# GASP XXXIX: MeerKAT hunts Jellyfish in A2626

Tirna Deb,1,2 Marc A.W. Verheijen1, Bianca M. Poggianti3, Alessia Moretti3,
J.M. van der Hulst1, Benedetta Vulcani3, Mpati Ramatsoku4,5, Paolo Serra4,
Julia Healy6, Marco Gullieuszik3, Cecilia Bacchini3, Alessandro Ignesti3,
Ancla Müller7, Nikki Zabel8, Nicholas Luber9,10, Yara L. Jaffé11, Myriam
Gitti12,13
1Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747
AV Groningen, The Netherlands
2Department of Physics and Astronomy, University of the Western Cape, Robert
Sobukwe Road, Bellville 7535, South Africa
3INAF- Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122
Padova, Italy
4INAF- Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047
Selargius (CA), Italy
5Department of Physics and Electronics, Rhodes University, PO Box 94,
Makhanda, 6140, South Africa
6ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA
Dwingeloo, The Netherlands
7Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical
Institute, Universitätsstr. 50, 44801 Bochum, Germany
8Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch
7701, South Africa
9Department of Physics and Astronomy, West Virginia University, P.O. Box 6315,
Morgantown, WV 26506, USA
10Center for Gravitational Waves and Cosmology, West Virginia University,
Chestnut Ridge Research Building, Morgantown, WV 26505
11Instituto de Física y Astronomía, Universidad de Valparaíso, Avda. Gran
Bretãna 1111 Valparaíso, Chile
12Dipartimento di Fisica e Astronomia, Università di Bologna, via Gobetti
93/2, 40129 Bologna, Italy
13INAF, Istituto di Radioastronomia di Bologna, via Gobetti 101, 40129
Bologna, Italy
E-mail<EMAIL_ADDRESS>
(Accepted 2022 August 24. Received 2022 August 24; in original form 2021
September 27)

###### Abstract

We present MeerKAT H i observations of six jellyfish candidate galaxies
(JFCGs) in the galaxy cluster, A2626. Two of the six galaxies JW100 and JW103,
that were identified as JFCGs from B-band images, are confirmed as jellyfish
galaxies (JFGs). Both of the JFGs have low H i content, reside in the cluster
core, and move at very high velocities ($\sim$ 3$\sigma_{cl}$). The other
JFCGs, identified as non-jellyfish galaxies, are H i rich, with H i
morphologies revealing warps, asymmetries, and possible tidal interactions.
Both the A2626 JFGs and three other confirmed JFGs from the GASP sample show
that these galaxies are H i stripped but not yet quenched. We detect H i,
H$\alpha$, and CO(2-1) tails of similar extent ($\sim$ 50 kpc) in JW100.
Comparing the multi-phase velocity channels, we do not detect any H i or
CO(2-1) emission in the northern section of the tail where H$\alpha$ emission
is present, possibly due to prolonged interaction between the stripped gas and
the ICM. We also observe an anti-correlation between H i and CO(2-1), which
hints at an efficient conversion of H i to H2 in the southern part of the
tail. We find that both RPS and H i-to-H2 conversion are significant depletion
channels for atomic gas. H i-to-H2 conversion is more efficient in the disc
than in the tail.

###### keywords:

Galaxies: clusters: intracluster medium – galaxies: evolution, ISM

††pubyear: 2021††pagerange: GASP XXXIX: MeerKAT hunts Jellyfish in A2626–GASP
XXXIX: MeerKAT hunts Jellyfish in A2626

##  1 Introduction

In the dense cluster environment, hydrodynamical interactions such as ram-
pressure stripping (RPS) between the inter-stellar medium (ISM) of galaxies
and intra-cluster medium (ICM) play an important role in transforming galaxies
(Gunn & Gott, 1972) from star forming (SF) and gas rich, to quiescent and gas
poor. ‘Jellyfish’ galaxies (JFGs) are extreme examples of RPS with ‘tentacles’
of material that stretch tens of kpcs beyond their stellar discs (Smith et
al., 2010; Fumagalli et al., 2014; Ebeling et al., 2014).For example, one of
the most studied jellyfish galaxy is ESO137-001 in the nearby Norma cluster,
with its long, extended tail observed in multi-wavelength (Sun et al., 2007;
Jáchym et al., 2014; Fossati et al., 2016; Jáchym et al., 2019).

The extended, fragile atomic hydrogen (H i) gas discs are not only the
reservoirs for star formation, but also serve as diagnostic tracers of
gravitational and hydrodynamic environmental processes (Bravo-Alfaro et al.,
2001; Chung et al., 2009; Jaffé et al., 2015). Images of the H i gas in
galaxies show that the extended H i discs are readily affected by the cluster
environment, resulting in H i stripped and H i deficient galaxies compared to
field galaxies (Verheijen & Sancisi, 2001; Oosterloo & van Gorkom, 2005; Chung
et al., 2009; Serra et al., 2013; Gogate et al., 2020; Loni et al., 2021).
This deficiency and stripping is less conspicuous for the centrally
concentrated and more tightly bound molecular hydrogen gas (H2). Studies of
cluster galaxies often show conflicting results that differ between clusters:
low SF in H i stripped but H2 normal galaxies (Virgo cluster, Kenney & Young
1989; Lee et al. 2017; Brown et al. 2021), and normal or slightly enhanced SF
in H i deficient but H2 rich galaxies (JFGs, Moretti et al. 2020a, b). In the
Virgo cluster, some galaxies are H i deficient and show signatures of RPS, but
are only moderately deficient in H2 (Chung et al. 2009; Kenney & Young 1989;
Brown et al. 2021). In Fornax, galaxies can be H i or H2 deficient, or
deficient in both gas phases (Zabel et al., 2019; Loni et al., 2021).

Until recently, observations and studies of the environmental impact on the
multi-phase ISM have been limited to nearby clusters such as Virgo (Chung et
al. 2009; Kenney & Young 1989; Brown et al. 2021), Fornax (Zabel et al., 2019;
Loni et al., 2021) or Coma (Boselli et al., 1997; Casoli et al., 1996; Healy
et al., 2021c). It is still not clear what the relative influence of the ICM
pressure is on the H i and H2 gas. It also remains a puzzle why H i stripped
galaxies appear to be quiescent while the H2 gas, which is the more direct
constituent for SF, is not deficient. There are still many other questions
without clear answers: How do environmental processes impact the baryon cycle
in galaxies? How does SF quench in galaxies: by gas removal or consumption?
Therefore, it is very timely to broaden our understanding of environmental
processes acting on the multi-phase ISM of galaxies in clusters. JFGs, extreme
examples of RPS and often located in the dense cluster core, are excellent
objects to investigate environmental effects on the multi-phase ISM.

The GAs Stripping Phenomena in galaxies survey (GASP; Poggianti et al. 2017b)
was carried out with the Multi Unit Spectroscopic Explorer (MUSE), observing a
statistically significant sample of JFGs in nearby clusters (z=0.04-0.07) over
a wide range of stellar masses, morphological asymmetries and environments.
The key scientific motivation is to investigate how, where and why gas removal
occurs, and to quantify the amount of star formation involved in these
processes (Poggianti et al., 2017b). Within GASP, JFGs are defined as the
galaxies with a H$\alpha$ tail at least as long as the diameter of the stellar
disc. In the RPS tails of these JFGs, often in situ SF occurs by efficient
conversion of H i to H2 followed by the collapse of molecular clouds due to
thermal instabilities, turbulent motion etc (Poggianti et al., 2019; Müller et
al., 2021). H i gas is also ionised by the young stars that are formed in situ
in the tail. Poggianti et al. (2017a) observed a strong correlation between
RPS and the presence of an Active Galactic Nucleus (AGNs), hinting at the
possibility that ram pressure is causing gas to flow towards the centre thus
triggering the AGN activity. Poggianti et al. (2016) identified possible
stripped galaxies as jellyfish candidates galaxies (JFCGs) purely based on
B-band imaging. JO201, JO204, JO206, and JW100 were confirmed stripped
galaxies i.e. JFGs with long tails by MUSE observations (Poggianti et al.,
2017b; Gullieuszik et al., 2017; Bellhouse et al., 2017, 2019; Poggianti et
al., 2019).

