11institutetext: Kapteyn Astronomical Institute, University of Groningen,
Landleven 12, 9747 AV Groningen, The Netherlands 22institutetext: Department
of Physics and Astronomy, University of the Western Cape, Robert Sobukwe Road,
Bellville 7535, South Africa

# An H i story of galaxies in Abell 2626 and beyond

T. Deb  M.A.W. Verheijen contact<EMAIL_ADDRESS>J.M. van der Hulst
1111

(Received xxx; accepted xxx)

###### Abstract

Context. To study the effects of environment on galaxies we use H i
observations of galaxies in and around the cluster Abell 2626 (A2626). The
cluster can effectively be divided in three different environments: the
cluster itself, a group environment in the periphery of the cluster (we call
it the Swarm) and substructure in the cluster itself. We use these to study
the dependence of galaxy properties on environment.

Aims. We have explored the relationship between H i deficiency, H i
morphology, and star formation deficiency for the galaxies in and around the
A2626 galaxy cluster to investigate the environmental effects on those
properties.

Methods. To quantify asymmetries of the outer H i disc of a galaxy, we used 1)
three visual classes based on the outermost reliable H i contour (settled,
disturbed, unsettled H i discs), 2) the offset between the H i centre and the
optical centre of a galaxy, and 3) the modified asymmetry parameter Amod as
defined by Lelli et al. (2014).

Results. The H i deficiency of a galaxy is strongly correlated with the
projected distance from the centre of A2626. Furthermore, substructure
galaxies tend to be more asymmetric than the isolated galaxies in A2626,
plausibly because of more efficient tidal interactions within substructures
than outside substructures. Moreover, asymmetric, offset, and smaller H i
discs are not necessarily the result of the cluster environment, as they are
also observed in substructures in A2626 and in the Swarm. This signifies that
‘pre-processing’ of the H i discs of galaxies in groups or substructures plays
an important role, together with the ‘processing’ in the cluster environment.
Finally, the galaxies in all three environments have slightly lower star-
formation rates (SFRs) than the typical SFR for normal galaxies as manifested
by their offset from the star formation main sequence, implying effective gas
removal mechanisms in all three environments.

###### Key Words.:

Galaxy: evolution – galaxies: clusters: intracluster medium

##  1 Introduction

###  1.1 General science background

The well-known morphology-density relation (Dressler, 1980) provides direct
evidence of the notion that the cosmic environment of galaxies influences
their constitutional properties and star formation activity – both during
their formation (nature) and subsequent evolution and interaction with their
surroundings (nurture). In the hierarchical structure formation scenario
(Toomre & Toomre, 1972), galaxy clusters grow by accreting galaxies and galaxy
groups and other clusters along the filaments of the cosmic web. Diverse
astrophysical processes impact the morphologies, gas content, and star
formation properties of the galaxies when they move into the dense cluster
environment. Within $R_{200}$ ($\approx 0.75R_{vir}$) of a cluster,
gravitational perturbations such as tidal galaxy-galaxy interactions and
mergers (e.g. Spitzer & Baade 1951; Tinsley & Larson 1979; Merritt 1983;
Springel 2000), tidal galaxy-cluster interactions (Byrd & Valtonen, 1990;
Valluri, 1993), and galaxy-galaxy interactions (sometimes called galaxy
‘harassment’, e.g. Moore et al. 1996; Jaffé et al. 2016), affect both the
stellar and gaseous components of the galaxies. The hydro-dynamical processes
such as ‘starvation’ (e.g. Larson et al. 1980; Balogh et al. 2000), ‘thermal
evaporation’ (Cowie & Songaila, 1977), ‘ram pressure stripping’ (e.g. Gunn &
Gott 1972) and ‘viscous stripping’ (Nelson, 1982) only affect the gas content
of galaxies. Moreover, sometimes these processes are effective in transporting
cold gas to the centers of galaxies and may trigger AGN activity (Baldry et
al., 2004; Balogh et al., 2009; Poggianti et al., 2017).

