text:
11institutetext: INAF - Osservatorio Astronomico di Cagliari, Via della
Scienza 5, 09047, Selargius, CA, Italy
11email<EMAIL_ADDRESS>22institutetext: Dipartimento di Fisica,
Università di Cagliari, Cittadella Universitaria, 09042, Monserrato, Italy
33institutetext: International Centre for Radio Astronomy Research (ICRAR),
The University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009,
Australia 44institutetext: ARC Centre of Excellence for All-Sky Astrophysics
in 3 Dimensions (ASTRO3D) 55institutetext: CSIRO Astronomy and Space Science,
Australia Telescope National Facility PO Box 76, Epping, NSW 1710, Australia
66institutetext: Department of Astronomy, University of Cape Town, Private Bag
X3, Rondebosch 7701, South Africa 77institutetext: School of Physics and
Astronomy, Cardiff University, Queens Buildings The Parade, Cardiff CF24 3AA,
UK 88institutetext: INAF-Astronomical observatory of Capodimonte, via
Moiariello 16, Naples 80131, Italy 99institutetext: Kapteyn Astronomical
Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The
Netherlands 1010institutetext: Australian Research Council, Centre of
Excellence for All-sky Astrophysics (CAASTRO), Australia 1111institutetext:
Department of Physics and Electronics, Rhodes University, PO Box 94, Makhanda,
6140, South Africa

# A blind ATCA H$I$1.2ex survey of the Fornax galaxy cluster

Properties of the H$I$1.2ex detections

A. Loni 1122 P. Serra 11 D. Kleiner 11 L. Cortese 3344 B. Catinella 3344 B.
Koribalski 55 T. H. Jarrett 66 D. Cs. Molnar 11 T. A. Davis 77 E. Iodice 88 K.
Lee-Waddell 55 F. Loi 11 F. M. Maccagni 11 R. Peletier 99 A. Popping 331010 M.
Ramatsoku 111111 M. W .L. Smith 77 N. Zabel 99

(Received October 30, 2020; accepted January 26, 2021)

We present the first interferometric blind H$I$1.2ex survey of the Fornax
galaxy cluster, which covers an area of 15 deg2 out to the cluster virial
radius. The survey has a spatial and velocity resolution of 67″
$\times\leavevmode\nobreak\ $95″($\sim 6\times 9$ kpc at the Fornax cluster
distance of 20 Mpc) and 6.6 km s-1 and a 3$\sigma$ sensitivity of
$N_{\text{H\scaleto{$I$}{1.2ex}}}$ $\sim$2$\leavevmode\nobreak\ \times$ 1019
cm-2 and $M_{\text{H\scaleto{$I$}{1.2ex}}}$ $\sim$2$\leavevmode\nobreak\
\times$ 107 M⊙, respectively. We detect 16 galaxies out of roughly 200
spectroscopically confirmed Fornax cluster members. The detections cover about
three orders of magnitude in H$I$1.2ex mass, from $8\times 10^{6}$ to
$1.5\times 10^{10}$ M⊙. They avoid the central, virialised region of the
cluster both on the sky and in projected phase-space, showing that they are
recent arrivals and that, in Fornax, H$I$1.2ex is lost within a crossing time,
$\sim 2$ Gyr. Half of these galaxies exhibit a disturbed H$I$1.2ex morphology,
including several cases of asymmetries, tails, offsets between H$I$1.2ex and
optical centres, and a case of a truncated H$I$1.2ex disc. This suggests that
these recent arrivals have been interacting with other galaxies, the large-
scale potential or the intergalactic medium, within or on their way to Fornax.
As a whole, our Fornax H$I$1.2ex detections are H$I$1.2ex-poorer and form
stars at a lower rate than non-cluster galaxies in the same $M_{\star}$ range.
This is particularly evident at $M_{\star}$$\lesssim 10^{9}$ M⊙, indicating
that low mass galaxies are more strongly affected throughout their infall
towards the cluster. The $M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ ratio
of Fornax galaxies is comparable to that in the Virgo cluster. At fixed
$M_{\star}$, our H$I$1.2ex detections follow the non-cluster relation between
$M_{\text{H\scaleto{$I$}{1.2ex}}}$ and the star formation rate, and we argue
that this implies that thus far they have lost their H$I$1.2ex on a timescale
$\gtrsim 1\mathrm{-}2$ Gyr. Deeper inside the cluster H$I$1.2ex removal is
likely to proceed faster, as confirmed by a population of H$I$1.2ex-undetected
but H21.2ex0pt-detected star-forming galaxies. Overall, based on ALMA data, we
find a large scatter in H21.2ex0pt-to-H$I$1.2ex mass ratio, with several
galaxies showing an unusually high ratio that is probably caused by faster
H$I$1.2ex removal. Finally, we identify an H$I$1.2ex-rich subgroup of possible
interacting galaxies dominated by NGC 1365, where pre-processing is likey to
have taken place.

###### Key Words.:

galaxies: cluster / galaxies: evolution / galaxies: ISM

##  1 Introduction

It is known that the evolution of galaxies is faster in denser environments
(Diaferio et al., 2001) and that, as a consequence, the relative abundance of
red early-type galaxies and blue late-type galaxies changes with environment
density (Hubble & Humason, 1931; Oemler, 1974; Dressler, 1980). In this
context, galaxy clusters are the most extreme environments within which
galaxies evolve. They are characterised by a high number density of galaxies
and by the presence of a dense intra-cluster medium (ICM), which galaxies move
through. Thus, in clusters, both hydrodynamical and gravitational interactions
such as ram-pressure stripping and tidal interactions, respectively, are
likely to happen (Gunn & Gott 1972; Toomre & Toomre 1972), as well as mergers
between galaxies (at least in the cluster outskirts - Sheen et al. 2012; Oh et
al. 2018). These interactions deplete the cold gas reservoirs of galaxies and,
therefore, affect their star formation activity. The balance between these
types of interactions depends both on galaxy properties – including their
orbits – and on cluster properties, such as the number density of galaxies and
the ICM density. Therefore, we expect galaxy evolution to proceed differently
in different clusters.

The aim of this work is to study galaxy evolution in the Fornax cluster, which
is a low mass cluster (7$\times 10^{13}$ M⊙ within a radius of 1.4 Mpc - twice
the virial radius, $R_{\mathrm{vir}}$; Drinkwater et al. 2001a), located in
the southern sky at a distance of 20 Mpc (Blakeslee et al., 2009). Fornax is
thus the second closest galaxy cluster to us after Virgo. Fornax has roughly
200 spectroscopically confirmed galaxies within $R_{\mathrm{vir}}\sim$700 kpc
(Maddox et al., 2019). NGC 1399 is the central dominant galaxy, which
coincides with the peak of the X-ray emission (Paolillo et al., 2002), and the
majority of the massive galaxies in the cluster central region are of an early
morphological type. Thus, Fornax shows a more dynamically evolved state than
Virgo (Grillmair et al., 1994; Jordán et al., 2007), but its growth is not
over at all.

The Fornax region includes two main substructures (Drinkwater et al., 2001a).
The first is the cluster itself centred on NGC 1399, whose interaction with
NGC 1404 is revealed by a perturbed ICM distribution (e.g. Sheardown et al.
2018). Here, three well defined groups of galaxies with different light and
colour distributions, kinematics, and stellar populations were found by
combining the Fornax Deep Survey imaging (Iodice et al., 2019b) with Fornax 3D
spectroscopy (Iodice et al., 2019a) in the two-dimensional projected phase
space: the core, the north-south clump and the infalling galaxies (see Fig. 7
in Iodice et al. 2019a). The core is still dominated by NGC 1399, which is one
of only two slow-rotators inside the virial radius (NGC 1427 is the other one,
located on the east side of the cluster). The NS-clump is located in the high-
density region of the cluster (within $0.4R_{\mathrm{vir}}\sim 0.3$ Mpc in
projection), where the X-ray emission is still bright. It hosts the reddest
and most metal-rich galaxies, all of them fast-rotating early-type galaxies.
The bulk of the gravitational interactions between Fornax galaxies as well as
most of the intra-cluster baryons (i.e. diffuse light, globular clusters and
planetary nebulae) (Cantiello et al., 2018; Spiniello et al., 2018; Iodice et
al., 2019b) are found in this NS clump, where galaxy growth is still ongoing
through the accretion of mass onto galaxies’ outer regions (Spavone et al.,
2020). The third group of objects in the cluster includes the infalling
galaxies, which are distributed nearly symmetrically around the core in the
low-density region outside $\sim 0.4R_{\mathrm{vir}}\sim 0.3$ Mpc in
projection. The majority of these galaxies are late-types with ongoing star
formation. Most of them exhibit signs of interaction with the environment
and/or minor mergers in the form of tidal tails and disturbed molecular gas
(Zabel et al., 2019; Raj et al., 2019). Previous works had also shown that
numerous dwarf galaxies are currently falling into the centre of the cluster
(Drinkwater et al., 2001b; Schröder et al., 2001; Waugh et al., 2002).
Finally, the other major structure in the Fornax volume is the infalling group
centred on NGC 1316 (Fornax A), located $\sim$ 1.5 Mpc south-west of the
cluster centre and experiencing ongoing interactions between its members
(Schweizer, 1980; Horellou et al., 2001; Iodice et al., 2017; Serra et al.,
2019).

The observation of galaxies’ atomic hydrogen through the 21 cm wavelength
emission line (H$I$1.2ex) gives us information on the evolutionary state of
galaxies as well as a global picture of the cluster. The evolution of galaxies
depends on the evolution of their atomic hydrogen reservoir, which is the
primary reservoir of fuel for star formation. Since it typically extends to
the outskirts of galaxies, H$I$1.2ex gas is the first component that is
affected by tidal interactions, ram-pressure stripping and mergers. Indeed,
cluster galaxies are usually deficient in atomic hydrogen with respect to non-
cluster galaxies (Giovanelli & Haynes, 1983; Haynes & Giovanelli, 1986;
Boselli & Gavazzi, 2006). H$I$1.2ex is, therefore, a crucial observable for
understanding galaxy evolution in dense environments (Hughes & Cortese, 2009;
Chung et al., 2009).

H$I$1.2ex emission in the Fornax cluster was studied in several works. Bureau
et al. (1996) used the Parkes radio telescope to measure the amount of atomic
hydrogen in 21 undisturbed galaxies with morphologies S0$/$a or later, located
within 6 deg from NGC 1399 and with cz $\leq$ 2520 km s-1. The eight galaxies
within $R_{\mathrm{vir}}$ did not show any peculiar value in the
$M_{\text{H\scaleto{$I$}{1.2ex}}}$/I$-$band infrared luminosity ratio with
respect to the rest of the sample. The same ratio evaluated for Ursa Major
galaxies, a lower density environment than Fornax, led them to conclude that
the observed Fornax galaxies are not H$I$1.2ex deficient.

The first blind H$I$1.2ex survey of Fornax was carried out by Barnes et al.
(1997), also with the Parkes telescope. They detected H$I$1.2ex in eight
galaxies within an area of 8 $\times$ 8 deg2. Of those, two galaxies are
within the cluster $R_{\mathrm{vir}}$: ESO 358G-063 and NGC 1365. Thanks to
the survey sensitivity, they excluded the existence of a significant
population of optically undetected H$I$1.2ex clouds with H$I$1.2ex mass
greater than 108 M⊙.

The number of H$I$1.2ex detections within the Fornax central region increased
with the targeted survey by Schröder et al. (2001). They found H$I$1.2ex in 37
out of 66 galaxies. Of those, 14 are within $R_{\mathrm{vir}}$, while the rest
lies within 5 deg from the centre of the cluster. They found a lack of
H$I$1.2ex in the cluster centre, and measured the H$I$1.2ex deficiency
parameter to be $0.38\pm 0.09$ (as defined in Solanes et al. 1996). This shows
a modest H$I$1.2ex depletion in Fornax (usually, galaxies with H$I$1.2ex
deficiency parameter ¿ 0.3 are considered to be H$I$1.2ex deficient, e.g.
Dressler 1986; Solanes et al. 2001). Furthermore, the
$M_{\text{H\scaleto{$I$}{1.2ex}}}$$-$to$-$blue light ratio of Fornax galaxies
- mean value ($0.68\pm 0.15$) M${}_{\odot}/$L⊙ \- is a factor 1.7 lower than
in the field. They also showed that the velocity dispersion of the sample of
H$I$1.2ex deficient galaxies is lower than that of the remaining H$I$1.2ex
detected galaxies. This difference in velocity dispersion agrees with the
deficient galaxies having more radial orbits (as shown in Dressler 1986),
which makes them good candidates for ram-pressure stripping (as discussed in
Solanes et al. 2001).

