# Unveiling the interplay between the GASP jellyfish galaxy JO194 and its
environment with Chandra

Chiara Bartolini  Dipartimento di Fisica e Astronomia, Università di Bologna,
via Piero Gobetti 93/2, 40129 Bologna, Italy  Alessandro Ignesti INAF-Padova
Astronomical Observatory, Vicolo dell’Osservatorio 5, I-35122 Padova, 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  Anna Wolter INAF - Osservatorio Astronomico di
Brera, via Brera, 28, 20121, Milano, 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  Bianca M. Poggianti INAF-Padova
Astronomical Observatory, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
Marco Gullieuszik INAF-Padova Astronomical Observatory, Vicolo
dell’Osservatorio 5, I-35122 Padova, Italy  Jacopo Fritz Instituto de
Radioastronomía y Astrofísica, UNAM, Campus Morelia, A.P. 3-72, C.P. 58089,
Mexico  Neven Tomičić Dipartimento di Fisica e Astronomia, Università di
Firenze, Via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy  INAF –
Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50127 Firenze, Italy

###### Abstract

X-ray studies of jellyfish galaxies opened a window in the physics of the
interplay between intracluster medium (ICM) and interstellar medium (ISM). In
this paper, we present the study of an archival Chandra observation of the
GASP jellyfish galaxy JO194. We observe X-ray emission extending from the
stellar disk to the unwinding spiral arms with an average temperature of
$kT=0.79\pm 0.03$ keV. To investigate the origin of the X-ray emission, we
compare the observed X-ray luminosities with those expected from the star
formation rates (SFR) obtained from H$\alpha$ emission. We estimate an X-ray
luminosity excess of a factor $\sim 2-4$ with respect to the SF, therefore we
conclude that SF is not the main responsible for the extended X-ray emission
of JO194. The metallicity in the spiral arms
($Z=0.24^{+0.19}_{-0.12}~{}$\mathrm{Z}_{\odot}$$) is consistent with that of
the ICM around JO194 ($Z=0.35\pm 0.07$), thus we suggest that the ICM
radiative cooling dominates the X-ray emission of the arms. We speculate that
the X-ray plasma results from the ISM-ICM interplay, although the nature of
this interplay is still mostly unknown. Finally, we observe that the X-ray
properties of JO194 are consistent with those of two other GASP galaxies with
different stellar mass, phase-space conditions in their hosting clusters, and
local ICM conditions. We suggest that the conditions required to induce
extended X-ray emission in jellyfish galaxies are established at the beginning
of the stripping, and they can persist on long time scales so that galaxies in
different clusters and evolutionary stages can present similar extended X-ray
emission.

Galaxy evolution (594); X-ray astronomy (1810); Galaxy clusters (584)

††software: CIAO (v4.13; Fruscione et al. (2006)), Sherpa (Freeman et al.,
2001; Doe et al., 2007; Burke et al., 2020), XSPEC (v12.11.1; Arnaud (1996)),
astropy (Astropy Collaboration et al., 2013, 2018), APLpy (Robitaille &
Bressert, 2012), PYQZ (Dopita et al., 2013; Vogt et al., 2015)

tablenum SIXtablenum

##  1 Introduction

The evolution of galaxies is strongly influenced by the environment in which
they live. Star-forming galaxies in high density environments, like clusters,
have a different gas content than their counterparts in the fields. In
particular, spiral galaxies in clusters have, on average, a lower gas content
(e.g., Davies & Lewis, 1973; Giovanelli & Haynes, 1988; Solanes et al., 2001;
Chung et al., 2009) than the galaxies in the field. This is due to a number of
physical interactions that can be particularly efficient in galaxy clusters,
and will eventually lead to the gas removal. As a consequence of the loss of
gas, the star formation rate (SFR) is reduced and the galaxies could become
passive (e.g., Werle et al., 2022). To explain the lack of gas-rich galaxies
in clusters, a mechanism that strips away the gas from galaxies was proposed
by Gunn & Gott (1972): ram pressure stripping. This mechanism acts when a
galaxy falls into the hot and dense ($n_{\text{ICM}}\sim
10^{-4}-10^{-2}~{}$\mathrm{c}\mathrm{m}^{-3}$$, Sarazin 1986a) intracluster
medium (ICM). When a galaxy falls into the ICM, it experiences a drag force in
the opposite direction of its relative motion and, if this force exceeds the
gravitational one, the cold interstellar medium (ISM) of the galaxy is
stripped away. The condition that expresses that the ram pressure overcomes
the gravitational force per unit of surface is $\rho_{\text{ICM}}v^{2}>2\pi
G\Sigma_{\ast}\Sigma_{g}$ (Gunn & Gott, 1972), where $\rho_{\text{ICM}}$ is
the ICM mass density, $v$ is the infall velocity of the galaxy,
$\Sigma_{\ast}$ and $\Sigma_{g}$ are the stellar and gas surface density. If
this condition is true, the whole ISM is stripped away and the galaxy becomes
passive.

The most spectacular examples of galaxies undergoing strong ram pressure are
the so-called “jellyfish galaxies”. These galaxies show extra-planar,
unilateral debris visible in the optical/UV light and long “tentacles” or
tails of diffuse ionized H$\alpha$ gas extending for dozens of
$\mathrm{k}\mathrm{p}\mathrm{c}$ beyond the galaxy disk, where new stars are
born in knots (e.g. Fumagalli et al., 2014; Poggianti et al., 2016). The ‘GAs
Stripping Phenomena in galaxies’ (GASP) survey aims at studying the gas
removal process and how ram pressure stripping affects star formation (e.g.,
Vulcani et al., 2018; Poggianti et al., 2019a; Vulcani et al., 2020; Bellhouse
et al., 2021), and AGN activities (Poggianti et al., 2017; Radovich et al.,
2019; Peluso et al., 2022) in jellyfish galaxies. This project is a European
Southern Observatory (ESO) Large Program, carried out with the Multi Unit
Spectroscopic Explorer (MUSE) of the Very Large Telescope. The 94 stripping
candidates (Poggianti et al., 2017) show an unilateral debris/disturbed
morphology and tails, they are at $z=0.04-0.07$, they have stellar masses in
the range $10^{9.2}-10^{11.5}~{}$\mathrm{M}_{\odot}$$ and they are located in
different environments, such as groups and clusters with different masses
(Poggianti et al., 2017).

With multi wavelength studies it is possible to investigate the process of ram
pressure stripping through the observation of stripped gas in different phases
(e.g., Poggianti et al., 2019b). With observations in the optical band it is
possible to reconstruct the star formation history of the galaxies (e.g.,
Fritz et al., 2017) and, analyzing them in the radio band, it is possible to
understand what is the role of magnetic fields during the stripping process
(e.g., Müller et al., 2020). In this context, X-ray studies of these galaxies
can probe the physics of the interplay between ISM and ICM. The high-energy
side of jellyfish galaxies has been the object of several studies in the past
decades (e.g., Cowie et al., 1977; Nulsen, 1982; Tschöke, D. et al., 2001; Sun
& Vikhlinin, 2005; Sun et al., 2006, 2009; Zhang et al., 2013; Boselli, A. et
al., 2016; Poggianti et al., 2019b; Campitiello et al., 2021; Sun et al.,
2021). These suggest that the observed X-ray plasma is the result of a complex
interaction between ICM and ISM which causes the heating of the ISM through
either shock and conduction, or the cooling of the ICM onto the tails of the
galaxy, or the ICM-ISM mixing. To improve the knowledge about the interaction
between ICM and ISM, it is necessary to extend these studies to additional
jellyfish galaxies.

JO194 ($z=0.041$), located in Abell 4059, is a jellyfish galaxy characterized
by an extended X-ray emission outside the disk. The galaxy is almost face-on
along the line-of-sight with an inclination angle between the galaxy and its
direction of motion through the ICM of $8^{\circ+16}_{-5}$ (Bellhouse et al.,
2021). For these reasons, it is a good candidate to study the interaction
between the ICM and ISM in the stripped arms in the X-ray energy band. JO194
is a spiral Sb galaxy (RA $23:57:00.680$ Dec $-34:40:50.10$ Varela et al.
2009) at a projected distance of $269~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$
North of ESO 349-G010, the central galaxy of the cluster. JO194 has a stellar
mass of $M_{\ast}=1.6\cdot 10^{11}~{}$\mathrm{M}_{\odot}$$ and a line-of-sight
velocity of $v_{los}=2030~{}$\mathrm{k}\mathrm{m}\mathrm{/}\mathrm{s}$$ with
respect to the mean velocity of the cluster (Poggianti et al., 2017). Its
projected vicinity to the center of the cluster and its high velocity makes
the stripping process very efficient. From this information, it was argued
that this is the first time that the galaxy is falling to the center of the
cluster (see Bellhouse et al., 2017; Jaffé et al., 2018). The star formation
of this galaxy ($9\pm 2$ M⊙ yr-1 Vulcani et al., 2018) is mainly located in
the galaxy disk with only the $5\%$ of the stars are currently formed in the
tails (Poggianti et al., 2019a). JO194 was studied in Bellhouse et al. (2021)
to understand the unwinding mechanism of the arms caused by the stripping
process. It was shown that the youngest stars
($<20~{}$\mathrm{M}\mathrm{y}\mathrm{r}$$) are born in the stripped arms while
older stars ($5.7\cdot 10^{9}-1.4\cdot 10^{10}~{}$\mathrm{y}\mathrm{r}$$) were
born in the inner part of the unwinding arms near the stellar disk. So the
youngest stars born in the arms trace the motion of the unwinding arms that
characterize the galaxy. This is an important result to better understand how
ram pressure is acting on the arms of the spiral galaxies and to reconstruct
the unwinding phenomenon as a function of the stellar age (Bellhouse et al.,
2021).