While the neutral atomic hydrogen or H i gas in galaxies provides the raw fuel
for star formation, the morphologies and kinematics of the extended,
collisional and fragile H i gas discs serve as sensitive diagnostic tracers of
the environment driven processes (e.g. Davies & Lewis 1973; Haynes et al.
1984; Williams & Rood 1987; Abramson et al. 2011; Serra et al. 2013; Jaffé et
al. 2015). One of the nice illustrations of H i in galaxies in a cluster
environment was by the VLA Imaging survey of Virgo galaxies in Atomic gas
(VIVA) survey (Chung et al., 2009): galaxies near the core of the Virgo
cluster show small or disturbed H i discs of low column density while in many
cases, the H i gas is displaced from the stellar body, trailing the galaxy
along its infall trajectory and providing evidence of ram pressure stripping.
Occasionally, star formation occurs in situ in these gas tails, ionising the
gas and giving rise to the ‘jellyfish’ phenomenon (e.g. Poggianti et al. 2017;
Ramatsoku et al. 2019, 2020; Deb et al. 2020). In two other nearby clusters,
Coma (e.g. Molnar et al. 2021) and Fornax (Loni et al., 2021), at least half
of the H i detections show a disturbed H i morphology, including several cases
of asymmetries, tails, offsets between H i and optical centres, and truncated
H i discs. While most of the H i-selected Coma galaxies have both enhanced
star formation rates and are H i deficient compared to field galaxies of the
same stellar mass (Molnar et al., 2021), Fornax galaxies are H i deficient and
have low star formation rate (Loni et al., 2021). This means that the cluster
environment affects both the H i gas content and the star formation rate in
galaxies. The extent of the disturbance on H i morphologies and star formation
depends on the properties of the cluster environment and the galaxies.
However, H i deficient and quenched galaxies are also observed in less dense
environments including cluster outskirts, galaxy groups and the filaments of
the cosmic web, both in observations and simulations (e.g. Solanes et al.
2001; Tonnesen et al. 2007; Yoon et al. 2017). So, the galaxies may already be
pre-processed, i.e. have lost their H i gas and halted their star formation
activity prior to entering the dense cluster environment. It is important to
understand the nature and efficiency of pre-processing that happens at larger
distances from the cluster centers, beyond $R_{200}$, where galaxies can be
relatively isolated or reside in groups with miscellaneous dynamic histories
and galaxy populations (e.g. Laigle et al. 2018; Kraljic et al. 2018; Alpaslan
et al. 2016; Sarron et al. 2018; Kleiner et al. 2017; Vulcani et al. 2019).
Thus, wide-area, volume limited, H i imaging surveys of galaxy clusters that
probe the galaxies beyond the virial radius of the cluster, are indispensable
to investigate different environmental mechanisms in different clusters for a
better understanding of the impact of the different environments on galaxies.

Figure 1: Redshift distribution of three different environments in and around
A2626. green, magenta, and orange histograms present non-substructure,
substructure galaxies in A2626 and galaxies in the Swarm respectively.

However, it is difficult to identify the ongoing astrophysical mechanisms for
individual galaxies within an observed volume around a cluster only from their
H i morphologies. Moreover, if the galaxies are not well resolved, it is even
difficult to unambiguously classify their H i morphologies. Though it is not
possible to confirm in detail the corresponding environmental mechanisms by
only investigating the H i morphologies of the galaxies, it is still possible
to broadly identify the plausible environmental mechanisms acting on galaxies
based on their H i morphologies. Moreover, if we also consider H i deficiency,
star formation rate, optical colours and morphologies, and the location of the
galaxies with respect to the cluster core, together with the H i morphologies,
we would gain further insight in the corresponding astrophysical interactions
with the environment.

In this work, we will explore the H i deficiencies and morphologies of
galaxies and relate those to the corresponding environmental mechanisms for
the H i detected galaxies in and around the A2626 cluster, recently observed
with MeerKAT (Healy et al., 2021a). For this purpose, we will employ different
methods to classify or quantify H i morphologies to understand which method is
the most suitable to identify a particular environmental process.

###  1.2 A2626 and its neighbourhood

A2626 is a fairly massive ($\sim$ 5 $\times$ 1014 $\text{M}_{\odot}$) galaxy
cluster located at a redshift of z=0.055 (Healy et al., 2021b). With MeerKAT
we surveyed a large cosmic volume, containing three distinct over-densities
that represent three different global environments. Galaxies within the A2626
cluster are in the redshift range $0.0475<z<0.0615$. Behind A2626, at the
redshift range $0.0615<z<0.0675$, we find a collection of galaxy groups that
we refer to as the Swarm. Behind the Swarm, we find another over-density, in
the redshift range $0.0675<z<0.0745$, associated with the cluster A2637 (see
Fig. 6 and 11 in Healy et al. 2021a and Fig. 5(a,b,c) in Healy et al. 2021b ).
This cluster is located to the north-east of A2626, near the FWHM of the
primary beam of MeerKAT, hence its galaxies suffer from significant primary
beam attenuation. Therefore we only consider galaxies at the redshifts in the
over-densities of A2626 and The Swarm (see Fig. 1). Table 1 presents different
properties of A2626 and the Swarm. Healy et al. (2021b) have identified six
different substructures in the A2626 cluster based on the Dressler-Shectman
(DS) test (see Fig. 13 in Healy et al. 2021b). We note that in one of the
substructures, close to the cluster centre, none of the galaxies are detected
in H i. Due to a relatively low number of H i detections in each substructure,
we have combined the H i detections of all the substructures together in order
to analyse the H i properties of the substructure galaxies.

Within these two over-densities (A2626 and the Swarm), we identify three
classes of galaxies, depending on their environment.

Table 1: Properties of A2626 and the Swarm. Environment | cz ($km/s$) |  $\sigma$ ($km/s)$ | R200 (Mpc)
---|---|---|---
A2626 | 16576 | 660 $\pm$ 26 | 1.59
The Swarm | 19247 | 397 $\pm$ 22 | 0.95

  1. 1.

non-substructure or ‘isolated’ galaxies in A2626: 57 galaxies that are in the
cluster environment but not a member of a substructure. However,we note that,
these non-substructure galaxies are still in a high density environment.

  2. 2.

Substructure galaxies in A2626: 34 galaxies that are within the substructures
in A2626. They are in groups within the cluster environment.

  3. 3.

The Swarm galaxies: 31 galaxies that are within the group environment
associated with the Swarm, a structure behind the cluster.

We will explore the environmental impact on the H i deficiencies and
morphologies of these 122 galaxies: processing due to the cluster environment
as well as pre-processing of galaxies before they fall into the cluster.