Five of the GASP JFGs (JO201, JO204, JO206, JO194, JO135) were also observed
with the Jansky Very Large Array (JVLA; Perley et al. 2009) to investigate
their H i gas content. Ramatsoku et al. (2019, 2020) observed that both JO201
and JO206 show a strong stripping of H i gas relative to the stellar disc (on
the sky and/or in velocity). Both the H i and H$\alpha$ tails are of similar
extent, and these galaxies have comparable H i deficiencies, with enhanced SF
compared to that of galaxies of similar stellar and H i mass. However, it is
worth noting that JO201 resides in a more massive and dense cluster than JO206
(Bellhouse et al., 2017, 2019; Poggianti et al., 2017b; Jaffé et al., 2018).
Deb et al. (2020) observed a much longer (90 kpc) H i tail compared to the RPS
H$\alpha$ tail (30 kpc, Gullieuszik et al. 2017) in JO204. Deb et al. (2020)
also investigated whether RPS is inducing the AGN in JO204 by funnelling H i
gas towards the centre of the galaxy, resulting in a redshifted H i absorption
profile, but the study was inconclusive. Luber et al. (2022) investigated the
H i properties of non-jellyfish galaxies in clusters with JFGs from GASP
sample. Our study will add the H i picture of the of the JFCGs in A2626, and
will complement the detailed studies of JW100 in other wavelengths (Poggianti
et al., 2019).

Healy et al. (2021b) have conducted a blind MeerKAT H i survey of the galaxy
cluster A2626, which hosts six JFCGs including JW100, signifying ongoing
environment driven evolution of the galaxies. In this paper, we compare the H
i properties and SF of the A2626 JFCGs with other JFGs from the GASP sample
residing in different clusters. In the next sections we will first address the
H i in all six JF(C)Gs and then concentrate on understanding the details of
the RPS process in JW100 for which all ISM phases are now available from
Poggianti et al. (2019), Moretti et al. (2020a) and our MeerKAT observations.
H i observations of JW100 are the last piece of the puzzle to complete the
picture of the interplay of the multi-phase (atomic, molecular, ionised) ISM
with itself and with the ICM.

The paper is organised as follows: Sec. 2 gives an overview of A2626 and the
JFCGs in it. H i observations and data processing are presented in Sec. 3. In
Sec. 4 we explore the H i properties of the JFCGs. In Sec. 5, we investigate
the relation between H i deficiency and star-formation rate (SFR) in the JFGs.
In sec. 6, we then focus on JFG JW100 and study the atomic, molecular, and
ionised gas phases in different velocity channels. In sec. 7, we calculate the
contribution of different depletion mechanisms to the stripping of the multi-
phase gas in JW100. Finally in sec. 8, we summarise the findings and
conclusions.

Throughout this paper, we adopt a Chabrier initial mass function (IMF;
Chabrier 2003), and assume a cold dark matter cosmology with $\Omega_{M}$ =
0.3, $\Omega_{\Lambda}$ = 0.7 and H0 = 70 km s-1 Mpc-1. At the cluster
redshift (z=0.055292, Healy et al. 2021a), this yields 1$\arcsec$=1.074 kpc.

Table 1: A2626 cluster properties

Cluster properties
---
RA | 23:36:31a
Dec | +21:09:36.3a
z | 0.055292b
$\sigma_{cl}$ | 660 $\pm$ 26 km/sb
M200 | 3.9 $\times$ 1014 $\text{M}_{\odot}$ c
R200 | 1.59 Mpcb
LX | 1.9 $\times$ 1044 erg/s d

a Cava et al. (2009), b Healy et al. (2021a), c Biviano et al. (2017), d Wong
et al. (2008)

##  2 A2626 and its jellyfish candidate galaxies

Figure 1: The location of jellyfish candidate galaxies (JFCGs) JW98, JW99,
JW100, JW101, JW102, and JW103 in the A2626 volume (Poggianti et al., 2016).
The dashed circle represents the R200 of A2626. The black contours represent
the X-ray emission in A2626 from ROSAT images. The coloured dots represent the
H i detections in different redshift over-densities in the A2626 volume (Healy
et al., 2021a). JClass=1 JFCGs (i.e. least probable JFGs from their optical
morphologies, JW98, JW99, JW101, JW102) are outside the cluster core, higher
JClass (JClass $\geq$ 2) galaxies JW100 and JW103 are residing close to the
cluster core in projection. On the side left and right panels we show the
optical Dark Energy Camera Legacy Survey (DECaLS, Dey et al. 2019) images of
JFCGs in A2626, including the $\Delta$cz velocity difference with respect to
the cluster core.

###  2.1 The galaxy cluster A2626

The focus of our study are the JFCGs identified in A2626, one of 77 clusters
imaged as part of the WIde-field Nearby Galaxy-cluster Survey (WINGS: Fasano
et al. 2006). A2626 is a moderately massive, X-ray emitting cluster at a
redshift of z=0.055292 (Healy et al., 2021a). The global properties of the
cluster are listed in Table 1. It is a cooling-core cluster (Wong et al.,
2008; McDonald et al., 2018; Ignesti et al., 2018; Kadam et al., 2019). The
X-ray surface brightness and thermodynamic maps show cold fronts and spiral
wakes, indicate sloshing and the disturbed state of the ICM of A2626 (Rizza et
al., 2000; Wong et al., 2008; McDonald et al., 2018; Kadam et al., 2019). The
cluster hosts a peculiar central Brightest Cluster Galaxy (BCG, IC 5338) that
displays an offset nucleus in an asymmetric stellar envelope. The BCG is a
radio-loud galaxy and the source of a peculiar and well-studied radio source
known as ‘The Kite’ (Rizza et al., 2000; Gitti et al., 2004; Gitti, 2013; Kale
& Gitti, 2017; Ignesti et al., 2017, 2018; Ignesti et al., 2020). Using a new
spectroscopic survey, Healy et al. (2021a) identified six galaxy
groups/substructure within and beyond the projected R200 radius. These
substructures indicate that A2626 is actively accreting galaxies and galaxy
groups from its surroundings, and confirm that A2626 is a dynamically active
cosmic environment.