Abell 4059 is a relaxed, cool-core cluster (Laganá et al., 2019) at $z=0.048$
(Abell et al., 1989), dominated by the cD galaxy ESO 349-G010 (RA 23: 57:
00.74 Dec -34: 45: 32.99, Huang & Sarazin 1998). In Schwartz et al. (1991) and
Huang & Sarazin (1998) are presented the X-ray analysis of the cluster using,
respectively, High Energy Astronomy Observatory (HEAO), EXOSAT, Einstein and
ROSAT observations. The previous X-ray analysis revealed that there is an
excess of surface brightness in the central region of the cluster. This excess
is the result of a cooling flow with a nominal cooling mass rate of
$\dot{M}=184^{+22}_{-25}~{}$\mathrm{M}_{\odot}\mathrm{/}\mathrm{y}\mathrm{r}$$.
The analysis of the X-ray properties of the ICM of Abell 4059 are presented in
Reynolds et al. (2008), Choi et al. (2004) and Mernier et al. (2015). From the
analysis of the radial profiles, the temperature of the ICM increases with
radius (in the first $\sim 100~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$ from the
center of the cluster the temperature grows from 1 to $\sim
5~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$), while electron density ($n_{e}$) and
pressure (P) decrease: $n_{e}$ goes from $0.1~{}$\mathrm{c}\mathrm{m}^{-3}$$
to $0.001~{}$\mathrm{c}\mathrm{m}^{-3}$$ and P goes from
$10^{-10}~{}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{3}$$
to
$10^{-11}~{}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{3}$$
in the first $150~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$ (see Section 3.1). Also
the abundance of different metals
($\ch{O},\ch{Ne},\ch{Si},\ch{S},\ch{Ar},\ch{Ca}$ and $\ch{Fe}$) decreases with
radius and it is peaked toward the center of the cluster. Also ram pressure
stripping might have contributed to a more recent enrichment in the core
(Mernier et al., 2015).

We carried out an X-ray analysis of the jellyfish galaxy JO194 to explore the
physical properties of the X-ray plasma in the galaxy, like average
temperature, metallicity and luminosity, and to investigate the contribution
of the SFR in producing this emission. The final purpose is to study the
origin of the X-ray emission produced in the arms of this galaxy and what are
the similarities between JO194 and the other GASP jellyfish galaxies that have
been studied in the X-ray energy band, JO201 and JW100. This paper is
structured as follows. In Section 2 we describe the procedures adopted for the
Chandra data reduction, for the surface brightness analysis and the spectra
extraction. In Section 3 we present the results of the spectral analysis of
the ICM spectra and JO194 spectra and we discuss them in Section 4. Summary
and conclusions are presented in Section 5.

Throughout this paper, we adopt a Chabrier initial mass function (IMF;
Chabrier 2003), a concordance $\Lambda$CDM cosmology of
$\Omega_{\text{M}}=0.3$, $\Omega_{\Lambda}=0.7$,
$H_{0}=70~{}$\mathrm{k}\mathrm{m}\mathrm{s}^{-1}\mathrm{M}\mathrm{p}\mathrm{c}^{-1}$$
($1~{}$\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}$=0.94~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$
at $z=0.048$) and all the errors are given at $1\sigma$ level.

##  2 Data analysis

###  2.1 Data preparation

The Chandra observation of the cluster A4059 was performed with the Advanced
CCD Imaging Spectrometer (ACIS) on January 26 2005 (obsID:5785, PI Reynolds
and $92.1~{}$\mathrm{ks}$$ exposure time). The observation was made in the
VFAINT mode with both ACIS-S (S1,S2, and S3 chips) and ACIS-I (I2 and I3
chips) instruments. In this work, we processed only the CCD including JO194
(S3 chip).

The dataset was downloaded from Chandra Data
Archive111https://cda.harvard.edu/chaser/ and was reprocessed with the
software package CIAO 4.13 and CALDB 4.9.4 to remove bad pixels and flares.
After the cleaning phase, the net exposure time was $91.36~{}$\mathrm{ks}$$.
To improve the absolute astrometry, we have identified the point sources using
the CIAO task WAVDETECT and we cross-matched them with the optical catalog
USNO-A2.0222http://tdc-www.harvard.edu/catalogs/ua2.html. To evaluate the
background signal (observational and cosmological) we used the CALDB Blank-Sky
files that match our data in the chip of interest. Then we normalized the
observed and background count-rate in the
$9-12~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ band rescaling the exposure time of
the background accordingly. The exposure-corrected, background-subtracted
Chandra image of the chip of interest in the
$0.5-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ energy band is shown in the Fig.
1. The resolution at the off-axis distance of JO194 is
$4.92~{}$\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}$.$

In addition to the Chandra data, in this work we made use of the MUSE
corrected for stellar absorption H$\alpha$ image of JO194 (Poggianti et al.,
2017), and the stellar disk mask presented in Gullieuszik et al. (2020).

Figure 1: Exposure-corrected, background-subtracted image of Abell 4059 in the
energy band $0.5-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ smoothed with a
$3\sigma$ Gaussian.

###  2.2 Surface brightness analysis

To characterize the X-ray morphology of JO194, we analyzed the X-ray surface
brightness profiles of the ICM of Abell 4059. We performed the analysis using
the sherpa333https://cxc.cfa.harvard.edu/sherpa/threads/ environment of the
software CIAO.

To analyze the 2D surface brightness profile, we defined an ellipsoidal region
that followed the morphology of the X-ray emission (Figure 1), centered on the
coordinates of the cluster center (RA (J2000) 23 : 57 : 01.065, Dec (J2000)
$-34:45:33.28$444These coordinates are shown in the ACCEPT archive:
https://web.pa.msu.edu/astro/MC2/accept/clusters/5785.html), with major axis
of $246.7~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$ and position angle of
$345^{\circ}$. To model only the ICM thermal emission, we excluded the point
sources found by the task WAVDETECT with circles of average radius of
$12~{}$\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}$$. With
sherpa, we performed a 2D fit of the surface brightness $I(r)$ inside this
ellipsoidal region using a Cash statistic (Cash, 1979) and an
elliptical555https://cxc.cfa.harvard.edu/sherpa/ahelp/beta2d.html
$\beta$-model (Cavaliere & Fusco-Femiano, 1976):

$I(r)=I_{0}\left[1-\left(\frac{r}{r_{c}}\right)^{2}\right]^{0.5-3\beta}$ (1)

where $I_{0}=(3.60\pm 0.02)\cdot
10^{-8}~{}$\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{p}\mathrm{i}\mathrm{x}\mathrm{e}\mathrm{l}^{2}\mathrm{/}\mathrm{s}$$
is the central surface brightness, $r_{c}=60.53\pm
0.50~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$ is the core radius and $\beta=0.53\pm
0.01$. The model’s ellipticity and inclination angle are $\epsilon=0.19$ and
$\theta=-5.29$ radians, respectively. The best-fit $\beta$ and $r_{c}$ values
are in agreement with the previous results presented in Huang & Sarazin
(1998), which reported $\beta=0.55^{+0.03}_{-0.03}$ and
$r_{c}=66^{+10}_{-9}~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$. Then, with the CIAO
routine dmimgcalc, we subtracted the 2D $\beta$-model to the exposure-
corrected background-subtracted image to obtain a residual image that
highlights the excesses of surface brightness of JO194 relative to the ICM
thermal emission.

###  2.3 Spectral Analisys

To understand the origin of the extended X-ray emission of JO194 and to study
the interaction between the ICM and ISM contained in the arms of the galaxy,
we performed a spectroscopic analysis of the Chandra data with the software
package XSPEC (version 12.11.1 Arnaud 1996).

To derive the ICM properties in the region in which the galaxy is located, we
assumed that the gas has a spherical symmetry and we considered 4 annular
sectors with an aperture of $93^{\circ}$ centered on the center of the
cluster. We adopted an average width of
$77.2~{}$\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}$=72.5~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$
to have more than 1000 counts in every sector (see Figure 2).

For each region, we masked all the point sources identified by WAVDETECT and
the extended emission of JO194 contained in the last annulus in the north
direction, to prevent their emission from contaminating that of the ICM. We
evaluated that the extended emission of JO194 is contained in an elliptical
region of major axis of $45.5~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$ and of minor
axis of $41.4~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$, shown in Figure 2.

Figure 2: (a) Exposure-corrected, background-subtracted, Gaussian-smoothed
image of Abell 4059 in the $0.5-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ energy
band. The sectors used to extract the ICM spectra (green) and the masked
regions (blue) are overlaid.(b) Exposure-corrected, background-subtracted,
Gaussian-smoothed image of JO194 in
$0.5-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ energy band with on top the
regions used for the spectral extraction based on the contours of the
H$\alpha$ emission and the stellar disk. The three blue sources are background
galaxies, according to their MUSE optical spectra. Therefore, we exclude them
from the analysis.