###  1.3 Description of available data products

The detailed description of H i observations, data reduction, and H i data
products are provided in Healy et al. (2021a). We have H i maps, H i signal-
to-noise maps, velocity fields, position velocity diagrams, and H i global
profiles for all 219 H i detections in the A2626 volume (see Healy et al.
2021a for detail). The sensitivity of MeerKAT, enables the detection of H i
emission well beyond the FWHM of the primary beam and, hence, we imaged an
area of 2 $\times$ 2 deg2 centred on A2626. For this work, we analysed the H i
data at an angular resolution of 15” which corresponds to a spatial resolution
of $\sim$15 kpc at the distance of A2626. At the centre of the field-of-view
and at the redshift of A2626, the $3\sigma$ H i mass detection limit for a
galaxy with a line width of $300\,\text{km}\,\text{s}^{-1}$ is
$\text{M}_{\textsc{H$\,$i}}=2\times 10^{8}\,\text{M}_{\odot}$, with a
5$\sigma$ column density sensitivity of
$\mathrm{N}_{\textsc{H$\,$i}}=2.5\times 10^{19}\,\text{cm}^{-2}$ per channel
(207 kHz wide corresponding to 45 km s-1) when the data are smoothed to
$30^{\prime\prime}$ resolution.

Photometric imaging data were taken from the DECam Legacy Survey (Dey et al.,
2019) and the Sloan Digital Sky Survey (SDSS, York et al. 2000; Aguado et al.
2019). Optical spectroscopic data were mainly taken from the SDSS, and a new
dedicated MMT/ Hectospec survey (Healy et al., 2021b). Stellar masses and star
formation rates are calculated from WISE photometry kindly provided by T.
Jarrett (private communication). Stellar masses are calculated from
W1$(3.4\mu$m) and W2 (4.6$\mu$m) fluxes and SFRs are based mainly on the 12
$\mu$m emission.

For our analysis, we have adopted optical centres from the SDSS and we have
measured the H i centres by fitting 2D Gaussians to the H i maps of the
galaxies. To quantify how well an H i map is resolved, we have divided the
number of pixels above a certain column density level in the H i map (e.g. the
column density corresponding to 3 times signal-to-noise) by the number of
pixels in the MeerKAT synthesized beam.

The remainder of the paper is structured as follows: Sec. 2 presents different
ways of classifying H i morphologies. In Sec. 3 we explore H i deficiency of
galaxies in A2626 and the Swarm. In Sec. 4, we investigate the star formation
rates and depletion times in the A2626 and the Swarm galaxies. In Sec. 5, we
showcase some interesting H i morphologies of selected galaxies. In Sec. 6, we
discuss our observational findings and address questions raised by our
results. Finally, in Sec. 7 we summarise our work. In this paper, we have used
${H}_{0}=70\,\text{km}\,\text{s}^{-1}$ and $\Omega_{m}=0.3$.

##  2 Classifying H i morphologies

H i asymmetries are thought to be indicative of environmental effects. We aim
to study them in relation with the local environment and then explore which
method(s) are robust enough to assess H i morphologies in the A2626 galaxies
given the limitations of the available MeerKAT data (good sensitivity though
variable across the field of view, moderate linear resolution). To be able to
do this, we first need to examine the detected sources carefully and prune the
sample so that only the objects with sufficient sensitivity and resolution are
considered for further analysis.

Figure 2: Histograms of three observational properties of galaxies in and
around A2626. (a) The distribution of the peak signal-to-noise of the H i
maps. (b) The number of beams enclosed by the 3$\sigma$ contours of the H i
maps. (c) The distribution of 3$\sigma$ column density levels in the H i maps.

In the observed volume, 219 H i sources identified by SoFiA (Serra et al.,
2015) have optical counterparts (see Healy et al. 2021a). Among those 219
sources, we first identified the uncertain H i detections with low signal-to-
noise, with only a few very high signal-to-noise pixels, or sources missing
one peak of the double horn profile in the SoFiA mask as revealed in the
position-velocity diagram, and excluded them from our analysis. We then were
left with 177 reliable H i sources with a peak signal-to-noise in their H i
map $\geq$ 5, as indicated in the signal-to-noise in the atlas pages in (Healy
et al., 2021a). Among those 177 galaxies, 108 galaxies are in the A2626
cluster and the Swarm, the number of H i sources in each of these environments
is given in Table 2. In the left panel of Fig. 2, we present a histogram with
a distribution of the peak signal-to-noise values for those 177 galaxies.
Therefore, this histogram gives an impression of the quality of the data. The
median peak signal-to-noise of the galaxies in the surveyed volume is 8.6. The
middle panel of Fig. 2 shows the histogram of the number of beams within the
3$\sigma$ contour of the H i map (see Healy et al. 2021a for details). The
median number of beams for these 177 galaxies is 3.1.

Table 2: H i sources in different environments. Environment | No. of H i sources
---|---
Non-substructure in A2626 | 46
Substructures in A2626 | 30
The Swarm | 32

###  2.1 Visual classifications

At first we have classified our entire sample visually based on their H i
morphologies. For visual classification, we have assessed H i morphologies
based on the 3$\sigma$ column density contours. We have visually classified in
three different classes: settled, one-sided / disturbed, and unsettled,
similar to the work by Molnar et al. (2021), who classified H i morphologies
of the galaxies in the Coma cluster.

