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11email<EMAIL_ADDRESS>22institutetext: University of Oulu, Space
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University of Tokyo, 2–21–1 Osawa, Mitaka, Tokyo 181–0015, Japan
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Institute, 44780 Bochum, Germany 1313institutetext: Dipartimento di Fisica,
Università di Cagliari, Cittadella Universitaria, 09042 Monserrato, Italy
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I-40129 Bologna, Italy

# A MeerKAT view of pre-processing in the Fornax A group

D. Kleiner 11 P. Serra 11 F. M. Maccagni 11 A. Venhola 22 K. Morokuma-Matsui
33 R. Peletier 44 E. Iodice 55 M. A. Raj 55 W. J. G. de Blok 667744 A. Comrie
88 G. I. G. Józsa 9910101111 P. Kamphuis 1212 A. Loni 111313 S. I. Loubser
1414 D. Cs. Molnár 11 S. S. Passmoor 99 M. Ramatsoku 101011 A. Sivitilli 77 O.
Smirnov 101099 K. Thorat 151588 F. Vitello 1616

(Received 13 November, 2020; accepted 25 January, 2021)

We present MeerKAT neutral hydrogen (H i) observations of the Fornax A group,
which is likely falling into the Fornax cluster for the first time. Our H i
image is sensitive to 1.4 $\times$ 1019 atoms cm-2 over 44.1 km s-1, where we
detect H i in 10 galaxies and a total of (1.12 $\pm$ 0.02) $\times$ 109 M⊙ of
H i in the intra-group medium (IGM). We search for signs of pre-processing in
the 12 group galaxies with confirmed optical redshifts that reside within the
sensitivity limit of our H i image. There are 9 galaxies that show evidence of
pre-processing and we classify each galaxy into their respective pre-
processing category, according to their H i morphology and gas (atomic and
molecular) scaling relations. Galaxies that have not yet experienced pre-
processing have extended H i discs and a high H i content with a H2-to-H i
ratio that is an order of magnitude lower than the median for their stellar
mass. Galaxies that are currently being pre-processed display H i tails,
truncated H i discs with typical gas fractions, and H2-to-H i ratios. Galaxies
in the advanced stages of pre-processing are the most H i deficient. If there
is any H i, they have lost their outer H i disc and efficiently converted
their H i to H2, resulting in H2-to-H i ratios that are an order of magnitude
higher than the median for their stellar mass. The central, massive galaxy in
our group (NGC 1316) underwent a 10:1 merger $\sim$ 2 Gyr ago and ejected 6.6
– 11.2 $\times$ 108 M⊙ of H i, which we detect as clouds and streams in the
IGM, some of which form coherent structures up to $\sim$ 220 kpc in length. We
also detect giant ($\sim$ 100 kpc) ionised hydrogen (H$\alpha$) filaments in
the IGM, likely from cool gas being removed (and subsequently ionised) from an
in-falling satellite. The H$\alpha$ filaments are situated within the hot halo
of NGC 1316 and there are localised regions that contain H i. We speculate
that the H$\alpha$ and multiphase gas is supported by magnetic pressure
(possibly assisted by the NGC 1316 AGN), such that the hot gas can condense
and form H i that survives in the hot halo for cosmological timescales.

###### Key Words.:

Galaxies: groups: general – galaxies: groups: individual: Fornax A – galaxies:
evolution – galaxies: interactions – galaxies: ISM – radio lines: galaxies

##  1 Introduction

Our current understanding of galaxy formation and evolution is that secular
processes and galaxy environment fundamentally shape the properties of
galaxies (e.g. Baldry et al., 2004; Balogh et al., 2004; Bell et al., 2004;
Peng et al., 2010; Driver et al., 2011; Schawinski et al., 2014; Davies et
al., 2019). In the local Universe (z $\sim$ 0) up to $\sim$ 50% of galaxies
reside in groups (Eke et al., 2004; Robotham et al., 2011), making it
essential to understand the group environment in the context of galaxy
evolution.

While there is no precise definition of a galaxy group, it generally contains
3 – 102 galaxies in a dark matter (DM) halo of 1012 – 1014 M⊙ (e.g. Catinella
et al., 2013). As the galaxy number density and DM halo mass of groups span a
wide range, there is no dominant transformation mechanism that galaxies are
subjected to, but rather multiple secular and external mechanisms working
together. The properties of group galaxies appear to correlate with group halo
mass and virial radius, implying that quenching paths in groups are different
from those in clusters (Weinmann et al., 2006; Haines et al., 2007; Wetzel et
al., 2012; Woo et al., 2013; Haines et al., 2015).

As galaxies fall towards clusters, there is sufficient time for external (i.e.
environmentally driven, such as tidal and hydro-dynamical) mechanisms to
transform and even quench the galaxies, prior to reaching the cluster (e.g.
Porter et al., 2008; Haines et al., 2013, 2015; Bianconi et al., 2018; Fossati
et al., 2019; Seth & Raychaudhury, 2020). This is called “pre-processing” and
refers to the accelerated, non-secular evolution of galaxies that occurs prior
to entering a cluster. As pre-processing requires external mechanisms to
transform the galaxies, this evolution commonly occurs in groups, where it is
generally thought that group galaxies follow a different evolutionary path
compared to galaxies of the same mass in the field (e.g. Fujita, 2004;
Mahajan, 2013; Roberts & Parker, 2017; Cluver et al., 2020). In particular,
pre-processing is likely to be most efficient in massive ($>$ 1010.5 M⊙)
galaxies residing in massive (1013 – 1014 M⊙) groups (Donnari et al., 2020).
It has also been shown that pre-processing is responsible for the decrease in
star formation activity for late-type galaxies at distances between 1 and 3
cluster virial radii (e.g Lewis et al., 2002; Gómez et al., 2003; Verdugo et
al., 2008; Mahajan et al., 2012; Haines et al., 2015).

Neutral hydrogen in the atomic form (H i) is ideal for tracing tidal and
hydro-dynamical processes in galaxies and the intra-group medium (IGM). H i is
the main component of the interstellar medium (ISM) and can show the effects
of ram pressure, viscous and turbulent stripping, thermal heating (e.g. Cowie
& McKee, 1977; Nulsen, 1982; Chung et al., 2007; Rasmussen et al., 2008; Chung
et al., 2009; Steinhauser et al., 2016; Ramatsoku et al., 2020), and moderate
and strong tidal interactions (e.g. Koribalski, 2012; de Blok et al., 2018;
Kleiner et al., 2019), long before these mechanism can be identified in the
stars.

In this paper we present a detailed analysis of the Fornax A galaxy group
based on H i and ancillary observations. The Fornax A group is an excellent
candidate to search for pre-processing signatures as it is likely in-falling
into the (low mass – 5 $\times$ 1013 M⊙) Fornax cluster (Drinkwater et al.,
2001) for the first time. The group galaxies span a variety of stellar masses
and morphological types, implying that tidal and hydro-dynamical interactions
are likely to affect the galaxies gas and stellar content (Raj et al., 2020).

Using Meer Karoo Array Telescope (MeerKAT) H i observations, deep optical
imaging from the Fornax Deep Survey (FDS: Iodice et al., 2016, 2017; Venhola
et al., 2018, 2019; Raj et al., 2019, 2020), wide-field H$\alpha$ imaging from
the VLT Survey Telescope (VST) and molecular gas observations from the Atacama
Large Millimetre Array (ALMA), we identify galaxies at different stages of
pre-processing following various types of interactions.

This paper is organised as follows: Section 2 describes the Fornax A group.
Section 3 describes the H i and H$\alpha$ observations and the data reduction
process used to produce our images. We present the results of our H i
measurements, H i images, and the relation to stellar and H$\alpha$ emission
in Section 4. In Section 5 we present the atomic-to-molecular gas ratios and
discuss the evidence and timescale of pre-processing in the group. Finally, we
summarise our results in Section 6. Throughout this paper we assume a
luminosity distance of 20 Mpc to the most massive galaxy (NGC 1316) in the
Fornax A group (Cantiello et al., 2013; Hatt et al., 2018) and assume all
objects in the group are at the same distance. At this distance, 1′
corresponds to 5.8 kpc.

##  2 The Fornax A group

The Fornax A galaxy group is the brightest group in the Fornax volume. It is
located on the cluster outskirts at a projected distance of $\sim$ 3.6 deg
(1.3 Mpc, or $\sim$ 2 $\times$ the Fornax cluster virial radius) from the
cluster centre and has a mass of 1.6 $\times$ 1013 M⊙, which is of the same
order of magnitude as the Fornax cluster (Mvir $\sim$ 5 $\times$ 1013 M⊙)
itself (Maddox et al., 2019). Within the virial radius of the group ($\sim$ 1
degree or 0.38 Mpc, as measured by Drinkwater et al., 2001), there are
approximately 70 galaxies (mostly dwarfs) that have been photometricially
identified as likely group members (Venhola et al., 2019), of which 13 have
confirmed spectroscopic redshifts (Maddox et al., 2019).

The brightest group galaxy (BGG), NGC 1316, is a peculiar early-type galaxy
with a stellar mass of 6 $\times$ 1011 M⊙ (Iodice et al., 2017). NGC 1316 is a
giant radio galaxy (Ekers et al., 1983; Fomalont et al., 1989; McKinley et
al., 2015; Maccagni et al., 2020), known merger remnant, and the brightest
galaxy in the Fornax cluster volume (even brighter than the brightest cluster
galaxy NGC 1399). There are a number of extended stellar loops and streams in
NGC 1316 that are a result of a 10:1 merger that occurred 1 – 3 Gyr ago,
between a massive early-type galaxy and a gas-rich, late-type galaxy
(Schweizer, 1980; Mackie & Fabbiano, 1998; Goudfrooij et al., 2001; Iodice et
al., 2017; Serra et al., 2019). The majority of the remaining bright ($m_{B}$
¡ 16) galaxies are late types that have stellar mass ranges of 8 $<$
log(M⋆/M⊙) $<$ 10.5 (Raj et al., 2020).