Table 2: Properties of the jellyfish candidate galaxies

Name | RA | DEC | Optical |  $\Delta$cz |  $\text{M}_{\star}$ d,e | SFRc,e | $\pm^{c,e}$ | $\text{M}_{\rm HI}$ | $\pm$ | JClassa
---|---|---|---|---|---|---|---|---|---|---
| (J2000)a | (J2000)a | redshiftsb,c | ($\text{km}\,\text{s}^{-1}$) |  $\times$ 109 |  $\text{M}_{\odot}$yr-1 |  $\text{M}_{\odot}$yr-1 |  $\times$ 109 |  $\times$ 109 |
| deg | deg |  |  | $\text{M}_{\odot}$ |  |  | $\text{M}_{\odot}$ | $\text{M}_{\odot}$ |
JW98 | 23:36:09.41 | +21:11:11.8 | 0.0681 | +3886 | 5.2${}^{+2.6}_{-1.7}$ | 0.31 | 0.17 | 11 | 0.4 | 1
JW99 | 23:36:18.55 | +21:04:04.4 | 0.05327 | -661 | 8.8${}^{+3.4}_{-2.5}$ | 2.7 | 0.9 | 24 | 0.5 | 1
JW100 | 23:36:25.01 | +21:09:03.6 | 0.06189 | +1978 | 320${}^{+310}_{-120}$ | 4 | 0.8 | 2.8 | 1.2 | 5
JW101 | 23:37:25.92 | +20:54:35.3 | 0.06983 | +4417 | 16${}^{+7}_{-5}$ | 4.2 | 1.7 | 8.9 | 0.7 | 1
JW102 | 23:35:40.01 | +21:08:44.5 | 0.0613 | +1781 | 2.7${}^{+48}_{-2.6}$ | 0.3 | 0.18 | 6.5 | 0.3 | 1
JW103 | 23:36:26.54 | +21:10:55.6 | 0.06189 | +1876 | 5.2${}^{+4.4}_{-2.8}$ | 2.2 | 0.9 | 1 | 0.3 | 2

aPoggianti et al. (2016), bHealy et al. (2021a) , cPoggianti et al. (2019), d
Vulcani et al. (2018), e based on WISE, provided via private communication

###  2.2 Jellyfish candidate galaxies in A2626

Figure 2: Atlas pages for JW98 and JW99. Clockwise from the top left panel:
DECaLS r-band image, the H i column density contours on DECaLS g-band image in
greyscale, total H i map in greyscale, signal-to-noise map as a colour map
with the different colours representing different levels of signal-to-noise, H
i global profile, H i position-velocity (PV) diagram, H i velocity field, and
radio continuum map on DECaLS g-band image. The H i column density contours
are plotted at the levels N${}^{3\sigma}_{\textsc{H$\,$i}}\times 2^{n}$ with
n=0,1,2,3,… with N${}^{3\sigma}_{\textsc{H$\,$i}}$ \- the average ${3\sigma}$
column density for each galaxy. The H i beam size (15") is shown in the bottom
left. In the velocity field, the lighter greyscales and blue contours show the
approaching side with contour levels 25, 50, 75, 100,..200
$\text{km}\,\text{s}^{-1}$. The systemic velocity vsys is shown by the white
contour. The darker greyscales and red contours represent the receding side of
the H i rotating disc with the contour levels -25, -50, -75, …, -200
$\text{km}\,\text{s}^{-1}$. The radio continuum contours are plotted at the
levels 2n of the RMS noise, where n=0,1,2,3,… Dashed contours are drawn at
-2$\sigma$ the RMS noise. The hatched elliptical beam in the radio continuum
map is the beam size of continuum map (14.6" $\times$ 9.30", 171.6∘). The
position-velocity (PV) diagrams are made along the major axis of the optical
image, starting at the receding side of the galaxy. The position angles are
mentioned on the top of each panel which are determined by visual inspection
of the optical image while the receding side with respect to the systemic
recession velocity of the galaxy is determined from the three-dimensional H i
datacube. The directions of the start and end points of the PV slices are
indicated in the bottom left and right corners of each panel respectively. The
orange contour outlines the manually made optimum mask. The vertical dashed
line indicates the optical centre of the galaxy from the SDSS. The horizontal
dashed line corresponds to the central frequency as derived from the global
profile. The H i redshift is calculated from this central frequency and
reported in the top right of the bottom right panel. The H i spectrum or
global profile of each galaxy is constructed by applying the dilated, optimum
H i mask for each galaxy to the 20" resolution H i datacube. The vertical
black downward arrow in the middle represents the central frequency,
corresponding to the systemic velocity, which is the midpoint of the 20%
velocity width. The measured optical redshifts are shown with an orange upward
arrow. The method to derive the errorbars as shown on the global profile is
described in Section 5.1 in Healy et al. (2021b). Figure 3: Atlas pages for
JW101 and JW102. The description of atlas pages is the same as for Fig. 2.

A2626 hosts six JFCGs as identified by Poggianti et al. (2016). Using WINGS
B-band images to search for optical evidence of gas stripping, the JFCGs were
identified based on their optical morphology. Poggianti et al. (2016) ranked
the galaxies on a scale of 1 to 5, JClass = 1 being the least plausible JFG
and JClass = 5 being the most plausible JFG. The six JFCGs identified in A2626
are JW98, JW99, JW100, JW101, JW102, and JW103. Their global properties are
listed in Table 2 and illustrated in Fig. 1 which also indicates their
projected locations in A2626. Of these galaxies, JW100 has the highest
JClass=5 and is known to be experiencing ram-pressure stripping (RPS) of its
ISM. In the top right panel of Fig. 1, JW100 clearly displays and extended RPS
tail towards the South West that appears as light blue in the optical image
that indicating extra-planar star formation activity, which was confirmed by
MUSE observations (Poggianti et al., 2019). JW100 has been the subject of many
detailed studies (as summarised below) while the other five JFCGs were not
part of the GASP MUSE sample. JW103 has a low JClass=2 and it is likely at a
later stage of RPS than JW100 due to its lower stellar mass. It is at a same
velocity and similar location at the cluster as JW100 with two extensions of
blue light towards the West in the optical image (see top right panel of 1).
The other four candidate JFGs have JClass=1 and are least likely to be
experiencing RPS (see Fig. 1).

It should be noted that JW100, JW102 and JW103 have a very similar radial
velocity offset with respect to A2626 of $\Delta$cz $\approx$+1800 km/s ($\sim
3\sigma_{\rm cl}$) indicating that these galaxies are entering A2626 as a
group. JW100 and JW103 with JClass$>$1 are close to the X-ray emitting cluster
core while JW102 is still at a substantial projected distance ($\sim$ 0.6
R200) and thus not strongly affected by RPS yet.

The optical morphologies of the JFCGs show a wide variety of characteristics,
ranging from irregular blue systems like JW98, which Poggianti et al. (2016)
suggest to be the result of harassment, to two-armed grand-design spirals with
a hint of lopsidedness like JW99, to bulge-dominated like JW101. Notably, the
optical image of JW100 shows some reddening of the stellar light just west of
its bulge, suggesting that dust is also being removed from the disk of this
galaxy. In the lower JClass galaxies, it is not always obvious which blue
features hint as ongoing RPS, but their overall disturbed or lopsided optical
morphologies classified these galaxies as JFCGs. The spatially resolved H i
observations presented in this paper will clarify whether these JClass=1
galaxies are actually experiencing RPS.

Using stellar masses and SFRs determined using WISE photometry (T. Jarrett,
private communication), we can compare the A2626 JFCGs to other galaxies in
A2626 and different reference samples (see Sec. 5). The stellar masses are
calculated from the 3.4$\rm\mu$m (W1) and 4.6$\rm\mu$m (W2) emission. The SFRs
are determined from the PAH emission in the 12$\rm\mu$m (W3) band; see Cluver
et al. (2014); Cluver et al. (2017); Jarrett et al. (2013); Jarrett et al.
(2019) for details.

All the above mentioned studies of JFCGs in A2626 lack observational
information on an important constituent of the ram-pressure stripped gas,
namely the H i gas, and that is what our current study contributes. H i
observations of JFCGs in A2626 will provide a diverse picture of H i
morphologies of such galaxies that are at different stages of stripping and
can confirm whether candidate JFGs are actually experiencing ongoing RPS or
not. Particularly, JW100 is one of the most well studied JFGs so far and
information about the atomic gas will complete the picture of the multi-phase
gas of JW100.