  1. 1.

Settled sources (VClass 1) - These are sources in which the H i distribution
is already ‘settled’ i.e. they have symmetric H i morphologies centred on a
stellar disc and a velocity gradient consistent with rotation. Spatially
unresolved H i sources were also included to this category as well.

  2. 2.

Disturbed sources (VClass 2) - These H i sources either have a regular disc
component with an additional one-sided asymmetry in their H i morphology, or
the H i distribution is regular with a significant excess of H i flux on one
side of the stellar disc.

  3. 3.

Unsettled sources (VClass 3) - These are H i sources with an irregular H i
morphology and/or kinematics or with H i flux extensions in multiple
directions from the stellar disc, or an extreme 3D asymmetry / displacement
relative to the optical light with an unclear H i disc component.

Table. 3 shows the number of galaxies with different visual classifications in
different environments. Thus, substructure galaxies are more disturbed /
unsettled than non-substructure galaxies in A2626 or the Swarm. Though the
statistics is limited for the galaxies in these three environments, in
substructures there are significantly more ($\sim$ 84%) disturbed / unsettled
galaxies than among the non-substructure galaxies (65%) in A2626 or the Swarm
galaxies (60%). In substructures, only 17% $\pm$ 8% of the galaxies have
settled H i discs while in the other two environments 37% $\pm$ 8% of the
galaxies have settled H i discs. This difference is at the 2$\sigma$ level.

Table 3: No. of galaxies with different Visual classifications in different environments. Environment | No. of | No. of | No. of
---|---|---|---
| VClass 1 sources | VClass 2 sources | VClass 3 sources
non-substructure in A2626 | 16 (35%) | 25 (54%) | 5 (11%)
Substructures in A2626 | 5 (16.7%) | 23 (76.7%) | 2 (6.6%)
The Swarm | 13 (40%) | 12 (38%) | 7 (22%)

###  2.2 H i offsets

Another effective way to quantify asymmetry in the H i distribution is to
measure the offset of the H i centre from the optical centre. H i centres are
calculated by fitting 2D Gaussians to the H i maps while optical centres are
taken from the SDSS. A significant offset of the H i centre from the optical
centre may signify an external environmentally induced disturbance in the H i
morphology. We note that with a small H i offset, a galaxy can still undergo
subtle environmental processes like thermal evaporation, or starvation or can
be at a late stage of ram-pressure stripping. Fig. 3 presents the
distributions of H i offset for non-substructure, substructure, and the Swarm
galaxies. All the galaxies in these three environments display a range of H i
offsets, signifying there is no obvious correlation of H i offset with the
different environments in and around A2626.

Figure 3: Distribution of H i offset in kpc presented as histograms for the
galaxies in and around A2626. Top, middle, and bottom panel shows H i offset
for the non-substructure (green), substructure (magenta) galaxies in A2626 and
galaxies in the Swarm (orange) respectively.

###  2.3 Quantifying asymmetries

Defining H i asymmetry
To investigate the environmental impact on the H i discs of galaxies, it is
important to quantify the severity of the asymmetry/ disturbance in the outer
H i distribution of individual galaxies. The optical/ infrared morphologies of
galaxies are generally quantified with the concentration, asymmetry, and
smoothness (CAS) parameters (Conselice et al., 2003) and the Gini and M20
parameters (Lotz et al., 2004). Holwerda et al. (2011, 2013) used these
parameters to quantify the H i morphologies of galaxies from multiple H i
surveys (WHISP, LITTLE-THINGS, VLA-ANGST). In addition, Holwerda et al. (2011,
2013) and Giese et al. (2016) used the definition of asymmetry from Conselice
et al. (2003). Giese et al. (2016) found that the asymmetry parameter is much
more robust than any of the other parameters for H i imaging data.

$A=\frac{\sum_{i,j}^{N}|{I(i,j)-I_{180}(i,j)}|}{\sum_{i,j}^{N}|{I(i,j)}|}$ (1)

where I(i, j) denotes the value of the pixel at the i, j position of the
original image of the galaxy and $I_{180}(i,j)$ is the value of the pixel in
the same position of the image rotated by 180∘ around the centre of the
galaxy. So, asymmetry is measured by summing the pixel-by-pixel difference of
the original and rotated image and normalising that by the total intensity in
the image. Hence, the asymmetry index can have a value between 0 and 1.
Asymmetries in the fainter outer regions of the H i discs give a negligible
contribution to the global asymmetry index compared to the brighter ($\sim$ 2
orders of magnitude or more) inner regions. Environmental processes, however,
influence the extended outer parts of the H i disc in a galaxy much more
easily than the inner parts. In order to give proper weight to the asymmetries
in the outer H i disc, Lelli et al. (2014) introduced a modified asymmetry
index ($A_{mod}$):

$A_{mod}=\frac{1}{N}\sum_{i,j}^{N}\frac{|{I(i,j)-I_{180}(i,j)}|}{|{I(i,j)+I_{180}(i,j)}|}$
(2)

where $N$ is the total number of pixels in the image. Thus, this definition of
$A_{mod}$ normalizes the intensity differences at the position $(i,j)$ with
the ‘local’ intensity of the pixels, contrary to the total intensity of all
the pixels in the H i map. For example, for a highly lopsided galaxy with H i
emission exclusively on one side, $A_{mod}$ will obtain the maximum value of
1. However, we recognize that the value of $A_{mod}$ depends on some selection
criteria.