There have been a variety of previous studies that have detected H i in the
Fornax A group. Horellou et al. (2001) and Serra et al. (2019) imaged the
central region of the Fornax A group in H i, where the more recent image of
Serra et al. (2019) detected NGC 1316, NGC 1317, NGC 1310, and ESO 301-IG 11,
along with four clouds at the outskirts of NGC 1316 (EELR, SH2, CN,1, and
CN,2), and two tails (TN and TS). The remaining six galaxies, which have
previously been detected, are NGC 1326, NGC 1326A ,and NGC 1326B in the H i
Parkes All Sky Survey (HIPASS; Meyer et al., 2004; Koribalski et al., 2004),
NGC 1316C with the Nançay telescope (Theureau et al., 1998), FCC 35 with the
Australian Telescope Compact Array (ATCA) and the Green Bank Telescope (Putman
et al., 1998; Courtois & Tully, 2015), and FCC 46 with the ATCA (De Rijcke et
al., 2013). Within NGC 1316, H i has been resolved in the centre and
correlates with massive amounts of molecular gas (Morokuma-Matsui et al.,
2019; Serra et al., 2019). H i has also been detected in the outer stellar
halo, within the regions defined by the H$\alpha$ extended emission line
region (EELR; originally discovered by Mackie & Fabbiano, 1998), in the
southern star cluster complex (SH2; Horellou et al., 2001) and in two northern
clouds (CN,1 and CN,2) (Serra et al., 2019). Lastly, $\sim$ 6 $\times$ 108 M⊙
of H i was detected in the IGM, defined as the northern and southern tails (TN
and TS). The tails are ejected H i gas from the NGC 1316 merger and extend up
to 150 kpc from the galaxy centre (Serra et al., 2019).

The Fornax A group is an ideal system to search for pre-processing. Evidence
suggests that the group is in the early stage of assembly (Iodice et al.,
2017; Raj et al., 2020) and is located at the cluster infall distance where
pre-processing is thought to occur (Lewis et al., 2002; Gómez et al., 2003;
Verdugo et al., 2008; Mahajan et al., 2012; Haines et al., 2015). The BGG is
massive enough to experience efficient pre-processing (Donnari et al., 2020)
and Raj et al. (2020) show that there are signatures of pre-processing in the
group; six of the nine late types have an up-bending (type III) break in their
radial light profile. This indicates that the star formation may be halting in
the outer disc of galaxies, although, what is driving the decline in star
formation is not yet clear.

##  3 Observations and data reduction

###  3.1 MeerKAT radio observation

Commissioned in July 2018, MeerKAT is a new radio interferometer and a
precursor for the Square Kilometre Array SKA1-MID telescope (Jonas, 2016;
Mauch et al., 2020). MeerKAT is designed to produce highly sensitive radio
continuum and H i images with good spatial and spectral resolution in a
relatively short amount of observing time. The MeerKAT Fornax Survey (MFS; PI:
P.Serra) is one of the designated Large Survey Projects (LSPs) of the MeerKAT
telescope. The MFS will observe the Fornax galaxy cluster in H i over a wide
range of environment densities, down to a column density of a few $\times$
1019 atoms cm-2 at a resolution of 1 kpc, equivalent to a H i mass limit of 5
$\times$ 105 M⊙ (Serra et al., 2016).

The Fornax A group was observed with MeerKAT in two different commissioning
observations in June 2018, which differ by the number of antennas (36 and 62,
respectively) connected to the correlator. We present the details of these
observations and of the H i cube in Table 1. The MeerKAT baselines range
between 29 m and 7.7 km and for both these observations, the SKARAB correlator
in the 4k mode was used, which consists of 4096 channels in full polarisation
in the frequency range 856-1712 MHz with a resolution of 209 kHz (equivalent
to 44.1 km s-1 for H i at the distance of the Fornax cluster).

The first observation (referred to as Mk-36) used 36 antennas and observed the
target for a total of 8 h. Results from this observation are presented both in
radio continuum (Maccagni et al., 2020) and in H i (Serra et al., 2019); these
papers provide a detailed description of the data reduction process. In this
work, we use the Mk-36 calibrated measurement set in combination with that
from the second observation (detailed below).

The second observation (Mk-62) used 62 antennas and observed the target for a
total of 7 h. PKS 1934-638 and PKS 0032-403 were observed, where the former
was observed for 20 min and used as the bandpass and flux calibrator while the
latter was observed for 2 min every 10 min and used as the gain calibrator.

Table 1: Observation and H i cube properties. The measurements of the H i cube RMS noise and column density (over a single channel of 44.1 km s-1) were taken in the pointing centre and restoring beam was taken from the centre channel. Property | Mk-36 observation | Mk-62 observation
---|---|---
Date | 2 June 2018 | 16 June 2018
ID | 20180601-0009 | 20180615-0039
Time on target | 8 hr | 7 hr
Number of antennas | 36 | 62
Pointing centre (J2000) | 03h 22m 41.7s, -37d 12′ 30.0″
Available bandwidth | 856 - 1712 MHz
H i cube frequency range | 1402 - 1420 MHz
H i cube spectral resolution | 209 kHz (44.1 km s-1 at z $\sim$ 0)
H i cube pixel size | 6.5″
H i cube weight | robust = 0.5 and 20″taper
H i cube RMS noise | 90 $\mu$Jy beam-1
H i cube restoring beam | 33.0″$\times$ 29.2″
3$\sigma$ H i column density | 1.4 $\times$ 1019 atoms cm-2

We used the Containerised Automated Radio Astronomical Calibration
(CARACal111https://caracal.readthedocs.io; Józsa et al., 2020) pipeline to
reduce the MeerKAT observations. The pipeline uses
Stimela222https://github.com/SpheMakh/Stimela, which containerises different
open-source radio interferometry software in a Python framework. This makes
the pipeline both flexible and highly customisable and has been used to reduce
MeerKAT and other (e.g. Jansky Very Large Array) interferometric observations
(e.g. see Serra et al., 2019; Maccagni et al., 2020; Ramatsoku et al., 2020;
Ianjamasimanana et al., 2020).

We used CARACal to reduce the Mk-62 observation end-to-end and include the
already reduced Mk-36 observation (Serra et al., 2019; Maccagni et al., 2020)
at the spectral line imaging step. For the Mk-62 observation, we used 120
(1330 - 1450) MHz of bandwidth to ensure adequate continuum imaging and
calibration. We used 18 (1402 - 1420) MHz, which easily covers the group
volume, for the (joint) spectral line imaging.

Our choice of data reduction techniques and steps is outlined using CARACal as
follows: First, we flag the radio frequency interference (RFI) in the
calibrators data based on the Stokes Q visibilites using AOflagger (Offringa
et al., 2012). Then, a time-independent, antenna-based, complex gain solution
was derived for the bandpass using CASA bandpass and the flux scale was
determined with CASA gaincal. A Frequency-independent, time-dependent,
antenna-based complex gains were determined using CASA gaincal. The gain
amplitudes were scaled to bootstrap the flux scale with CASA fluxscale, and
the bandpass and complex gain solutions were applied to the target
visibilities using CASA applycal. The RFI in the target data was then flagged
based on the Stokes Q visibilites, using AOflagger (Offringa et al., 2012). We
imaged and self-calibrated the continuum emission of the target with WSclean
(Offringa et al., 2014; Offringa & Smirnov, 2017) and CUBICAL (Kenyon et al.,
2018), respectively. This process was repeated two more times, in which each
self-calibration iteration was frequency-independent and solved only for the
gain phase, with a solution interval of 2 min. The final continuum model was
subtracted from the visibilities using CASA msutils. The visibilities from
both the Mk-36 and Mk-62 calibrated measurement sets were then Doppler
corrected into the barycentric rest frame using CASA mstransform. Residual
continuum emission in the combined measurement set was removed by fitting and
subtracting a second order polynomial to the real and imaginary visibility
spectra with CASA mstransform. Then, we created a H i cube by imaging the H i
emission with WSclean (Offringa et al., 2014; Offringa & Smirnov, 2017) and
made a 3D mask through source finding with the Source Findina Application
(SoFiA; Serra et al., 2015). This was then used as a clean mask to image a new
H i cube with higher image fidelity. Finally, we applied the primary beam
correction of Mauch et al. (2020) down to a level of 2%, which corrects for
the sensitivity response pattern of MeerKAT.

Our H i cube was imaged333The deep H i imaging revealed periodic, artefacts
caused by the correlator during this time of commissioning. The artefacts were
apparent at the sky position of bright continuum emission. We were able to
remove the artefacts by excluding baselines less than 50 m in the cube and 85
m for the single, worst channel. While short baselines are essential for
diffuse emission, this equates to 5 and 22 baselines out of 1891. using an 18
MHz sub-band (centred on NGC 1316) and the basic properties are presented in
Table 1. The root mean square (RMS) noise is 90 $\mu$Jy beam-1, which equates
to a 3$\sigma$ H i column density of 1.4 $\times$ 1019 atoms cm-2 over a
single channel of 44.1 km s-1 at the angular resolution of 33.0″$\times$
29.2″. Compared to Serra et al. (2019), we present an image that is
approximately twice as large and more than twice as sensitive and has
comparable spatial and velocity resolutions.

We searched for H i sources using SoFiA outside the CARACal pipeline. To
ensure that we properly captured H i emission that is diffuse or far from the
pointing centre, we tested different combinations of smoothing kernels and
detection thresholds in the SoFiA smooth + clip algorithm, per-source
integrated signal-to-noise ratio (S/N) thresholds, and reliability thresholds.
Pixels in the H i cube are detected if their value is above a smooth + clip
detection threshold of 3.5 (in absolute value and relative to the cube noise)
for spatial smoothing kernels equal to 1, 2, and 3 times the synthesised beam
in combination with velocity smoothing kernels over a single (i.e. no
smoothing) and three channels. The mean, sum, and maximum pixel value of each
detected source (normalised to the local noise) create a parameter space that
can separate real H i emission from noise peaks (Fig. 1; Serra et al., 2012).
The reliability of each source (defined as the local ratio of positive-to-
negative source density within this 3D parameter space) as well as the
integrated S/N are then used to identify statistically significant, real H i
sources. Our catalogue was created by retaining only sources with an
integrated S/N above 4 and a reliability above 0.65. As shown in Fig. 1, this
selection is purposefully designed to be conservative, ensuring that detected
diffuse H i emission (i.e clouds in the IGM) is clearly real emission and does
not include noise peaks.