##  3 H i observations and data processing

A2626 was observed with the MeerKAT telescope (Jonas & MeerKAT Team, 2016) as
one of the first MeerKAT-64 open time observations (project code
SCI-20190418-JH-01). A2626 was observed during the nights of 15-17 July 2019
for a total of 15 (3 $\times$ 5) hours. The large field-of-view of MeerKAT
(61$\arcmin$ $\times$ 61$\arcmin$ at a frequency of 1346 MHz where H i
emission from z = 0.055 is detected) is adequate to cover the cluster beyond
R200 and encompasses all the JFCGs. The sensitivity of MeerKAT enables a
3$\sigma$ H i mass detection limit of 2 $\times$ 108 $\text{M}_{\odot}$ in the
field centre with a linewidth of 300 $\text{km}\,\text{s}^{-1}$, and a
corresponding 5$\sigma$ column density of 2.5 $\times$ 1019 cm-2 at an angular
resolution of 30$\arcsec$. The data calibration and imaging was carried out
using CARACal, (version 1.0, previously known as MeerKATHI, Józsa et al. 2020)
to reduce the data and SoFiA-2 (Serra et al., 2015) to detect the H i sources
within the MeerKAT cube. For the complete details on the data calibration,
imaging, and source finding, see Healy et al. (2021b).

The four low class (JClass = 1) candidate galaxies turned out to be gas rich
and were identified by SoFiA-2 (H i ID=76 (JW98), 88 (JW99), 156 (JW101), 45
(JW102) in Healy et al. 2021b). The atlas pages for JW98 and JW99, and JW101
and JW102 are shown in Fig. 2 and 3 respectively. JW103 (JClass=2) and JW100
(JClass=5) were not identified by SoFiA-2 with the general parameter settings
used in Healy et al. (2021b). Both galaxies have very faint H i emission, and
could only be identified visually with the knowledge of their coordinates and
redshifts from optical observations. The data were smoothed using Gaussian
kernels to angular resolutions of 15″, 20″, 25″, 30″, and with boxcar kernels
to velocity resolutions of $\sim$135, $\sim$225, and $\sim$315
$\text{km}\,\text{s}^{-1}$ to enhance the sensitivity of the data. An angular
resolution of 20″ and a velocity resolution of $\sim$135
$\text{km}\,\text{s}^{-1}$ resulted in the highest signal-to-noise H i
emission for both JW100 and JW103. Masks were constructed manually for each
velocity channel containing H i emission with an H i flux of at least three
times the rms noise in that channel, and visually detecting spatial coherence
in the distribution of the H i emission compared to the neighbouring channels.
We have then CLEAN-ed (Högbom, 1974) the ‘dirty’ channels of the 20″ H i
datacube using the mask made, down to the 0.3$\sigma$ level. The clean
components were restored with a 20″circular Gaussian beam. The pixels in the
cleaned 20″ datacube that are outside the mask were set to zero and pixels
inside the mask were summed up along the part of the frequency axis contained
within the mask to obtain the total H i maps for JW100 and JW103.

##  4 H i properties of the jellyfish candidate galaxies

Figure 4: Atlas pages for JW100 and JW103. All the data products except the
second and third panel in the bottom row are the same as in Fig. 2. The second
and third panels of the second row are a H i Position-Velocity (PV) diagram
along the optical major axis and a H i position-velocity (PV) diagram across
the tail respectively. The H i column density contours are plotted at the
levels N${}^{3\sigma}_{\textsc{H$\,$i}}\times 2^{\rm n}$ with n=0,1,2,3,…
while N${}^{3\sigma}_{\textsc{H$\,$i}}$, the average ${3\sigma}$ column
density for each galaxy and the H i beam size (20″) is shown in the bottom
left of the H i maps. The cyan contour in the PV diagrams outlines the
manually made optimum mask as described in the section 3.

The H i morphologies of the gas-rich JFCGs JW98, JW99, JW101, and JW102 do not
reveal any obvious stripped jellyfish-like gas tails at the angular resolution
of our MeerKAT observations (15$\arcsec$ corresponding to 16 kpc at the
redshift of A2626, see Fig 2 and Fig 3). The detailed description of the
methods used to derive the H i properties of JW98, JW99, JW101, and JW102 can
be found in Healy et al. (2021b) and the description of atlas pages of these
galaxies are in Fig 2 and 3. JW98 and JW101 are at higher redshifts (see Table
2) than the redshift range of A2626 (0.0475 $<$ z $<$ 0.0615, Healy et al.
2021a) and thus are not likely members of the cluster. According to Poggianti
et al. (2016), JW98 is possibly a harassed galaxy, the radio continuum shows
an extension to the west which may support the harrassment scenario. JW99 is a
somewhat lopsided galaxy, both in the optical and in H i, and displays a mild
kinematic asymmetry (see the velocity field in Fig. 2). JW101 and JW102 both
seem to have warped H i discs, likely due to interactions with their
neighbouring galaxies. All of these JFCGs have significant H i masses
($\sim$1010 M⊙), often exceeding their stellar mass.

Fig. 4 shows the H i properties of the JFGs JW100 and JW103 in the form of
atlas pages. Due East of JW100, at the centre of A2626, is the peculiar,
extended radio continuum source known as the “kite” source. For both JW100 and
JW103, the optical and HI redshifts are slightly offset (H i redshift is
within the error of the optical redshift). MeerKAT H i observations of JW100
reveal a striking H i tail extending south-westwards up to $\sim$ 20 kpc from
the nucleus of the galaxy. Most of the H i emission associated with JW100 is
in the tail, which is evident in the PV diagram along the tail (see Fig. 4).
For JW103 the signal-to-noise of the H i emission is very low and we do not
observe any H i tail in the direction of the optical tentacles which are
visible in the B-band image. However, the H i emission is mostly on one side
of the galaxy (north) and blue-shifted in JW103. The total H i masses of JW100
and JW103 (see equation 5 in Healy et al. 2021b) seen in emission are
2.8$\times$109 $\text{M}_{\odot}$ and 1$\times$109 $\text{M}_{\odot}$,
respectively. Interestingly, in both JW100 and JW103, we observe an uni-
directional radio-continuum tail in the direction of the optical tail. The
radio tail is produced by synchrotron emission of cosmic rays accelerated in
the disc that have been advected in the tail by the ram pressure wind (Ignesti
et al., 2022). Evidence of non-thermal radio tails have been frequently
reported in jellyfish galaxies (e.g. Chen et al. 2020; Roberts et al. 2021).

So, with the combination of H i and optical morphologies of the six the JFCGs
in A2626, we infer that only JW100 is an actual JFG while JW103 is a ram-
pressure stripped galaxy at a later stage of stripping. Other JFCGs namely
JW98, JW99, JW101, and JW102 are identified as non-jellyfish galaxies from
their H i observations. They are H i rich ($\text{M}_{\rm HI}$ $\sim$1010 M⊙)
and do not show signs of RPS, but reveal mildly disturbed H i morphologies
consistent with harassment or tidal interactions.

##  5 H i deficiency and SFR enhancement in jellyfish galaxies

The relation between stellar masses of galaxies and star formation rates (the
‘star formation main sequence’; SFMS) is a well-known scaling relation (e.g.
Brinchmann et al. 2004; Noeske et al. 2007; Elbaz et al. 2007; Speagle et al.
2014; Tomczak et al. 2016). We investigate the location of the JFGs and JFCGs
with respect to the SFMS. For that purpose, we use the scaling relation from
Cluver et al. (2020). Both the SFMS and quenching threshold relations in
Cluver et al. (2020) are calibrated using the the WISE data and the same
methodologies that is used for stellar mass and SFR calculations for our
sample galaxies.