(i) The value of $A_{mod}$ depends on the H i column density above which it is
measured. Depending on the observational setup and the sensitivity of the
telescope, different H i observations have different column density
sensitivities. Moreover, even with the same observational settings, galaxies
may have diverse 3$\sigma$ column density thresholds, depending on the local
noise in the vicinity of the galaxy and the location of the galaxy with
respect to the pointing centre which affects the local primary beam
attenuation.

(ii) The choice of the galaxy centre around which the H i disc is rotated also
effects the measurement of $A_{mod}$. For disturbed and unsettled H i
distributions, the H i centre often does not coincide with the optical centre.
Moreover, the determination of the optical or the H i centre also depends on
the adapted method of calculation.

(iii) The reliability of the $A_{mod}$ measurement also depends on how well
resolved the galaxy is, both in terms of the beam size in kpc and the number
of beams across the H i map. If the galaxy is only marginally resolved and
detected at low signal-to-noise, the asymmetry in the outer H i disc might be
dominated by noise, rather than the asymmetry induced by environmental
processes.

Bilimogga et al. (2022) have investigated the dependence of $A_{mod}$ on
signal-to-noise, column density threshold and angular resolution of the H i
observations. Using mock galaxies from the EAGLE simulations (Schaye et al.,
2015; Crain et al., 2015), they suggest an optimal combination of the
observational constraints that are required for a robust measurement of the
$A_{mod}$ value of the outer H i disc of a galaxy: a column density threshold
of 5 $\times$ 1019cm-2 or lower at a minimal signal-to-noise of 3 and a galaxy
resolved with at least 11 beams. These are not ‘hard’ limits as they depend on
what one considers to be acceptable deviations of the measured $A_{mod}$ from
the intrinsic $A_{mod}$ of a galaxy. Our observations do not reach this column
density sensitivity and most galaxies are not resolved by 11 beams.
Consequently, we measure H i asymmetries of the inner H i discs.

Measuring asymmetry in galaxies in and around A2626

To compute $A_{mod}$, we need to adopt the position of the centre of the
galaxy as well as a column density threshold above which we would consider the
intensity of the H i emission. All 219 H i detected galaxies in our survey
have optical counterparts within the footprints of the Sloan Digital Sky
Survey (SDSS, York et al. 2000; Aguado et al. 2019) and the DECam Legacy
Survey (DECaLS, Dey et al. 2019). For our analysis, we adopted the optical
centres from the SDSS for all 219 galaxies. We did not consider the H i
kinematic centres since most of the galaxies in our sample have disturbed/
unsettled H i discs. Moreover, optical centres are generally better tracers of
the dynamical centre of a galaxy, compared to the H i centres. Using the
optical centre sometimes results in a high $A_{mod}$ value for the galaxies
with a strong offset between the H i distribution and the stellar body, that
may indicate a recent interaction or accretion event.

It is crucial to calculate $A_{mod}$ above a specific column density for all
the galaxies in the sample to make a fair comparison between different
$A_{mod}$ values. But that specific column density will correspond to
different signal-to-noise levels for different galaxies, depending on the
local noise properties and the location of the galaxy within the primary beam.
We have a range of 3$\sigma$ column density sensitivities (9-65) $\times$ 1019
cm-2 in our MeerKAT observations (Healy et al., 2021a). To measure $A_{mod}$
reliably, we need adequate signal-to-noise images and we need as low an
$N_{HI}$ limit as possible to measure the outer parts of galaxies well. The
bottom panel of Fig. 2 shows the distribution of the 3$\sigma$ H i column
density levels for the reliable H i detections with a minimum signal-to-noise
$\geq$ 5 (177 galaxies among the 219 galaxies).

The vertical dotted line in the bottom panel of Fig. 2 represents the H i
column density level of 25 $\times$ 1019 cm-2. Galaxies to the left of this
dotted line have 3$\sigma$ H i column densities below 25 $\times$ 1019 cm-2.
Hence, their $A_{mod}$ values will be reliable when including pixels above 25
$\times$ 1019 cm-2 in the calculation of $A_{mod}$. The galaxies to the right
of the dotted line have 3$\sigma$ H i column density levels above 25 $\times$
1019 cm-2, thus, will have noisy pixels (below their own 3$\sigma$ contour)
included in the $A_{mod}$ calculation when including pixels with values as low
as 25 $\times$ 1019 cm-2. By adopting a threshold of 25 $\times$ 1019, we
retain 71% of the galaxies for which the $A_{mod}$ calculation only includes
pixels with signal-to-noise ¿3 in the H i map. Hence we consider the
corresponding $A_{mod}$ values as reliable, provided the galaxy is
sufficiently resolved (see below).