However, we found some real H i emission below these thresholds that should be
included in the detection mask. We thus operated on the detection mask using
the virtual reality (VR) software iDaVIE-v (Sivitilli et al. in press) from
the Institute for Data Intensive Astronomy (IDIA) Visualisation Lab (Marchetti
et al., 2020; Jarrett et al., 2020). This allowed us to use a ray marching
renderer (Comrie et al. in prep) to view and interact with our H i cube, while
making adjustments to the mask within a 3D digital immersive setting. We were
able to inspect the mask for any spurious H i emission that was included or
identify real H i emission that was missed. This was accomplished by importing
the detection mask from SoFiA, overlaying it with the H i cube in the VR
environment, and then adjusting the mask using the motion-tracking hand
controllers. As part of this process, we added two sources to the detection
mask within the VR environment by marking zones where emission was clearly
present.

The two sources added in VR were originally excluded from the detection mask
because they are below the reliability threshold of 0.65 (but above the
integrated S/N threshold of 4). These sources are deemed real because they
either coincide with emission at other wavelengths (see below) or are part of
large, coherent H i emission. Following these edits to the detection mask in
VR, we created H i intensity and velocity maps that are presented in the next
section.

Figure 1: Sum of the pixel values as a function of the mean pixel value for
all sources detected with SoFiA. The blue points indicate the positive
detections and the red points indicate the negative detections (Serra et al.,
2012). Detected H i clouds are shown as black crosses. The dotted line shows
the per-source integrated S/N of 4. Only positive sources above this threshold
and with a reliability $>$ 0.65 are retained in our final catalogue. The
chosen integrated S/N of 4 is a conservative threshold as it is closer to area
of parameter space occupied by the most statistically significant detections
(i.e. the positive sources with a high sum/noise for their mean/noise value)
and is clearly above the edge of non-statistically significant detections
(i.e. where the density of positive sources is approximately the same as the
density of negative sources). Owing to this conservative threshold, the
detected H i clouds, while often diffuse, occupy the parameter space of real,
reliable H i emission.

###  3.2 VST H$\alpha$ observation

To generate the H$\alpha$-emission images, we used a combination of H$\alpha$
narrow-band images and $r^{\prime}$ broad-band images both collected using the
OmegaCAM attached to the VST at Cerro Paranal, Chile (PID: 0102.B-0780(A)).
The OmegaCAM is a 32 CCD wide-field camera with a 1$\deg\times$ 1$\deg$ field
of view and a pixel size of 0.21″. We used the NB 659 H$\alpha$ filter with 10
nm throughput, bought by Janet Drew for the VST Photometric H$\alpha$ Survey
(VPHAS; Drew et al., 2014). The imaging was done using large $\approx$ 1 deg
pointings and short 150 s and 180 s exposures in $r^{\prime}$ and H$\alpha$
bands, respectively. This strategy allows us to make accurate sky background
removal by subtracting averaged background models from the science exposures,
and it also reduces the amount of imaging artefacts (such as satellite tracks)
in the final mosaics because those are averaged out when the images are
stacked. The total exposure times in the $r^{\prime}$-band and H$\alpha$-band
were 8 250s and 31 140s, respectively. Similar data reduction and calibration
was done for both $r^{\prime}$-band and H$\alpha$-images. Details of the used
reduction steps are given by Venhola et al. (2018).

As the H$\alpha$ narrow-band images are sensitive both to H$\alpha$ emission
and flux coming from the continuum, we needed to subtract the continuum flux
from the H$\alpha$ images before they can be used for H$\alpha$ analysis. As
the flux in the $r^{\prime}$ band is dominated by the continuum, we use scaled
$r^{\prime}$-band images to subtract the continuum from the H$\alpha$. The
optimal scaling of the $r^{\prime}$-band image was selected by visually
determining the scaling factor that results in a clean subtraction of the
majority of stars and early-type galaxies.

However, there are some caveats in this procedure, which leaves some
systematic over- and under-subtraction in the H$\alpha$ images. If the seeing
conditions or point spread functions (PSFs) differ between the broad- and
narrow-band images there will be some residuals in the continuum subtracted
image. In addition to these residuals caused by the inner parts of the PSF
($\lesssim$ 5″), also the extended outer parts (see Venhola et al., 2018) and
reflection rings of the PSF may leave some features in the images. In the case
of bright, extended, and peaked galaxies such as NGC 1316, these PSF features
are also significant. As the positions of the reflection rings are dependent
on the position of the source on the focal plane they do not overlap precisely
in the narrow- and broad-band images and thus leave some systematic over- and
under- subtractions in the images. These kinds of features are apparent in the
reduced H$\alpha$ emission images.

The over- and under-subtraction artefacts dominate in and around objects with
bright stellar emission. Therefore, NGC 1316 is significantly affected to the
extent that the artefacts obscure real H$\alpha$ emission. To rectify this, we
select a sub-region that includes NGC 1316, NGC 1317, and NGC 1310 and create
a model of the background that is ultimately subtracted from the original
image.

The background model was created by masking the visible, real H$\alpha$
emission, and replaced with the background local median. The masked image is
then filtered with a median filter to eliminate sharp features in the image.
Lastly, the (masked, filtered) background model is subtracted from the
original image.

We repeat this process using the residual image to create an improved mask,
which is then subtracted from the original image. We use a conservative
approach to mask the H$\alpha$ emission, as the aim is to remove the dominant
artefacts and achieve a uniform background throughout the image. We present a
comparison of the images and additional detail in Appendix A.

##  4 H i distribution in the group

In Fig. 2, we present the primary beam-corrected H i column density map as
detected by MeerKAT, overlaid on a $gri$ stacked optical image from the FDS
(Iodice et al., 2016, 2017; Venhola et al., 2018). Our H i image (Fig. 2) is
sensitive to a column density of N${}_{H\,\textsc{i}}$ = 1.4 $\times$ 1019
atoms cm-2 in the most sensitive part (pointing centre), equating to a
3$\sigma$ H i mass lower detection limit 1.7 $\times$ 106 M⊙ for a point
source 100 km s-1 wide.

As a result of the improved sensitivity of our image, in H i we detect 10
galaxies out of the 13 spectroscopically confirmed galaxies (Maddox et al.,
2019), all the previously known clouds and streams, and a new population of
clouds and streams in the IGM. Eleven of our H i detections (10 galaxies and
SH2) have corresponding optical redshifts (Maddox et al., 2019). NGC 1341, FCC
19, and FCC 40 are the 3 galaxies with optical redshifts in which we do not
detect any H i. NGC 1341 is a late-type (SbcII) galaxy with a stellar mass of
5.5 $\times$ 109 M⊙ (Raj et al., 2020), in which H i has beeen previously
detected (Courtois & Tully, 2015). However, NGC 1341 is outside our H i image
field of view and we do not include it in our sample. FCC 40 is a low surface
brightness dwarf (dE4) elliptical (Iodice et al., 2017) and is unlikely to
contain massive amounts of H i. It is also located in a region of the image in
which the sensitivity is 75% worse than the pointing centre, such that we do
not detect H i below 5.6 $\times$ 1019 atoms cm-2. FCC 19 is a dS0 with a
stellar mass of 3.4 $\times$ 108 M⊙ (Iodice et al., 2017; Liu et al., 2019).
As it is near the pointing centre (70 kpc in projection from NGC 1316), we
would expect to detect H i if there were any. However, no H i is detected in
FCC 19 and we discuss the implications of this in section 5.2.

We present the three-colour (constructed using the individual $g$, $r$, and
$i$ images) FDS (Iodice et al., 2016) optical image cutout for each group
galaxy in our sample, which has been overlaid with the H i contours at their
respective column density sensitivity (or upper limit) in Fig. 3. The
integrated H i flux and mass of the H i detections and the basic properties of
the group galaxies within the H i image field of view are presented in Table
2. The velocity field is presented in Fig. 4 and highlights some new large-
scale coherent H i structures, which extend up to $\sim$ 220 kpc in length.