Figure 1: Layout and detections of our ATCA H$I$1.2ex survey. The green
outline includes the 15-deg2 region where the average noise is 2.8 mJy beam-1
(see section 2). The red contours represent the lowest reliable H$I$1.2ex
column density – 3$\sigma$ over 25 km s-1 – of our 16 detections. The grey
dashed circle is $R_{\mathrm{vir}}$. The background optical image comes from
the Digital Sky Survey (blue band). The yellow contours show the X-ray
emission in the Fornax cluster detected with XMM-Newton (Frank et al., 2013)
and convolved with a 3 arcmin FWHM gaussian kernel. These contours are spaced
by a factor of 2, with the lowest level at 3.7 counts deg-2 s-1. Figure 2:
ATCA H$I$1.2ex contours overlaid on an optical image for all our H$I$1.2ex
detections, sorted according to increasing H$I$1.2ex mass. The g-band optical
images come from the Fornax Deep Survey (Iodice et al., 2016; Venhola et al.,
2018; Peletier et al., 2020) for all galaxies except FCC 323, whose g-band
optical image comes from the DESI Legacy Imaging Surveys, DR8 release, (Dey et
al., 2019). In each panel we show the 3$\sigma$ column density sensitivity -
values reported in the top right corner - with white colour, while cyan
contours represent steps of 3n from it (n = 0, 1, 2, …). We show the PSF on
the bottom-left corner of each panel, and a 5 kpc scale bar in the bottom-
right corner. Figure 3:  Integrated H$I$1.2ex spectra of our Fornax H$I$1.2ex
detections (in red) sorted according to increasing H$I$1.2ex mass as in Fig.
2. We compare our spectra to spectra from the literature (shown in black; see
top-left corner for details). We also show the barycentric velocity obtained
from our H$I$1.2ex spectra (vertical red line) and from optical spectra
(vertical blue line; Maddox et al., 2019).

Waugh et al. (2002) presented a blind survey of Fornax based on the H$I$1.2ex
Parkes All Sky Survey (HIPASS $-$ Barnes et al. 2001) data with a
$M_{\text{H\scaleto{$I$}{1.2ex}}}$ limit of 1.4 $\times$ 108 M⊙. They detected
110 galaxies within an area of $\sim$620 deg2 around the cluster. Of those,
nine H$I$1.2ex detections are within $R_{\mathrm{vir}}$. The authors confirmed
a H$I$1.2ex depletion in the H$I$1.2ex detections (all late types) near the
centre of the cluster, and suggested that H$I$1.2ex-rich galaxies detected in
the outer parts of the cluster are infalling towards the cluster for the first
time. H$I$1.2ex detections are arranged in a large scale sheet-like structure
with a negative velocity gradient from south-east to north-west.

Waugh (2005) presented the result of the deepest blind H$I$1.2ex survey of the
cluster so far: the Basketweave survey of Fornax carried out with the Parkes
telescope. This survey used a new scanning technique, which improved the
sampling and the noise level compared to HIPASS. H$I$1.2ex was detected in 53
galaxies within 100 deg2 down to a detection limit of 108 M⊙. Of those, 15
were new detections, which confirmed the results of Waugh et al. (2002). In
addition, the author presented higher resolution H$I$1.2ex observations (2
arcmin, 3.3 km s-1) of 28 individual Fornax galaxies carried out with the
Australia Telescope Compact Array (ATCA). Of those, six are within
$R_{\mathrm{vir}}$. Within this region, ESO LV-3580611 was marked as having an
intriguing H$I$1.2ex morphology with an elongation to the north-east of the
system. As suggested in Schröder et al. (2001), also Waugh (2005) pointed out
that this galaxy may be moving towards us.

The only other Fornax galaxy within $R_{\mathrm{vir}}$ with resolved H$I$1.2ex
imaging is NGC 1365 (van der Hulst et al., 1983; Jorsater & van Moorsel,
1995). The latter study observed an elongated H$I$1.2ex distribution to the
west of the system. They suggested that it may be caused by the interaction
with the ICM.

All previous blind H$I$1.2ex surveys of Fornax were carried out with the
Parkes telescope. Their angular resolution of 15 arcmin was insufficient to
study the H$I$1.2ex morphology of the detected galaxies, which is a powerful
tracer of environmental effects. Better angular resolution was achieved with
the interferometric observation of a few selected galaxies in Fornax (see
references above), but those observations covered only a small portion of the
cluster volume. Here we present the first, blind, interferometric H$I$1.2ex
survey of the Fornax cluster.

Our survey was carried out with the ATCA and covers an area of 15 deg2 centred
on NGC 1399 with a spatial and velocity resolution of 67″ $\times$ 95″($\sim
6\leavevmode\nobreak\ \times\leavevmode\nobreak\ 9$ kpc at a distance of 20
Mpc) and 6.6 km s-1, respectively. The average column density sensitivity
within the survey area is 2$\times$1019 cm-2 (3$\sigma$ over 25 km s-1) and
the $M_{\text{H\scaleto{$I$}{1.2ex}}}$ sensitivity is 2$\times 10^{7}$ M⊙
(3$\sigma$ over 100 km s-1). Initially, a case study by Lee-Waddell et al.
(2018) revealed the tidal origin of NGC 1427A using spatially resolved
H$I$1.2ex images from a subregion of our ATCA mosaic. Here we present the
results of the full survey. In Sect.2 we describe observations and data
reduction. In Sect.3 we present the H$I$1.2ex detections, their H$I$1.2ex
images and spectra. We also compare their H$I$1.2ex mass, H21.2ex0pt mass and
SFR relative to non-cluster control samples, and compare their spatial and
velocity distribution with those of the general Fornax population. In Sect. 4
we discuss our results, which we then summarise in Sect.5. We include
supplementary material on each galaxy in Appendix A.

##  2 ATCA observations and data reduction

Our blind Fornax survey covers an area of 15 deg2 (defined at a sensitivity
level 3 $\times$ higher than in the best region of the H$I$1.2ex cube),
spanning from the centre of the cluster to a distance slightly further than
$R_{\mathrm{vir}}$. The observations were carried out with the ATCA in the
750B configuration, from December 2013 to January 2014 (project code
C2894)111Data available on https://atoa.atnf.csiro.au/query.jsp. The cluster
was observed for 336 hrs using 756 different pointings with a spacing of 8.6
arcmin (1/4 of the primary beam FWHM at 1.4 GHz)

The 64 MHz bandwidth, centred at 1396 MHz, was divided into 2048 channels,
providing a velocity resolution of 6.6 km s-1. We reduced the data using the
MIRIAD software (Sault et al., 1995). PKS B1934-638 and PKS 0332-403 were
chosen as the bandpass calibrator and the phase calibrator, respectively. The
latter was observed at 1.5 hr intervals between on-source scans. We flagged
strong radio frequency interference based on Stokes V visibilities. After
flagging and calibration we further processed a restricted frequency range
1407.2 MHz - 1419.6 (which corresponds to the velocity range of 166 - 2783 km
s-1), which includes all spectroscopically confirmed Fornax galaxies (Maddox
et al., 2019). Within this range we used the UVLIN task of MIRIAD to fit and
subtract continuum emission using 2nd-order polynomials.

We obtained the dirty cube with the INVERT task using natural weights to
maximise surface-brightness sensitivity. We used MOSMEM and RESTOR to clean
and restore H$I$1.2ex emission, respectively. The restoring Gaussian PSF has a
major and minor axis FWHM of 95 and 67 arcsec, respectively, and a position
angle of 0.4 deg. The root mean square (RMS) noise level of the final cube
goes down to 2.0 mJy beam-1 in the most sensitive region. Within the survey
area the RMS noise is $\leq 6.0$ mJy beam-1 and, on average, 2.8 mJy beam-1.
This corresponds to a $3\sigma$ H$I$1.2ex column density sensitivity of
$N_{\text{H\scaleto{$I$}{1.2ex}}}$ $\sim 2\times$1019 cm-2 assuming a line
width of 25 km s-1 and a $3\sigma$ $M_{\text{H\scaleto{$I$}{1.2ex}}}$
sensitivity of $\sim$2$\times$107 M⊙ over a linewidth of 100 km s-1. We
searched for H$I$1.2ex sources with the SoFiA source-finding package (Serra et
al., 2015) within the survey footprint (see the green outline in Fig. 1). By
smoothing and clipping, we convolve the input cube with a set of kernels and
detect emission above 3.5$\sigma$ of the local noise level of each kernel.
Reliable detections are identified based on the reliability algorithm
presented in (Serra et al., 2012), which assumes that true sources have
positive total flux and that the noise is symmetric around 0. For one faint
source, NGC 1436, visual inspection was necessary to improve the SoFiA
detection mask used to estimate galaxy parameters. Lastly, our final list of
H$I$1.2ex detections includes a faint source which did not pass the
reliability test, but which we consider a genuine detection given its spatial
correspondence with the known optical source FCC 323.

##  3 H$I$1.2ex detections in the Fornax cluster

###  3.1 H$I$1.2ex detection properties

We detect H$I$1.2ex in the 16 galaxies listed in Table 1. Of these, three are
new H$I$1.2ex detections: FCC 090, FCC 102, FCC 323. The last is the only
galaxy with no previous redshift measurement. In Fig. 1 we show the location
of our H$I$1.2ex detections on the sky and in Fig. 2 we show the H$I$1.2ex
morphology of each galaxy. In these figures, red and white contours,
respectively, represent the lowest reliable H$I$1.2ex column density contour,
defined as 3 times the local RMS assuming a typical H$I$1.2ex linewidth of
$\sim$25 km s-1 (see Table 1 and top-right corner of each panel in Fig. 2).

Fig. 3 shows the integrated ATCA H$I$1.2ex spectra of our 16 detections. We
calculated the error bars by summing in quadrature the statistical uncertainty
$-$ derived from the local RMS (Table 1) and the number of independent pixels
detected in each channel $-$ and the flux-scale uncertainty. We find the
latter to be $\pm 20$% based on the flux ratio calculated for a sample of
selected bright point sources between our radio continuum image and the
Northern VLA Sky Survey (Condon et al. 1998).

We compare our spectra to those obtained from previous observations. In
particular, we use HIPASS spectra from the BGC catalogue (Koribalski et al.,
2004) or HIPASS data reprocessed by us. Since HIPASS data do not show any
emission at the position of NGC 1436 and ESO 358-G016, we used Green Bank
Telescope data (GBT - Courtois et al. 2009) and Parkes data (Bureau et al.,
1996) as comparison, respectively. On the other hand, HIPASS spectra of ESO
358-G015 and ESO 358-G051 are noisy, so we used comparison spectra from
Matthews et al. (1998) and Theureau et al. (1998), respectively, based on
Nanacy data. Furthermore the literature spectra of ESO 358-G015 and ESO
358-G016 were rebinned to our ATCA channel-width. For consistency, we
calculated the uncertainties in the comparison spectra by combining the noise
in the spectrum and the flux-scale uncertainty of each survey, except for ESO
358-G016 for which the flux-scale uncertainty was not provided. For this
galaxy, the error bars are shown as the RMS of the spectrum. In each panel of
Fig. 3 we also show the velocity $v_{\mathrm{opt}}$ derived from optical
spectroscopy (Maddox et al., 2019) and the barycentric H$I$1.2ex velocity
$v_{\mathrm{HI}}$ derived from our ATCA spectra.

We estimated the H$I$1.2ex mass ($M_{\text{H\scaleto{$I$}{1.2ex}}}$) of our
detections from the integrated H$I$1.2ex flux using eq.50 in Meyer et al.
(2017) and adopting the same distance of 20 Mpc for all galaxies (Mould et
al., 2000). The uncertainty on $M_{\text{H\scaleto{$I$}{1.2ex}}}$ is obtained
from the error bars of the spectrum (Fig. 3). We report H$I$1.2ex fluxes and
masses in Table 1. Our H$I$1.2ex fluxes agree with those in the literature
within 1$\sigma$ for nine out of 13 galaxies, and within 2$\sigma$ for 11 of
them. The two cases with a discrepancy larger than 2.5$\sigma$ are NGC 1436
and ESO 358-G051. The comparison spectrum for NGC 1436 comes from GBT data
(Courtois et al., 2009) and shows that we are most likely missing H$I$1.2ex
flux from the blue-shifted part of the system. For this galaxy, the total
H$I$1.2ex flux recovered by ATCA is lower than the GBT flux by 2.5$\sigma$
(corresponding to a factor of 2.8). The reason of this discrepancy arises from
a combination of low S/N and the presence, in at least some of the blue-
shifted channels, of artefacts in this part of the ATCA cube. The case of ESO
358-G051 is less clear since the ATCA cube does not show any obvious artefacts
and the galaxy well detected. However the total H$I$1.2ex flux recovered by
ATCA is lower than the Nancay flux by 3$\sigma$. Based on the current data it
is possible that some of the emission is spread over multiple ATCA beams and
therefore is too weak to be detected. Future MeerKAT data will clarify this
issue Serra et al. (2016). Finally, the H$I$1.2ex mass of FCC 323 is below the
typical sensitivity of our data quoted in Sec. 2 because of the narrow
linewidth as well as the low value of the local noise (Table 1). Our
detections cover about three of magnitude in
$M_{\text{H\scaleto{$I$}{1.2ex}}}$, from FCC 323
($M_{\text{H\scaleto{$I$}{1.2ex}}}$ = $8\times 10^{6}$ M⊙) to NGC 1365
($M_{\text{H\scaleto{$I$}{1.2ex}}}$ = $1.5\times 10^{10}$ M⊙). Fig. 4 shows
the cumulative histogram of the H$I$1.2ex masses of our sample. A future study
will analyse the H$I$1.2ex mass function inferred from our data.