Figure 5: Left panel: H i mass as a function of stellar mass for jellyfish
candidate galaxies (JFGs and JFCGs) compared to reference sample galaxies. The
field galaxies are the ALFALFA sample from Maddox et al. 2015, the other
coloured circles are H i detected galaxies from the PP, UMa and A2626 volumes.
The star markers are identified as JFGs in our MeerKAT sample and the previous
JVLA studies of GASP galaxies (Ramatsoku et al., 2019, 2020; Deb et al.,
2020). The pentagon markers are jellyfish candidate galaxies (JFCGs) selected
from optical B-band images which are identified as non-jellyfish galaxies from
their H i morphologies. Clearly, the JFGs have lower H i content compared to
field galaxies or other reference sample galaxies at fixed stellar mass. Right
panel: SFMS for JFGs and JFCGs. The SFMS relation and quenching threshold are
taken from Cluver et al. (2020) which is calibrated using the WISE data and
the same methodologies that is used for stellar mass and SFR calculations for
our sample galaxies. JFGs are scattered around the SFMS. The downarrows are
2${\sigma}$ upper limits on the SFR from WISE.  Figure 6: SFR vs H i mass for
JFGs and JFCGs compared with the reference sample galaxies with the same
stellar mass range. Grey hexagons are galaxies from the xGASS sample
(Catinella et al., 2010) and the star, and pentagon markers represent the same
JFGs and JFCGs galaxies as in Fig. 5. JFGs are H i deficient but their star
formation is not quenched yet.

We have also included the galaxies detected in the volume-limited H i surveys
of the Ursa Major (UMa, Verheijen & Sancisi 2001; Busekool et al. 2021),
Perseus-Pisces (PP, Bilimogga et al. in prep), and the entire A2626 volumes as
reference samples. The UMa volume is defined as a ‘supergroup’ that lacks any
central concentration while still being gravitationally bound (Wolfinger et
al., 2013). At a distance of 18.6 Mpc, the volume is characterised by mostly
late-type galaxies with a normal gas content (Tully & Fisher, 1977). We use
the H i detections from a blind H i imaging survey of the volume with the
JVLA-D configuration (for more details see Busekool et al. 2021) supplemented
by targeted observations of 50 UMa galaxies (described in Verheijen & Sancisi
2001). The PP volume is a section of a ridge embedded in the PP supercluster.
A blind H i imaging survey of this volume has been performed with the JVLA-C
configuration (Bilimogga et al. in prep). At a distance of 66 Mpc, the volume
consists of several loose groups of galaxies that follow the large-scale
structure of the PP supercluster (see Trasarti-Battistoni 1998). Since the UMa
and PP observations are also volume limited H i surveys like the MeerKAT A2626
survey, we used those galaxies to compare with our JFCGs. For A2626, 97 H i
sources other than JW100 and JW103 are detected in our MeerKAT obervations
(see Healy et al. 2021b), and they are also used in this paper as a reference
sample. Stellar masses and SFRs for PP and UMa galaxies have also been
calculated from WISE photometry using the same prescription as for the A2626
galaxies as mentioned in Sec. 2.2.

The left panel of Fig 5 shows H i vs stellar masses for the JFCGs and the
reference sample galaxies. Three other H i detected JFGs namely JO201, JO204,
and JO206 (Ramatsoku et al., 2019, 2020; Deb et al., 2020) from the GASP
sample (Poggianti et al., 2017b) are also plotted with JW100 and JW103 in
coloured star markers. Four non-stripping JFCGs are plotted with pentagon
coloured star markers. Together with PP, UMa, and A2626 galaxies, we have also
plotted the results from the Arecibo Legacy Fast ALFA- Sloan Digital Sky
Survey (ALFALFA–SDSS) galaxy sample of field galaxies with errorbars from
Maddox et al. (2015), which spans a large range in stellar and H i masses. All
five galaxies that are under the influence of RPS (JO201, JO204, JO206, JW100,
and JW103) have relatively low H i masses for their stellar masses when
compared to the reference sample galaxies (also found by Luber et al. 2022).
Especially, when compared with the ALFALFA field galaxies, all five JFGs are
almost 1 dex below the typical H i masses of the galaxies of similar stellar
masses.

The right panel of Fig 5 shows SFR vs stellar masses for the JFGs, the
reference sample galaxies and the SFMS relation. For all but JW103, the SFRs
are measured with MUSE (Vulcani et al., 2018). The SFR calculations from MUSE
and WISE observations are verified to be consistent (within $\sim$ 10%
scatter). For JW103, we use the SFR calculated from the 12$\mu$m (W3)
emission. Interestingly, both JFGs and JFCGs which are later identified as
non-jellyfish galaxies do not stand out with respect to the SFMS of Cluver et
al. (2020), i.e. they do not necessarily have enhanced SFR compared to their
stellar masses, this is contrary to what is found by Vulcani et al. (2018).
However, we note that JW100 lies below the quenching threshold as defined by
Cluver et al. (2020), signifying a strongly reduced star formation activity
for its stellar mass.

Finally, we inspect how the atomic gas content is linked to the global star
formation activity of the JFGs as compared to usual field or cluster galaxies
following Fig 6 in Ramatsoku et al. (2020). We have prepared a reference
sample of field galaxies with the same stellar mass range as the five JFGs
(9.7 $\leq$ log$M_{*}$ $\leq$11.5) from the Extended GALEX Arecibo SDSS Survey
(xGASS; Catinella et al. 2010). Fig. 6 displays SFR vs MHI for the xGASS
sample (Doyle & Drinkwater, 2006; Huang et al., 2012; Saintonge et al., 2016)
in addition to the PP, UMa and A2626 cluster galaxies. The coloured star and
pentagon markers represent the JFGs and other JFCGs respectively.
Intriguingly, all the five JFGs lie above the distribution of the reference
samples. In other words, the JFGs have higher star formation rates compared to
their H i masses for galaxies with similar stellar masses (Ramatsoku et al.,
2019, 2020). This suggests that these galaxies are being H i stripped but are
not yet quenched. Furthermore, this also implies that the little H i gas these
galaxies retain, is more efficiently transformed into H2 gas, as concluded by
Moretti et al. (2020b), resulting in a high SFR for their H i masses, compared
to normal galaxies of similar H i masses. Given that these JFGs are H i
deficient and often do not have enhanced SFR for their $\text{M}_{\star}$,
they must have evolved from right to left in Fig. 6, and not so much from
bottom to top. We note in the right panel of Fig. 5, that the JFGs do not lie
significantly above the SFMS and therefore, do not currently have an enhanced
SFR for their stellar mass. Therefore, we conclude that the JFGs have not
moved upwards significantly in Fig. 6. However, we cannot entirely rule out
that a mild enhancement of the SFR may have occurred during a short period of
time since these galaxies entered the clusters.