Figure 2: Primary beam-corrected constant H i contours from MeerKAT (blue) overlaid on a FDS (Iodice et al., 2016) $gri$ stacked optical image. The lowest contour represents the 3$\sigma$ column density level of N${}_{H\,\textsc{i}}$ = 1.4 $\times$ 1019 atoms cm-2 over a 44.1 km s-1 channel, where the contours increase by a factor of 3n ($n$ = 0, 1, 2, …). The group galaxies are labelled and the galaxies not detected in H i are outlined by a dashed black ellipse. The grey circles indicate the sensitivity of the primary beam (Mauch et al., 2020) at 50%, 10%, and 2%. The red dashed circle denotes the 1.05 degree (0.38 Mpc) virial radius of the group as adopted in Drinkwater et al. (2001), where the restoring beam (33.0″$\times$ 29.2″) is shown in the bottom left corner and a scale bar indicating 20 kpc at the distance of Fornax A in the bottom right corner. The direction to the Fornax cluster is shown by the black arrow. In H i, we detect 10 (out of 12) galaxies, previously known clouds and streams in the IGM and a population of new H i clouds in the IGM. The previously known IGM H i structures are labelled in Fig. 4 for clarity. Figure 3: Optical three-colour composite of each group galaxy in our sample with overlaid H i contours. The colour image is comprised of the $g$-, $r$-, and $i$-band filters from the FDS (Iodice et al., 2016); the white dashed contour shows the most sensitive, constant column density of N${}_{H\,\textsc{i}}$ = 1.4 $\times$ 1019 atoms cm-2 from Fig. 2 and the blue contours start from the local column density sensitivity (i.e. 1.4 $\times$ 1019 atoms cm-2 scaled by the primary beam response; see top left corner of each cutout) and increase by a factor of 3n with $n$ = 0, 1, 2, …, at each step. For non-detections, the 3$\sigma$ H i column density upper limit over a single channel is shown in red in the top left of the cutout. The restoring beam (33.0″$\times$ 29.2″) is shown in orange in the bottom left corner and a 5 kpc scale bar is shown in the bottom right corner. Table 2: Basic properties of the group galaxies and H i detected sources within the H i image field of view. The primary beam-corrected integrated H i flux, mass, and upper limits are included for all sources while the morphological type, stellar mass, and $g$ – $r$ colour is included for all the galaxies. The H i mass was calculated using a distance of 20 Mpc and the statistical uncertainty of the flux was measured and propagated to the H i mass. The 3$\sigma$ upper limits of the H i flux and mass are calculated for non-detections using the local RMS and a 100 km s-1 wide integrated flux for a point source. All previously known sources are individually identified and the remaining H i IGM detections are summed into the remaining clouds category. The galaxy morphologies are classified in Ferguson (1989), the photometry is used to estimate the stellar mass (with the method of Taylor et al., 2011), and $g$ \- $r$ colours are measured in Raj et al. (2020) for the majority of the galaxies and in Venhola et al. (2018) for FCC 19 and FCC 40. The photometry, $g$ \- $r$ colour, and stellar mass of NGC 1316 are measured independently in Iodice et al. (2017). Source | Integrated flux | H i mass | Morphological type | Stellar mass |  $g$ – $r$
---|---|---|---|---|---
| (Jy km s-1) | (107 M⊙) |  | (109 M⊙) | (mag)
NGC 1310 | 5.13 $\pm$ 0.07 | 48.1 $\pm$ 0.6 | SBcII | 4.7 | 0.6 $\pm$ 0.1
NGC 1316 | 0.72 $\pm$ 0.04 | 6.8 $\pm$ 0.4 | SAB0 | 600 | 0.72 $\pm$ 0.01
NGC 1316C | 0.18 $\pm$ 0.02 | 1.7 $\pm$ 0.2 | SdIII pec | 1.4 | 0.7 $\pm$ 0.1
NGC 1317 | 2.96 $\pm$ 0.02 | 27.8 $\pm$ 0.2 | Sa pec | 17.1 | 0.77 $\pm$ 0.02
NGC 1326 | 24.3 $\pm$ 0.5 | 228 $\pm$ 4 | SBa(r) | 29.4 | 0.62 $\pm$ 0.04
NGC 1326A | 15.2 $\pm$ 0.8 | 142 $\pm$ 8 | SBcIII | 1.7 | 0.5 $\pm$ 0.1
NGC 1326B | 49 $\pm$ 1 | 455 $\pm$ 9 | SdIII | 1.8 | 0.3 $\pm$ 0.1
ESO 301-IG 11 | 1.52 $\pm$ 0.04 | 14.3 $\pm$ 0.4 | SmIII | 2.9 | 0.57 $\pm$ 0.04
FCC 19 |  $<$ 0.03 |  $<$ 0.17 | dS0 | 0.18 | 0.62 $\pm$ 0.04
FCC 35 | 3.51 $\pm$ 0.09 | 33.0 $\pm$ 0.8 | SmIV | 0.17 | 0.2 $\pm$ 0.1
FCC 40 |  $<$ 0.15 |  $<$ 0.72 | dE4 | 0.002 | 0.61 $\pm$ 0.04
FCC 46 | 0.13 $\pm$ 0.03 | 1.2 $\pm$ 0.2 | dE4 | 0.58 | 0.46 $\pm$ 0.01
TN | 2.24 $\pm$ 0.07 | 21.0 $\pm$ 0.7 | - | - | -
TS | 4.86 $\pm$ 0.08 | 45.6 $\pm$ 0.7 | - | - | -
CN,1 | 0.75 $\pm$ 0.05 | 7.0 $\pm$ 0.5 | - | - | -
CN,2 | 0.35 $\pm$ 0.03 | 3.3 $\pm$ 0.3 | - | - | -
EELR | 0.49 $\pm$ 0.02 | 4.6 $\pm$ 0.2 | - | - | -
SH2 | 0.31 $\pm$ 0.02 | 2.9 $\pm$ 0.2 | - | - | -
Remaining clouds | 3.0 $\pm$ 0.2 | 28 $\pm$ 2 | - | - | -
Figure 4: H i velocity field, where the known galaxies and previously detected
clouds and tails in the IGM are labelled. As in Fig. 2, the two galaxies not
detected in H i are outlined by black, dashed ellipses and the direction to
the Fornax cluster is shown by the black arrow. The velocity colour bar is
centred on the systemic velocity of the BGG (NGC 1316) at 1760 km s-1. The
grey circles indicate the sensitivity of the primary beam (Mauch et al., 2020)
at 50%, 10%, and 2%. The red dashed circle denotes the 1.05 degree (0.38 Mpc)
virial radius of the group as adopted in Drinkwater et al. (2001), where the
restoring (33.0″$\times$ 29.2″) beam and scale bar are shown in the bottom
corners. The clouds that make up TN have a new, extended component,
effectively doubling the size compared to its original discovery in Serra et
al. (2019).

###  4.1 Newly detected H i

Our H i image is the widest and deepest interferometric image of the Fornax A
group to date. Naturally, we detect new H i sources, additional H i in known
sources and resolved H i in previously unresolved sources. All the sources are
presented in Table 2, Fig. 2, and 4. As described in Section 2, several
sources in the Fornax A group have been previously detected. The new H i
sources detected in this work are as follows: resolved H i tails associated
with FCC 35, NGC 1310, and NGC 1326; an extension of TN in the form of
additional, coherent clouds; an additional component to TS in the form of a
western cloud; and a population of clouds in the IGM (unlabelled in Fig. 4).

###  4.2 H i in galaxies

We detect H i in ten galaxies, where the H i is well resolved in eight of them
(Fig. 3). Out of those, two galaxies have H i that is confined to the stellar
disc, while the remaining 6 have H i emission that extend beyond the stellar
disc. The two galaxies with unresolved H i are NGC 1316C and FCC 46.

The two well-resolved galaxies with H i confined within the stellar discs are
NGC 1316 and NGC 1317 (Fig. 3). We detected 6.8 $\times$ 107 M⊙ of H i in the
centre of NGC 1316, 60% more H i in the centre than previously detected in
(Serra et al., 2019). The H i has complex kinematics (also seen in the
molecular gas and dust) beyond a uniformly rotating disc. The H i in NGC 1317
is sharply truncated at the boundary of the stellar disc. Given its stellar
mass and morphology, NGC 1317 is H i deficient by at least an order of
magnitude (discussed in detail in section 5.2).

There are six galaxies in the group that have extended H i discs. Three
galaxies (NGC 1326A/B and ESO 301-IG 11) have slightly extended and mostly
symmetric H i discs, while the other three galaxies (FCC 35, NGC 1326, and NGC
1310) have extended H i features that are significantly disturbed and
asymmetric (Fig. 3).

NGC 1326A and B have extended H i discs and although they overlap in
projection, they are separated by $\sim$ 800 km s-1 in velocity. There is no H
i connecting these two galaxies along the line of sight down to a column
density of 2.8 $\times$ 1020 atoms cm-2, which is also confirmed through
visual inspection in virtual reality. Future, more sensitive data from the MFS
(Serra et al., 2016) will unambiguously show whether these galaxies are
interacting or not.

The collisional ring galaxy ESO 301-IG 11 has a slightly extended H i disc,
where the extension is in the south-east direction (away from the group
centre). As suggested by its classification, the H i is likely to have been
tidally disturbed in the collision that formed the ring.

In the three galaxies with disturbed or asymmetric H i discs (detailed below),
strong tidal interactions can be reasonably excluded as the cause, as the deep
$g$-band FDS images show no stellar emission associated with the extended H i
down to a surface brightness of 30 mag arcsec-2. The H i tails and asymmetries
all differ in these galaxies, likely because each galaxy is affected by
different processes, such as gentle tidal interactions, ram pressure, and
accretion.

The dwarf late-type galaxy FCC 35 has a long, asymmetric (kinematically
irregular) H i tail pointing away from the group centre. The two closest
galaxies (spatially with confirmed redshifts) are NGC 1316C and FCC 46, a
dwarf late type and dwarf early type. These two galaxies have unresolved H i
and are more H i deficient than the majority of the group galaxies. Neither a
dynamical interaction between these galaxies nor a hydrodynamical mechanism
(such as ram pressure) can be ruled out as the cause for the long, H i tail of
FCC 35.

NGC 1326 is a barred spiral galaxy with a ring and has clumpy, extended, and
asymmetric H i emission in the south, pointing towards the group centre. The
one-sided H i emission could be indicative of a tidal interaction. However,
this could also be an instrumental effect, as the galaxy is located very far
from the pointing centre and is subjected to a variable sensitivity response.
The southern side (where the H i tail is) is sensitive down to $\sim$ 6.1
$\times$ 1019 atoms cm-2, while the northern side has a lower sensitivity of
$\sim$ 2.3 $\times$ 1020 atoms cm-2. As the tails are diffuse ($<$ 1 $\times$
1020 atoms cm-2), more sensitive observations are needed to determine if NGC
1326 has extended H i emission on the northern side.

Finally, the massive late-type galaxy NGC 1310 is surrounded by H i extensions
and clouds of different velocities, which is unusual, because it is a
relatively (compared to NGC 1317) isolated galaxy, with an undisturbed optical
spiral morphology and a uniformly rotating H i disc. Despite the coarse
velocity resolution, we can determine from our observations that the majority
of the H i extensions and clouds (except for the extended component of the
disc to the south) are not rotating with the disc (Fig. 4) and cover a broad
range ($\sim$ 1450 – 1950 km s-1) in velocity, suggesting that it may be
anomalous H i gas from an external origin. Future data from the MFS (Serra et
al., 2016) with better velocity resolution will clarify this point.