For the spectral analysis of JO194, we considered different regions: a region
that follows the contours of the H$\alpha$ emission (Poggianti et al., 2019)
and resembles the X-ray emission (named “Total”), a region that traces the
contour of the stellar disk (named “Disk”) (Gullieuszik et al., 2020) and a
region that is the difference between the Total and the Disk regions, from now
on we will call this one “Arms of JO194” because it covers the unwinding arms
of the galaxy. Figure 2 shows the regions in which the spectra of JO194 are
extracted in the $0.5-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ energy band. The
extraction of different regions was possible thanks to the high number of
counts in all regions. Table 1 presents the total counts of the extracted
spectra of JO194. The Total and Disk regions were chosen to compare the X-ray
luminosities, obtained from the spectral analysis, with those obtained from
the SFR found in a previous study on the H$\alpha$ emission of JO194 (Vulcani
et al., 2018) (see section 4.2).

Table 1: Total data counts and rate of counts that do not belong to the background counts of the spectra that represent the Total, the Disk and the Arms of JO194. | Total Data Counts | Rate
---|---|---
Total | $2881$ | $84.3\%$
Disk | $1461$ | $88.5\%$
Arms | $1448$ | $81.7\%$

In the spectral analysis we excluded the point sources in the field of view.
Concerning the three point sources located $\sim
14~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$ south of JO194 (Figure 2), the MUSE
optical spectra revealed that they are background galaxies at redshift
$z>0.63$, thus they were masked.

We extracted a spectrum from each region using the CIAO task specextract and
we binned them to give 25 counts in each energy bin. Background spectra were
extracted using blanksky files in each region. All spectra of the X-ray
emission of JO194 and of the ICM were extracted separately and then were
analyzed in the $0.5-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ range.

###  2.4 Spectral models

To take into account that the X-ray emission of the ICM along the line of
sight contaminates that of JO194, we investigated the properties of the ICM
surrounding the galaxy. The spectra of the four ICM regions were fitted
jointly to obtain the deprojected profiles of the ICM thermal properties. We
estimated deprojected quantities by using the model
projct$\cdot$(phabs$\cdot$apec) where projct is the component that executes
the deprojection, phabs represents the galactic absorption and apec describes
the emission of a single temperature plasma666For more information consult the
website: https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/node133.html. The
Galactic line-of-sight absorption was fixed to a column density value of
$n_{H}=1.08\cdot 10^{20}~{}$\mathrm{c}\mathrm{m}^{-2}$$ as computed in the
HI4PI survey (HI4PI Collaboration: et al., 2016). The parameters of the apec
component, temperature, metallicity, and normalization, were left free to
vary, while the redshift was fixed to the cluster value $z=0.048$ (Abell et
al., 1989). The results of this analysis are presented in Section 3.

We modeled separately the spectra of the regions of JO194 (see Figure 2) in
order to investigate the nature and the origin of the extended X-ray emission
in every region. In order to have a reliable fit of the galaxy spectra, we had
to take into account the ICM contamination along the line of sight.
Specifically, by rescaling the ICM spectral counts of the fourth sector in the
$0.5-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ energy band (18500 net counts),
and by comparing it with the net counts extracted on the galaxy in the same
band (Table 1), we estimated that the ICM composes the 77$\%$ of the total
counts, and the 60$\%$ of the disk’s ones. For this reason, we tested complex
models made of two components: the first apec${}_{\texttt{ICM}}$ component
accounts for the ICM emission along the line of sight that we modeled to the
properties of the surrounding ICM ($kT=4.44\pm 0.16$ keV, $Z=0.35\pm 0.07$
$Z_{\odot}$, see Section 3.1). The second component of the model represents
JO194 emission. For the galactic emission, that here after is indicated with
the subscript GAL, we tested different models:

  * •

an apec component that describes the X-ray emission of a single temperature
plasma and can represent the SF in the galaxy;

  * •

both an apec and a pow component, this latter one describes a non-thermal
emission. This model models a possible AGN emission and the contribution of
unresolved High mass X-ray Binaries (HXRB);

  * •

a mkcflow777For more information see the
website:https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/node193.htmlcomponent
that describes the emission of a radiative cooling multi-phase plasma from a
temperature $T_{\textsc{HIGH}}$ to a temperature $T_{\textsc{low}}$ and
returns the mass accretion rate ($\dot{M}$). In our analysis this model
represents the ICM that cools onto the ISM;

  * •

a cemekl888For more information see the website:
https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/node149.html component that
describes a multi-temperature plasma where the emission measure $EM=\int
n_{e}n_{H}dV$ scales with the temperature of the plasma $EM\propto T^{\alpha}$
and T has a maximum, T${}_{\textsc{MAX}}$. From T${}_{\textsc{MAX}}$ and the
index $\alpha$, it is possible to derive the mass-weighted temperature of the
plasma, $T_{mw}=T_{\textsc{MAX}}\cdot(\alpha+1)/(\alpha+2)$ (Sun et al.
(2010)).

In all models we included the Galactic line-of-sight absorption using the
phabs component with $n_{H}=1.08\cdot 10^{20}~{}$\mathrm{c}\mathrm{m}^{-2}$$
(HI4PI Collaboration: et al., 2016).

For the metallicities of the ISM we used two different approaches: the first
one was to set free the metallicity, the second one was to fix it to specific
values. With the first approach, in some cases the resulting best-fit
parameters were unreliable (see Table 4). In these cases, to model the ISM
emission, we used the second approach and we adopted the metallicities of the
ionized gas reported in Franchetto et al. (2020) and the metallicity of the
ICM ($Z=0.35\pm 0.07~{}$\mathrm{Z}_{\odot}$$). For the Total and Disk regions,
two estimates of the oxygen abundance are provided based on different
estimators: $Z_{\text{O3N2}}$, which is based on the O3N2 indicator, and
$Z_{\text{PYQZ}}$ which was obtained with the PYQZ code (e.g., Dopita et al.,
2013; Vogt et al., 2015). The corresponding solar metallicities were derived
by the following relation:

$Z=10^{(12+\log O/H)-8.69}~{}$\mathrm{Z}_{\odot}$$ (2)

where $8.69$ is the oxygen abundance in the sun (Asplund et al., 2009). The
metallicities values obtained for the Total region are presented in the Table
2.

Metallicity | Total | Disk
---|---|---
$Z_{\text{O3N2}}$ | $1.24~{}$\mathrm{Z}_{\odot}$$ | $1.26~{}$\mathrm{Z}_{\odot}$$
$Z_{\text{PYQZ}}$ | $2.81~{}$\mathrm{Z}_{\odot}$$ | $2.90~{}$\mathrm{Z}_{\odot}$$
Table 2: Metallicities of the Total and Disk regions obtained from the results
presented by Franchetto et al. (2020) for JO194 using the equation 2.

Finally, we used C-statistic and the abundance tables presented in Asplund et
al. (2009).

##  3 Results

In this section, we describe the main results of the spectral analysis of the
X-ray emission of the ICM of Abell 4059 and the ISM of JO194. Figure 3 shows a
multi-wavelength field of JO194, in the optical, H$\alpha$ (only contours) and
X-ray energy bands.

Figure 3: Multi-wavelength view of JO194 in a $1.5^{\prime}\times
1.7^{\prime}$ region centered on the galaxy. (a) MUSE white-light image
($4650-9300~{}$\r{A}$$) with on top the contours of the stellar disk (yellow)
and H$\alpha$ emission (light blue), (b)Residual X-ray image of JO194 in the
$0.5-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ band (see Section 2.2) with on
top the contours of the stellar disk (yellow) H$\alpha$ (red) and positive
residuals (white); (c) Exposure-corrected, background-subtracted image in the
$2.0-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ band, smoothed with a $3\sigma$
pixel gaussian. For reference, we show the stellar disk (blue) and the lowest
contour of the H$\alpha$ emission (white).

###  3.1 ICM spectral analysis results

We have first estimated the properties of the ICM around JO194. The parameters
that were returned from the spectral analysis were the temperature and the
metallicity of the plasma. From these, we have also estimated the deprojected
electron density and pressure through the following relations:

$n_{e}=\sqrt{\frac{N\cdot 4\pi D_{L}^{2}}{V\cdot 0.82\cdot 10^{-14}}}$ (3)

and

$P=1.9\cdot n_{e}\cdot kT$ (4)

in which $N$ is the apec component’s normalisation, $D_{L}=6.58\cdot
10^{26}~{}$\mathrm{c}\mathrm{m}$$ is the luminosity distance,
$V=\frac{4}{3}\pi(R_{\textsc{ext}}^{3}-R_{\textsc{int}}^{3})$ is the volume of
the spherical shell where $R_{\textsc{ext}}$ and $R_{\textsc{int}}$ are the
sectors’ external and internal radii, $n_{e}n_{H}=0.82n_{e}^{2}$ and the total
density of particles is $n=1.9n_{e}$.

In Table 3 we report the deprojected properties of the ICM for every annulus.
In Figure 4 the deprojected radial profiles of the properties of the ICM are
shown. The electron density profile was fitted with a $\beta$-model to
constrain the deprojected ICM density profile. The best-fit parameters are:
$\beta=0.46\pm 0.06$ and core radius $r_{c}=74\pm
14~{}$\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}$=70\pm
13~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$. These values are consistent with those
reported in Section 2.2.