##  6 The Multi-phase ISM of JW100

###  6.1 Previous studies on JW100

JW100 is the proto-typical JFG. It is a nearly edge-on spiral galaxy with an
AGN in its centre (Poggianti et al., 2019; Radovich et al., 2019). JW100
resides in A2626 under optimal conditions for ram-pressure stripping (Jaffé et
al., 2018; Gullieuszik et al., 2020), with a particularly high line-of-sight
velocity of 2000 $\text{km}\,\text{s}^{-1}$ with respect to the cluster rest
frame and a projected distance of 83 kpc from the cluster centre (Poggianti et
al., 2019). It is the most massive JFG in the entire GASP sample with a
stellar mass of 3.2 $\times$ 1011 $\text{M}_{\odot}$ (Poggianti et al., 2019).
JW100 is one of the most spectacular JFGs with extra-planar RPS tails of
ionised and molecular gas, UV stellar light, X-ray and radio continuum
emission (Poggianti et al., 2019; Moretti et al., 2020a; Ignesti et al.,
2022).

In the northern part of the tail, there is a lack of extra-planar clumpy CO
gas and star formation while only diffuse H$\alpha$ is observed, with a high
[OI]/H$\alpha$ ratio and X-ray emission. This suggests that the excess
H$\alpha$ emission in the northern part of the tail is possibly a result of a
complex ICM-ISM interplay triggered by RPS (e. g. Ferland et al. 2009; Fossati
et al. 2016; Cramer et al. 2019; Campitiello et al. 2020). We note here that
the northern, diffuse H$\alpha$ emission has a recession velocity that is
inconsistent with the rotational velocity of JW100, suggesting that the
ionised gas is decelerated along the orbit and longer exposed to interaction
with ICM (see Sec. 6.2 for more details). The radio continuum emission from
the tail of JW100 is mainly synchrotron emission caused by relativistic
electrons ejected by supernovae explosions, indicating the presence of
magnetic fields pervading the stripped gas (as in e.g. Chen et al. 2020;
Müller et al. 2021; Roberts et al. 2021).

Based on ALMA observations, Moretti et al. (2020a, b) found a large amount of
molecular gas ($\sim$ 2.5 $\times$ 1010 $\text{M}_{\odot}$), i.e. 8% of the
stellar mass, a value more than an order of magnitude higher than that found
in typical Virgo cluster galaxies (Corbelli et al., 2012; Brown et al., 2021)
and in the local xCOLD GASS sample (Saintonge et al., 2017) for galaxies of
similar stellar mass. The ALMA data reveal a 35 kpc long RPS tail of clumpy
and diffuse molecular gas from the centre of JW100 (Moretti et al., 2020a).
The high values of the CO line ratio (r21 $\geq$ 1) in the southern part of
the tail, the narrow, single-component CO emission lines, and low velocity
dispersion hint that the tail in that region is composed of dense star forming
molecular gas. This signifies that these clouds are formed in situ, either
from the stripped H i or the diffuse molecular gas (Moretti et al., 2020a, b).

###  6.2 The distribution of H i in JW100 in comparison with H${\alpha}$ and
CO

Figure 7: Multi-phase gas in the RPS tail in JW100. Top panel: H$\alpha$
(white, 1$\arcsec$ resolution) and H i (red, 20$\arcsec$ resolution) contours
overlayed on the DECaLS color image. Bottom panel: CO(2-1) (blue, 1$\arcsec$
resolution) and H i (red, 20$\arcsec$ resolution) contours overlayed on DECaLS
color image. 1$\arcsec$ corresponds to 1.074 kpc at a distance of JW100.
Figure 8: H$\alpha$, CO(2-1), and H i channel map overlays for JW100. In the
bottom two rows, the CO(2-1) emission is smoothed to the same angular
resolution as the H i emission (20$\arcsec$) to compare the atomic and
molecular gas phases on the same spatial scales. The red solid contours
represent the H i emission with contour levels of (2, 3, 4, 5,..)
$\times\sigma$ or RMS and the dotted red contour represents the $-2\sigma$
level. The blue contours indicate CO(2-1) emission with contour levels (2, 4,
8, 16,..) $\times\sigma$ where $\sigma$= 0.28 mJy/beam for the high resolution
ALMA cube and 0.67 mJy/beam for the 20$\arcsec$ ALMA cube. The central light-
grey rectangle represents the area in which H$\alpha$ emission is observed
with MUSE, after substraction of the [NII] emission lines and the stellar
continuum emission. The image is larger than the field of view of MUSE, so the
DECaLS g-band image is shown in the background. The black contour represents
the stellar disc and the hatched circle on the bottom left of each panel shows
the size of the 20$\arcsec$ H i beam. H$\alpha$ emission is seen in the
northern part of the RPS tail but no CO(2-1) or H i emission while both
H$\alpha$ and CO are anti-correlated with H i on small scales.

Fig. 7 shows the multi-phase gas in the RPS tail of JW100. The top and bottom
panel of Fig. 7 show the H i emission (red, 20$\arcsec$ resolution) on
H$\alpha$ (white, 1$\arcsec$ resolution) and H i emission (red, 20$\arcsec$
resolution) on CO(2-1) (blue, 1$\arcsec$ resolution) respectively with the
DECaLS (Dey et al., 2019) color image in the background. The atomic,
molecular, and ionised gas tails are almost of similar extent ($\sim$ 50 kpc),
this small difference of the tail lengths could be also due to the different
sensitivities and resolutions of the H i, H$\alpha$, and CO(2-1) data.

We then inspect the distribution of gas at different recession velocities
through channel maps in Fig. 8. We compare the H i contours (red), CO(2-1)
contours (blue), and the H$\alpha$ emission (greyscale) at different
velocities in JW100 in ten panels. The top two rows of Fig. 8 show H$\alpha$
emission at the original MUSE angular and velocity resolution (1$\arcsec$
$\times$ 1$\arcsec$, 57.1 $\text{km}\,\text{s}^{-1}$, Poggianti et al. 2019),
CO(2-1) emission at the original ALMA resolution (1.4$\arcsec$ $\times$
1.1$\arcsec$, 44.1 $\text{km}\,\text{s}^{-1}$, Moretti et al. 2020a) and H i
emission smoothed to 20$\arcsec$ and $\sim$ 135 $\text{km}\,\text{s}^{-1}$.
The high resolution ALMA channel maps indicate the location of small CO(2-1)
clumps. In the bottom two rows of Fig. 8, the CO(2-1) emission is also
smoothed to an angular resolution of 20$\arcsec$ to compare the atomic and
molecular gas phases on the same spatial scales.

H$\alpha$ emission is visible already in the top-left panels (17,420-17,762
$\text{km}\,\text{s}^{-1}$) where H i and CO(2-1) are not yet visible. CO(2-1)
and H i emission are detectable from the 4th and 5th panel respectively. In
the 6th, 7th, and 8th panel of Fig. 8, H i emission is found further down the
tail (away from the centre of JW100) than the CO(2-1) clumps. In the bottom
two rows of Fig. 8, though the CO(2-1) clumps are smoothed in those panels,
the centres of the CO(2-1) emission are still offset from the H i emission.
The 3$\sigma$ H i ‘appendix’ at the bottom left in the 8th panel, the
3$\sigma$ H i peaks in the 9th and 10th panels, and the -2$\sigma$ H i peaks
in the 1st and 2nd panels are consistent with the noise behaviour and
plausibly not associated with the H i emission or absorption of JW100. These
peaks are not included in the H i mask that is used to make the H i map and
the global profile.

The H i and CO(2-1) emissions in the channel maps (4-8th panel) are not
exactly co-located, or anti-correlated, which suggests several possibilities.
It could be because of a high rate of ionisation of the H i gas by the young
massive/OB stars, or due to an efficient conversion of H i to H2.