Figure 4: Cumulative histogram of the ATCA H$I$1.2ex masses of our Fornax
sample. Figure 5: H$I$1.2ex disc size as a function of ATCA
$M_{\text{H\scaleto{$I$}{1.2ex}}}$ for our resolved H$I$1.2ex detections.
Dashed line and the blue shaded area show the scaling relation and 3$\sigma$
scatter respectively in (Wang et al., 2016).

Due to the improved resolution of ATCA over a single dish, we detected, in
half of the sample, a variety of H$I$1.2ex morphologies (see Fig. 2) including
offsets between optical and H$I$1.2ex centres, truncated discs, asymmetries
and H$I$1.2ex tails, which we describe in this section and in Appendix A,
following the same order in which galaxies are shown in Fig. 2 (from the
lowest to the highest H$I$1.2ex mass): the H$I$1.2ex distribution in FCC 102
is offset with respect to the optical centre towards the north. We also notice
a large difference between $v_{\mathrm{opt}}$ and
$v_{\text{H\scaleto{$I$}{1.2ex}}}$, $\sim$100 km s-1. However, this is
consistent with the large uncertainty on $v_{\mathrm{opt}}$ given that the
latter was measured from absorption lines for this galaxy (Natasha Maddox,
priv. comm.); the H$I$1.2ex peak of FCC 090 corresponds to the optical centre
but the H$I$1.2ex distribution has an elongation to the south; H$I$1.2ex in
ESO-LV 3580611 (FCC 306) is more extended to the north although there is no
offset between optical and H$I$1.2ex centre (Waugh, 2005); the H$I$1.2ex
morphology of NGC 1437B (FCC 308) is asymmetric and more extended to the
south; H$I$1.2ex in NGC 1351A (FCC 067) shows an elongation towards the south;
the H$I$1.2ex of ESO 358-G063 (FCC 312) is more extended to the east side of
the disc and the H$I$1.2ex contours appear to be compressed on the west side;
In NGC 1427A (FCC 235), we detected a H$I$1.2ex tail which points to the
south-east, away from the cluster centre, consistent with the tidal origin of
this galaxy discussed in Lee-Waddell et al. (2018); the H$I$1.2ex distribution
in NGC 1365 (FCC 121) is extended to the north and appears to be compressed in
the south-west part of the disc (see also Jorsater & van Moorsel 1995). We
mark these H$I$1.2ex disturbed galaxies with star markers in all subsequent
figures.

The remaining half of the galaxies are either unresolved (or nearly so) and
centred on the stellar body, or do not show noticeable asymmetries. One of
them hosts a H$I$1.2ex disc unusually truncated within the stellar disc, NGC
1436. This case will be discussed in detail in later sections.

Figure 6: $M_{\text{H\scaleto{$I$}{1.2ex}}}$ to $M_{\star}$ ratio as a
function of $M_{\star}$. Fornax galaxies (red+blue markers) are compared with
non-cluster galaxies from VSG+HRS (grey circles). Red and blue colours show
Fornax H$I$1.2ex deficient and normal galaxies, respectively. We show Fornax
galaxies with a distorted H$I$1.2ex morphology with star-shaped markers (see
Sect- 3). We show with a black solid line the xGASS scaling relation. The
black dashed line is the linear extrapolation of this trend for $M_{\star}$
$<$ 1.4 $\times$ 109M⊙. The orange dashed line shows the ATCA average
sensitivity evaluated as 3 $\times$ 2.8 mJy beam-1 with a linewidth of 100 km
s-1. The vertical dashed line at 3 $\times$ 109 M⊙ separates low- from high-
mass galaxies.

A final case worth commenting on is FCC 120. This galaxy has been detected
with Parkes and the spectrum in Schröder et al. (2001) shows an evident double
horn profile that is $\sim$100 km s-1 wide, while we detect a single-peak
profile (within the uncertainties - see Fig. 3). Careful visual inspection of
our H$I$1.2ex cube did not reveal any H$I$1.2ex emission missing from our
SoFiA detection mask. The regular H$I$1.2ex morphology of this galaxy in Fig.
2 and the good agreement between $v_{\text{H\scaleto{$I$}{1.2ex}}}$ and
$v_{\mathrm{opt}}$ in Fig. 3 suggest that our H$I$1.2ex characterisation of
this galaxy is correct. Future, deeper data from the Meerkat Fornax survey may
confirm this (Serra et al., 2016).

We estimated the H$I$1.2ex size for all our resolved H$I$1.2ex detections
using the method of Wang et al. 2016 from the H$I$1.2ex intensity maps of our
galaxies. We considered a galaxy resolved if its surface brightness profile
deviates from the shape of the point-spread function. We measured the HI
diameter where the surface density is 1 M⊙ pc-2 and is then deconvolved with
the H$I$1.2ex beam. In Fig. 5 we show that both disturbed and regular galaxies
follow the H$I$1.2ex size-mass scaling relation of Wang et al. (2016) within
the 3$\sigma$ scatter. This can be understood as asymmetric H$I$1.2ex features
usually have a low surface brightness and do not significantly contribute to
the total $M_{\text{H\scaleto{$I$}{1.2ex}}}$ of galaxies. In Fig.5 we also
show the H$I$1.2ex unresolved galaxies. The upper limit on their size were set
equal to the ATCA beam minor axis.

Figure 7: Expected $M_{\text{H\scaleto{$I$}{1.2ex}}}$ versus measured
$M_{\text{H\scaleto{$I$}{1.2ex}}}$. The former were evaluated with the Haynes
& Giovanelli’s (1984) method with the coefficient summarised by Boselli &
Gavazzi (2009). The latter are the ATCA $M_{\text{H\scaleto{$I$}{1.2ex}}}$
calculated as described in Sect.3.1. The dashed line, a factor of 3 below the
1:1 relation, shows the typical threshold below which galaxies are considered
deficient.

###  3.2 $M_{\text{H\scaleto{$I$}{1.2ex}}}$ to $M_{\star}$ ratio

Given the abundant (albeit subtle) H$I$1.2ex asymmetries and offsets in our
Fornax sample, which might be tracing environmental interactions within or on
their way to the Fornax cluster (whether with other galaxies, the large-scale
potential or the intergalactic medium), we study the amount of H$I$1.2ex in
these galaxies in search of signs of H$I$1.2ex depletion. In order to do so,
we evaluated the ratio between $M_{\text{H\scaleto{$I$}{1.2ex}}}$ and stellar
mass ($M_{\star}$) for each galaxy in our sample. The $M_{\star}$ values are
derived using the WISE W1 (3.4 $\mu$m) in$-$band luminosity, W1$-$W2 colour
and the prescription given by Cluver et al. (2014a), with the custom
photometry further defined in Jarrett et al. (2019), and assuming a common
distance of 20 Mpc for all galaxies. We show the distribution of our sample on
the $M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$$-$vs$-$$M_{\star}$ plane in
Fig. 6.

We compare galaxies in Fornax with a sample consisting of void galaxies from
the Void Galaxy Survey (VGS - Kreckel et al. 2012) and field galaxies from the
Herschel Reference Survey (HRS - Boselli et al. 2014). We also used the xGASS
$M_{\star}$-$M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ scaling relation,
which shows the weighted median of
$\log_{10}$($M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$) as a function of
$M_{\star}$. This was obtained from 1177 galaxies selected only by stellar
mass ($M_{\star}$ = 109–1011.5 M⊙) and redshift (0.01 $<$ z $<$ 0.05;
Catinella et al. 2018). In Fig. 6 we extrapolate the xGASS trend down to 107
M⊙ (black dashed line) and show that it is consistent with the non-cluster
sample of galaxies.

Our sample of Fornax H$I$1.2ex detections appears to be systematically offset
with respect to our VGS+HRS comparison sample and to the xGASS scaling
relation. A two-sample Kolmogorov–Smirnov test on the distribution of off sets
from the xGASS scaling relation rejects the null hypothesis that Fornax and
VGS+HRS galaxies are drawn from the same parent sample (p-value = 0.004, KS
statistic = 0.43). Indeed, ten out of 16 Fornax galaxies are below or at the
lower edge of the distribution of non-cluster galaxies. Their offset from the
xGASS scaling relation is larger than the RMS deviation of VGS+HRS galaxies
from it, thus indicating H$I$1.2ex deficiency. For illustrative purposes we
henceforth label these HI deficient galaxies in red colours in Fig. 6 and in
all other upcoming figures in this paper.

H$I$1.2ex deficiencies measured from plots like our Fig. 6 should be taken
with caution because of the large $M_{\text{H\scaleto{$I$}{1.2ex}}}$ scatter
at at fixed $M_{\star}$ (e.g. Maddox et al. 2015). This scatter is to first
order driven by Hubble type or, equivalently, by galaxy properties that
correlate with Hubble type, such as the star formation rate (SFR). For this
reason, in Sect. 3.3 we further analyse how the H$I$1.2ex deficiency at fixed
$M_{\star}$ relates to the H21.2ex0pt and SFR of these galaxies. Furthermore,
in the present section we estimate H$I$1.2ex deficiencies as proposed by
Haynes & Giovanelli (1984) and recently revisited by Jones et al. (2018),
where the measured $M_{\text{H\scaleto{$I$}{1.2ex}}}$ is compared with an
expected $M_{\text{H\scaleto{$I$}{1.2ex}}}$ calculated based on galaxies’
optical size and Hubble type. For this purpose, we use the coefficients
summarised by Boselli & Gavazzi (2009) and originally given in Haynes &
Giovanelli (1984), Solanes et al. (1996), Boselli & Gavazzi (2009), and adopt
the optical sizes and Hubble types listed in Table 1 for our galaxies. Fig. 7
shows the comparison between the expected and the measured
$M_{\text{H\scaleto{$I$}{1.2ex}}}$. The dashed line, a factor of 3 below the
1:1 relation, is the typical threshold below which galaxies are considered
H$I$1.2ex deficient using the Haynes & Giovanelli’s (1984) method (e.g.
Cortese et al. 2011). Also in this case, Fornax galaxies appear offset towards
lower H$I$1.2ex masses and there is a good match between galaxies labelled as
H$I$1.2ex deficient based on our Fig. 6 and those below the dashed line in
Fig. 7.

From these figures, we see that not all galaxies with a disturbed H$I$1.2ex
morphology are H$I$1.2ex deficient. Thus, H$I$1.2ex morphological
disturbances, whatever their exact nature (e.g. tidal or hydrodynamical, which
is difficult to establish with the current data), allow us to identify cases
of environmental interactions before a significant fraction of the cold
interstellar medium is removed. Combining H$I$1.2ex morphological information
and $M_{\text{H\scaleto{$I$}{1.2ex}}}$-$M_{\star}$ ratio may reveal likely new
members of the cluster (we come back to this point in Sect.4).

Fornax galaxies with a disturbed H$I$1.2ex morphology cover the full
$M_{\star}$ range. However, we measure a stronger H$I$1.2ex depletion for low
mass galaxies: the average offset from the xGASS scaling relation in Fig. 6 is
$-0.86$ dex and $-0.33$ dex for $M_{\star}$ below and above 3$\times$109,
respectively. We show the threshold of 3$\times$109 M⊙ with a vertical dashed
blue line in Fig. 6.

We already mentioned the problematic detection of NGC 1436. Although we are
probably missing some flux, it remains a deficient galaxy even if we estimate
the H$I$1.2ex mass from the GBT flux (Table 1) with a
log10($M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$) = $-2.0$ and an offset
from the xGASS scaling relation of $-0.93$ dex.

Fig. 6 shows also eight galaxies where we did not detect H$I$1.2ex emission
but H21.2ex0pt was detected with with the Atacama Large
Millimeter/submillimeter Array (ALMA) (Zabel et al., 2019). For these galaxies
we calculated the $M_{\text{H\scaleto{$I$}{1.2ex}}}$ upper limit as $3\times$
the local noise (Table 1) of the cube and assuming the CO line width of these
galaxies estimated by the PV diagrams in Zabel et al. (2019). Some of these
galaxies appear to have too little H$I$1.2ex given the molecular gas content
and star formation rate, as we discuss in following sections. We show the
optical morphology of these galaxies in Appendix B.