###  4.3 H i in the intra-group medium

We detect a total of (1.12 $\pm$ 0.02) $\times$ 109 M⊙ of H i in the IGM. All
of the previous clouds in Serra et al. (2019) were detected as well as
additional H i in some of these features. We detect new clouds, the majority
residing in the north, with some forming large, contiguous 3D structures.

We searched for any association between the new H i in the IGM and stellar
emission. In particular, as more H i has been detected within the stellar halo
of NGC 1316, we checked for any correlation between the H i and known stellar
loops (Fig. 5). Overall, there is very little, clear association between the H
i in the NGC 1316 halo and its stellar loops. The major exceptions are TS and
its newly detected cloud, as they are fully contained within the SW stellar
loop. The H i in SH2 and EELR may potentially correlate with the stellar loop
L1 and there are some H i cloud (e.g. CN,2) in the north that partially
overlap with the stellar loop L7. Other than examples above, all the remaining
H i in the IGM shows no association with stellar emission.

Figure 5: Low surface brightness (star removed) image of NGC 1316 in $g$-band,
observed with the VST (Iodice et al., 2017). The known (Schweizer, 1980;
Richtler et al., 2014; Iodice et al., 2017) stellar loops are labelled and
outlined by the dashed green lines. The H i is shown by the solid blue
contours and the previously known H i clouds are labelled. The clouds that
make up TS (including the new western H i cloud) overlap with the stellar SW
loop. There is some overlap with some H i clouds in the north (e.g. CN,2 and
the clouds to the west) and the optical loop L7. Overall, there is no
consistent correlation between the stellar loops and the distribution of H i
clouds.

We detect an extension in TN, effectively doubling its length and mass. The
extension smoothly connects in velocity with the previously known emission and
now extends up to $\sim$ 220 kpc from NGC 1316 (Fig. 4), which is where the H
i originated from Serra et al. (2019). The clouds that make up TN now contains
(2.10 $\pm$ 0.07) $\times$ 108 M⊙ of H i. The north and south tails contain
60% (6.7 $\pm$ 0.1 $\times$ 108 M⊙) of the total IGM H i mass. The remaining
clouds in the IGM mostly reside to the north of NGC 1316, with the majority of
these existing over a narrow (90 km s-1) velocity range. It is possible some
of these clouds form large coherent H i structures, although it is not clear
compared to the case of TN and TS. While TN and TS originate from a single
pericentric passage of the NGC 1316 merger (Serra et al., 2019), the remaining
clouds in the IGM are more likely to be the remnants of recently accreted
satellites onto NGC 1316, which is consistent with Iodice et al. (2017).

The clouds immediately to the north-west of NGC 1317 may be a remnant of its
outer disc. These clouds are within a projected distance of 10 kpc from NGC
1317 and the cloud and the galaxy have the same velocity. The H i-to-stellar
mass ratio of the galaxy is low by at least an order of magnitude (see below)
and these clouds alone are not enough to explain the H i deficiency. However,
these are the only clouds that show potential evidence that they originated
from NGC 1317.

All the H i in the IGM located north of the group centre (NGC 1316) and the
clouds to the south-east of ESO 301-IG 11 appear to be decoupled from the
stars. The H i in the south (SH2, TS) has stellar emission associated with it.
Additionally, there are a few H i clouds near to the group centre that contain
multiphase gas.

###  4.4 Multiphase gas in the intra-group medium

In Figure 6, we show the ionized H$\alpha$ gas emission detected in the
vicinity of NGC 1316 (i.e. the group centre). H$\alpha$ is detected in NGC
1316, NGC 1317, and NGC 1310. However, the most striking features are the
H$\alpha$ complexes detected in the IGM.

Figure 6: OmegaCAM H$\alpha$ emission showing the ionised gas in the vicinity
of NGC 1316. The blue contour shows the majority of the western lobe of NGC
1316 in radio continuum at a (conservative) level of 1.3 mJy beam-1 from
Maccagni et al. (2020). The white contours show the 3$\sigma$ H i column
density of 1.4 $\times$ 1019 atoms cm-2 (over 44.1 km s-1) from this work.
Known sources (i.e. galaxies and IGM H i) and multiphase (MP) gas clouds that
contain H$\alpha$ and H i as well as the Ant-like feature from Fomalont et al.
(1989) are labelled. This image reveals long filaments of ionised gas in the
IGM.

There are giant filaments of H$\alpha$ in the IGM stretching between galaxies
of the group. H i is directly associated with some of the ionised gas, showing
the coexistence of multiphase gas in the IGM. These occur in EELR, CN,1, the
cloud directly below CN,1 and in five newly detected clouds containing H i
that we label MP in Fig. 6. Additionally, we detect the “Ant” (or ALF; Ant-
like feature) first detected as a depolarising feature in Fomalont et al.
(1989) and later in H$\alpha$ by Bland-Hawthorn et al. (1995). The H$\alpha$
emission is thought to provide the intervening turbulent magneto-ionic medium
required to depolarize the radio continuum emission Fomalont et al. (1989).
There is no optical continuum emission nor any H i emission currently
associated with the Ant.

While there are a number of multiphase gas clouds in the IGM, the brightest
case is EELR. It is clear that EELR has a complex multiphase nature, with H i,
H$\alpha$, and dust all previously detected in it (Mackie & Fabbiano, 1998;
Horellou et al., 2001; Serra et al., 2019). We detect 50% more H i than the
previous study (Serra et al., 2019) and H i is only present in the region of
the bright, more ordered ionised gas morphology. Given that our H i image is
sensitive to a column density of 1.4 $\times$ 1019 atoms cm-2, it is unlikely
that there is any H i in the less ordered (and likely turbulent) part of EELR.
Currently, the origin of EELR is unclear, and we will present a detailed
analysis of it and its multiphase gas in future work.

##  5 Pre-processing in the group

The Fornax A group is at a projected distance of $\sim$ 1.3 Mpc (approximately
2 virial radii) from the Fornax Cluster centre. Redshift independent distances
are too uncertain to establish whether the group is falling into the cluster.
However, the intact spiral morphologies of group galaxies imply that the group
has not passed the cluster pericentre as spiral morphologies do not typically
survive more than one pericentric passage (e.g. Calcáneo-Roldán et al., 2000).
At this distance, the intra-cluster medium (ICM) of the Fornax cluster should
not have a significant impact on the group galaxies, meaning that quenched
galaxies are a result of pre-processing within the group.

An optical analysis of the radial light profiles of the group galaxies and the
intra-group light (IGL) concluded that the Fornax A group is in an early stage
of assembly (Raj et al., 2020). This is evident from the low level (16%) of
IGL and from the group being dominated by late types with undisturbed
morphologies and comparable stellar masses (Raj et al., 2020).

In this work, we detect H i throughout the Fornax A group both in the galaxies
and the IGM. While the galaxies range from being H i rich to extremely H i
deficient, the majority of the galaxies contain a regular amount of H i for
their stellar mass. This is consistent with the group being in an early phase
of assembly, as the majority of galaxies would be H i deficient for a group in
the advanced assembly stage. The H i detections show evidence of pre-
processing in the form of (2.8 $\pm$ 0.2) $\times$ 108 M⊙ of H i in the IGM, H
i deficient galaxies, truncated H i discs, H i tails, and asymmetries. The
diversity of galaxy H i morphologies suggest that we are observing galaxies at
different stages of pre-processing, as we detail below.

###  5.1 NGC 1316 merger

The most obvious case of pre-processing in the group is NGC 1316, the BGG. It
is a peculiar early type that is the brightest galaxy in the entire Fornax
cluster volume and the result of a 10:1 merger that occurred 1 – 3 Gyr ago
between a massive early-type galaxy and a gas-rich late-type galaxy
(Schweizer, 1980; Mackie & Fabbiano, 1998; Goudfrooij et al., 2001; Iodice et
al., 2017; Serra et al., 2019). There are large stellar loops and streams, an
anomalous amount of dust and molecular gas (2 $\times$ 107 and 6 $\times$ 108
M⊙, respectively) in the centre, as well as H i in the centre and in the form
of long tails (Draine et al., 2007; Lanz et al., 2010; Galametz et al., 2012;
Morokuma-Matsui et al., 2019; Serra et al., 2019).

The H i mass budget for a 10:1 merger to produce the features observed in NGC
1316 requires the progenitor to contain $\sim$ 2 $\times$ 109 M⊙ of H i (Lanz
et al., 2010; Serra et al., 2019). Recently, Serra et al. (2019) detected 4.3
$\times$ 107 M⊙ of H i in the centre of NGC 1316, overlapping with the dust
and molecular gas, and a total H i mass of 7 $\times$ 108 M⊙ when including
the tails and nearby H i clouds. While these authors detected an order of
magnitude more H i than previous studies, this is a factor of $\sim$ 3 lower
than expected. In this work, we detect a H i mass in the centre of (6.8 $\pm$
0.4) $\times$ 107 M⊙ and a total H i mass 0.9 – 1.2444The lower limit was
determined by only including the same H i sources as Serra et al. (2019) and
the TN extension, while the upper limit includes the remaining H i clouds in
the IGM $\times$ 109 M⊙ associated with NGC 1316 in the form of streams and
clouds. This brings the observed H i mass budget even closer to the expected
value under the 10:1 lenticular + spiral merger hypothesis – just within a
factor 1.7 - 2.2, which is well within the uncertainties.

Since the merger 1 – 3 Gyr ago, NGC 1316 has been accreting small satellites
(Iodice et al., 2017). The satellites may have contributed to the build up of
H i, however, we do not observe any H i correlated with dwarf galaxies within
150 kpc of NGC 1316. Any contributed H i is second order compared to the
initial merger, which is supported by the H i mass of NGC 1316 being dominated
by the tails. Tidal forces from the initial merger ejected 6.6 $\times$ 108 M⊙
of H i into the IGM in the TN and TS tails alone. The remaining H i in the IGM
is likely to be a combination of gas decoupled from stars in the initial
merger and gas from more recently accreted satellites. H i tidal tails that
span hundreds of kpc in galaxy groups have been shown to survive in the IGM
for the same timescale (1 – 3 Gyr) from when this merger took place (Hess et
al., 2017).