H$\alpha$ emission is extended over larger velocity range than H i and
CO(2-1). The systemic velocity of JW100 is $\sim$ 18500
$\text{km}\,\text{s}^{-1}$ (z= 0.06189, Poggianti et al. 2019). And based on
the TF relation, we would expect a rotational velocity of 400 km/s. Therefore,
due to pure rotation of the galaxy, we would expect gas emission at velocities
in the range of 18100-18900 km/s. Indeed, the channel maps show the maximum
recession velocity of 18800 km/s, consistent with the rotation of the galaxy.
However, the lowest velocities where the H$\alpha$ emission becomes visible,
is roughly 17400 km/s. This difference of 1100 km/s from the systemic velocity
of the galaxy can not be accommodated by the rotation of the galaxy.
Furthermore, this low recession velocity is closer to the systemic velocity of
the cluster, suggesting that the H$\alpha$ emission is lagging behind the
galaxy along its orbit through the cluster. This is observed in other GASP
galaxies falling into clusters, such as JO201 in A85 (Bellhouse et al., 2017,
2019).

##  7 H i depletion channels

Figure 9: Cartoon of H i depletion channels. There are three principal
depletion channels: ram-pressure stripping of the diffuse H i gas, H i to H2
conversion in the disc and tail, and ionisation of H i gas. Figure 10:
H$\alpha$/H i and H2/H i ratio maps of JW100. The maps are constructed by
smoothing the H i, H2, and H$\alpha$ column density maps to 20$\arcsec$
resolution and regriding the maps to the same pixel sizes (0.2$\arcsec$). The
black curved contour inside represents the stellar disc. The black solid
contours represent the H i emission with contour levels (3, 4, 5,..)
$\times\sigma$ or RMS. We limit the ratio map to within the 1 sigma contour of
the H i column density map. The beam size is shown in the bottom left. The
negative gradient in both the maps show that there is an abundance of H i over
H2 or H$\alpha$ along the extended tail. Considering the MeerKAT beam size, we
note that there are only 2-3 resolution elements in these ratio maps.

In what follows, we sketch a qualitative picture of possible H i depletion
channels. We estimate the relative importance of several potential H i
depletion mechanisms that act on a galaxy undergoing RPS. We identify three
main depletion channels for H i gas in JW100 (see schematic diagram Fig. 9).
The relevance of each depletion channel depends on the amount of atomic or
molecular gas that JW100 harboured before the onset of RPS (see Table 3).

We note here already that simulations suggest that up to 50% of the gas can be
re-accreted (Schulz & Struck, 2001; Vollmer et al., 2001; Roediger et al.,
2006; Kapferer et al., 2009; Quilis et al., 2017; Tonnesen, 2019) and gas
fallback has been observed in at least one galaxy (Cramer et al., 2021). Our
data do not evidence the presence of redshifted H i gas. This is not
surprising, as JW100 is moving at an exceptional high velocity with respect to
the cluster, which makes the gas difficult to fall back onto the galaxy.
Therefore, we ignore in our calculations the gas that might be falling back to
the disc.

###  7.1 Stripping (ram pressure)

One of the pivotal depletion mechanisms is ram-pressure stripping of the
diffuse H i gas from the disc while JW100 is falling into the centre of A2626
and interacting with the hot, X-ray emitting ICM.

If we assume that the only effect of the RPS is the displacement of the
initial H i gas content, then we can infer the amount of ram-pressure stripped
gas based on the mass of H i gas in the tail of JW100. The mass of H i gas in
the tail is derived by subtracting the amount of H i gas inside the stellar
disc from the total H i gas seen in emission. The stellar disc is derived from
the MUSE observations (Poggianti et al., 2019). Hence, the total H i mass in
the tail is: (2.8 $\times$ 109 – 0.8 $\times$ 109) $\text{M}_{\odot}$ = 2
$\times$ 109 $\text{M}_{\odot}$. Thus, at least 70% of the H i gas has been
removed from the disc.

Table 3: Expected and observed gas masses in JW100

Expected massa Observed mass Gas phase value range total disc tail (in units
of 109 $\text{M}_{\odot}$) H i 19 8.6-42a 2.8 0.8 2.0 H2 3.5 1-10b 6-25c 5-24c
1.9 Stellar 320

a based on scaling relations from Dénes et al. (2014),b based on Saintonge et
al. (2017),c based on Moretti et al. (2020b), the range corresponds to
different values of $\alpha_{CO}$ varying from 0.9 (Amorín et al. 2016) to 4.3
(Bolatto et al. 2013).

###  7.2 H i to H2 conversion

For field galaxies in the xGASS sample (Catinella et al., 2018), depending on
their stellar masses (9.18 $\leq$ log$\text{M}_{\star}$/$\text{M}_{\odot}$
$\leq$ 11.2), the H i-to-H2 mass ratio varies in the range of 0.03-1.3 . In
the Fornax cluster, however, Loni et al. (2021) found
M${}_{\text{H${}_{2}$}}$/$\text{M}_{\rm HI}$ in the range of 0.1-10 for
galaxies with stellar masses 9 $\leq$ log$\text{M}_{\star}$/$\text{M}_{\odot}$
$\leq$ 11 (Zabel et al., 2019; Loni et al., 2021). In the case of JW100,
M${}_{\text{H${}_{2}$}}$/$\text{M}_{\rm HI}$ is 8.9, which is higher than the
field galaxies in the xGASS sample, but near the high end of the
M${}_{\text{H${}_{2}$}}$/$\text{M}_{\rm HI}$ range for Fornax galaxies (Zabel
et al., 2019; Moretti et al., 2020a; Loni et al., 2021). From this
observation, Moretti et al. (2020b) inferred an efficient conversion of atomic
to molecular gas in JW100, reinforcing the notion that H i to H2 conversion is
a significant H i depletion channel.

We aim to investigate the relative efficiency of H i-to-H2 conversion in the
disc and the tail of the JFG JW100. In Table 3, we summarise the estimated
amount of H i and H2 in the disc and the tail with the uncertainties. In the
discussion that follows, we assume the maximum masses from Table 3. This may
introduce some uncertainty in the final calculation, but the overall
conclusion of our considerations remains unaffected.

####  7.2.1 H i to H2 conversion in the disc

The current amount of H i in the disc is an order of magnitude less than the
expected amount of H i with which the galaxy entered into the cluster (see
Table 3). The missing H i in the disc is either stripped from the disc or
locally converted into H2. However, the amount of H2 in the disc is roughly a
factor 5 larger than the estimated amount of H2 before the onset of RPS. The
amount of H2 gas that the disc gained is: observed H2 mass in the disc minus
expected H2 mass which results in 14 $\times$ 109 $\text{M}_{\odot}$.
Therefore, the excess of H2 in the disc might have formed from H i that was
present in the disc before the onset of RPS. Thus, if all the H2 in the disc
is formed from H i gas then (14/42=) 1/3 of H i is converted into H2 in the
disc.

####  7.2.2 H i to H2 conversion in the tail

A significant fraction of the H i gas in the disc is pushed out into the tail
where it can also be converted into H2. Interestingly, combining LOFAR,
MeerKAT, and JVLA radio continuum observations of JW100, Ignesti et al. (2022)
estimated the strength of the magnetic field which could be strong enough to
shield the stripped ISM gas from the ICM (Müller et al., 2021). The draping of
the magnetized plasma around the tail is possibly preventing thermal
conduction and hydrodynamic turbulence by the hot ICM, enabling the stripped H
i gas to cool and condense into molecular clumps (e.g., Sparre et al. 2020; Ge
et al. 2021; Müller et al. 2021). The fact that we observe ongoing star
formation associated with these clumps suggests that these clumps fragment
further into giant molecular clouds (Poggianti et al., 2019; Moretti et al.,
2020b).