Notably, in a statistical sense, for a sample of galaxies with random
inclinations, 25 $\times$ 1019 cm-2 corresponds to 1 $\text{M}_{\odot}$/pc2 in
a face-on orientation, which is the typical column density at which the
diameters of H i discs are measured. 1 $\text{M}_{\odot}$/pc2 surface density
corresponds to H i column density of 12.5 $\times$ 1019 cm-2. Since the
galaxies in our sample cover the full range of inclinations, by considering
the observed column density along the line-of-sight, we do not always consider
the H i column density perpendicular to the plane of those galaxies. Since
most of the galaxies in our survey are barely resolved, we can not make
meaningful inclination corrections for the H i column densities of individual
galaxies. So, we have adopted a statistical approach. The median inclination
of a randomly oriented sample of galaxies is i = 60∘. This means that a face-
on column density of 12.5 $\times$ 1019 cm-2 increases to a line-of-sight
column density of 12.5/cos(60∘) = 25 $\times$ 1019 cm-2. Thus, 25 $\times$
1019 cm-2 is also the practical level adopted for measuring H i diameters.

Another important aspect for a reliable $A_{mod}$ measurement is angular
resolution. For poorly resolved galaxies, $A_{mod}$ is not very meaningful.
So, in addition to signal-to-noise, we need to restrict the sample to
sufficiently resolved galaxies. We have considered the following criteria to
identify the galaxies for which the $A_{mod}$ value is reliable: galaxies with
a 3$\sigma$ column density level of $\leq$ 25 $\times$ 1019 cm-2, with a
minimum peak signal-to-noise of $\geq$5, and resolved by more than 3 beams.
Consequently, we are left with 33 galaxies for which the calculation of
$A_{mod}$ is meaningful.

Figure 4: Distribution of $A_{mod}$ and H i offset for the galaxies in and
around A2626. The galaxies above the horizontal dashed line ($A_{mod}$ ¿0.4)
are considered as H i disturbed / unsettled galaxies. Galaxies on the right
side of the vertical dashed line are considered as high H i offset galaxies. H
i maps of a few example galaxies (see Section 2.3) are included as insets in
the top panel. In the bottom right corner we reported the Pearson’s cofficient
and p-value for the relation between $A_{mod}$ and H i offset. $A_{mod}$ and H
i offset seem to have a very strong correlation.

The relation between visual classifications, offsets, and $A_{mod}$

We have explored the relation between our different methods of classifying H i
morphologies : visual classes, H i offset, and $A_{mod}$. Fig. 4 shows
$A_{mod}$ as a function of offset between the H i and the optical centres. We
observe a strong correlation between $A_{mod}$ and offset for these 33
galaxies. The Spearman correlation coefficient is 0.81 which is shown in the
bottom-left of Fig. 4. This correlation is expected since a high offset
between the H i and optical centres would result in a high $A_{mod}$ value,
though a galaxy with a high $A_{mod}$ value does not necessarily have a
strongly offset H i disc. For practical purposes, we are classifying a galaxy
as H i asymmetric when its $A_{mod}$ value is $\geq$0.4. Such an H i
asymmetric galaxy will be located above the horizontal dashed line in Fig. 4.
Similarly, we have defined galaxies to the right of the vertical dashed line
in Fig. 4 as the high H i offset galaxies.

These thresholds as indicated by the horizontal and vertical dashed lines have
separated the galaxies in different classes. In particular, we found three
different areas in the plot representing three different types of galaxies.

  1. 1.

In the first quadrant (q1), we find high $A_{mod}$, low H i offset galaxies.
For these galaxies, a high value of $A_{mod}$ is driven by the asymmetry in
the outer H i disc. So, these are H i asymmetric galaxies. For example, in
Fig. 4, we observe an asymmetry in the outer H i contour of galaxy 184 which
results in a high $A_{mod}$ value.

  2. 2.

In the second quadrant (q2), we find high $A_{mod}$, high H i offset galaxies.
For these galaxies, a high $A_{mod}$ is driven by the high H i offset
(sometimes due to the combination of an H i offset and an outer disc H i
asymmetry). For example, galaxy 105 in Fig. 4 has a strongly offset H i disc
with respect to the optical centre.

  3. 3.

In the fourth quadrant (q4), we find low $A_{mod}$, low H i offset galaxies.
These galaxies have an H i disc that is neither asymmetric, nor offset. Hence,
these are H i normal galaxies (e.g. galaxy 76 in Fig. 4), although their H i
discs can be small with respect to the stellar disc.

We note that there are no galaxies in the third quadrant (q3), i.e. galaxies
with low $A_{mod}$ and high H i offset. This is expected since a high H i
offset would automatically cause a high $A_{mod}$ value.

Figure 5: Distribution of $A_{mod}$ and H i offset as a function of visual
classes for the galaxies in and around A2626. Left panel: settled galaxies (in
circles), in top panel H i maps of settled galaxies with high $A_{mod}$. Right
panel: disturbed (diamonds) and unsettled (stars) galaxies, in top panel H i
maps of disturbed and unsettled galaxies with low $A_{mod}$.