###  5.2 Pre-processing status of the group galaxies

In this section, we identify galaxies at different stages of pre-processing
according to their H i morphology and cool gas (H i and H2) ratios. The
categories are as follows: i) early, where a galaxy has yet to experience
significant pre-processing; ii) ongoing, for galaxies that currently show
signatures of pre-processing; and iii) advanced, for galaxies that have
already experienced significant pre-processing.

There are a total of 12 galaxies in the sample, which are all the
spectroscopically confirmed galaxies within the H i image field of view. In
our sample, 10 galaxies have H i detections and 2 galaxies (FCC 19 and FCC 40)
have H i upper limits (Fig. 3). There are 7 galaxies that have been observed
with ALMA. The 5 galaxies that were not observed are ESO 301-IG 11, FCC 19,
FCC 35, FCC 40, and FCC 46 (Morokuma-Matsui et al., 2019, Morokuma-Matsui et
al. in prep). We measure the molecular gas mass of the observed galaxies using
the standard Milky Way CO-to-H2 conversion factor of 4.36 (M⊙ K km s-1 pc-2)-1
(Bolatto et al., 2013) as well as estimated stellar masses (Table 2) from Raj
et al. (2020) and Venhola et al. (2018), which are derived from the $g$ and
$i$ photometric relation in Taylor et al. (2011). We remove the helium
contribution from our molecular gas masses so that we are measuring the
molecular-to-atomic hydrogen gas mass (except in the total gas fraction, shown
below) and can directly compare our findings to Catinella et al. (2018).

We present the H i and H2 scaling ratios in Fig. 7. We measure the H i gas
fraction FHI $\equiv$ log(MHI/M⋆), the total gas fraction Fgas $\equiv$
log(1.3(MHI \+ MH2)/M⋆) where the 1.3 accounts for the helium contribution,
the molecular-to-atomic gas mass ratio Rmol $\equiv$ log(MH2/MHI), and the H2
gas fraction FH2 $\equiv$ log(MH2/M⋆). We compare the H i fraction of our
galaxies to those in the Herschel Reference Survey (HRS; Boselli et al., 2010,
2014) and the Void Galaxy Survey (VGS; Kreckel et al., 2012), which span a
comparable stellar mass range of our galaxies. We also compare FHI to the
median trend of the extended GALEX Arecibo SDSS Survey (xGASS; Catinella et
al., 2018). Furthermore, we compare our molecular gas scaling relations to the
median trends of xGASS-CO (Fig. 7), which are xGASS galaxies with CO
detections (Catinella et al., 2018). The xGASS and xGASS-CO trends provide a
good reference for the H i and H2 scaling relations in the local Universe as
the median FHI trend was derived from 1179 galaxies selected with 109 $<$ M⋆
(M⊙) $<$ 1011.5 and 0.01 $<$ z $<$ 0.05, and the H2 mass and scaling relations
derived using a subset 477 galaxies from the parent sample that have CO
detections.

Figure 7: Atomic and molecular gas scaling ratios. In all figures, the early,
ongoing, and advanced pre-processing categories are shown as blue circles,
green squares, and red diamonds, respectively and H2 upper limits are depicted
by arrows. Solid markers indicate H i detections and open markers are non-
detections. FCC 40 is not assigned to any pre-processing category and is shown
as the open black star. _Top left panel_ : The H i gas fraction compared to
galaxies from the HRS (Boselli et al., 2010, 2014) and VGS (Kreckel et al.,
2012) (grey points) that show the typical scatter in FHI. The orange shaded
region indicates the median trend from xGASS (Catinella et al., 2018). _Top
right panel_ : The total gas fraction of our galaxies compared to the median
xGASS-CO trend (Catinella et al., 2018) (orange shaded region). _Bottom left
panel:_ The molecular-to-atomic-gas ratio of our galaxies compared to the
median xGASS-CO trend (Catinella et al., 2018) (orange shaded region). _Bottom
right panel_ : The H2 gas fraction as a function of H i gas fraction, showing
constant ratios of 100%, 30%, 10%, and 3%. Overall, the galaxies in the early
category are H i rich, the galaxies in the ongoing category typically follow
the xGASS and xGASS-CO median scaling relations (Catinella et al., 2018),
while galaxies in the advanced category have no H i or are H i-deficient with
irregularly high H2-to-H i ratios.

The two galaxies that show no signatures (i.e. in the early phase) of pre-
processing are NGC 1326A and NGC 1326B. They are H i rich galaxies with
typical extended H i discs and a low molecular gas content. Both galaxies were
observed with ALMA (Morokuma-Matsui et al., 2019, Morokuma-Matsui et al. in
prep), although no CO was detected, placing upper limits on the H2 mass. They
have the highest H i fraction and lowest H2-to-H i ratios given their stellar
mass (Fig. 7). The galaxies are just within the virial radius of the group,
making them furthest from the group centre in projected distance. This
increases the likelihood that the galaxies have not undergone pre-processing
yet.

The galaxies that show current signatures of pre-processing (i.e. the ongoing
category) are FCC 35, ESO 301-IG 11, NGC 1310, NGC 1316, and NGC 1326. In
general, these galaxies have H i tails or asymmetric extended H i emission,
typical H i and H2 ratios (for the galaxies with H2 observations) that follow
the median xGASS trends in Fig. 7. The exception to this is NGC 1316. As this
galaxy is the BGG, it has a unique formation and evolution history (discussed
in section 5.1) that displays both an ongoing (e.g. tidal tails) and advanced
state (giant elliptical with a lack of H i contained in the stellar body) of
pre-processing. In this work, we include this galaxy in the ongoing category,
although the H i mass range calculated in section 5.1 reflects that it could
also be part of the advanced category.

FCC 35 is the bluest galaxy (Fig. 3 and Table 2) in the group (Raj et al.,
2020) and has extremely strong and narrow optical emission lines that classify
it as either a blue compact dwarf or an active star-burst HII galaxy (Putman
et al., 1998). Previous studies (i.e. Putman et al., 1998; Schröder et al.,
2001) detected a H i cloud associated with FCC 35 and suggested it may be a
result of a tidal interaction with the nearest (projected separation of 50
kpc) neighbour NGC 1316C. This is a plausible scenario as FCC 35 has an up-
bending (Type-III) break in the stellar radial profile, and a bluer outer
stellar disc (Raj et al., 2020), which could be tidally induced star
formation. However, the star formation could also be compression/shock induced
(Raj et al., 2020). We detect the H i cloud of FCC 35 as part of a long tail
pointing away from the group centre, making it the most likely galaxy to show
evidence of ram pressure stripping. The lower IGM (compared to the ICM)
density means that ram pressure stripping is less prevalent in groups. Despite
the observational challenges, a few cases have been reported (e.g. Westmeier
et al., 2011; Rasmussen et al., 2012; Vulcani et al., 2018; Elagali et al.,
2019) and ram pressure is thought to play an important role in the pre-
processing of galaxies in groups. FCC 35 is not H i deficient (Fig. 7),
implying that the gas has recently been displaced, similar to other galaxies
showing early signs of gas removal (e.g. Ramatsoku et al., 2020; Moretti et
al., 2020).

ESO 301-IG 11 is a collisional ring galaxy with a H i gas fraction below the
median trend, although it is not the most H i deficient galaxy for its stellar
mass. There is clear evidence of a tidal interaction in the form of irregular
optical morphology, an up-bending (Type-III) break in the stellar radial
profile and a slightly extended and asymmetric H i disc. The galaxy is blue in
colour, although the outer stellar disc is redder than the inner disc (Raj et
al., 2020), implying that the tidal interaction may have restarted star
formation in the centre.

The asymmetric H i tail of NGC 1326 is diffuse ($<$ 1 $\times$ 1020 atoms
cm-2) and only detected on one side of the galaxy. The sensitivity of the
opposing side prevents us from detecting H i that diffuse, and we are
therefore unable to distinguish whether the extended H i is part of a regular
extended H i disc or a signature of pre-processing. With the current H i
content, it follows the same H i and H2 trends as the other galaxies in the
ongoing category.

The optical morphology and gas scaling relations of NGC 1310 suggest that it
is not being pre-processed. The stellar spiral structure is completely intact
(Fig. 3), ruling out strong tidal interactions and the H i gas fraction and
molecular-to-atomic gas ratios are close to the median trends. However, the H
i morphology appears complex and incoherent, with many asymmetric extensions
and nearby clouds at different velocities. It is clear that the anomalous H i
clouds and extensions are not rotating with the main H i disc (Fig. 4),
suggesting external origins. The H i extension in the north-west may be
emission from a dwarf satellite galaxy, although a spectroscopic redshift
would be required to confirm this. Given the presence of the H$\alpha$
filaments in the vicinity of NGC 1310, the remaining clouds may be a result of
hot gas, cooling in the IGM (and hot halo of NGC 1316) and being captured or
accreted onto this galaxy.

Finally, the galaxies that are in the advanced stage of pre-processing are NGC
1316C, NGC 1317, FCC 19, and FCC 46. There is no H i detected in FCC 19, and
the other three galaxies have truncated H i discs and are H i deficient as
their FHI is more than 3$\sigma$ from the xGASS median trend (Fig. 7).