Table 3: Internal Radius ($R_{\text{INT}}$), External Radius ($R_{\text{EXT}}$), Deprojected Temperature, Metallicity, Electronic density and Pressure of the four ICM annuli. Property | Annulus 1 | Annulus 2 | Annulus 3 | Annulus 4
---|---|---|---|---
$R_{\text{INT}}$ \- $R_{\text{EXT}}$ | $12.0-82.9~{}$\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}$$ | $82.9-148.0~{}$\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}$$ | $148.0-220.7~{}$\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}$$ | $220.7-325.0~{}$\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}$$
$11.3-77.9~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$ | $77.9-139.1~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$ | $139.1-207.4~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$ | $207.4-305.5~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$
kT(keV) | $3.55\pm 0.10$ | $4.30\pm 0.14$ | $4.51\pm 0.21$ | $4.44\pm 0.16$
Z($Z_{\odot}$) | $0.93\pm 0.09$ | $0.84\pm 0.10$ | $0.57\pm 0.11$ | $0.35\pm 0.07$
$n_{e}(10^{-3}~{}$\mathrm{c}\mathrm{m}^{-3}$)$ | $6.87\pm 0.07$ | $3.74\pm 0.04$ | $2.10\pm 0.03$ | $1.39\pm 0.01$
P($10^{-11}~{}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{-3}$$) | $7.42\pm 0.25$ | $4.88\pm 0.17$ | $2.88\pm 0.15$ | $1.87\pm 0.07$
Figure 4: Deprojected radial profiles of (from top to bottom): temperature,
metallicity, fitted electron density and pressure in log scale. In red we show
the best-fit $\beta$-model profile ($\beta=0.46\pm 0.06$ and $r_{c}=70\pm
13~{}$\mathrm{k}\mathrm{p}\mathrm{c}$$) which describe the deproject electron
density radial profile.

###  3.2 Galaxy spectral analysis results

As previously introduced in Section 2.4, we modeled the spectrum extracted
over the galaxy as the combination of the ICM, represented by an
apec${}_{\texttt{ICM}}$ component, and galactic emissions, indicated by the
subscript GAL. Therefore the temperature, the metallicity and the redshift
parameters of the first apec${}_{\texttt{ICM}}$ component were set to those
measured in the region including JO194 (i.e., the fourth sector of the ICM
analysis) that are $kT=4.44\pm 0.16~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$,
$Z=0.35\pm 0.07~{}$\mathrm{Z}_{\odot}$$, and $z=0.048$ (see Table 3). The
second component of the model, that is the galactic emission, was tested among
all the components shown in the list in Section 2.3.

The best-fit results are reported in Table 4. We selected the best-fitting
model based on the final statistics (i.e., best agreement with the data) and
the physical significance (i.e., we rejected those models which provided a
good fit with unphysical parameters, such as a temperature significantly
higher than the one of the ICM, or an extremely low metallicity). In summary,
we observe that:

  * •

The “Total” spectrum is well reproduced by two different models. The first one
is the phabs$\cdot$(apec${}_{\texttt{ICM}}$+cemekl${}_{\texttt{GAL}}$) with
the cemekl${}_{\texttt{GAL}}$ maximum temperature fixed to the ICM temperature
$T_{\textsc{MAX}}=4.44~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ and metallicity
fixed to the values shown in Section 2.4. This model yields a mass weighted
temperature of $T_{mw}=2.65~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$. The second
model is phabs$\cdot$(apec${}_{\texttt{ICM}}$+apec${}_{\texttt{GAL}}$) that
measured an average X-ray temperature of the galactic component of
$kT=0.79~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$, which is consistent with those
of other jellyfish galaxies (see Section 4.4);

  * •

The best-fitting models for the galactic disk are
phabs$\cdot$(apec${}_{\texttt{ICM}}$+apec${}_{\texttt{GAL}}$) with the second
apec${}_{\texttt{GAL}}$ metallicity fixed to the values shown in Section 2.4
and the model phabs$\cdot$(apec${}_{\texttt{GAL}}$+pow${}_{\texttt{GAL}}$)
with the metallicity of the apec${}_{\texttt{GAL}}$ component fixed to the
values of the stellar disk. Due to the low statistics, the Disk spectrum
showed large uncertainties above 2 keV that resulted in small values of the
reduced chi-square ($\chi_{R}^{2}\sim 0.7$). In the
phabs$\cdot$(apec${}_{\texttt{GAL}}$+pow${}_{\texttt{GAL}}$) model the ICM
emission contribution seems to be negligible, but this is at odds with the
previous results because the ICM contributes far more than the 60$\%$ of the
emission in the disk (see Section 2.4). In addition, the resulting luminosity
of the powGAL component is not consistent with that we expect from the current
SFR. By adopting the $L_{X}$-SFR relation for point sources, in the
$0.5-8.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ energy band, presented in Mineo
et al. (2012) ($L_{X,\text{non
thermal}}($\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$)=2.6\times
10^{39}\cdot SFR($\mathrm{M}_{\odot}\mathrm{/}\mathrm{y}\mathrm{r}$)$), the
current SFR in the disk of JO194 ($8\pm 2$ M⊙ yr-1 Vulcani et al., 2018)
entails a luminosity of $2.3\times 10^{40}$ erg s-1, that is more than an
order of magnitude lower than we measured from the spectral analysis.
Therefore the powGAL model is not a reliable description of the disk emission,
and we dismiss this model in the following analysis. We argue that the powGAL
model fit goodness turns out solely from the fact that, due to the poor
statistics of the current data, it tentatively fits the spectrum above
$2~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$.

  * •

The spectrum of JO194 arms is well reproduced by two different models. The
first one is the phabs$\cdot$(apec${}_{\texttt{ICM}}$+apec${}_{\texttt{GAL}}$)
model with the metallicity of the two apec component fixed to
$Z=0.35~{}$\mathrm{Z}_{\odot}$$. This model provides an estimate of the
average temperature of arms’ X-ray plasma ($kT=0.70\pm
0.03~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$). The second one is the
phabs$\cdot$(apec${}_{\texttt{ICM}}$+mkcflow${}_{\texttt{GAL}}$) model with
$t_{\textsc{HIGH}}$ fixed to the value of the surrounding ICM ($kT=4.44$ keV).
The best-fit model yields a mass flow of $0.24^{+0.06}_{-0.05}$ $M_{\odot}$
yr-1 and a metallicity of $Z=0.24^{+0.19}_{-0.12}Z_{\odot}$, which may be the
first direct measure of the metallicity of the X-ray plasma in the arms of a
jellyfish galaxy. The implications of these results are discussed in Section
4.

Table 4: Best fit results extracted from the region that follows the contours
of the H$\alpha$ emission and resembles the X-ray emission (“Total”), the disk
(“Disk”) and all the galaxy emission without the disk (“Arms”). The first
apec${}_{\texttt{ICM}}$ component of the models refers to the ICM emission
while the second refers to the galactic emission. The only exception is the
fourth model in which the apec${}_{\texttt{GAL}}$ component is referred to the
emission of the Disk. The best-fit parameters shown in the fourth column refer
only to the galactic component (namely apec${}_{\texttt{GAL}}$,
cemekl${}_{\texttt{GAL}}$, pow${}_{\texttt{GAL}}$ and
mkcflow${}_{\texttt{GAL}}$). The degree of freedom are indicated with the word
“dof”.

Region | Model | Fixed Parameters | Parameters of the galactic component | $\chi^{2}$
---|---|---|---|---
dof
$\chi^{2}_{R}$
Total | phabs$\cdot$(apec${}_{\texttt{ICM}}$+apec${}_{\texttt{GAL}}$) | $Z_{\text{O3N2}}=1.24Z_{\odot}$ | $kT=0.79^{+0.03}_{-0.04}~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ | $\chi^{2}=77.98$
$F=(2.72\pm 0.22)\cdot 10^{-14}~{}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$ | $dof=86$
$L=(1.49\pm 0.12)\cdot 10^{41}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=1.15$
$Z_{\text{PYQZ}}=2.81Z_{\odot}$ | $kT=0.79^{+0.03}_{-0.03}~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ | $\chi^{2}=78.35$
$F=(2.60\pm 0.22)\cdot 10^{-14}~{}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{s}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$ | $dof=86$
$L=(1.42\pm 0.12)\cdot 10^{41}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=0.91$
phabs$\cdot$(apec${}_{\texttt{ICM}}$+cemekl${}_{\texttt{GAL}}$) | $Z_{\text{O3N2}}=1.24Z_{\odot},T_{max}=4.44$\mathrm{k}\mathrm{e}\mathrm{V}$$ |  $\alpha=0.48_{-0.18}^{+0.18}$, |  $\chi^{2}=85.12$,
$F=(5.76\pm 2.04)\cdot 10^{-14}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$ |  $dof=85$,
$L=(3.17\pm 1.11)\cdot 10^{41}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=1.00$
$Z_{\text{PYQZ}}=2.81Z_{\odot},T_{max}=4.44$\mathrm{k}\mathrm{e}\mathrm{V}$$ | $\alpha=0.46_{-0.17}^{+0.17}$ |  $\chi^{2}=83.36$,
$F=(4.80\pm 0.17)\cdot 10^{-14}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$ |  $dof=85$,
$L=(2.64\pm 0.90)\cdot 10^{41}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=0.98$
Disk | phabs$\cdot$(apec${}_{\texttt{ICM}}$+apec${}_{\texttt{GAL}}$) | $Z_{\text{O3N2}}=1.26Z_{\odot}$ | $kT=0.85^{+0.04}_{-0.04}$\mathrm{k}\mathrm{e}\mathrm{V}$$ |  $\chi^{2}=29.77$,
$F=(1.86\pm 0.19)\cdot 10^{-14}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$, |  $dof=43$,
$L=(1.02\pm 0.10)\cdot 10^{41}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=0.69$
$Z_{\text{PYQZ}}=2.90Z_{\odot}$ | $kT=0.84^{+0.04}_{-0.04}~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ |  $\chi^{2}=30.01$,
$F=(1.77\pm 0.18)\cdot 10^{-14}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$, |  $dof=43$,
$L=(9.70\pm 0.97)\cdot 10^{40}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=0.70$
phabs$\cdot$(apec${}_{\texttt{GAL}}$+pow${}_{\texttt{GAL}}$) | $Z_{\text{PYQZ}}=1.26Z_{\odot}$ |  $kT=0.90^{+0.04}_{-0.05}$\mathrm{k}\mathrm{e}\mathrm{V}$$, $\Gamma=1.77^{+0.10}_{-0.10}$ | $\chi^{2}=30.30$
$F=(1.01\pm 0.18)\cdot 10^{-13}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$ | $dof=42$
$L=(5.48\pm 0.99)\cdot 10^{41}~{}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=0.72$
$Z_{\text{PYQZ}}=2.90Z_{\odot}$ |  $kT=0.90^{+0.04}_{-0.05}$\mathrm{k}\mathrm{e}\mathrm{V}$$, $\Gamma=1.80^{+0.10}_{-0.10}$ | $\chi^{2}=30.36$
$F=(1.01\pm 0.08)\cdot 10^{-13}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$ | $dof=42$
$L=(5.46\pm 0.45)\cdot 10^{41}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=0.72$
Arms | phabs$\cdot$(apec${}_{\texttt{ICM}}$+apec${}_{\texttt{GAL}}$) | $Z=0.35Z_{\odot}$ | $kT=0.70^{+0.08}_{-0.10}~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ | $\chi^{2}=49.21$
$F=(9.18\cdot 1.68)\cdot 10^{-15}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$ | $dof=47$
$L=(5.07\pm 0.92)\cdot 10^{40}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=1.05$
phabs$\cdot$(apec${}_{\texttt{ICM}}$+mkcflow${}_{\texttt{GAL}}$) | $T_{high}=4.44$\mathrm{k}\mathrm{e}\mathrm{V}$$ |  $T_{low}=0.17^{+0.23}_{-0.14}$\mathrm{k}\mathrm{e}\mathrm{V}$$, $Z=0.24^{+0.19}_{-0.12}Z_{\odot}$ | $\chi^{2}=56.56$,
$\dot{M}=0.24^{+0.06}_{-0.05}$\mathrm{M}_{\odot}\mathrm{/}\mathrm{y}\mathrm{r}$$ | $dof=46$,
$F=(3.91\pm 0.91)\cdot 10^{-14}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{s}$$ | $\chi^{2}_{R}=1.23$
$L=(2.15\pm 0.49)\cdot
10^{41}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$