Next, we have explored the same offset-$A_{mod}$ relation with our visual
classifications of H i asymmetries as an additional parameter. The left and
right panel of Fig. 5 show the galaxies with settled H i discs as circles and
the galaxies with disturbed / unsettled H i discs as diamonds and stars
respectively, based on our visual classifications as mentioned in Sec. 2.1.
So, if there is a complete overlap of our visual classification with the H i
asymmetries based on the $A_{mod}$ values, all the diamond and star symbols
should be above the $A_{mod}$ = 0.4 horizontal dashed line and all the circle
symbols should be below the $A_{mod}$ = 0.4 horizontal dashed line. However,
in the bottom left corner in the right panel of Fig. 5, two galaxies (31 and
45) that are visually classified as galaxies with a disturbed or an unsettled
H i disc, have nevertheless a low $A_{mod}$ value. These galaxies have a low
$A_{mod}$ value since $A_{mod}$ is calculated within the green contour that
corresponds to 25 $\times$ 1019 cm-2, which is above the 3$\sigma$ level
(because it is inside the outermost black contour). These galaxies, however,
have disturbed / unsettled H i discs if we consider the outer H i contour that
corresponds to the 3$\sigma$ column density. This illustrates that the choice
of 25 $\times$ 1019 cm-2 means that for some galaxies, the visual
classification based on the outer H i disc will not correspond to the
$A_{mod}$ value calculated with pixels above the 25 $\times$ 1019 cm-2
contour. Furthermore, in the left panel of Fig. 5, we find three galaxies with
$A_{mod}$ $>$0.4 yet they are visually classified to have settled H i discs.
For galaxies 147 and 120, the H i disc seems somewhat offset from the optical
centre. Galaxy 191 has an H i cloud detached from the H i disc, yet it was
included in the calculation of $A_{mod}$. However, the visual impression of
the H i discs of these galaxies are settled. This illustrates that the
$A_{mod}$ values and the visual classifications both are valuable tracers of H
i morphologies and largely follow each other.

Figure 6: Distribution of $A_{mod}$ and H i offset, colour coded with three
different environments for the galaxies residing in and around A2626. The
green, magenta, and orange circles represent non-substructure, substructure
galaxies in A2626 and galaxies in the Swarm respectively. Substructure
galaxies seem to have more asymmetric H i discs with high $A_{mod}$ compared
to non-substructure galaxies in A2626.

As a next step, we are considering the environment as a parameter in the same
$A_{mod}$ vs H i offset plot. We have used three different colours to
represent the three different environments as described in Sec. 1.2. In Fig.
6, green, magenta and the orange circles represent the non-substructure
galaxies in A2626, the substructure galaxies in A2626, and the Swarm galaxies
respectively. Although the statistics is limited, interestingly, all the
galaxies in the substructure for which we have reliable $A_{mod}$ values are H
i asymmetric galaxies. At the same time, only a fraction of the non-
substructure galaxies in A2626 have an H i asymmetric disc. The occurrence of
more H i asymmetric galaxies in substructures of A2626 (all magenta symbols
are above the dashed line) is possibly due to more effective tidal
interactions in the substructure environment than the non-substructure
galaxies in A2626 experience. However, galaxies in the Swarm and non-
substructure galaxies in A2626 are found to have both disturbed and
undisturbed H i discs.

##  3 H i deficiency in A2626 and the Swarm galaxies

In addition to exploring different H i morphological classifications and their
significance, we have explored the H i deficiency as another indicator of
environmentally induced gas depletion and removal processes. We examined a
galaxy’s H i deficiency as a function of its projected distance from the
centre of A2626, its position in the phase-space diagram of A2626, and its H i
morphology.

H i deficiency is defined as a logarithmic quantity (Haynes & Giovanelli
1983), a difference between the log of expected H i mass and the log of
observed H i mass of a galaxy.

$HIdef=log[M_{HI}^{exp}]-log[M_{HI}^{obs}]$ (3)

Figure 7: Top left: H i deficiency vs projected distance normalised by R200
for the non-substructure (green circles) and substructure (magenta circles) in
A2626. Horizontal dashed lines presents the range of H i deficiencies of the
field galaxies. There is a strong correlation between H i deficiency and
projected distance - H i deficient galaxies seem to reside close to the
cluster core. Top right: Histogram of the distribution of H i deficiencies of
galaxies in the Swarm. Bottom left: The distribution of the non-substructure
(green) and substructure (magenta) galaxies in A2626 in the projected phase-
space. The black dashed lines indicate the escape velocity. There is no clear
difference between the distribution of galaxies in the two different
environments apart from the previously observed trend of the proximity of non-
substructure galaxies to the cluster core. Bottom right: Six H i maps overlaid
on DECaLS colour images for the most H i deficient (top three) and the most H
i rich (bottom three) galaxies. The H i deficient galaxies seem to be bright
and yellowish with offset or truncated H i discs. The H i rich galaxies are
fainter and bluish, with extended H i discs.

Thus H i deficiency is positive for H i deficient galaxies and negative for
galaxies with excess H i gas. $M_{HI}^{exp}$ is generally calculated from H
i-optical scaling relations (e.g. Haynes et al. 1984; Chamaraux et al. 1986;
Batuski & Burns 1985; Solanes et al. 1996; Dénes et al. 2014). We used the
scaling relation from Dénes et al. (2014) (see their Table 3), a
multiwavelength scaling relation between the H i content and the optical
diameter of the galaxies. The scaling relation of Denes 2014 is based on the
r-band diameters reported by HyperLEDA.

$log[M_{HI}^{exp}]=\alpha_{\lambda}+\beta_{\lambda}log[D_{\lambda}]$ (4)

where $M_{HI}^{exp}$ is the expected H i mass, $D_{\lambda}$ is the optical
diameter (in kpc) in a particular band, $\alpha_{\lambda}$ and
$\beta_{\lambda}$ are the parameters for this band. To calculate the expected
H i mass, we have measured the diameters ($D_{r}$=2R25) from the DECaLS r-band
images where R25 is the radius of the 25th mag/arcsec2 isophote.