NGC 1316C and NGC 1317 have a low H i mass fraction and regular H2 mass
fraction. The total gas fraction of these galaxies is low and is driven by the
lack of H i. Hence, they have significantly more H2 than H i and a molecular-
to-atomic fraction an order of magnitude higher (the highest in our sample)
than the median trend (Fig. 7). Both these galaxies have no break (Type-I) in
their stellar radial profile (Raj et al., 2020), showing no sign of disruption
to their stellar body and their H i confined to the stellar disc, implying
that the outer H i disc has been removed. Ram pressure or gentle tidal
interactions are likely to be responsible for removing the outer H i disc of
these galaxies. The less dense (compared to the ICM) IGM combined with the
group potential allows galaxies to hold on to their gas more effectively than
in clusters (Seth & Raychaudhury, 2020). The retained atomic gas within the
stellar body can then be converted into molecular gas. This scenario is
consistent with the findings of the GAs Stripping Phenomena in galaxies with
MUSE (GASP; Moretti et al., 2020) project, where pre-processed galaxies in
groups (and clusters) have their outer H i removed (via ram pressure) and the
remaining H i is efficiently converted into H2. These galaxies in the advanced
stage of pre-processing with truncated H i discs and regular amounts of H2 are
similar to some galaxies in the Virgo (Cortese et al., 2010) and Fornax
cluster (Loni et al., 2021). This suggests that late-type galaxies that have
been sufficiently processed lose their outer H i disc and end up with more H2
than H i.

Despite the similarities between NGC 1316C and NGC 1317, these galaxies have
likely been pre-processed on different timescales. The stellar mass of NGC
1316C is more than an order of magnitude lower than that of NGC 1317 and
according to Raj et al. (2020), NGC 1316C only recently ($<$ 1 Gyr) became a
group member while NGC 1317 may have been a group member for up to 8 Gyr.
There is no star formation beyond the very inner ($<$ 0.5′) disc of NGC 1317
(Raj et al., 2020) and even though there is only a projected separation of
$\sim$ 50 kpc between NGC 1316 and NGC 1317, a strong tidal interaction can be
reasonably excluded due to the intact spiral structure of NGC 1317 (Richtler
et al., 2014; Iodice et al., 2017). The outer H i disc has been removed and
possibly lost to the IGM (i.e. potentially identified as the adjacent clouds
at the same velocity) as a result of gentle tidal or hydrodynamical
interactions. Alternatively, the outer disc may have been converted to other
gaseous phases on short timescales ($<$ 1 Gyr). While we are unable to
identify the exact mechanisms that are responsible for the truncated H i disc
of NGC 1317, it is evident that the galaxy has not had access to cold gas over
long timescales.

Out of all the galaxies with H i, FCC 46 is the most H i deficient given its
stellar mass. It is a dwarf elliptical with a recent star formation event and
H i was first detected as a polar ring orbiting around the optical minor axis
by De Rijcke et al. (2013). As the H i is kinematically decoupled from stellar
body, the gas was likely accreted from an external source (De Rijcke et al.,
2013). Our measured H i mass (Table 2) is consistent with that from De Rijcke
et al. (2013), although, as a result of our sensitivity at that position, we
do not detect the diffuse H i component that shows the minor axis rotation. A
minor merger event (e.g. with a dwarf late type) is consistent with the
morphology and $\sim$ 107 M⊙ of H i found in FCC 46.

FCC 19 is a dwarf lenticular galaxy (Fig. 3) with a stellar mass of 3.4
$\times$ 108 M⊙ (Liu et al., 2019). It has a $g$ – $r$ colour of 0.58 (Iodice
et al., 2017), which is similar to the colour of NGC 1310, NGC 1326, NGC
1326A, and ESO 301-IG 11 (Table 2), which have regular H i fractions and are
likely forming stars. However, no H i is detected in FCC 19 and we measure a
3$\sigma$ FHI upper limit of -2.3 (Fig. 7) assuming a 100 km s-1 line width.
FCC 19 is situated in the most sensitive part of the image, meaning that the
galaxy truly does not contain H i. Being so close (70 kpc in projection) to
NGC 1316, the tidal field and hot halo of NGC 1316 are likely to have played
significant roles in removing the H i from FCC 19. The H i has likely been
stripped from the galaxy and lost to the IGM. The stripped H i may also be
potentially heated and prevented from cooling.

Lastly, we refrain from assigning a category to FCC 40 because we are unable
to ascertain whether the galaxy properties are a result of secular evolution
or have been influenced by pre-processing. This galaxy is a low surface
brightness (Fig. 3), low-mass (M⋆ = 2.3 $\times$ 106 M⊙) blue dwarf elliptical
(Table 2) with no H i detected. We place an upper limit on the H i mass (and H
i fraction), although it is currently unknown if galaxies of this mass,
colour, and morphology are expected to contain H i.

We show the spatial distribution of each group galaxy and their pre-processing
status in Fig. 8. The distribution shows a variety of pre-processing stages
mixed throughout the group, with no clear radial dependence. The majority of
on-going and advanced pre-processing are $<$ 0.5 of the group virial radius,
although there are galaxies (i.e. FCC 46 and NGC 1326) that have the same pre-
processing status and are located closer to edge of the group, $>$ 0.5 of the
group virial radius. At a distance of $\sim$ 2 (cluster) virial radii from the
Fornax cluster, the Fornax group is located at the distance where pre-
processing is thought to be the most efficient (Lewis et al., 2002; Gómez et
al., 2003; Verdugo et al., 2008; Mahajan et al., 2012; Haines et al., 2015).
In general, it is not clear whether the pre-processing at this infall distance
is driven by the group interacting with the cluster, or by local (e.g. tidal
and hydrodynamical) interactions within the group. In this instance, it
appears that pre-processing is driven by local interactions within the Fornax
A group for the following reasons: i) The massive, central galaxy is at least
one order of magnitude more massive (Table 2) than the satellite galaxies. ii)
This central galaxy underwent a merger 1 – 3 Gyr ago (discussed in Section
5.1). iii) The majority of galaxies close to the group centre ($<$ 0.5 of the
group virial radius) show evidence of pre-processing, while the two galaxies
(NGC 1326 A/B) closest to the Fornax cluster (and furthest from the group
centre) show no evidence of pre-processing. In addition to these points, there
are four galaxies (NGC 1310, NGC 1317, ESO 301-IG 11, and FCC 19) that
spatially overlap (in projection) with the radio lobes of NGC 1316 (Fig. 8)
and therefore may be influenced by the AGN (e.g Johnson et al., 2015).

Figure 8: Pre-processing map of the Fornax A group. The background image shows
the 1.44 GHz MeerKAT radio continuum emission (Maccagni et al., 2020) and the
position of each group galaxy are overlaid with the same markers as Fig. 7.
The filled markers represent H i detections, the open markers indicate H i
non-detections, where the early, ongoing, advanced, and unclassified pre-
processing categories are shown as blue circles, green squares, red diamonds,
and black stars, respectively. The red dashed circle denotes the 1.05 degree
(0.38 Mpc) virial radius of the group as adopted in Drinkwater et al. (2001).
A 20 kpc scale bar is shown in the bottom right corner and the direction to
the Fornax cluster is shown by the black arrow. There is no consistent trend
between projected position and pre-processing status, although the majority of
group galaxies show evidence of pre-processing. The extent of the NGC 1316 AGN
lobes show that it may be playing a role in the pre-processing of neighbouring
galaxies and the magnetic field could help the containment of multiphase gas.

###  5.3 Gas in the IGM

The H i tails and clouds in the IGM are a direct result of galaxies having
their H i removed through hydrodynamical and tidal interactions over the past
few Giga-years. As described in sections 4.3 and 5.1, the majority (if not
all) of the H i in the IGM is due to the Fornax A merger and the recent
accretion of satellites.

The amount (1.12 $\pm$ 0.2 $\times$ 109 M⊙) of detected H i in the IGM is not
enough to account for all of the missing H i in the H i deficient group
galaxies. However, the outer parts of the image are subject to a large primary
beam attenuation and some of the IGM H i may be hiding in the noise. We
estimate the amount of H i potentially missed by assuming that we detect all H
i in the IGM in the inner 0.1 deg2 (primary beam response $>$ 90%) and that
the IGM H i in this area is representative of the IGM throughout the entire
group both in terms of amount of H i per unit area and H i column density
distribution. Under this assumption, the primary beam attenuation reduces the
detected H i by a factor of $\sim$ 2.3, implying that we may be missing up to
$\sim$ 1.5 $\times$ 109 M⊙ of H i in the IGM. All the (including the missed) H
i in IGM is still not enough to explain all the H i deficient galaxies in the
group and clearly gas exists in other phases (i.e. H2 and H$\alpha$). Some of
the H i in the galaxies has been converted into H2, which explains why the
more advanced pre-processed galaxies that have H i, display high molecular-to-
atomic gas ratios, and there is H$\alpha$ in galaxies and the IGM.

Currently, the origin of giant ionised gas filaments in the IGM is not well
understood. However, they are typically observed in high-mass groups or low-
mass clusters (e.g. halo masses $>$ 1013.5 M⊙), for example the Virgo cluster
(Kenney et al., 2008; Boselli et al., 2018b, a; Longobardi et al., 2020) and
the Blue Infalling Group (Cortese et al., 2006; Fossati et al., 2019); see
Yagi et al. (2017) for a list of clusters that contain long ionised gas
filaments. A likely scenario is that cool gas is stripped from an in-falling
galaxy, and subsequently ionised, possibly from ionising photons originating
from star-forming regions (Poggianti et al., 2018; Fossati et al., 2019) or
through non-photo-ionisation mechanisms such as shocks, heat conduction, and
magneto-hydrodynamic waves (Boselli et al., 2016). We use the relation in
Barger et al. (2013) to estimate the total H$\alpha$ mass in the IGM (i.e.
EELR, SH2, and the filaments) from our H$\alpha$ photometry (Fig. 6). Assuming
a typical H$\alpha$ temperature of 104 K and electron density of 1 cm-3, we
estimate the total H$\alpha$ mass in the IGM to be $\sim$ 2.6 $\times$ 106 M⊙,
which does not significantly contribute to the total gas budget in the IGM.

