comprehension of the result obtained in this work, as well as their
comparison with what revealed from previous studies, is crucial to determine
the direction of future analysis. The emerging scenario is that the
interaction between the ICM and the ISM originates a warm galactic plasma
which is responsible for both the X-ray emission and the [OI]/H$\alpha$ excess
in the optical spectrum. Furthermore, the constrain on the low metallicity of
the ionising plasma provided by our CLOUDY simulations could indicate that the
origin of the warm plasma is strictly connected to the ICM. This suggests that
the nature of the interaction between the ICM and the ISM is either the
cooling of the ICM or the mixing of these two plasma. On the basis of the
results obtained from the radio study of the jellyfish galaxy JO206 (Müller et
al., 2021), we speculate that this process could be a direct consequence of
the ICM draping and thus that joint radio and X-ray analysis can provide
complementary insights of this mechanism. The similarities observed in JW100
and ESO 137-001 (regarding X-ray temperatures, thermal pressure of the
surrounding ICM and in the case of JO201 and JW100 also of Mach numbers and
excesses of [OI]/H$\alpha$) suggest that the emerging mechanism could be a
common feature of jellyfish galaxies. Extending the procedure adopted in this
paper and in Poggianti et al. (2019a) to other objects of the GASP sample is
crucial to confirm this scenario. We also evince that a better overview on the
ram pressure stripping process could be reached with both deeper Chandra
observations, which provide the opportunity to characterize the thermal
properties of jellifish galaxies, and numerical simulations, which, starting
from observational constrains could allow to determine the balance between
cooling and heating in the ongoing stripping process.

We thank the Referee for the useful suggestions that improved the presentation
of the work. A.I. thanks C. Stuardi and G. Sabatini for the insightful
discussions. Based on observations collected at the European Organization for
Astronomical Research in the Southern Hemisphere under ESO programme
196.B-0578. This project has received funding from the European Research
Council (ERC) under the Horizon 2020 research and innovation programme (grant
agreement N. 833824). A.I., B.V., M.G. acknowledge financial contribution from
the grant PRIN MIUR 2017 n.20173ML3WW_001 (PI Cimatti). We acknowledge
financial contribution from the INAF main-stream funding programme (PI
Vulcani) and from the agreement ASI-INAF n.2017-14-H.0 (P.I. Moretti). YJ
acknowledges financial support from CONICYT PAI (Concurso Nacional de
Inserción en la Academia 2017) No. 79170132 and FONDECYT Iniciación 2018 No.
11180558. JF acknowledges financial support from the UNAM- DGAPA-PAPIIT
IA103520 grant, México. ACCL acknowledges the financial support of the
National Agency for Research and Development (ANID) / Scholarship Program /
DOCTORADO BECA NACIONAL/2019-21190049

##  Appendix A H$\alpha$ mask

Figure 6: Zoomed-in image of JO201, in green is represented the region used to
extract spectra from the H$\alpha$ mask, while in silver is reported the
contour of the H$\alpha$ emission (Bellhouse et al., 2019, used as reference).

We present here the results of the analysis of the X-ray spectrum extracted in
a region resembling the H$\alpha$ emission (Figure 6). The procedure followed
for the extraction and the fit of the spectra is the same used for the stellar
disk mask (see §4.2.2). In Table 5 we report the results of the fits obtained
combining the galaxy and the additional components as explained in section
§3.4.2; for two of these models we fixed the T${}_{\text{MAX}}$ parameter to
the values of the ICM. In particular, the model 5), apec+apec, reveals a
temperature of kT=0.79${}^{+0.11}_{-0.08}$ keV for the galactic component,
which is lower than the ICM temperature. A similar result is also obtained
with the model 6), apec+cemekl, where the k$T_{\text{MAX}}$ detected is
kT${}_{\text{MAX}}$=0.9${}^{+0.3}_{-0.3}$ keV, which is lower than the value
of the ICM and is compatible with the value found in case 1). For the other
two models, 7) apec+cemekl and 8) apec+mkcflow, we set the $T_{\text{MAX}}$
parameter to match the temperature of the ICM.