Notes: kT is the average temperature of the galactic plasma, $\alpha$ is the
index of the powerlaw that describes how the emission measure scales with the
temperature of the galaxy, $\Gamma$ is the photon index of the power law,
$T_{\text{LOW}}$ is the lowest temperature of the galactic plasma and
$\dot{M}$ is the mass accretion rate. The fluxes and the luminosities refer to
the galactic component and were measured in
$0.5-10.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ energy band.

##  4 Discussion

###  4.1 Characteristics of the galaxy at different wavelengths

From the analysis of JO194 in the optical and H$\alpha$ energy bands, it was
argued that in the center of the galaxy is located a Low Ionization Nuclear
Emission-line Region (LINER) AGN (Poggianti et al., 2017). From our analysis,
we concluded that in the $2.0-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$ energy
band (see Figure 3) there is no hard X-ray emission in the center of JO194.
For reference, we derive an upper limit of its luminosity in the 0.5-10 keV
band of $L<6\times 10^{40}$ erg s-1. This evidence challenges the hypothesis
of an AGN sitting at the center of JO194, as already discussed in Radovich et
al. (2019). For the conclusive analysis of the core region of JO194, we refer
to Moretti et al., in preparation.

Concerning the disk in the three different energy bands, an arc of emission is
visible on the right side of the disk due to an intense star formation (see
Figure 3).

Figure 3 shows the comparison between the morphology of JO194 in the optical,
H$\alpha$ (only contours) and X-ray energy band. JO194 is characterized by an
extended emission outside the stellar disk visible in each of these energy
bands. In Figure 3, we used the residual image of the galaxy. The morphology
of the galaxy is more distinguishable in the residual image because the
surface brightness excess relative to the ICM emission is highlighted. Figure
3 shows the hard X-ray emission ($2.0-7.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$)
of the galaxy for the analysis of the core region of JO194. This image was
used to test if there is an excess of hard X-ray emission due to the AGN. The
lack of the emission at the center of this galaxy challenges the hypothesis of
the presence of the AGN.

###  4.2 Origin of the X-ray emission

The X-ray extended emission of late-type, star-forming galaxies is produced by
the thermal emission of gas heated by young, massive stars, and the non-
thermal X-ray emission of High Mass X-ray Binaries (HMXB) (e.g. Mineo et al.,
2013). To investigate if this is the case also in JO194, we compared the
observed X-ray luminosities with those expected from the ongoing star
formation rates (SFR) measured in Vulcani et al. (2018). In Vulcani et al.
(2018) the SFR were estimated from the H$\alpha$ emission using the relation
SFR ($M_{\odot}$ yr-1) = $4.6\cdot 10^{-42}(L_{H\alpha}/(\text{erg s}^{-1}))$.
The SFR values for the disk and all the entire galaxy are
SFR${}_{\textsc{tot}}=(9\pm
2)~{}$\mathrm{M}_{\odot}\mathrm{/}\mathrm{y}\mathrm{r}$$ and
SFR${}_{\textsc{disk}}=(8\pm
2)~{}$\mathrm{M}_{\odot}\mathrm{/}\mathrm{y}\mathrm{r}$$. Following the
relation presented in Poggianti et al. (2019b):

$L_{X(0.5-10.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$)}($\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$)=7.6\times
10^{39}\text{SFR}($\mathrm{M}_{\odot}\mathrm{/}\mathrm{y}\mathrm{r}$),$ (5)

we calculated the corresponding X-ray luminosity, $L_{\text{X,SFR}}$, in the
energy band $0.5-10.0~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$, which are,
respectively, $(6.8\pm 1.5)$ and $(6.1\pm 1.5)\times
10^{40}~{}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$ for the total
and disk region.

Figure 5: Ratio between the observed X-ray luminosities, $L_{\text{X,obs}}$,
of the total and disk region shown in the Table 4, and the X-ray luminosity
associated to the SFR, $L_{\text{X,SFR}}$ (Equation 5). The first four models
refer to the Total region of the galaxy and the others to the Disk. The first
apec${}_{\texttt{ICM}}$ represent the contribution of the ICM emission, except
for the last two ones that represent the contribution of the galaxy’s disk
emission. The apec${}_{\texttt{GAL}}$ component describes the X-ray emission
of a single temperature plasma, cemekl${}_{\texttt{GAL}}$ represents a multi-
temperature plasma and mkcflow${}_{\texttt{GAL}}$ represents a multi-phase gas
in radiative cooling. The dashed grey line points $L_{\text{X,obs}}$/
$L_{\text{X,SFR}}=1$.

The comparison between the observed and estimated X-ray luminosities is shown
in Figure 5. It emerges that, in the Total region, the observed X-ray
luminosities are in excess with respect to what expected from the current SFR.
Specifically, the excesses are by a factor of $\sim\times 2$ and $\times 4$
for the different spectral models. This indicates that the X-ray emission of
JO194 cannot be the result of the star formation activity alone, and instead
is produced by a different process. However in the disk, where most of the
star formation takes place (Poggianti et al., 2019a), the luminosities are
marginally consistent with each other
($L_{\text{X,obs}}/L_{\text{X,SFR}}=1.7\pm 0.7$ and $1.6\pm 0.6$, where
$L_{\text{X,obs}}$ are the luminosities of the model
phabs$\cdot$(apec${}_{\texttt{ICM}}$+apec${}_{\texttt{GAL}}$)). Therefore, we
speculate that this additional process is more pronounced in the total galaxy
than in the disk. Due to the fact that the total region actually includes the
disk itself, we suggest that this additional channel to form the X-ray plasma
is taking place mostly in the regions of the galaxy outside of the disk, that
are the unwinding arms.

We note that similar X-ray luminosity excesses ($\sim 10$) have been observed
also in Poggianti et al. (2019b) for JW100, and Campitiello et al. (2021) for
JO201. These discrepancy in X-ray luminosities further corroborate the idea
that the star formation alone cannot provide the total X-ray luminosity, so,
as already suggested in the literature, additional processes must concur in
the origin of the X-ray emission in jellyfish galaxies.

The spectral analysis results (Table 4) allows to speculate on the origin of
the X-ray plasma. To begin with, our results rule out the hypothesis that the
X-ray emission could be powered by the SFR alone, either the nonthermal
emission from HMXB or the thermal emission. On the other hand, the average
temperature and the X-ray luminosity excess resemble the results previously
reported in literature for other ram pressure stripped galaxies (e.g., Sun et
al., 2010; Zhang et al., 2013; Poggianti et al., 2019; Campitiello et al.,
2021; Sun et al., 2021). Therefore, as concluded by the previous studies, we
argue that the X-ray plasma in JO194 may be the outcome of the interaction
between ICM and ISM triggered by RPS. However, the nature of the interaction
is poorly understood.