Figure 8: We compare Fornax galaxies (same colour coding of Fig.6) with Virgo
cluster galaxies from HRS (light purple circles). Dark purple circles show the
average scaling relation obtained from Virgo cluster galaxies (Cortese et al.,
2011)

Finally, Fig. 8 shows the comparison between Fornax and Virgo galaxies (from
HRS - Boselli et al. 2014) belonging to Virgo clouds A, B, N, E and S (as
defined by Gavazzi et al. 1999) with
10${}^{9}\lesssim$$M_{\star}$$\lesssim$1011 M⊙. Here we show the Virgo cluster
galaxies and the average scaling relation obtained from them in the same
$M_{\star}$ range (Cortese et al., 2011). Since Virgo is populated by
H$I$1.2ex poor galaxies with respect to field galaxies (Davies & Lewis, 1973;
Chamaraux et al., 1980; Cayatte et al., 1994; Hughes & Cortese, 2009; Chung et
al., 2009), this scaling relation is shifted towards lower gas fractions with
respect to xGASS (Fig. 6). Although Fornax and Virgo galaxies experience
different cluster environments, the distribution of Fornax galaxies cover the
whole range of $M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ of Virgo
galaxies, reaching the same level of H$I$1.2ex deficiency.

###  3.3 Linking H$I$1.2ex properties with H21.2ex0pt and SFR

Figure 9: $M_{\text{H}_{2}}$$/$$M_{\text{H\scaleto{$I$}{1.2ex}}}$ as a
function of $M_{\star}$. We use the same colour coding as Fig. 6. Blue shadow
shows $1\times\sigma$ scatter from the xGASS weighted average of
$\log_{10}$($M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$). We show
upper limits with downward arrows. We show lower limits with upward arrows.
Figure 10: SFR as a function of $M_{\star}$. We use the same colour coding as
Fig. 6. We show upper limits in SFR with downward arrows. The dashed black
line represents the SFR scaling relation in (Whitaker et al., 2012).

The ratio between molecular and atomic gas mass
($M_{\text{H}_{2}}$$/$$M_{\text{H\scaleto{$I$}{1.2ex}}}$) can be useful to
identify anomalous galaxies where, for example, only the atomic phase is
affected by the environment or H$I$1.2ex is not efficiently converted to
H21.2ex0pt. We thus compared the atomic and molecular gas reservoirs of our
detections (see Fig. 9). For this purpose, we used H21.2ex0pt masses from
Zabel et al. (2019) for all our H$I$1.2ex detections except for FCC 323 (no
molecular gas data available). We also include in this analysis the eight
galaxies detected with ALMA (Zabel et al., 2019) that are not detected in
H$I$1.2ex (upper limits in Fig. 6 and Fig. 8; see Sec. 3.2). We also scaled
the molecular upper limit from Zabel et al. (2019) to be consistent with a
line width of 100 km s-1. As a comparison, we used the xGASS
$M_{\star}$-$M_{\text{H}_{2}}$$/$$M_{\text{H\scaleto{$I$}{1.2ex}}}$ scaling
relation in Catinella et al. (2018), which describes the typical
$M_{\text{H}_{2}}$$/$$M_{\text{H\scaleto{$I$}{1.2ex}}}$ ratio as a function of
$M_{\star}$ at $z$ $=$ 0.

For $M_{\star}$$>$109 M⊙, 55$\%$ of all H$I$1.2ex detected galaxies are also
H21.2ex0pt detected, while this fraction drops to zero for $M_{\star}$$<$109
M⊙ (likely because the lower metallicity of these objects makes CO
progressively harder to detect). In the rest of this section we therefore
focus on the higher $M_{\star}$ range. Here, we see that about half of the
galaxies are consistent with the xGASS sample, while most of the remaining
galaxies are above the xGASS scaling (see Fig. 9). In particular, galaxies
with a distorted H$I$1.2ex morphology are compatible with the xGASS trend with
the exception of NGC 1427A whose lack of molecular gas is puzzling (see Zabel
et al., 2019). Although this galaxy has a normal (and large) H$I$1.2ex mass
for its stellar mass, no molecular gas was detected with ALMA. The agreement
with the scaling relation holds also for NGC 1437B, which is the only
H$I$1.2ex deficient galaxy with a distorted morphology in this range of
$M_{\star}$.

The detection with the highest
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ ratio is NGC 1436.
Although we might be missing H$I$1.2ex flux (Sect. 3.2), this mass ratio
remains high even if we use the $M_{\text{H\scaleto{$I$}{1.2ex}}}$ value
estimated from the GBT flux (with a
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ = 1.7). Other three
galaxies, NGC 1386, NGC 1387 and NGC 1380, may be even more peculiar as the
lower limit on their $M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$
ratio is already an order of magnitude above the xGASS scaling relation. High
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ ratios have also been
measured in Virgo galaxies (Cortese et al., 2016).

Given the broad distribution of
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ ratio in Fornax, we
further investigate whether their star formation rate follows standard scaling
with $M_{\star}$ and $M_{\text{H\scaleto{$I$}{1.2ex}}}$. In particular, we are
interested in understanding whether the general offset of Fornax galaxies
towards low $M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ ratios in Fig. 6 is
associated with a decrease of SFR at fixed $M_{\star}$.

Figure 11:  SFR deviation from the Whitaker et al. (2012) scaling relation
(Fig. 10) plotted against the $M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$
deviation from the xGASS scaling relation (Fig. 6). We use the same colour
coding as Fig. 6. We show upper limits in $M_{\text{H\scaleto{$I$}{1.2ex}}}$
with leftward arrows. The horizontal blue line represents no deviation from
the SFR scaling relation, while the vertical blue line represents no deviation
from the $M_{\star}$-$M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ scaling
relation.

We start by comparing Fornax galaxies with the same sample of non-cluster
galaxies used in Sect. 3.2. SFR of both samples of Fornax galaxies and non-
cluster galaxies are evaluated using eq. 2 in Boquien et al. (2016). We set
the scaling coefficient for the infrared 24 $\mu$m band (W4) equal to 6.17. We
adopted the calibration factor for near UV 231 nm band (NUV) to be log10C =
-43.17 (Kennicutt & Evans, 2012). NUV fluxes come from GCAT (Seibert et al.,
2012) and IRSA catalogues (Leroy et al., 2019). Since the latter catalogue is
more reliable for galaxies at $z$ = 0, we used it to get the NUV flux from
Fornax galaxies. W4 was obtained using custom software optimised for
performing aperture photometry on resolved galaxies (Jarrett et al. 2013;
Cluver et al. 2014b; Jarrett et al. 2019). Then, we removed the contribution
from the evolved stellar populations using the method of Helou et al. (2004)
which consists of scaling and subtracting the W1 light (a proxy of evolved
stellar population) from W4 as described in Cluver et al. (2017). For a
fraction of galaxies, NUV and W4 fluxes were not available (3/212 non-cluster
galaxies; 11/24 Fornax galaxies). In these cases, we calculate the SFR from
the W3 (12$\mu$m) luminosity – with stellar emission subtracted (as for W4)–
and the TIR-to-MIR relation using eq. 4 in Cluver et al. (2017), where L12
$\mu$m is the continuum-subtracted spectral ($\nu\leavevmode\nobreak\ \times$
Lν) luminosity. Our conclusions below do not change if we calculate SFR from
WISE data alone.

Fig. 10 shows that the majority of the Fornax galaxies have a SFR below the
values predicted by the scaling relation of Whitaker et al. (2012).
Furthermore, the difference between the SFR of Fornax galaxies and that
predicted from Whitaker et al. (2012) increases towards lower $M_{\star}$.
That is, the SFR-$M_{\star}$ relation in Fornax is steeper than that of non-
cluster galaxies. This might indicate a stronger SFR decrease in low-mass
galaxies, similar to what observed for their H$I$1.2ex reservoirs (as
discussed in Sect. 3.2) and their H21.2ex0pt reservoirs (Zabel et al., 2019).
Also when compared to the HRS+VGS comparison sample, Fornax galaxies appear to
be offset towards lower SFR values, mirroring the results in Fig. 6.

Among the four galaxies with the highest
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ ratio in Fig. 9, NGC
1386 is the only one with a higher SFR than expected. This is likely due to
the presence of an AGN (Rodríguez-Ardila et al., 2017) which affects the SFR
measurement. The other two H$I$1.2ex undetected galaxies, NGC 1380 and NGC
1387, reside at the lower edge of the comparison sample. We notice that NGC
1380 is also highly H21.2ex0pt deficient in Zabel et al. (2019).

Figure 12: Distribution of our Fornax H$I$1.2ex detections on the sky (blue
and red markers) compared to that of all Fornax galaxies in our footprint
(grey circles; Maddox et al., 2019). The green and gray contours are the same
as Fig. 1.

In Fig. 11 we investigate whether, in Fornax, the low SFR (at fixed
$M_{\star}$; Fig. 10) can be related to the H$I$1.2ex deficiency (Fig. 6).
This figure plots the SFR and $M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$
deviations from the respective scaling relations (Fig. 10 and Fig. 6) against
one another. We find that Fornax galaxies mostly populate the area of the plot
of H$I$1.2ex deficient galaxies with low SFR, following the trend of non-
cluster galaxies in the comparison sample. That is, in Fornax, a decrease in
$M_{\text{H\scaleto{$I$}{1.2ex}}}$ is accompanied by a decrease in SFR just
like in non-cluster galaxies. In Sec. 4 we present an interpretation of this
result.

On top of this general trend we see a few outliers. The most evident ones are
ESO 358-G060 and the H$I$1.2ex undetected NGC 1386. While the high SFR in NGC
1386 might be an artefact caused by the presence of an AGN, the case of ESO
358-G060 is more complicated. We carefully inspected this galaxy’s WISE images
and no SFR from W3/W4 with old stellar population subtraction was detectable.
Therefore, we calculated its SFR the from NUV flux alone using eq. 12 in
Kennicutt & Evans (2012). Although ESO 358-G060 is H$I$1.2ex rich, its SFR is
lower than that any galaxy of the comparison sample with a similar
$M_{\star}$. It is worth noting the cases of NGC 1436 which lies at the upper
edge of the comparison sample of H$I$1.2ex deficient galaxy (bottom-left
quadrant of Fig. 11) and NGC 1427A which shows a low SFR although its large
H$I$1.2ex reservoir. All these cases will be discussed in Sect. 4.

###  3.4 H$I$1.2ex properties as a function of 3D location in Fornax cluster

In Fig. 12, we compare the 2D distribution of our Fornax H$I$1.2ex detections
with that of all spectroscopic Fornax members (Maddox et al., 2019) included
within our survey footprint. Keeping in mind that the area of the sky that we
observed is not symmetric, most of the galaxies in our sample are located
south of the centre of the cluster, while only four out of 16 detections are
located north of it.

Figure 13: Phase space diagram of the Fornax cluster. Here, we used the same
colour coding as Fig. 12. We show the caustic curves of the cluster with black
dashed lines. Histograms: black and gray colours represent H$I$1.2ex detected
galaxies and all Fornax members, respectively. The area within the black
triangle shows the virialised area of the cluster

Almost all our H$I$1.2ex detections (88$\%$ of the sample) are located farther
than 0.5 $R_{\mathrm{vir}}$ in projection (the only exceptions being NGC 1427A
at 0.2 $R_{\mathrm{vir}}$, and ESO 358-G051 at 0.4 $R_{\mathrm{vir}}$). In
contrast, only 33$\%$ of all Fornax galaxies in our footprint are outside 0.5
$R_{\mathrm{vir}}$.

The same effect, with the addition of the kinematical information, can be seen
in the projected phase-space diagram shown in Fig. 13. We used a cluster
systemic velocity of $v_{\mathrm{sys}}$ = 1442 km s-1 and a cluster velocity
dispersion of $\sigma_{\mathrm{cl}}\leavevmode\nobreak\ =\leavevmode\nobreak\
318$ km s-1 (Maddox et al., 2019). In order to draw caustic curves we
proceeded as in Jaffé et al. (2015) using $M_{\mathrm{vir}}$ = 5$\times$1013
M⊙, $R_{\mathrm{vir}}$ = 700 kpc (Drinkwater et al., 2001a) and a cluster halo
concentration parameter equal to 6 (Navarro et al. 1996; the exact value of
this parameter does not change the result significantly). The inner triangle
shows the virialised area where it is more likely to find old members of the
Fornax cluster (Rhee et al., 2017). From the right histogram in Fig. 13, we
see that the spectroscopic Fornax members have a peak at the cluster systemic
velocity, while velocities of our H$I$1.2ex detections cover all velocity
range without any preferred velocity. Thus, both Fig. 12 and Fig. 13 show that
the H$I$1.2ex galaxy sample does not follow the distribution of spectroscopic
galaxies.

In Fig. 13, we also see that 11 out of 16 galaxies are within the escape
velocity boundary in projection (black lines). It is worth noting that the
H$I$1.2ex deficient galaxies FCC 120 and ESO-LV 3580611 are located outside
the caustic curves. Among H$I$1.2ex disturbed galaxies, six out of eight are
redshifted with respect to the recessional velocity of the cluster.