Simulations show that $\sim$ 104 K (i.e. relatively cool) gas clouds can
survive in hot haloes (such as NGC 1316) for cosmological timescales (Nelson
et al., 2020). The clouds originate from satellite mergers, and are not in
thermal equilibrium, but rather magnetically dominated. Cooling is triggered
by the thermal instability and the cool gas is surrounded by an interface of
intermediate temperature gas (Nelson et al., 2020). These ingredients can
explain how multiphase gas clouds are present in the hot halo of NGC 1316
(Fig. 6), such that the H$\alpha$ filaments are a result of satellite
accretion and the H i has rapidly cooled from these structures, with the
ability to survive in the IGM for cosmological timescales.

Recently, Müller et al. (2020) suggest that magnetic fields of the order of 2
– 4 $\mu$G can shield H$\alpha$ and H i in the ICM / IGM such that the gas
clouds do not dissipate. As the H$\alpha$ filaments and multiphase gas clouds
are within the radio lobes (in projection) of NGC 1316, the magnetic field of
the lobes (measured to be $\sim$ 3 $\mu$G by McKinley et al., 2015; Anderson
et al., 2018; Maccagni et al., 2020) may be providing additional stability for
the H$\alpha$ and H i to survive. Indeed, the Ant detected by Fomalont et al.
(1989) and Bland-Hawthorn et al. (1995) is a small portion of the giant
H$\alpha$ filaments in the IGM. Even though there is currently no H i
associated with the Ant, other sections of the H$\alpha$ filaments show that
neutral and ionised gas can coexist in some regions of the IGM, possibly
transform into one another, and accrete onto group galaxies (e.g. NGC 1310).

##  6 Conclusions

We present results from MeerKAT H i commissioning observations of the Fornax A
group. Our observations are reduced with the CARACal pipeline and our H i
image is sensitive to a column density of 1.4 $\times$ 1019 atoms cm-2 in the
field centre. Out of 13 spectroscopically confirmed group members, we detect H
i in 10 and report an H i mass upper limit for 2 (the remaining galaxy is
outside the field of view of our observation). We also detect H i in the IGM,
in the form of clouds, some distributed along coherent structures up to 220
kpc in length. The H i in the IGM is the result of a major merger occurring in
the massive, central galaxy NGC 1316, 1 – 3 Gyr ago, combined with H i being
removed from satellite galaxies as they are pre-processed.

We find that 9 out of the 12 galaxies show some evidence of pre-processing in
the form of H i deficient galaxies, truncated H i discs, H i tails, and
asymmetries. Using the H i morphology and the molecular-to-atomic gas ratios
of the galaxy, we classify whether each galaxy is in the early, ongoing, or
advanced stage of pre-processing.

Finally, we show that there are giant H$\alpha$ filaments in the IGM, within
the hot halo of NGC 1316. The filaments are likely a result of molecular gas
being removed from a satellite galaxy and then ionised. We observe a number of
H i clouds associated with the ionised H$\alpha$ filament, indicating the
presence of multiphase gas. Simulations show that hot gas can condense into
cool gas within hot haloes and survive for long periods of time on a
cosmological timescale, which is consistent with the cool gas clouds we detect
within the hot halo of NGC 1316. The multiphase gas is supported by magnetic
pressure, implying that the magnetic field in the lobes of the NGC 1316 AGN
might be playing an important role in maintaining these multiphase gas clouds.
The cycle of AGN activity and cooling gas in the IGM could ultimately result
in the cool gas clouds falling back onto the central galaxy. We summarise our
main findings as follows:

  1. 1.

We present new, resolved H i in FCC 35, NGC 1310, and NGC 1326.

  2. 2.

There is a total of(1.12 $\pm$ 0.02) $\times$ 109 M⊙ of H i in the IGM, which
is dominated by TN and TS (combined H i mass of 6.6 $\times$ 108 M⊙). We
detect additional components in both tails, an extension in TN, effectively
doubling its length, and a cloud in TS that shows coherence with the stellar
south-west loop.

  3. 3.

The H i in the IGM is decoupled from the stars, other than in TS and SH2.

  4. 4.

We measure 0.9 – 1.2 $\times$ 109 M⊙ of H i associated with NGC 1316, bringing
the observed H i mass budget within a factor of $\sim$ 2 of the expected value
for a 10:1 lenticular + spiral merger occurring $\sim$ 2 Gyr ago.

  5. 5.

Out of the 12 group galaxies in our sample, 2 (NGC 1326A and NGC 1326B) are in
the early phase of pre-processing, 5 (FCC 35, ESO 301-IG 11, NGC 1310, NGC
1316, and NGC 1326) are in the ongoing phase of pre-processing, 4 (NGC 1316C,
NGC 1317 FCC 19, and FCC 46) are in the advanced stage of pre-processing, and
1 (FCC 40) remains unclassified.

  6. 6.

Galaxies that are yet to be pre-processed have a typical extended H i disk,
high H i content, and molecular-to-atomic gas ratios at least an order of
magnitude below the median trend for their stellar mass. Galaxies that are
currently being pre-processed typically display H i tails or asymmetric
extended disks, while containing regular amounts of H i and H2. Galaxies in
the advanced stage of pre-processing have no H i or have lost their outer H i
and are efficiently converting their remaining H i to H2.

  7. 7.

We detect the Ant first observed by Fomalont et al. (1989) as a depolarising
feature and later in H$\alpha$ by Bland-Hawthorn et al. (1995), which turns
out to be a small part of long, ionised H$\alpha$ filaments in the IGM.
Localised cooling (potentially assisted by the magnetic field in the lobes of
the NGC 1316 AGN) can occur in the H$\alpha$ filaments to condense and form H
i.

In this work, our deep MeerKAT H i image shows many examples of pre-processing
in the Fornax A group, such as galaxies with a variety of atypical
morphologies and massive amounts of H i in the IGM. The improved sensitivity
and resolution of the MFS (Serra et al., 2016) will likely reveal more H i
throughout the group and provide kinematic information for the H i in galaxies
and the IGM.

###### Acknowledgements.

The MeerKAT telescope is operated by the South African Radio Astronomy
Observatory, which is a facility of the National Research Foundation, an
agency of the Department of Science and Innovation. We are grateful to the
full MeerKAT team at SARAO for their work on building and commissioning
MeerKAT. This paper makes use of the following ALMA data:
ADS/JAO.ALMA#2017.1.00129.S. ALMA is a partnership of ESO (representing its
member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST
and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the
Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO,
and NAOJ. This work also made use of the Inter-University Institute for Data
Intensive Astronomy (IDIA) visualisation lab (https://vislab.idia.ac.za). IDIA
is a partnership of the University of Cape Town, the University of Pretoria
and the University of Western Cape. 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). The research of OS is supported by the South African Research Chairs
Initiative of the Department of Science and Innovation and the National
Research Foundation. KT acknowledges support from IDIA. The work of KMM is
supported by JSPS KAKENHI Grant Number of 19J40004. RFP acknowledges financial
support from the European Union’s Horizon 2020 research and innovation program
under the Marie Skłodowska-Curie grant agreement No. 721463 to the SUNDIAL ITN
network. AV acknowledges the funding from the Emil Aaltonen foundation. PK is
partially supported by the BMBF project 05A17PC2 for D-MeerKAT. AS
acknowledges funding from the National Research Foundation under the Research
Career Advancement and South African Research Chair Initiative programs
(SARChI), respectively. FV acknowledges financial support from the Italian
Ministry of Foreign Affairs and International Cooperation (MAECI Grant Number
ZA18GR02) and the South African NRF (Grant Number 113121) as part of the ISARP
RAIOSKY2020 Joint Research Scheme.

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##  Appendix A H$\alpha$ image comparison

In Fig.9, we show the H$\alpha$ image after the standard reduction and the
image we used that modelled and subtracted the background of the original
image using a median filter. The giant H$\alpha$ filaments can be seen in the
original image, however, it is dominated by the over- and under-subtracted
artefacts and has a variable background.

The success of our median smoothing and model background subtraction is
dependent on how well the real H$\alpha$ is masked. Anything that is included
in the mask, by definition, is included in the final image. This is especially
challenging for diffuse H$\alpha$ emission located in areas with high
background noise. The converse is also true: if spurious H$\alpha$ emission is
included in the mask, it will also be in the final image.

To mitigate these issues as best as possible, we used a conservative approach
to carefully mask the real H$\alpha$ that was clearly visible in the original
image. It is particularly difficult to mask real H$\alpha$ emission in areas
with a highly variable background and where the background is significantly
under subtracted. The result is that some of the diffuse H$\alpha$ emission is
lost and not reproduced in the final image. As this is an iterative process,
we were able to recover H$\alpha$ emission in the most over-subtracted regions
of the image. Even though we cannot conserve 100% of the H$\alpha$ emission in
this process, the purpose of this is to present the underlying structure of
the new, giant H$\alpha$ filaments detected in the IGM.

Figure 9: Comparison of the original and filtered H$\alpha$ images. _Top
image_ : H$\alpha$ image after the standard data reduction process. _Bottom
image_ : H$\alpha$ image we present in our work that iteratively modelled and
subtracted (described in section 3.2) the background of the original image.
Both images are presented on the same scale. The original image is clearly
dominated by over- and under-subtracted artefacts, while the new image has a
smooth and uniform background, which retains the majority of the real
H$\alpha$ emission. Some diffuse H$\alpha$ emission is lost in this process,
however, the new image is a significant improvement that shows the underlying
structure of the giant H$\alpha$ filaments in the IGM.

license: Creative Commons - Attribution - https://creativecommons.org/licenses/by/4.0/

authors:
D. Kleiner, P. Serra, F. M. Maccagni, A. Venhola, K. Morokuma-Matsui,
  R. Peletier, E. Iodice, M. A. Raj, W. J. G. de Blok, A. Comrie, G. I. G.
  J\'ozsa, P. Kamphuis, A. Loni, S. I. Loubser, D. Cs. Moln\'ar, S. S.
  Passmoor, M. Ramatsoku, A. Sivitilli, O. Smirnov, K. Thorat and F. Vitello

submitter: Dane Kleiner