In this work we add a new piece to the puzzle. The X-ray spectrum of J0194’s
arms is reasonably described by the model
phabs$\cdot$(apec${}_{\texttt{ICM}}$+mkcflow${}_{\texttt{GAL}}$) that is the
sum of the ICM thermal emission along the line-of-sight, and the emission of
the hot plasma cooling onto the galaxy. Although this fitting model has a
reduced $\chi^{2}$ slightly larger than the competing
phabs$\cdot$(apec${}_{\texttt{ICM}}$+apec${}_{\texttt{GAL}}$) model (see Table
4), it allows the first solid metallicity measurement of the hot plasma
generated by the interaction between ICM and ISM
($Z=0.24^{+0.19}_{-0.12}~{}$\mathrm{Z}_{\odot}$$). This value is compatible
within the uncertainties with the ICM abundance ($Z=0.35\pm
0.07~{}$\mathrm{Z}_{\odot}$$). This result would support the idea that the
X-ray emission is produced by the radiative cooling of the metal-poor ICM onto
JO194. The ICM cooling in cool-core clusters has been observed to be
associated with extended H$\alpha$ emission (Voit & Donahue, 1997). Therefore,
we tested if the H$\alpha$ luminosity expected from the ICM radiative cooling
was consistent with the upper limit imposed by the observed H$\alpha$
luminosity of JO194 ($L=(1.24\times 10^{41})\pm(1.12\times 10^{37})$ erg s-1).
The cooling H$\alpha$ luminosity is:

$L_{H\alpha}=3.8\times 10^{39}\dot{M}_{100}=9.1\cdot 10^{36}\text{erg s}^{-1}$
(6)

where $\dot{M}_{100}=0.25^{0.06}_{-0.05}$ is the mass cooling rate in units of
100 solar masses per year. The resulting luminosity is negligible with respect
to the observed H$\alpha$ luminosity due to star formation ($L=(5.66\pm
0.01)\cdot 10^{40}~{}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}$$).
We conclude that the ICM radiative cooling scenario does not violate the
observations, and the warm gas in JO194 is mostly photoionized by the star
formation.

However, the observed physical parameters of the ICM ($kT=4.44\pm 0.16$ keV,
$n_{e}=1.39\pm 0.01~{}$\mathrm{c}\mathrm{m}^{-3}$$) imply that the ICM cooling
time (e.g., Sarazin, 1986b) is of the order of $\sim
30~{}$\mathrm{G}\mathrm{y}\mathrm{r}$$, thus resulting much larger than the
Hubble time. Therefore, the pure ICM radiative cooling cannot be the only
origin of the X-ray emission. A previous phase of ISM-ICM mixing seems to be
necessary to rise the density and lower the temperature of the ICM, which
entail a decrease of its cooling time (e.g., Gronke & Oh, 2018; Fielding et
al., 2020), and stimulates the radiative cooling, that results in the extended
X-ray emission. Indeed Franchetto et al. (2021) already reported on evidence
of ISM-ICM mixing, traced by metallicity gradients, in the stripped tails of
jellyfish galaxies.

###  4.3 Connections between X-ray and H$\alpha$ surface brightness

In order to unfold the connection between X-ray emitting plasma and the warm
gas in the galaxy, we studied the ratios between the X-ray ($SB_{X}$) and the
H$\alpha$ ($SB_{H\alpha}$) surface brightness in the arms, the disk and the
total region that represent JO194. Previous evidence of a spatial correlation
between X and H$\alpha$ have been presented in Poggianti et al. (2019b);
Campitiello et al. (2021); Sun et al. (2021). The $SB_{X}/SB_{H\alpha}$ ratio
can be a signature of the physical process that produce the warm and hot gas.
In case of X-ray and H$\alpha$ emission both powered by star formation, the
expected $SB_{X}/SB_{H\alpha}$ ratio can be estimated by combining Equation 5
with the SFR(H$\alpha$) relation reported in Section 4.2 resulting in:

$SB_{X}/SB_{H\alpha}=3.48\cdot 10^{-2}$ (7)

In the case of jellyfish galaxies, Sun et al. (2021) observed that in their
tails holds a different, higher ratio of:

$SB_{X}/SB_{H\alpha}=3.48\pm 0.25$ (8)

which could be evidence of a different origin of the X-ray plasma with respect
to the star formation, that is the interaction between the ISM and the ICM.

In the case of JO194, we computed the average X-ray surface brightness by
dividing the fluxes of the galactic component in the 0.5-10.0 keV band
corrected for the absorption (see Table 4) by the surface of the respective
regions (Figure 2). The corresponding H$\alpha$ surface brightness derived by
dividing the H$\alpha$ flux measured in the Total, Disk and Arms regions from
the emission-only, dust-corrected MUSE image in the spaxels with $S/R>3$
(Figure 3) by the total area of the corresponding regions. The resulting
$SB_{H\alpha}$ of total, disk and arms are, respectively, 0.9, 2.2 and
$0.1\cdot
10^{-16}$\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{s}\mathrm{/}\mathrm{c}\mathrm{m}^{2}\mathrm{/}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{s}\mathrm{e}\mathrm{c}^{2}$$.
The $SB_{X}/SB_{H\alpha}$ are shown in Figure 6, together with the ratios
reported in Equation 7 (magenta) and 8 (blue).

Figure 6: Total, disk and arms’ surface brightness ratios. The magenta and the
blue vertical ranges are respectively the ratio predicted by the SFR
calibrators (Equation 7) and the one presented in Sun et al. (2021).

The $SB_{X}/SB_{H\alpha}$ measured for JO194 are in between the values
reported in in Equation 7 and 8, with the disk and total being closer to the
former, and the arms’ mkcflow${}_{\texttt{GAL}}$ model being closer to the
latter. Nevertheless, the $SB_{X}/SB_{H\alpha}$ measured for the arms is not
in agreement with the ratio presented in Sun et al. (2021). This result may
suggest that the X-ray emission in the disk is mainly powered by the local
star formation, whereas this is not true for the arms. In this case, the fact
that the observed $SB_{X}/SB_{H\alpha}$ ratios are lower than the values
observed in jellyfish galaxies’ tails (see Equation 8) might indicate that the
stripped arms are not fully evolved into a tail, at least in terms of X-ray
properties. We speculate that the galaxy is still in an early stage of its
evolution into a jellyfish galaxy, so in the stripped arms the ICM and ISM
have not reach yet the balance observed in the other, more evolved jellyfish
galaxies.

###  4.4 Comparison with other GASP jellyfish galaxies

Table 5: Comparison between the properties of different jellyfish galaxies: JO104, JO201, JW100. $R_{\text{cl}}$ is the cluster-centric distance and $R_{200}$ is the virial radius. The Mach number and the Ram Pressure of JW100 are lower limits (Poggianti et al. (2019b)). The properties of JO201 are shown in Bellhouse et al. (2017) and Campitiello et al. (2021). Properties | JO194 | JO201 | JW100
---|---|---|---
$M_{\ast}($\mathrm{M}_{\odot}$)$ | $1.6\cdot 10^{11}$ | $3.6\cdot 10^{10}$ | $3.2\cdot 10^{11}$
$R_{\text{cl}}$ ($\mathrm{k}\mathrm{p}\mathrm{c}$) | $269$ | $360$ | $83$
$R_{\text{cl}}/R_{200}$ | 0.17 | 0.18 | 0.06
$kT_{\textsc{average}}($\mathrm{k}\mathrm{e}\mathrm{V}$)$ | $0.79\pm 0.03$ | $0.76\pm 0.08$ | $0.82\pm 0.11$
$kT_{\text{ICM}}($\mathrm{k}\mathrm{e}\mathrm{V}$)$ | $4.44\pm 0.16$ | $7.10\pm 0.20$ | $3.50\pm 0.10$
$P_{\text{ICM}}($\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{3}$)$ | $1.87\cdot 10^{-11}$ | $2.16\cdot 10^{-11}$ | $3.41\cdot 10^{-11}$
$\rho_{\text{ICM}}($\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{3}$)$ | $2.63\cdot 10^{-27}$ | $1.89\cdot 10^{-27}$ | $5.8\cdot 10^{-27}$
$n_{e}$($\mathrm{c}\mathrm{m}^{-3}$) | $1.39\cdot 10^{-3}$ | $1.00\cdot 10^{-3}$ | $3.20\cdot 10^{-3}$
Mach Number | $1.9$ | $2.4$ | $>2.0$
Ram Pressure ($\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{/}\mathrm{c}\mathrm{m}^{3}$) | $1.08\cdot 10^{-10}$ | $2.11\cdot 10^{-10}$ | $>1.90\cdot 10^{-10}$

We compare the results of our study with the previous studies of the GASP
galaxies JO201 (Poggianti et al., 2019) and JW100 (Poggianti et al., 2019b)
(see Table 5). Although these galaxies are different in terms of stellar mass,
velocity and environment properties, they show several similarities in their
X-ray emission. These can help understanding the origin of the X-ray emitting
plasma and the evolution of the galaxies subjected to ram pressure stripping.

Specifically, these galaxies share the following properties:

  * •

The observed X-ray luminosity for the three galaxies exceeds by a factor $\sim
3-10$ that expected from the current SFR inferred from the H$\alpha$ emission;

  * •

The galactic average X-ray temperatures derived using a single
apec${}_{\texttt{GAL}}$ model is in between $0.7$ and $1.0$ keV. This value is
neither consistent with the temperature of the surrounding ICM ($4.44\pm
0.16$), nor the usual temperature of the X-ray emitting plasma in star-forming
spiral galaxy ($\sim 0.2$ and $\sim 0.65~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$,
e.g., Owen & Warwick, 2009). In addition, this value is in agreement with the
typical temperatures of X-ray tails of cluster late-type galaxies ($\sim 0.9$
keV, Sun et al., 2021). The fact that these systems, despite different
environmental conditions and evolutionary stage, have consistently developed
extended X-ray plasma with similar properties suggests that the process which
originated this hot plasma is both fast and stable.