Both Fig. 12 and Fig. 13 show that some galaxies in our sample are not just
close by one another on the sky but also in velocity. The clearest case is
that of NGC 1365 and its three neighbours: FCC 102, FCC 090 and ESO 358-G016.
They are all within a region of 250 kpc and a velocity range of 200 km s-1.
These galaxies might be part of a substructure which is accreting onto the
cluster (as suggested by Drinkwater et al. 2001a). In addition, most galaxies
in this substructure exhibit indications of ongoing interactions with one
another and/or with the intergalactic medium. Both H$I$1.2ex elongations in
FCC 102 and NGC 1365 point in projection to the north, while H$I$1.2ex in FCC
090 is elongated towards the south. FCC 090, FCC 102 and NGC 1365 form a
triplet of galaxies which lies along a line that points towards the centre of
the cluster. ESO 358-G016 has a regular H$I$1.2ex distribution and it lies in
projection to the north of NGC 1365. All but NGC 1365 are H$I$1.2ex deficient
galaxies. The existence of a sub-group centred on NGC 1365 is also consistent
with the large scale structure around Fornax (the Fornax-Eridanus
supercluster), which is mainly made by several groups of galaxies that are
assembling to form the cluster along the filament (Nasonova et al., 2011).

On the east of the cluster centre, we also see that FCC 323 and NGC 1437B are
near each other on the sky in projection ($\sim$120 kpc apart), as well as in
velocity ($\sim$10 km s-1 difference). The unresolved H$I$1.2ex morphology of
FCC 323 does not reveal, at this time, whether there is an ongoing interaction
between those galaxies. We mark it as a potential subgroup.

Another intriguing galaxy is ESO 358-G060. Among the galaxies with
$M_{\star}$$\leq 10^{9}$ M⊙ it is the only one with a normal H$I$1.2ex content
compared to non-cluster galaxies (see Fig. 6), and it shows no evidence of on-
going interactions with the Fornax environment (see Fig. 2). Given its
position just outside the caustic curves in Fig. 13, we discuss in Sect. 4
whether it might not have entered the cluster yet. Another, similar case is
NGC 1437A. It is a H$I$1.2ex rich galaxy which shows a quite regular H$I$1.2ex
morphology. It is also just outside the caustic curves in Fig. 13. However,
unlike the undisturbed ESO 358-G060, the optical appearance of NGC 1437A is
peculiar, similar to that of NGC 1427A, since they both exhibit an arrow
shaped optical morphology as pointed out in Raj et al. (2019). The MeerKAT
Fornax Survey (Serra et al., 2016) will deliver a higher resolution H$I$1.2ex
image of this object and will be able to reveal any H$I$1.2ex asymmetries that
might be hidden by projection effects within the ATCA beam.

##  4 Discussion

The population of H$I$1.2ex detected Fornax galaxies exhibits several
interesting features. Despite the limited resolution of our data, half of all
detections reveal H$I$1.2ex asymmetries and offsets relative to the stellar
body (Fig. 2). Furthermore, the H$I$1.2ex sample as a whole is gas-poorer and
is forming stars at a lower rate than samples of non$-$cluster galaxies in the
same $M_{\star}$ range (Fig. 6 and Fig. 10), and half of the galaxies with
$M_{\star}$$>$109 M⊙ have an anomalous
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ ratio. Finally, the
H$I$1.2ex detections are distributed in a noticeably different way with
respect to the majority of cluster galaxies both on the sky and in projected
phase space (Fig. 12 and Fig. 13). This body of evidence suggests that the
Fornax environment is influencing the evolution of these galaxies, which may
be the most recent arrivals in the cluster. In particular, the difference
between the 3D distribution of H$I$1.2ex detected galaxies compared with that
of spectroscopically confirmed Fornax members (Maddox et al. 2019), indicates
that H$I$1.2ex is a crucial observable to test the volume of the cluster where
H$I$1.2ex rich galaxies become H$I$1.2ex deficient, before a complete
H$I$1.2ex removal in the inner part of the cluster.

An outstanding question is how long it takes for a galaxy to lose its
H$I$1.2ex – the dominant component of the interstellar medium – as it falls
into a cluster. In the case of Fornax we can gain some insight through a joint
analysis of Fig. 11 and Fig. 13. If we assume that H$I$1.2ex is being actively
removed from within galaxies in Fornax (as suggested by the frequently
disturbed H$I$1.2ex morphologies - see Fig. 2), the lack of H$I$1.2ex
detections in the virialised region of Fig. 13 implies that H$I$1.2ex removal
happens on a time scale $\tau_{\mathrm{HI,loss}}$ shorter than the cluster’s
crossing time:
$\tau_{\mathrm{HI,loss}}\leq\tau_{\mathrm{cross}}\leavevmode\nobreak\
\sim\leavevmode\nobreak\
R_{\mathrm{vir}}/\sigma_{\mathrm{cl}}\leavevmode\nobreak\
\sim\leavevmode\nobreak\ 2$ Gyr (see Sect. 3.4; this $\tau_{\mathrm{cross}}$
is similar to that estimated from simulations, 1.2$\leavevmode\nobreak\
\pm\leavevmode\nobreak\ $0.5 Gyr independent of cluster mass; Rhee et al.
2017.) Hence, we expect that our 16 detections will have lost most of their
H$I$1.2ex by the time they reach the pericentre.

On the other hand, Fig. 11 suggests that, so far, H$I$1.2ex has been lost
slowly in our H$I$1.2ex detections (having been removed and/or consumed, and
not replenished). In that figure, our Fornax H$I$1.2ex detections are
distributed along the same correlation defined by non-cluster galaxies. Thus,
for those Fornax galaxies, the SFR has so far had sufficient time to respond
to a variation in H$I$1.2ex mass – just like outside clusters. Since the
transition from H$I$1.2ex to new stars happens through the intermediate phase
of H21.2ex0pt, this ‘equilibrium’ between H$I$1.2ex and SFR implies that
H21.2ex0pt is depleted faster than H$I$1.2ex is lost in galaxies currently at
the cluster’s outskirts, giving the entire cycle of H$I$1.2ex-to-H21.2ex0pt-
to-SFR enough time to ‘see’ the varying H$I$1.2ex content. Thus, while for
Fornax as whole $\tau_{\mathrm{HI,loss}}\leq\tau_{\mathrm{cross}}\sim 2$ Gyr
(see above), for our H$I$1.2ex detections in the outer regions of Fornax
$\tau_{\mathrm{HI,loss}}$ has so far been $\geq\leavevmode\nobreak\
\tau_{\mathrm{H_{2},depl}}\leavevmode\nobreak\ \sim\leavevmode\nobreak\
1\text{-}2$ Gyr (Bigiel et al., 2008). (The H21.2ex0pt depletion time
$\tau_{\mathrm{H_{2},depl}}$ is only marginally shorter in Fornax, in
particular at its outskirts, and anyway with large variations from galaxy to
galaxy; Zabel et al. 2020.)

It is likely that, further inside Fornax, H$I$1.2ex is removed faster than
what we estimated above for galaxies at the cluster’s outskirts. Indeed,
simulations indicate that for a galaxy within $0.5\leavevmode\nobreak\
R_{\mathrm{vir}}$, $\tau_{\mathrm{HI,loss}}\leavevmode\nobreak\
<\leavevmode\nobreak\ 0.5$ Gyr (Marasco et al., 2016). Such a rapid H$I$1.2ex
removal may not leave sufficient time for the SFR to ‘track’ the decrease in
H$I$1.2ex mass in Fig. 11, resulting in galaxies moving to the left of the
non-cluster sample. A confirmation of this effect may come from the eight
H21.2ex0pt-detected galaxies (most of them H21.2ex0pt-deficient; Zabel et al.
2019) where H$I$1.2ex has already been removed at least down to the ATCA
$M_{\text{H\scaleto{$I$}{1.2ex}}}$ sensitivity (left-pointing arrows in Fig.
11), and possibly by some H$I$1.2ex detections closer to the cluster centre
(e.g. NGC 1436). Indeed, these galaxies occupy a region to the left of the
comparison sample, showing that their SFR is still significant despite their
low H$I$1.2ex content. For these galaxies, H$I$1.2ex is likely to have been
removed faster than H21.2ex0pt is depleted:
$\tau_{\mathrm{HI,loss}}\leq\tau_{\mathrm{H_{2},depl}}$. This conclusion is
supported by the anomalously high
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ ratio of these galaxies
in Fig. 9 (in the $M_{\star}$ range where we have a reliable comparison).

Within the picture discussed above, H$I$1.2ex-detected galaxies in the outer
regions of Fornax are thus first infallers, which are starting to interact
with the Fornax environment and will lose most of their H$I$1.2ex by the time
they reach the pericentre. Even at the current early stage of infall, they
already show a relatively large diversity in H$I$1.2ex morphology (Fig. 2) and
mass (Fig. 6). More specifically, we found two morphologically undisturbed
H$I$1.2ex rich galaxies ($12\%$ of the H$I$1.2ex detections); four
morphologically disturbed H$I$1.2ex rich galaxies (25$\%$ of the H$I$1.2ex
detections); four morphologically disturbed H$I$1.2ex deficient galaxies
(25$\%$ of the H$I$1.2ex detections); six morphologically undisturbed - within
the ATCA resolution - H$I$1.2ex deficient galaxies (38$\%$ of the H$I$1.2ex
detections).

The morphologically undisturbed H$I$1.2ex-rich galaxies are ESO 358-G060 and
NGC 1437A. They both reside outside the caustic curves in projection (Fig. 13)
and, given their low $M_{\star}$, they should be easily perturbed by the
Fornax environment. Thus, we speculate that they are recent Fornax members
which have not yet had enough time to be significantly affected by the
cluster. One possible caveat is the low resolution of our images, which may
hide H$I$1.2ex disturbances in particular in the case of the optically
peculiar galaxy NGC 1437A. Another possibility is that they are outside the
cluster volume or in a region with lower ICM density. ESO 358-G060 is also an
outlier in Fig. 11, where the relatively high
$M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ does not correspond to a high
SFR. In this galaxy no SFR was detectable from WISE W3/W4 bands (after
subtracting the old stellar population light), and the NUV contribution to
star formation is low. Several physical processes might account for the low
SFR with respect to the large H$I$1.2ex reservoir: for example, an inefficient
H$I$1.2ex to H21.2ex0pt conversion due to a small amount of dust, a large
angular momentum which prevents the H$I$1.2ex from collapsing, and/or an
H$I$1.2ex external origin (e.g. see Geréb et al. 2016, 2018).

The morphologically disturbed H$I$1.2ex-rich galaxies are ESO 358-G063, NGC
1351A, NGC 1427A, NGC 1365. We discuss the last galaxy when we focus our
attention on the H$I$1.2ex detected subgroup of interacting galaxies. ESO
358-G063 and NGC 1351A, are both disc galaxies north of NGC 1399. They are
both quite isolated from all other Fornax Galaxies in RA, Dec and velocity in
projection (see Fig. 12 and Fig. 13). Thus, their slightly H$I$1.2ex disturbed
morphologies described in Sect.3 might be due to the interaction with the ICM
of the Fornax cluster. Furthermore, the agreement of
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ of ESO 358-G063 and NGC
1351A with the xGASS scaling relation in Fig. 9 suggests that they are recent
Fornax members, thus the cluster environment has not had enough time to
significantly deplete their H$I$1.2ex reservoirs. The last morphologically
disturbed H$I$1.2ex rich galaxy is NGC 1427A. This is the galaxy with the
second-highest H$I$1.2ex mass in our sample of Fornax H$I$1.2ex detections. In
Fig. 12 and Fig. 13, we see that NGC 1427A is the closest H$I$1.2ex detected
galaxy to the centre of the cluster in projection, but it has also the highest
velocity. Thus, it may be a new Fornax member which is infalling from the
foreground. As already mentioned, Lee-Waddell et al. (2018) studied the origin
of the H$I$1.2ex tail using the same data we present here. They concluded
that, unlike previously suggestions, ram-pressure is unlikely to be the main
process shaping the galaxy’s optical appearance. Instead, NGC 1427A is most
likely a recent merger remnant, thus shaped by tidal forces. The recent merger
might be the cause of the low molecular column density and/or its low
metallicity, resulting currently undetected by ALMA (Zabel et al. 2019; it is
the upper limit with the highest $M_{\star}$ in Fig. 9). NGC 1427A is also the
second H$I$1.2ex rich outlier with low SFR in Fig. 11. The H$I$1.2ex-
to-H21.2ex0pt conversion might be inefficient to have a SFR consistent with
its $M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ ratio.

The only H$I$1.2ex deficient and morphologically disturbed galaxies which do
not belong to the NGC 1365 subgroup are: ESO-LV 3580611 and NGC 1437B. They
are very close to one another on the sky but have significantly different
velocities. The former is outside the caustic curve in projection (Fig. 13),
which supports the hypothesis of an new infalling Fornax member made by
Schröder et al. (2001). The latter has a velocity similar to the recessional
velocity of the cluster. H$I$1.2ex and molecular morphologies (the latter
detected by Zabel et al. 2019) are elongated in the same direction. This
suggests that both gas phases are experiencing the same environmental
interaction. Raj et al., 2019 detected a tidal tail in NGC 1437B, which may be
due to a recent fly-by of another galaxy. As mentioned in Sect.3.4, although
the evidence is not strong, NGC 1437B might be part of a subgroup of
interacting galaxies which includes FCC 323. Thus, FCC 323 might be the fly-by
galaxy that NGC 1437B has interacted with.