Following the calculation in the previous subsection, the H i gas that is
pushed out of the disc into the tail, eventually into the ICM is : expected
amount of H i mass minus the amount of H i in the disc minus the amount of H i
converted into H2 in the disc which results in 27 $\times$ 109
$\text{M}_{\odot}$. Thus, by assuming that H2 in the tail is formed from H i,
we infer that 1.9/27 $\sim$ 7% of the diffuse H i gas is converted into H2 gas
in the tail. In the previous subsection, we concluded that 1/3 of the H i is
locally converted into H2. Hence, the conversion of H i into H2 in the disc is
several times more efficient than in the tail. Thus, 30+7=37% of the H i is
converted into H2 and 60% is blown out of the galaxy.

####  7.2.3 H2 vs. H i ratio map

We have calculated the spatially resolved atomic to molecular gas fraction for
both the disc and the tail. The relative amounts of both H i and H2 in the
disc and the tail are based on the stellar disc defined by the MUSE
observations (Poggianti et al., 2019; Moretti et al., 2020a). The bottom panel
in Fig. 10 presents the ratio map of H2 vs H i surface mass density (both
measured in units of $\text{M}_{\odot}$/pc2). The map is constructed by
smoothing the H i and H2 column density maps to 20$\arcsec$ resolution and
regriding the maps to the same pixel sizes (0.2$\arcsec$). Though we are
limited by the resolution of the H i data, evidently, there is a negative
gradient along the tail, implying there is more H i compared to H2 towards the
tip of the tail.

###  7.3 H i to H$\alpha$ conversion

Ionisation of H i owing to radiation from young stars or shock ionisation is
another depletion channel for H i gas. We have investigated the spatially
resolved ionised to atomic gas fraction for both the disc and the tail of
JW100 using the same strategy we used for the molecular to atomic gas
fraction. The top panel in Fig. 10 the shows the ratio map for H$\alpha$ vs H
i surface mass density (surface mass density is measured in units of
$\text{M}_{\odot}$/pc2 and H$\alpha$ emissivity in erg/s/cm2/pc2) for JW100.
In this case also, there is a gradient along the tail, implying that the
ionisation efficiency of neutral gas is decreasing along the tail. The lower
limit of H$\alpha$ gas estimated from the MUSE observations is 4.6 $\times$
106 $\text{M}_{\odot}$. Thus, we note that the H i mass in the tail is an
order of magnitude more than the ionised gas mass from which we infer that the
ionisation of neutral gas plays a minor role in the depletion of the neutral
atomic gas.

##  8 Summary

In this work, we have presented MeerKAT H i observations of the JFCGs in the
A2626 galaxy cluster. Among the six JFCGs as identified by Poggianti et al.
(2016) from their optical morphologies in WINGS B-band images, only JW100 is
identified as a JFG from its optical and H i morphologies. JW103 is probably a
ram-pressure stripped galaxy at a later stage of stripping. Both JW100 and
JW103 are moving at a very high similar velocity (cz $\sim$ +1800
$\text{km}\,\text{s}^{-1}$, $\sim 3\sigma_{cl}$ ) with respect to the ICM of
A2626, suggesting that these galaxies are entering as a group, confirmed by
Healy et al. (2021a). Both of these galaxies reside close to the X-ray
emitting cluster core, thus in the most favourable conditions for RPS. In our
upcoming work, taking advantage of these MeerKAT H i observations, we expect
to identify more RPS galaxies in the A2626 volume that could not be identified
from their optical morphologies.

\- The four JFCGs namely JW98, JW99, JW101, JW102 have significant H i masses
($\sim$ 1010 $\text{M}_{\odot}$). Their H i imaging reveals warps,
morphological and kinematic asymmetries, possible tidal interactions and no
convincing signature of RPS.

\- JW103 is barely detected in H i and very H i deficient ($\text{M}_{\rm HI}$
= 109 $\text{M}_{\odot}$). While in the optical image of JW103, we observe
blue tentacles towards the west, we do not detect any H i tails in the
direction of the optical tail, possibly because the H i emission is below the
detection limit. Interestingly, we see a radio continuum tail in the direction
of the optical tail, which is possibly due to advection of relativistic
electrons by the ram pressure wind (Ignesti et al., 2022).

\- We investigated H i vs stellar masses for JFCGs compared to a reference
sample of galaxies from the PP, UMa, A2626 and ALFALFA surveys of field
galaxies (Maddox et al., 2015). All JFGs have lower H i masses for their
stellar masses compared to most field or cluster galaxies while other JFCGs
are H i rich. We then plotted SFR vs stellar masses for the JFCGs, the
reference sample galaxies and the SFMS relations. None of the JFCGs and
previously studied GASP JFGs stand out with respect to the SFMS of Cluver et
al. (2020).

\- Finally, when we compared the H i content vs the SFR in the JFCGs and the
reference sample, all five JFGs stand out in the relation because of the lower
H i masses for their SFRs. It means that these galaxies are H i stripped but
not yet quenched.

\- H i observations of JW100 reveal an extended H i tail towards the south-
west up to $\sim$ 50 kpc from the stellar body of the galaxy. Comparing the
H$\alpha$, CO(2-1), and H i channel maps in JW100, we detect H$\alpha$
emission in the northern part of the RPS tail with no CO(2-1) or H i
counterpart (but where X-ray emission is present, Poggianti et al. 2019)
possibly because of complex ISM-ICM interaction due to prolonged RPS.
Moreover, both H$\alpha$ and CO are anti-correlated with H i, hinting at an
efficient conversion of H i to H2 in the tail. We hypothesize that the draping
of the magnetic plasma around the tail is preventing thermal conduction and
hydrodynamic turbulence by the hot ICM, enabling the stripped H i gas to cool
and condense into H2 clumps (e.g. Müller et al. 2021).

\- The spatially resolved H$\alpha$/H i and H2/H i ratio maps of JW100 show a
gradient along the tail, implying there is more H i compared to H2 or
H$\alpha$ toward the tip of the extended tail. We estimated the relative
importance of the different depletion mechanisms of the H i gas in JW100 and
we identify three main depletion channels: RPS removal, conversion of H i to
H2, and ionisation of the H i. We found that both RPS and H i-to-H2 conversion
are significant depletion channels. H i-to-H2 conversion is more efficient in
the disc than in the tail.

##  9 Data Availability

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

## Acknowledgements

We thank Tom Jarett for kindly providing the SFRs and setllar masses for the
A2626, PP, and UMa galaxies from WISE observations. TD thanks Pooja Bilimogga
for providing H i data on PP and UMa galaxies. This paper makes use of the
MeerKAT data (Project ID: SCI-20190418-JH-01). The MeerKAT telescope is
operated by the South African Radio Astronomy Observatory, which is a facility
of the National Research Foundation, an agency of the Department of Science
and Innovation. MV acknowledges the Netherlands Foundation for Scientific
Research support through VICI grant 016.130.338 and the Leids Kerkhoven-
Bosscha Fonds (LKBF) for travel support. 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). JMvdH
acknowledges support from the European Research Council under the European
Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement nr.
291531. JH has received funding from the European Research Council (ERC) under
the European Union’s Horizon 2020 research and innovation programme (grant
agreement No 882793/MeerGas). AI acknowledges the Italian PRIN-Miur 2017 (PI
A. Cimatti). Y.J. acknowledges financial support from CONICYT PAI (Concurso
Nacional de Insercion en la Academia 2017) No. 79170132 and FONDECYT
Iniciacion 2018 No. 11180558.

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