  * •

The Mach number of the GASP galaxies are similar: $1.9-2.4$. Though the
considered jellyfish galaxies are located in different environments (i.e.,
different ICM sound speed), this similarity implies that the galaxies are
moving at supersonic velocities and are subjected to similar hydrodynamical
phenomena like mixing or shock heating;

  * •

The ICM thermal pressure surrounding these galaxies ranges between 1.87 and
3.41$\times 10^{-11}$ erg cm-3. These values are consistent with the threshold
of $\sim 0.9\times 10^{-11}$erg cm-2 defined in Tonnesen et al. (2011) for the
formation of bright H$\alpha$ and X-ray filaments. Also the ram pressure felt
by these galaxies appears to be similar ($1-2\times 10^{-10}$ erg cm-3),
however these estimates are limited by the fact that the the total velocities
of these galaxies are still unknown;

  * •

These galaxies show extended, nonthermal radio emission (Ignesti et al., 2022,
, Müller et al., in preparation). As previously outlined in Müller et al.
(2021) and Ignesti et al. (2022), the co-existence of radio and X-ray extended
emission could be explained by the ICM draping (e.g., Pfrommer & Dursi, 2010).
By accreting hot, magnetized ICM, the magnetic field in jellyfish galaxies’s
tails can be amplified thus resulting in extended radio emission. In the case
of JO194, the accretion and cooling of hot ICM onto the spiral arms would be
consistent with the result of the spectral analysis (see Section 3.2).

##  5 Summary and conclusions

We analyzed an archival Chandra observation of the cluster Abell 4059,
focusing on the spectral analysis of the X-ray emission associated to
jellyfish galaxy JO194. We investigated different regions of the galaxy, named
Total, Disk, and Arms of JO194, where the latter indicates the Total region
without the Disk. We found that the X-ray emission comes from a plasma with a
temperature of $0.79^{+0.03}_{-0.04}~{}$\mathrm{k}\mathrm{e}\mathrm{V}$$,
which is in line with the previous results reported for similar galaxies. The
spectral analysis of the arms estimate of the hot plasma metallicity, that is
$Z=0.24^{+0.19}_{-0.21}~{}$\mathrm{Z}_{\odot}$$. This value is consistent with
that of the surrounding ICM, therefore we argue that the X-ray emission could
be produced by the ICM that is cooling onto the cold ISM.

We compared the X-ray luminosities obtained by the spectral analysis of the
disk and total with the X-ray luminosities associated with the current SFR,
finding an excess of factor $\sim 3$ over the entire galaxy. This indicates
that the star formation cannot be the only responsible for the X-ray emission
of the galaxy. Moreover, we showed that the excess is predominant in the arms
of the galaxy. Therefore, we suggest that the X-ray plasma pervading JO194 may
be produced by the interplay between ICM and ISM, which results in the cooling
of the former onto the galaxy. In order to explain the observed temperature of
the X-ray plasma ($kT=0.79$ keV), we conclude that ICM cooling likely takes
place initially via mixing between the ICM and ISM, which stimulates the
subsequent phase of radiative cooling that, in turn, might result in the
extended X-ray emission observed by Chandra.

In order to probe the nature of the interaction between ISM and ICM, we
compared the ratio between the observed X-ray and the H$\alpha$ surface
brightness with that measured in the tails of jellyfish galaxies. We found
that the surface brightness ratios in the arms of JO194 are smaller than what
is observed in the other ram pressure stripped galaxies. This might indicate
that, at least in terms of X-ray properties and ISM-ICM balance, the arms of
JO194 are still in an early stage of the evolution into a stripped tail.

Finally, we compared the X-ray properties of GASP jellyfish galaxies, finding
them in agreement with each other. Therefore, we suggest that the conditions
required to induce extended X-ray emission in jellyfish galaxies, that is the
production of hot plasma via ICM-ISM interplay (either via ICM cooling or ISM
heating), are established at the beginning of the stripping, and they can
persist on long time-scales, so that galaxies in different environments and
evolutionary stage can present similar thermal properties.

## Acknowledgements

We thank the Referee for the suggestions that improved the presentation of the
results. A.I., B.V. acknowledge the Italian PRIN-Miur 2017 (PI A. Cimatti).
A.W. acknowledges financial support from ASI through the ASI-INAF agreements
2017-14-H.0. J.F. acknowledges financial support from the UNAM- DGAPA-PAPIIT
IN111620 grant, México. We acknowledge funding from the INAF main-stream
funding programme (PI B. Vulcani). We acknowledge financial contribution from
the agreement ASI-INAF n.2017-14- H.0 (PI A. Moretti). 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 European Union’s
Horizon 2020 research and innovation programme (grant agreement No. 833824).
This research made use of Astropy, a community-developed core Python package
for Astronomy (Astropy Collaboration et al., 2013, 2018), and APLpy, an open-
source plotting package for Python (Robitaille & Bressert, 2012). C.B. thanks
Chinotto for all the constant, purrfect distractions he made during the
preparation of the paper. A.I. thanks the Tokyo Ska Paradise Orchestra’s music
for providing the inspiration during the preparation of the draft.