It is difficult to comment on the morphologically undisturbed H$I$1.2ex
deficient galaxies, since their symmetric H$I$1.2ex distribution may be a
consequence of the ATCA resolution. However, a very peculiar case is the
truncated H$I$1.2ex disc of NGC 1436. It is the closest spiral galaxy to the
centre of the cluster detected in H$I$1.2ex. Raj et al. (2019) observed an
ongoing morphological transition into lenticular: the spiral structure is
found only in its inner region, while the outer disc has the smooth appearance
typically found in S0 galaxies.

The inner part of NGC 1436 appears regular also in H21.2ex0pt, and the galaxy
is just moderately H21.2ex0pt deficient (Zabel et al., 2019). It is also the
only galaxy detected both in H$I$1.2ex and H21.2ex0pt which shows a high
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ ratio in Fig. 9. These
results and the evidence of morphological distortions only in the outer part
of the galaxy suggest that this galaxy may have gone through a quick
interaction with the cluster environment, which did not affect the inner
spiral structure yet. This idea is corroborated by the fact that NGC 1436 lies
on the upper edge on the comparison sample in Fig. 11, which means that
despite its H$I$1.2ex deficiency it is still forming stars at a significant
rate.

Finally, we focus on the NGC 1365 subgroup. We discussed all the H$I$1.2ex
morphologies and the 3D distribution of the subgroup members in Sect.3.1 and
Sect.3.4, respectively. As mentioned earlier, NGC 1365 is the only H$I$1.2ex
rich galaxy of the subgroup (Fig. 6) with a $M_{\text{H\scaleto{$I$}{1.2ex}}}$
at least two orders of magnitude larger than the other members. The H$I$1.2ex
distribution both in NGC 1365 and in FCC 102 is elongated to the north, while
it is elongated to the south in FCC 090. ESO 358-G016 is the only galaxy with
a regular H$I$1.2ex morphology. It is also the only galaxy located north of
NGC 1365. We propose two scenarios in order to explain the properties of the
subgroup and its members: the former is a case of interaction between galaxies
and the cluster environment which is responsible of the high
$M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ ratio of the low mass members.
In contrast, due to to its deep gravitational potential, NGC 1365 has been
able to retain its H$I$1.2ex, although some of it has been perturbed. The
latter scenario we propose, it is a case of preprocessing in a group of
galaxies where the local environment of the subgroup was able to affect the
$M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ ratio of the low mass members
before the group began to interact with the cluster environment.

In general, although we found a large variety of H$I$1.2ex properties in our
sample of H$I$1.2ex detections, we detected an overall trend towards H$I$1.2ex
disturbances and deficiency in Fornax. Fig. 6 makes evident an already evolved
state of Fornax H$I$1.2ex galaxies where $\sim$2/3 of the galaxies are
H$I$1.2ex deficient. Fig. 6 also shows that the Fornax environment is more
effective in altering the gas content of galaxies with $M_{\star}$ $<$ 3
$\times$ 109 M⊙) (see Sect.3.2).

Zabel et al. (2019) presented a similar study, where they compare Fornax and
field galaxies based on their molecular gas properties. They found some
molecular deficiency in all their detections except NGC 1365. However,
galaxies with $M_{\star}$ $<$ 3$\times$109 M⊙ are both more H21.2ex0pt-
deficient and morphologically disturbed with respect to more massive galaxies,
whose molecular gas morphology is always regular. We do not observe such a
clear difference in H$I$1.2ex. Indeed, we observe disturbed H$I$1.2ex
morphologies across our entire $M_{\star}$ range, confirming that atomic
hydrogen is the best tracer of early interactions. Despite this difference
between H$I$1.2ex and H21.2ex0pt morphologies, we also note some similarities
between our results and those in Zabel et al. (2019). Indeed, the mass range
of molecular disturbed galaxies is also characterised by a stronger H$I$1.2ex
depletion. Conversely, the H$I$1.2ex depletion is weaker in the mass range in
which galaxies show regular molecular-gas morphologies ($M_{\star}$ $>$
3$\times$109 M⊙; Sect.3.2). Thus, both gas phases show that the Fornax
environment is more effective in altering the gas content of low-mass galaxies
compared to high-mass galaxies.

Finally, our results are in agreement with the FDS$\&$F3D results (Iodice et
al., 2019a, b). Indeed, almost all galaxies with a disturbed H$I$1.2ex
morphology are of late type and belong to the group of the infalling galaxies
in (Iodice et al., 2019a), which are symmetrically distributed around the
cluster’s central region. These galaxies have active star formation and are
located in the low-density region of the cluster, where the X-ray emission is
faint or absent. Our results show that they are interacting with the cluster
environment. Deeper into the cluster, the lack of H$I$1.2ex detections is
consistent with the result that this region is dominated by evolved early-type
galaxies. Some of these galaxies have been able to retain part of their
H21.2ex0pt reservoirs (Zabel et al., 2019), but not their H$I$1.2ex.

##  5 Summary

The blind ATCA H$I$1.2ex survey of the Fornax galaxy cluster covers a field of
15 deg2 out to a distance of $\sim R_{\mathrm{vir}}$ from the cluster centre.
It has a spatial and velocity resolution of 67″ $\times$ 95″and 6.6 km s-1,
respectively, and a 3$\sigma$ $N_{\text{H\scaleto{$I$}{1.2ex}}}$ and
$M_{\text{H\scaleto{$I$}{1.2ex}}}$ sensitivity of $\sim$2$\leavevmode\nobreak\
\times$ 1019cm-2 and $\sim$2$\leavevmode\nobreak\ \times\leavevmode\nobreak\ $
107M⊙, respectively. The survey revealed H$I$1.2ex emission from 16 Fornax
galaxies covering a mass range of about three orders of magnitude, from
$8\times 10^{6}$ to $1.5\times 10^{10}$ M⊙. These galaxies exhibit a variety
of disturbances of the H$I$1.2ex morphology, including asymmetries, tails,
offsets between H$I$1.2ex and optical centres and a case of a truncated
H$I$1.2ex disc (Fig. 2). This suggests environmental interactions within or on
their way to Fornax (whether with other galaxies, the large-scale potential or
the intergalactic medium), supported by the offset of Fornax galaxies towards
low $M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ ratios with respect to the
xGASS $M_{\star}$-$M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ scaling
relation (Fig. 6), and resulting in H$I$1.2ex deficiencies similar to those
observed in the Virgo cluster (Fig. 8). The H$I$1.2ex sample of Fornax
galaxies is also forming stars at a lower rate than samples of non-cluster
galaxies at fixed $M_{\star}$ (Fig. 10). This deficit of SFR is consistent
with the deficit of H$I$1.2ex when compared to non-cluster galaxies (Fig. 11).

Our 16 detections reside outside the virialised region of the cluster – where
the distribution of the general population of Fornax galaxies is clustered –
both on the sky and in the projected phase space diagram (Fig. 12 and Fig.
13). This result implies that H$I$1.2ex is lost down to the ATCA sensitivity
within a crossing time ($\tau_{\mathrm{HI,loss}}\leavevmode\nobreak\
\leq\leavevmode\nobreak\ \tau_{\mathrm{cross}}\leavevmode\nobreak\ \sim 2$
Gyr), and that our H$I$1.2ex detections are recent arrivals in the cluster.
They still reside at the outskirts of Fornax, where their H$I$1.2ex and SFR
properties suggest that H$I$1.2ex has so far been lost on a time scale longer
than the H21.2ex0pt depletion time
($\tau_{\mathrm{HI,loss}}\leavevmode\nobreak\ \geq\leavevmode\nobreak\
\tau_{\mathrm{H_{2},depl}}\leavevmode\nobreak\ \sim 1\mathrm{-}2$ Gyr). In the
cluster’s central regions H$I$1.2ex removal is likely to proceed faster
($\tau_{\mathrm{HI,loss}}\leavevmode\nobreak\ <\leavevmode\nobreak\
\tau_{\mathrm{H_{2},depl}}$). This is supported by the relatively high SFR of
H$I$1.2ex-undetected, H21.2ex0pt-detected galaxies and by the anomalously high
$M_{\text{H}_{2}}$/$M_{\text{H\scaleto{$I$}{1.2ex}}}$ ratios of galaxies in
those regions (Fig. 9, Fig. 12 and Fig. 13). These are galaxies where SFR is
likely to be proceeding relatively unperturbed after rapid removal of the
H$I$1.2ex.

This picture is enriched by the new detection of the NGC 1365 subgroup – where
both pre-processing and early interaction with the cluster environment are
plausible scenarios to account for the H$I$1.2ex properties of its members –
and by the detection of several galaxies with peculiar ISM properties, such as
some H$I$1.2ex-rich but H21.2ex0pt-poor and low-SFR galaxies (NGC 1427A, ESO
358-G060). The future MeerKAT Fornax Survey (Serra et al., 2016) will observe
this cluster with a better resolution and sensitivity than those of our ATCA
survey, enabling a further step forward in the study of the evolution of
Fornax galaxies.

###### Acknowledgements.

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. 679627; project name FORNAX). Parts of this research were
supported by the Australian Research Council Centre of Excellence for All Sky
Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013.
LC is the recipient of an Australian Research Council Future Fellowship
(FT180100066) funded by the Australian Government. This publication has
received funding from the European Union Horizon 2020 research and innovation
programme under the Marie Skłodowska-Curie grant agreement number 721463 to
the SUNDIAL ITN network. NZ acknowledges support from the European Research
Council (ERC) in the form of Consolidator Grant CosmicDust
(ERC-2014-CoG-647939). This work made use of the Digitized Sky Surveys, which
were produced at the Space Telescope Science Institute under U.S. Government
grant NAG W-2166. The Australia Telescope Compact Array is part of the
Australia Telescope National Facility which is funded by the Australian
Government for operation as a National Facility managed by CSIRO. We
acknowledge the Gomeroi people as the traditional owners of the Observatory
site.