Regarding the metallicity, Z, we note that, when the temperature parameter is
free (models 5 and 6), the Z values obtained are lower than that of the ICM.
We considered these values implausible, because the X-ray emitting plasma is
either the ICM, or the ISM or a mixing of these two components and thus its
metallicity is expected to vary between the values of the ICM metallicity
(Z${}_{\text{ICM}}$=0.21 Z⊙, see § 3.4.2) and of the ISM metallicity (typical
values are Z$\sim$1 Z⊙). On the contrary, when the temperature is fixed to the
value of the ICM (T${}_{\text{ICM}}$=7.1 keV, see § 3.4.2) we reveal plausible
values of the metallicity, with Z=0.4${}^{+0.5}_{-0.2}\,Z_{\odot}$ in model 7
and Z=0.6$\pm 0.2\,Z_{\odot}$ in model 8. This behaviour attests the presence
of a temperature-metallicity degeneracy: the statistic at our disposal does
not allow us to reliably estimate at the same time both the temperature and
the metallicity parameter of the models used in our analysis.

Statistically, it is not possible to discern which emission model is preferred
by the fitting procedure because the $\chi^{2}_{\text{R}}$ values obtained
from the four fits are acceptable at 68 %.

Table 5: Fit results for spectra extracted from the H$\alpha$ mask (Figure 6. For this region we resort to the 1B extraction configuration, which models the ICM emission with an additional apec component (see §3.4.2). The best-fit parameters shown in the second column refer only to the galactic component indicated in the first column (namely 5) apec, 6) and 7) cemekl, 8) mkcflow). Model | Parameters |  |  $\chi^{2}$/dof
---
$\chi^{2}_{\text{R}}$
| 5) tbabs$\cdot$(apec+apec)
---
| kT=0.79${}^{+0.11}_{-0.08}$ Z=0.07${}^{+0.05}_{-0.03}$
---
F=(3.4$\pm$0.4)$\cdot$10-14 L=(2.0 $\pm$ 0.2)$\cdot$1041
| 57.02/49
---
1.164
| 6) tbabs$\cdot$(apec+cemekl)
---
| kT${}_{\text{MAX}}$=0.9${}^{+0.3}_{-0.3}$
$\alpha$=0.016${}^{+3.909}_{-0.017}$
---
Z=0.06${}^{+0.05}_{-0.02}$
F=(2.9$\pm$0.3)$\cdot$10-14 L=(1.6$\pm$ 0.2)$\cdot$1041
| 54.67/48
---
1.139
| 7) tbabs$\cdot$(apec+cemekl)
---
kT${}_{\text{MAX}}$=7.1 keV (fixed)
|  $\alpha$=0.01${}^{+0.27}_{-0.01}$
---
Z=0.4${}^{+0.5}_{-0.2}$ F=(2.7$\pm$0.2)$\cdot$10-14 L=(1.3 $\pm$
0.1)$\cdot$1041
| 53.57/49
---
1.093
| 8) tbabs$\cdot$(apec+mkcflow)
---
kT${}_{\text{MAX}}$=7.1 keV (fixed)
| kT${}_{\text{min}}$=0.08${}^{+0.17}_{-0.08}$ Z=0.6${}^{+0.2}_{-0.2}$
---
$\dot{M}=2.4_{-1.0}^{+3.4}$
F=(6.6$\pm$0.4)$\cdot$10-14 L=(3.19 $\pm$ 0.17)$\cdot$1041
| 67.78/49
---
1.383
Notes: The unit measure used are: keV for temperature (kT), Z⊙ for metallicity
(Z), M⊙ yr-1 for the mass accretion rate ($\dot{\text{M}}$), erg
s${}^{\text{-1}}$ cm ${}^{\text{-2}}$ for flux (F) and erg s${}^{\text{-1}}$
for luminosity (L). The unabsorbed flux and luminosity refer only to the
galactic component and were measured in the 0.5-10.0 keV energy band.

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