## References

  * Abell et al. (1989) Abell, G. O., Corwin, H. G., & Olowin, R. P. 1989, IA The Astrophysical Journal Supplement Series, 70, 1
  * Arnaud (1996) Arnaud, K. A. 1996, in Astronomical Society of the Pacific Conference Series, Vol. 101, Astronomical Data Analysis Software and Systems V, ed. G. H. Jacoby & J. Barnes, 17
  * Asplund et al. (2009) Asplund, M., Grevesse, N., Sauval, A. J., & Scott, P. 2009, ARA&A, 47, 481, doi: 10.1146/annurev.astro.46.060407.145222
  * Astropy Collaboration et al. (2013) Astropy Collaboration, Robitaille, T. P., Tollerud, E. J., et al. 2013, A&A, 558, A33, doi: 10.1051/0004-6361/201322068
  * Astropy Collaboration et al. (2018) Astropy Collaboration, Price-Whelan, A. M., Sipőcz, B. M., et al. 2018, AJ, 156, 123, doi: 10.3847/1538-3881/aabc4f
  * Bellhouse et al. (2017) Bellhouse, C., Jaffé, Y. L., Hau, G. K. T., et al. 2017, ApJ, 844, 49, doi: 10.3847/1538-4357/aa7875
  * Bellhouse et al. (2021) Bellhouse, C., McGee, S. L., Smith, R., et al. 2021, Monthly Notices of the Royal Astronomical Society, 500, 1285, doi: 10.1093/mnras/staa3298
  * Boselli, A. et al. (2016) Boselli, A., Cuillandre, J. C., Fossati, M., et al. 2016, A&A, 587, A68, doi: 10.1051/0004-6361/201527795
  * Burke et al. (2020) Burke, D., Laurino, O., wmclaugh, et al. 2020, sherpa/sherpa: Sherpa 4.12.1, 4.12.1, Zenodo, doi: 10.5281/zenodo.3944985
  * Campitiello et al. (2021) Campitiello, M. G., Ignesti, A., Gitti, M., et al. 2021, ApJ, 911, 144, doi: 10.3847/1538-4357/abec82
  * Cash (1979) Cash, W. 1979, ApJ, 228, 939, doi: 10.1086/156922
  * Cavaliere & Fusco-Femiano (1976) Cavaliere, A., & Fusco-Femiano, R. 1976, Astronomy and Astrophysics, 49, 137
  * Chabrier (2003) Chabrier, G. 2003, Publications of the Astronomical Society of the Pacific, 115, 763
  * Choi et al. (2004) Choi, Y. Y., Reynolds, C. S., Heinz, S., et al. 2004, ArXiv e-prints, 1795, 1, doi: 10.1086/382941
  * Chung et al. (2009) Chung, E. J., Rhee, M.-H., Kim, H., et al. 2009, The Astrophysical Journal Supplement Series, 184, 199, doi: 10.1088/0067-0049/184/2/199
  * Cowie et al. (1977) Cowie, L. L., Songaila, A., Cowie, L. L., & Songaila, A. 1977, Natur, 266, 501, doi: 10.1038/266501A0
  * Davies & Lewis (1973) Davies, R. D., & Lewis, B. M. 1973, MNRAS, 165, 231, doi: 10.1093/mnras/165.2.231
  * Doe et al. (2007) Doe, S., Nguyen, D., Stawarz, C., et al. 2007, in Astronomical Society of the Pacific Conference Series, Vol. 376, Astronomical Data Analysis Software and Systems XVI, ed. R. A. Shaw, F. Hill, & D. J. Bell, 543
  * Dopita et al. (2013) Dopita, M. A., Sutherland, R. S., Nicholls, D. C., Kewley, L. J., & Vogt, F. P. A. 2013, The Astrophysical Journal Supplement Series, 208, 10, doi: 10.1088/0067-0049/208/1/10
  * Fielding et al. (2020) Fielding, D. B., Ostriker, E. C., Bryan, G. L., & Jermyn, A. S. 2020, ApJ, 894, L24, doi: 10.3847/2041-8213/ab8d2c
  * Franchetto et al. (2020) Franchetto, A., Vulcani, B., Poggianti, B. M., et al. 2020, The Astrophysical Journal, 895, 106, doi: 10.3847/1538-4357/ab8db9
  * Franchetto et al. (2021) Franchetto, A., Tonnesen, S., Poggianti, B. M., et al. 2021, ApJ, 922, L6, doi: 10.3847/2041-8213/ac3664
  * Freeman et al. (2001) Freeman, P., Doe, S., & Siemiginowska, A. 2001, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 4477, Astronomical Data Analysis, ed. J.-L. Starck & F. D. Murtagh, 76–87, doi: 10.1117/12.447161
  * Fritz et al. (2017) Fritz, J., Moretti, A., Gullieuszik, M., et al. 2017, ApJ, 848, 132, doi: 10.3847/1538-4357/aa8f51
  * Fruscione et al. (2006) Fruscione, A., McDowell, J. C., Allen, G. E., et al. 2006, in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, Vol. 6270, Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, ed. D. R. Silva & R. E. Doxsey, 62701V, doi: 10.1117/12.671760
  * Fumagalli et al. (2014) Fumagalli, M., Fossati, M., Hau, G. K. T., et al. 2014, Monthly Notices of the Royal Astronomical Society, 445, 4335, doi: 10.1093/mnras/stu2092
  * Giovanelli & Haynes (1988) Giovanelli, R., & Haynes, M. P. 1988, in Galactic and Extragalactic Radio Astronomy, ed. K. I. Kellermann & G. L. Verschuur, 522–562
  * Gronke & Oh (2018) Gronke, M., & Oh, S. P. 2018, MNRAS, 480, L111, doi: 10.1093/mnrasl/sly131
  * Gullieuszik et al. (2020) Gullieuszik, M., Poggianti, B. M., McGee, S. L., et al. 2020, The Astrophysical Journal, 899, 13, doi: 10.3847/1538-4357/aba3cb
  * Gunn & Gott (1972) Gunn, J. E., & Gott, J. Richard, I. 1972, ApJ, 176, 1, doi: 10.1086/151605
  * HI4PI Collaboration: et al. (2016) HI4PI Collaboration:, Ben Bekhti, N., Flöer, L., et al. 2016, A&A, 594, A116, doi: 10.1051/0004-6361/201629178
  * Huang & Sarazin (1998) Huang, Z., & Sarazin, C. L. 1998, The Astrophysical Journal, 496, 728, doi: 10.1086/305406
  * Ignesti et al. (2022) Ignesti, A., Vulcani, B., Poggianti, B. M., et al. 2022, ApJ, 924, 64, doi: 10.3847/1538-4357/ac32ce
  * Jaffé et al. (2018) Jaffé, Y. L., Poggianti, B. M., Moretti, A., et al. 2018, Monthly Notices of the Royal Astronomical Society, 476, 4753, doi: 10.1093/MNRAS/STY500
  * Laganá et al. (2019) Laganá, T. F., Souza, G. S., MacHado, R. E., Volert, R. C., & Lopes, P. A. 2019, Monthly Notices of the Royal Astronomical Society, 487, 3922, doi: 10.1093/mnras/stz1575
  * Mernier et al. (2015) Mernier, F., de Plaa, J., Lovisari, L., et al. 2015, Astronomy & Astrophysics, 575, A37, doi: 10.1051/0004-6361/201425282
  * Mineo et al. (2013) Mineo, S., Gilfanov, M., Lehmer, B. D., Morrison, G. E., & Sunyaev, R. 2013, Monthly Notices of the Royal Astronomical Society, 437, 1698, doi: 10.1093/mnras/stt1999
  * Mineo et al. (2012) Mineo, S., Gilfanov, M., & Sunyaev, R. 2012, MNRAS, 419, 2095, doi: 10.1111/j.1365-2966.2011.19862.x
  * Müller et al. (2021) Müller, A., Ignesti, A., Poggianti, B., et al. 2021, Galaxies, 9, 116, doi: 10.3390/galaxies9040116
  * Müller et al. (2020) Müller, A., Poggianti, B. M., Pfrommer, C., et al. 2020, Nature Astronomy, 5, 159–168, doi: 10.1038/s41550-020-01234-7
  * Nulsen (1982) Nulsen, P. E. J. 1982, Monthly Notices of the Royal Astronomical Society, 198, 1007, doi: 10.1093/mnras/198.4.1007
  * Owen & Warwick (2009) Owen, R. A., & Warwick, R. S. 2009, MNRAS, 394, 1741, doi: 10.1111/j.1365-2966.2009.14464.x
  * Peluso et al. (2022) Peluso, G., Vulcani, B., Poggianti, B. M., et al. 2022, ApJ, 927, 130, doi: 10.3847/1538-4357/ac4225
  * Pfrommer & Dursi (2010) Pfrommer, C., & Dursi, L. 2010, Nature Physics, 6, 520, doi: 10.1038/nphys1657
  * Poggianti et al. (2016) Poggianti, B. M., Fasano, G., Omizzolo, A., et al. 2016, AJ, 151, 78, doi: 10.3847/0004-6256/151/3/78
  * Poggianti et al. (2017) Poggianti, B. M., Jaffé, Y. L., Moretti, A., et al. 2017, Nature, 548, 304, doi: 10.1038/nature23462
  * Poggianti et al. (2017) Poggianti, B. M., Moretti, A., Gullieuszik, M., et al. 2017, The Astrophysical Journal, 844, 48, doi: 10.3847/1538-4357/aa78ed
  * Poggianti et al. (2019a) Poggianti, B. M., Gullieuszik, M., Tonnesen, S., et al. 2019a, Monthly Notices of the Royal Astronomical Society, 482, 4466, doi: 10.1093/mnras/sty2999
  * Poggianti et al. (2019b) Poggianti, B. M., Ignesti, A., Gitti, M., et al. 2019b, The Astrophysical Journal, 887, 155, doi: 10.3847/1538-4357/ab5224
  * Poggianti et al. (2019) Poggianti, B. M., Gullieuszik, M., Tonnesen, S., et al. 2019, MNRAS, 482, 4466, doi: 10.1093/mnras/sty2999
  * Radovich et al. (2019) Radovich, M., Poggianti, B., Jaffé, Y. L., et al. 2019, MNRAS, 486, 486, doi: 10.1093/mnras/stz809
  * Reynolds et al. (2008) Reynolds, C. S., Casper, E. A., & Heinz, S. 2008, The Astrophysical Journal, 679, 1181, doi: 10.1086/587456
  * Robitaille & Bressert (2012) Robitaille, T., & Bressert, E. 2012, APLpy: Astronomical Plotting Library in Python.  http://ascl.net/1208.017
  * Sarazin (1986a) Sarazin, C. L. 1986a, Reviews of Modern Physics, 58, 1, doi: 10.1103/RevModPhys.58.1
  * Sarazin (1986b) —. 1986b, Reviews of Modern Physics, 58, 1, doi: 10.1103/RevModPhys.58.1
  * Schwartz et al. (1991) Schwartz, D. A., Bradt, H. V., Remillard, R. A., et al. 1991, ApJ, 376, 424, doi: 10.1086/170291
  * Solanes et al. (2001) Solanes, J. M., Manrique, A., García-Gómez, C., et al. 2001, ApJ, 548, 97, doi: 10.1086/318672
  * Sun et al. (2009) Sun, M., Donahue, M., Roediger, E., et al. 2009, The Astrophysical Journal, 708, 946–964, doi: 10.1088/0004-637x/708/2/946
  * Sun et al. (2010) Sun, M., Donahue, M., Roediger, E., et al. 2010, ApJ, 708, 946, doi: 10.1088/0004-637X/708/2/946
  * Sun et al. (2006) Sun, M., Jones, C., Forman, W., et al. 2006, ApJ, 637, L81, doi: 10.1086/500590
  * Sun & Vikhlinin (2005) Sun, M., & Vikhlinin, A. 2005, The Astrophysical Journal, 621, 718–724, doi: 10.1086/427728
  * Sun et al. (2021) Sun, M., Ge, C., Luo, R., et al. 2021
  * Tonnesen et al. (2011) Tonnesen, S., Bryan, G. L., & Chen, R. 2011, ApJ, 731, 98, doi: 10.1088/0004-637X/731/2/98
  * Tschöke, D. et al. (2001) Tschöke, D., Bomans, D. J., Hensler, G., & Junkes, N. 2001, A&A, 380, 40, doi: 10.1051/0004-6361:20011354
  * Varela et al. (2009) Varela, J., D’Onofrio, M., Marmo, C., et al. 2009, A&A, 497, 667, doi: 10.1051/0004-6361/200809876
  * Vogt et al. (2015) Vogt, F. P. A., Dopita, M. A., Borthakur, S., et al. 2015, Monthly Notices of the Royal Astronomical Society, 450, 2593, doi: 10.1093/mnras/stv749
  * Voit & Donahue (1997) Voit, G. M., & Donahue, M. 1997, ApJ, 486, 242, doi: 10.1086/304509
  * Vulcani et al. (2018) Vulcani, B., Poggianti, B. M., Gullieuszik, M., et al. 2018, ApJ, 866, L25, doi: 10.3847/2041-8213/aae68b
  * Vulcani et al. (2020) Vulcani, B., Poggianti, B. M., Tonnesen, S., et al. 2020, ApJ, 899, 98, doi: 10.3847/1538-4357/aba4ae
  * Werle et al. (2022) Werle, A., Poggianti, B., Moretti, A., et al. 2022, ApJ, 930, 43, doi: 10.3847/1538-4357/ac5f06
  * Zhang et al. (2013) Zhang, B., Sun, M., Ji, L., et al. 2013, The Astrophysical Journal, 777, 122, doi: 10.1088/0004-637x/777/2/122