Name | FCC | RA | Dec | $v_{\mathrm{opt}}$ | $v_{\text{H\scaleto{$I$}{1.2ex}}}$ | Flux | $M_{\text{H\scaleto{$I$}{1.2ex}}}$ |  M⋆ | RMS | Flux | Morph. |  D25(B) | Notes
---|---|---|---|---|---|---|---|---|---|---|---|---|---
|  | (J2000) | (J2000) |  |  |  |  |  |  | (literature) |  |  |
|  | (hh:mm:ss.ss) | (dd:mm:ss.ss) | $\left(\frac{\mathrm{km}}{\mathrm{s}}\right)$ | $\left(\frac{\mathrm{km}}{\mathrm{s}}\right)$ | $\left(\frac{\mathrm{Jykm}}{\mathrm{s}}\right)$ |  (108M$\odot$) |  (108M⊙) |  ($\frac{\mathrm{mJy}}{\mathrm{beam}}$) | $\left(\frac{\mathrm{Jykm}}{\mathrm{s}}\right)$ |  | (arcsec) |
ESO 358-G015 | 113 | 03:33:06.85 | -34:48:29.19 | 1365 | 1389 |  1.0$\pm$0.2 |  0.9$\pm$0.2 |  8$\pm$2 | 2.4 |  1.4$\pm$0.2∗ | Scd | 72.10 |  H$I$1.2ex def.
ESO 358-G016 | 115 | 03:33:09.19 | -35:43:06.69 | 1686 | 1694 |  0.8$\pm$0.2 |  0.7$\pm$0.2 |  1.2$\pm$0.2 | 2.5 |  1.2$\pm$0.5△ | Im | 48.42 |  H$I$1.2ex def.; N1365 s.group
ESO 358-G051 | 263 | 03:41:32.59 | -34:53:17.99 | 1733 | 1731 |  1.1$\pm$0.2 |  1.0$\pm$0.2 |  21$\pm$5 | 2.5 |  2.8$\pm$0.4∘ | Scd | 92.90 |  H$I$1.2ex def.
ESO 358-G060 | 302 | 03:45:12.14 | -35:34:15.26 | 806 | 803 |  12$\pm$2 |  11$\pm$2 |  1.0$\pm$0.6 | 2.4 |  10.6$\pm$0.6▼ | Scd | 104.30 |
ESO 358-G063 | 312 | 03:46:19.00 | -34:56:36.80 | 1932 | 1940 |  18$\pm$4 |  17$\pm$3 |  110$\pm$30 | 2.6 |  18.7$\pm$0.9▲ | Scd | 280.60 |  H$I$1.2ex dist;
ESO-LV 3580611 | 306 | 03:45:45.39 | -36:20:47.50 | 891 | 894 |  1.1$\pm$0.2 |  1.1$\pm$0.2 |  1.0$\pm$0.4 | 2.4 |  2.7$\pm$0.7▽ | Im | 31.99 |  H$I$1.2ex def.; H$I$1.2ex dist;
FCC 090 | 090 | 03:31:08.26 | -36:17:24.50 | 1827 | 1823 |  0.6$\pm$0.2 |  0.6$\pm$0.1 |  13 $\pm$3 | 2.3 | # | E | 62.80 |  H$I$1.2ex def.; H$I$1.2ex dist; N1365 s.group
FCC 102 | 102 | 03:32:10.73 | -36:13:14.91 | 1722 | 1631 |  0.5$\pm$0.1 |  0.5$\pm$0.1 |  1.6$\pm$0.5 | 2.5 | # | Im | 33.50 |  H$I$1.2ex def.,H$I$1.2ex dist; N1365 s.group
FCC 120 | 120 | 03:33:34.22 | -36:36:21.29 | 849 | 846 |  0.5$\pm$0.1 |  0.4$\pm$0.1 |  1.6$\pm$0.5 | 2.4 |  1.9$\pm$0.9▽ | Im | 48.13 |  H$I$1.2ex def.
FCC 207 | 207 | 03:38:19.27 | -35:07:44.69 | 1393 | # |  $<$0.1 |  $<$0.1 |  5 $\pm$ 1 | 2.3 | # | E | 36.65 |  H$I$1.2ex undet.
FCC 261 | 261 | 03:41:21.52 | -33:46:09.19 | 1710 | # |  $<$0.1 |  $<$0.1 |  3.6 $\pm$ 0.9 | 2.3 | # | E | 39.75 |  H$I$1.2ex undet.
FCC 282 | 282 | 03:42:45.31 | -33:55:13.80 | 1225 | # |  $<$0.2 |  $<$0.2 |  18 $\pm$ 4 | 2.5 | # | E | 54.39 |  H$I$1.2ex undet.
FCC 323 | 323 | 03:47:37.52 | -36:21:46.83 | # | 1502 |  0.08$\pm$0.03 |  0.08$\pm$0.03 |  0.6 $\pm$ 0.6 | 2.4 | # | E | # |  H$I$1.2ex def.
FCC 332 | 332 | 03:49:49.02 | -35:56:44.09 | 1326 | # |  $<$0.1 |  $<$0.1 |  7 $\pm$ 2 | 2.3 | # | E | 43.90 |  H$I$1.2ex undet.
FCC 335 | 335 | 03:50:36.73 | -35:54:33.59 | 1367 | # |  $<$0.1 |  $<$0.1 |  27 $\pm$ 6 | 2.6 | # | E | 80.90 |  H$I$1.2ex undet.
NGC 1351A | 067 | 03:28:48.72 | -35:10:41.30 | 1336 | 1337 |  5$\pm$1 |  5$\pm$1 |  35$\pm$8 | 3.2 |  6.1$\pm$0.9▲ | Sc | 161.50 |  H$I$1.2ex dist;
NGC 1365 | 121 | 03:33:36.37 | -36:08:25.44 | 1638 | 1641 |  160$\pm$30 |  150$\pm$30 |  600$\pm$100 | 2.8 |  146$\pm$3▼ | Sb | 673.20 |  H$I$1.2ex dist.; N1365 s.group
NGC 1380 | 167 | 03:36:27.59 | -34:58:34.41 | 1877 | # |  $<$0.6 |  $<$0.5 |  1400 $\pm$ 300 | 2.6 | # | S0a | 287.20 |  H$I$1.2ex undet.
NGC 1386 | 179 | 03:36:46.18 | -35:59:57.86 | 972 | # |  $<$0.5 |  $<$0.5 |  120 $\pm$ 30 | 2.9 | # | Sa | 203.30 |  H$I$1.2ex undet.
NGC 1387 | 184 | 03:36:57.06 | -35:30:23.90 | 1302 | # |  $<$0.3 |  $<$0.3 |  900 $\pm$ 200 | 2.1 | # | E | 169.10 |  H$I$1.2ex undet.
NGC 1427A | 235 | 03:40:09.30 | -35:37:27.99 | 2023 | 2026 |  22$\pm$4 |  21$\pm$4 |  23$\pm$5 | 2.3 |  22.5$\pm$0.8▼ | Im | 140.70 |  H$I$1.2ex dist;
NGC 1436 | 290 | 03:43:37.08 | -35:51:10.90 | 1392 | 1414 |  0.6$\pm$0.2 |  0.6$\pm$0.2 |  160$\pm$40 | 2.5 |  1.7$\pm$0.2⋄ | Sc | 177.10 |  H$I$1.2ex def.; truncated H$I$1.2ex disc
NGC 1437A | 285 | 03:43:02.19 | -36:16:24.14 | 891 | 887 |  6$\pm$1 |  6$\pm$1 |  10$\pm$3 | 2.4 |  7$\pm$1▲ | Scd | 111.70 |
NGC 1437B | 308 | 03:45:54.85 | -36:21:25.09 | 1515 | 1501 |  2.6$\pm$0.5 |  2.4$\pm$0.5 |  50 $\pm$10 | 2.5 |  3.2$\pm$0.6▲ | Scd | 157.80 |  H$I$1.2ex def.; H$I$1.2ex dist;
Table 1: We report parameters on the H$I$1.2ex detected galaxies. RA and Dec
coordinates correspond to the optical centres from NED. Errors on the total
H$I$1.2ex fluxes are evaluated as described in Sect.3. ‘Flux (literature)’
shows the total fluxes measured from other surveys as follows: $\diamond$
Courtois et al. (2009); $\triangle$ Bureau et al. (1996); $\ast$ Matthews et
al. (1998); $\circ$ Theureau et al. (1998); $\triangledown$ Schröder et al.
(2001); $\blacktriangledown$ Koribalski et al. (2004); $\blacktriangle$ HIPASS
data reprocessed by us. The total flux uncertainty of the comparison spectra
are calculated as described in Sect. 3 combining the noise in the spectrum and
the flux-scale uncertainty of each survey except for ESO 358-G016 for which
the flux-scale uncertainty was not provided. For this galaxy we show the
uncertainty on the total flux given in Bureau et al. (1996). Columns ‘Morph.’
and ‘D25(B)’ show the optical morphology and the optical size of our galaxies
used in Sect. 3.2 to evaluate the expected H$I$1.2ex content of our galaxies
with the Haynes & Giovanelli’s (1984) method. D25(B) is the optical isophotal
diameter measured at 25 mag/arcsec2 in B-band; we collected these values from
NED for 17 out of 24 galaxies. Almost all of the optical diameters comes from
RC3 catalogue (de Vaucouleurs et al., 1991), two out of 15 are from Lauberts &
Valentijn (1989). We estimated D25(B) for all the remaining galaxies except
for FCC 323 as follows: from the optical multi-component decomposition made by
Su et al. (2021), we scaled the r-band radial profile to B-band. In order to
do this, we previously converted g-r from Venhola et al. (2019) to obtain B-r
using the intermediate conversion formula g-r to B-g (Lupton, 2005). In column
‘Notes’: H$I$1.2ex def, H$I$1.2ex dist, N 1365 s.group stand for H$I$1.2ex
deficient, H$I$1.2ex morphologically disturbed galaxy, and galaxy included in
the NGC 1365 subgroup, respectively.

## References

  * Barnes et al. (2001) Barnes, D. G., Staveley-Smith, L., de Blok, W. J. G., et al. 2001, MNRAS, 322, 486
  * Barnes et al. (1997) Barnes, D. G., Staveley-Smith, L., Webster, R. L., & Walsh, W. 1997, MNRAS, 288, 307
  * Bigiel et al. (2008) Bigiel, F., Leroy, A., Walter, F., et al. 2008, AJ, 136, 2846
  * Blakeslee et al. (2009) Blakeslee, J. P., Jordán, A., Mei, S., et al. 2009, The Astrophysical Journal, 694, 556
  * Boquien et al. (2016) Boquien, M., Kennicutt, R., Calzetti, D., et al. 2016, A&A, 591, A6
  * Boselli et al. (2014) Boselli, A., Cortese, L., & Boquien, M. 2014, A&A, 564, A65
  * Boselli & Gavazzi (2006) Boselli, A. & Gavazzi, G. 2006, PASP, 118, 517
  * Boselli & Gavazzi (2009) Boselli, A. & Gavazzi, G. 2009, A&A, 508, 201
  * Bureau et al. (1996) Bureau, M., Mould, J. R., & Staveley-Smith, L. 1996, ApJ, 463, 60
  * Cantiello et al. (2018) Cantiello, M., D’Abrusco, R., Spavone, M., et al. 2018, A&A, 611, A93
  * Catinella et al. (2018) Catinella, B., Saintonge, A., Janowiecki, S., et al. 2018, MNRAS, 476, 875
  * Cayatte et al. (1994) Cayatte, V., Kotanyi, C., Balkowski, C., & van Gorkom, J. H. 1994, AJ, 107, 1003
  * Chamaraux et al. (1980) Chamaraux, P., Balkowski, C., & Gerard, E. 1980, A&A, 83, 38
  * Chung et al. (2009) Chung, A., van Gorkom, J. H., Kenney, J. D. P., Crowl, H., & Vollmer, B. 2009, AJ, 138, 1741
  * Cluver et al. (2017) Cluver, M. E., Jarrett, T. H., Dale, D. A., et al. 2017, ApJ, 850, 68
  * Cluver et al. (2014a) Cluver, M. E., Jarrett, T. H., Hopkins, A. M., et al. 2014a, ApJ, 782, 90
  * Cluver et al. (2014b) Cluver, M. E., Jarrett, T. H., Hopkins, A. M., et al. 2014b, ApJ, 782, 90
  * Condon et al. (1998) Condon, J. J., Cotton, W. D., Greisen, E. W., et al. 1998, AJ, 115, 1693
  * Cortese et al. (2016) Cortese, L., Bekki, K., Boselli, A., et al. 2016, MNRAS, 459, 3574
  * Cortese et al. (2011) Cortese, L., Catinella, B., Boissier, S., Boselli, A., & Heinis, S. 2011, MNRAS, 415, 1797
  * Courtois et al. (2009) Courtois, H. M., Tully, R. B., Fisher, J. R., et al. 2009, AJ, 138, 1938
  * Davies & Lewis (1973) Davies, R. D. & Lewis, B. M. 1973, MNRAS, 165, 231
  * de Vaucouleurs et al. (1991) de Vaucouleurs, G., de Vaucouleurs, A., Corwin, Herold G., J., et al. 1991, Third Reference Catalogue of Bright Galaxies
  * Dey et al. (2019) Dey, A., Schlegel, D. J., Lang, D., et al. 2019, AJ, 157, 168
  * Diaferio et al. (2001) Diaferio, A., Kauffmann, G., Balogh, M. L., et al. 2001, MNRAS, 323, 999
  * Dressler (1980) Dressler, A. 1980, ApJ, 236, 351
  * Dressler (1986) Dressler, A. 1986, ApJ, 301, 35
  * Drinkwater et al. (2001a) Drinkwater, M. J., Gregg, M. D., & Colless, M. 2001a, ApJ, 548, L139
  * Drinkwater et al. (2001b) Drinkwater, M. J., Gregg, M. D., Holman, B. A., & Brown, M. J. I. 2001b, MNRAS, 326, 1076
  * Ferguson (1989) Ferguson, H. C. 1989, AJ, 98, 367
  * Frank et al. (2013) Frank, K. A., Peterson, J. R., Andersson, K., Fabian, A. C., & Sanders, J. S. 2013, ApJ, 764, 46
  * Gavazzi et al. (1999) Gavazzi, G., Boselli, A., Scodeggio, M., Pierini, D., & Belsole, E. 1999, MNRAS, 304, 595
  * Geréb et al. (2016) Geréb, K., Catinella, B., Cortese, L., et al. 2016, MNRAS, 462, 382
  * Geréb et al. (2018) Geréb, K., Janowiecki, S., Catinella, B., Cortese, L., & Kilborn, V. 2018, MNRAS, 476, 896
  * Giovanelli & Haynes (1983) Giovanelli, R. & Haynes, M. P. 1983, AJ, 88, 881
  * Grillmair et al. (1994) Grillmair, C. J., Freeman, K. C., Bicknell, G. V., et al. 1994, ApJ, 422, L9
  * Gunn & Gott (1972) Gunn, J. E. & Gott, J. Richard, I. 1972, ApJ, 176, 1
  * Hamraz et al. (2019) Hamraz, E., Peletier, R. F., Khosroshahi, H. G., et al. 2019, A&A, 625, A94
  * Haynes & Giovanelli (1984) Haynes, M. P. & Giovanelli, R. 1984, AJ, 89, 758
  * Haynes & Giovanelli (1986) Haynes, M. P. & Giovanelli, R. 1986, ApJ, 306, 466
  * Horellou et al. (2001) Horellou, C., Black, J. H., van Gorkom, J. H., et al. 2001, A&A, 376, 837
  * Hubble & Humason (1931) Hubble, E. & Humason, M. L. 1931, ApJ, 74, 43
  * Hughes & Cortese (2009) Hughes, T. M. & Cortese, L. 2009, MNRAS, 396, L41
  * Iodice et al. (2016) Iodice, E., Capaccioli, M., Grado, A., et al. 2016, ApJ, 820, 42
  * Iodice et al. (2019a) Iodice, E., Sarzi, M., Bittner, A., et al. 2019a, A&A, 627, A136
  * Iodice et al. (2017) Iodice, E., Spavone, M., Capaccioli, M., et al. 2017, ApJ, 839, 21
  * Iodice et al. (2019b) Iodice, E., Spavone, M., Capaccioli, M., et al. 2019b, A&A, 623, A1
  * Jaffé et al. (2015) Jaffé, Y. L., Smith, R., Candlish, G. N., et al. 2015, MNRAS, 448, 1715
  * Jarrett et al. (2019) Jarrett, T. H., Cluver, M. E., Brown, M. J. I., et al. 2019, ApJS, 245, 25
  * Jarrett et al. (2013) Jarrett, T. H., Masci, F., Tsai, C. W., et al. 2013, AJ, 145, 6
  * Jones et al. (2018) Jones, M. G., Espada, D., Verdes-Montenegro, L., et al. 2018, A&A, 609, A17
  * Jordán et al. (2007) Jordán, A., Blakeslee, J. P., Côté, P., et al. 2007, ApJS, 169, 213
  * Jorsater & van Moorsel (1995) Jorsater, S. & van Moorsel, G. A. 1995, AJ, 110, 2037
  * Kennicutt & Evans (2012) Kennicutt, R. C. & Evans, N. J. 2012, ARA&A, 50, 531
  * Koribalski et al. (2004) Koribalski, B. S., Staveley-Smith, L., Kilborn, V. A., et al. 2004, AJ, 128, 16
  * Kreckel et al. (2012) Kreckel, K., Platen, E., Aragón-Calvo, M. A., et al. 2012, AJ, 144, 16
  * Lauberts & Valentijn (1989) Lauberts, A. & Valentijn, E. A. 1989, The surface photometry catalogue of the ESO-Uppsala galaxies
  * Lee-Waddell et al. (2018) Lee-Waddell, K., Serra, P., Koribalski, B., et al. 2018, MNRAS, 474, 1108
  * Leroy et al. (2019) Leroy, A. K., Sandstrom, K. M., Lang, D., et al. 2019, ApJS, 244, 24
  * Lupton (2005) Lupton, P. 2005
  * Maddox et al. (2015) Maddox, N., Hess, K. M., Obreschkow, D., Jarvis, M. J., & Blyth, S. L. 2015, MNRAS, 447, 1610
  * Maddox et al. (2019) Maddox, N., Serra, P., Venhola, A., et al. 2019, MNRAS, 490, 1666
  * Marasco et al. (2016) Marasco, A., Crain, R. A., Schaye, J., et al. 2016, MNRAS, 461, 2630
  * Matthews et al. (1998) Matthews, L. D., van Driel, W., & Gallagher, J. S., I. 1998, AJ, 116, 1169
  * Meyer et al. (2017) Meyer, M., Robotham, A., Obreschkow, D., et al. 2017, PASA, 34, 52
  * Mould et al. (2000) Mould, J. R., Hughes, S. M. G., Stetson, P. B., et al. 2000, ApJ, 528, 655
  * Nasonova et al. (2011) Nasonova, O. G., de Freitas Pacheco, J. A., & Karachentsev, I. D. 2011, A&A, 532, A104
  * Navarro et al. (1996) Navarro, J. F., Frenk, C. S., & White, S. D. M. 1996, ApJ, 462, 563
  * Oemler (1974) Oemler, Augustus, J. 1974, ApJ, 194, 1
  * Oh et al. (2018) Oh, S., Kim, K., Lee, J. H., et al. 2018, ApJS, 237, 14
  * Ondrechen & van der Hulst (1989) Ondrechen, M. P. & van der Hulst, J. M. 1989, ApJ, 342, 29
  * Paolillo et al. (2002) Paolillo, M., Fabbiano, G., Peres, G., & Kim, D. W. 2002, ApJ, 565, 883
  * Peletier et al. (2020) Peletier, R., Iodice, E., Venhola, A., et al. 2020, arXiv e-prints, arXiv:2008.12633
  * Raj et al. (2019) Raj, M. A., Iodice, E., Napolitano, N. R., et al. 2019, A&A, 628, A4
  * Rhee et al. (2017) Rhee, J., Smith, R., Choi, H., et al. 2017, ApJ, 843, 128
  * Rodríguez-Ardila et al. (2017) Rodríguez-Ardila, A., Prieto, M. A., Mazzalay, X., et al. 2017, MNRAS, 470, 2845
  * Sault et al. (1995) Sault, R. J., Teuben, P. J., & Wright, M. C. H. 1995, Astronomical Society of the Pacific Conference Series, Vol. 77, A Retrospective View of MIRIAD, ed. R. A. Shaw, H. E. Payne, & J. J. E. Hayes, 433
  * Schröder et al. (2001) Schröder, A., Drinkwater, M. J., & Richter, O. G. 2001, A&A, 376, 98
  * Schweizer (1980) Schweizer, F. 1980, ApJ, 237, 303
  * Seibert et al. (2012) Seibert, M., Wyder, T., Neill, J., et al. 2012, in American Astronomical Society Meeting Abstracts, Vol. 219, American Astronomical Society Meeting Abstracts #219, 340.01
  * Serra et al. (2016) Serra, P., de Blok, W. J. G., Bryan, G. L., et al. 2016, in MeerKAT Science: On the Pathway to the SKA, 8
  * Serra et al. (2012) Serra, P., Jurek, R., & Flöer, L. 2012, PASA, 29, 296
  * Serra et al. (2019) Serra, P., Maccagni, F. M., Kleiner, D., et al. 2019, A&A, 628, A122
  * Serra et al. (2015) Serra, P., Westmeier, T., Giese, N., et al. 2015, MNRAS, 448, 1922
  * Sheardown et al. (2018) Sheardown, A., Roediger, E., Su, Y., et al. 2018, ApJ, 865, 118
  * Sheen et al. (2012) Sheen, Y.-K., Yi, S. K., Ree, C. H., & Lee, J. 2012, ApJS, 202, 8
  * Solanes et al. (1996) Solanes, J. M., Giovanelli, R., & Haynes, M. P. 1996, ApJ, 461, 609
  * Solanes et al. (2001) Solanes, J. M., Manrique, A., García-Gómez, C., et al. 2001, ApJ, 548, 97
  * Spavone et al. (2020) Spavone, M., Iodice, E., van de Ven, G., et al. 2020, A&A, 639, A14
  * Spiniello et al. (2018) Spiniello, C., Napolitano, N. R., Arnaboldi, M., et al. 2018, MNRAS, 477, 1880
  * Su et al. (2021) Su, A. H., Salo, H., Janz, J., et al. 2021, arXiv e-prints, arXiv:2101.05699
  * Theureau et al. (1998) Theureau, G., Bottinelli, L., Coudreau-Durand, N., et al. 1998, A&AS, 130, 333
  * Toomre & Toomre (1972) Toomre, A. & Toomre, J. 1972, ApJ, 178, 623
  * van der Hulst et al. (1983) van der Hulst, J. M., Ondrechen, M. P., van Gorkom, J. H., & Hummel, E. 1983, in IAU Symposium, Vol. 100, Internal Kinematics and Dynamics of Galaxies, ed. E. Athanassoula, 233–234
  * Venhola et al. (2018) Venhola, A., Peletier, R., Laurikainen, E., et al. 2018, A&A, 620, A165
  * Venhola et al. (2019) Venhola, A., Peletier, R., Laurikainen, E., et al. 2019, A&A, 625, A143
  * Wang et al. (2016) Wang, J., Koribalski, B. S., Serra, P., et al. 2016, MNRAS, 460, 2143
  * Waugh (2005) Waugh, M. 2005, PhD thesis, University of Melbourne.
  * Waugh et al. (2002) Waugh, M., Drinkwater, M. J., Webster, R. L., et al. 2002, MNRAS, 337, 641
  * Whitaker et al. (2012) Whitaker, K. E., van Dokkum, P. G., Brammer, G., & Franx, M. 2012, ApJ, 754, L29
  * Zabel et al. (2020) Zabel, N., Davis, T. A., Sarzi, M., et al. 2020, MNRAS
  * Zabel et al. (2019) Zabel, N., Davis, T. A., Smith, M. W. L., et al. 2019, MNRAS, 483, 2251

## Appendix A

In this section, we describe in detail each galaxy of our sample. Whenever
possible we compare the information estimated from our H$I$1.2ex data with
observations at other wavelengths. The galaxies are sorted according to
increasing H$I$1.2ex mass as in Table 1, Fig. 2 and Fig. 3.

### FCC 323

According to Ferguson (1989), FCC 323 is a dE2/ImV. This is the faintest among
our H$I$1.2ex detections and, for the first time, we detected it in H$I$1.2ex
and thus measured its recessional velocity of
$v_{\text{H\scaleto{$I$}{1.2ex}}}$ $=$ 1502 km s-1. FCC 323 has a
$M_{\text{H\scaleto{$I$}{1.2ex}}}$ = (7.6$\pm$3.9)$\times$106 M⊙ and a
$M_{\star}$ = (6.3$\pm$5.8)$\times$107 M⊙. The Fornax environment might have
strongly affected the H$I$1.2ex reservoir of FCC 323. It has the highest
$M_{\text{H\scaleto{$I$}{1.2ex}}}$/$M_{\star}$ offset from the (extrapolated)
xGASS scaling relation in Fig. 6. Its closest galaxy both on the sky and
velocity is NGC 1437B (120 kpc and 12 km s-1 away), leading us to think that
FCC 323 and NGC 1437B might be part of a subgroup or a pair of galaxies.

### FCC 120

FCC 120 is classified as ImIV by Ferguson (1989). The H$I$1.2ex emission
associated with the galaxy seems to be quite regular. The ATCA spectrum in
Fig. 3 shows one peak (within the uncertainties), which is different from the
100 - km s-1 \- wide double horn spectrum detected by Schröder et al. (2001)
based on Parkes data. Even if the latter data shows strong RFI at a velocity
of 1250 km s-1, this RFI does not affect the detection of the galaxy. However,
$v_{\mathrm{opt}}$ from Maddox et al. (2019), agrees with our
$v_{\text{H\scaleto{$I$}{1.2ex}}}$. Deeper H$I$1.2ex data (Serra et al. 2016)
may confirm this. FCC 120 has a $M_{\text{H\scaleto{$I$}{1.2ex}}}$ =
(4.5$\pm$1.5)$\times$107 M⊙ and a $M_{\star}$ = (1.6$\pm$0.5)$\times$108 M⊙.

### FCC 102

FCC 102 is an irregular ImIV classified by Ferguson (1989). We detect
H$I$1.2ex emission associated with FCC 102 for the first time. Although the
H$I$1.2ex morphology is not resolved, the H$I$1.2ex is offset towards the
north with respect to the optical centre. The difference between
$v_{\mathrm{opt}}$ from Maddox et al. (2019), and
$v_{\text{H\scaleto{$I$}{1.2ex}}}$ from ATCA data is almost 100 km s-1.
However, those values are still consistent due to the large uncertainty on
$v_{\mathrm{opt}}$, which was measured from absorption lines for this galaxy
(Natasha Maddox, priv. comm

authors:
A. Loni (1 and 2), P. Serra (1), D. Kleiner (1), L. Cortese (3 and 4),
  B. Catinella (3 and 4), B. Koribalski (5), T. H. Jarrett (6), D. Cs. Molnar
  (1), T. A. Davis (7), E. Iodice (8), K. Lee-Waddell (5), F. Loi (1), F. M.
  Maccagni (1), R. Peletier (9), A. Popping (3 and 10), M. Ramatsoku (1 and
  11), M. W .L. Smith (7), and N. Zabel (9) ((1) INAF - Osservatorio
  Astronomico di Cagliari, Selargius, CA, Italy, (2) Dipartimento di Fisica,
  Universit\`a di Cagliari, Cittadella Universitaria, Monserrato, Italy, (3)
  International Centre for Radio Astronomy Research (ICRAR), The University of
  Western Australia, Crawley, WA, Australia, (4) ARC Centre of Excellence for
  All-Sky Astrophysics in 3 Dimensions (ASTRO3D), (5) CSIRO Astronomy and Space
  Science, Australia Telescope National Facility, Epping, NSW, Australia, (6)
  Department of Astronomy, University of Cape Town, Rondebosch, South Africa,
  (7) School of Physics and Astronomy, Cardiff University, Queens Buildings The
  Parade, Cardiff, UK, (8) INAF-Astronomical observatory of Capodimonte,
  Naples, Italy, (9) Kapteyn Astronomical Institute, University of Groningen,
  Groningen, The Netherlands, (10) Australian Research Council, Centre of
  Excellence for All-sky Astrophysics (CAASTRO), Australia, (11) Department of
  Physics and Electronics, Rhodes University, Makhanda, South Africa)

submitter: Alessandro Loni