###  3.1 H i deficiency in substructures

The top left panel of Fig. 7 shows the H i deficiency of galaxies vs. their
projected distance from the center of A2626, normalised by R200 of A2626. R200
is the projected radius of a sphere with mean density 200 times the critical
density of the universe. Light and magenta symbols represent the non-
substructure and the sub-structure galaxies in A2626 respectively. The area
between the three horizontal dashed lines represents the H i deficiency range
for field galaxies. The vertical dashed line indicates R200 of A2626. The H i
deficiencies of the Swarm galaxies are presented in the orange histogram in
the top right panel of Fig. 7. First of all, we observe a correlation between
H i deficiency and the projected distance from the center of A2626 (Spearman’s
coefficient= -0.377, p-value= 0.0012). This clearly illustrates that galaxies
become H i deficient towards the cluster core. Moreover, within 1.5 R200, the
most H i deficient galaxies seem to be the non-substructure galaxies. On
average, non-substructure galaxies are more H i deficient ($<HIdef>_{non-
substructure}$ = 0.27, $<HIdef>_{ss}$=0.15) than sub-structure galaxies in
A2626. We note, however, that the substructure galaxies are more prevalent at
larger cluster centric radii. The six panels in the bottom-right of Fig. 7
illustrate H i maps overlaid on DECaLS colour images for the most H i
deficient (top three) and the most H i rich (bottom three) galaxies. Focusing
on them, we observe that the H i deficient galaxies are bright and have a
yellowish colour, and have small or truncated H i discs. On the contrary, the
H i rich galaxies are fainter with a bluish colour and have extended H i
discs.

Figure 8: HI deficiency vs projected distance normalised by R200 (similar to
Fig. 7), with symbols and colour codes to include additional information
regarding H i morphologies of galaxies in A2626. Top panel: Different symbols
(shown in the top right corner) to represent non-substructure (green) and
substructure (magenta) galaxies of different visual classes. Middle panel: The
colour scale (colourbar shown in the right) represents H i offsets of non-
substructure (filled circles) and substructure (open circles) galaxies
respectively. Bottom panel: The colour scale (colourbar shown in the right)
represents $A_{mod}$ value of non-substructure (filled circles) and
substructure (open circles) galaxies respectively.

###  3.2 H i deficiency in phase-space

In the bottom left panel of Fig. 7, we show the phase-space diagram for non-
substructure and sub-structure galaxies in A2626. The black dashed lines
indicate the escape velocity for A2626 galaxies, calculated using the
formalism from Jaffé et al. (2015). We assumed a concentration index C = 6 and
the mass enclosed by R200 is calculated using:

$M_{200}=\frac{4}{3}\pi R^{3}_{200}200\rho_{c}$ (5)

where M200 is the mass enclosed by R200 and $\rho_{c}$ is the critical
density. The vertical grey dashed line in the phase-space diagram represents
R200 of A2626. In the phase-space diagram, there is no clear additional trend
in the distribution of non-substructure/ sub-structure galaxies.

###  3.3 H i deficiency and H i morphology

As a next step, in Fig. 8, we have explored the same H i deficiency vs.
projected distance plot with colour codes and symbols to include additional
information regarding the H i morphologies of those 76 galaxies.

H i deficiency and visual classifications

In the top panel of Fig. 8, we present the H i detected galaxies of the three
visual classes as defined in Sec. 2.1 with circle, triangle, and star symbols
respectively. The light and magenta symbols in each class stand for the non-
substructure and sub-structure galaxies in the A2626 cluster. Interestingly,
the VClass 3 or unsettled galaxies (star symbols) seem to have H i
deficiencies similar to the field galaxies (i.e. within the three horizontal
dashed lines). Moreover, substructures contain more disturbed / unsettled
galaxies compared to the non-substructure galaxies in A2626 as we have seen in
Sec. 2.1. However, there is no obvious correlation of the visual classes of
the H i detected galaxies with their H i deficiencies or with their projected
distance from the cluster centre. Thus, the visually classified H i
morphologies are not directly related to the H i deficiencies of the galaxies
or their location in the A2626 cluster.

H i deficiency and H i offsets

In the middle panel of Fig. 8, we have again plotted H i deficiency vs.
projected distance from the center of A2626 for the H i detected galaxies,
with a colour coding according to the offset (in units of kpc) of the H i
centers of those galaxies with respect to the optical centers. The filled and
open circles are non-substructure and substructure galaxies respectively.
There is no correlation between H i deficiency and H i offset for the galaxies
in A2626. This implies that the H i deficient galaxies are not necessarily H i
offset and vise versa.

H i deficiency and $A_{mod}$

In the bottom panel of Fig. 8, we examine the H i deficiency vs projected
distance, with a colour coding indicating the $A_{mod}$ values of the H i
discs of galaxies in A2626. Since $A_{mod}$ is meaningful for only 11 well-
resolved, high signal-to-noise galaxies in A2626, we have plotted only those
galaxies. The filled and open symbols are non-substructure and sub-structure
galaxies in A2626. Similar to what we see in two other panels in Fig. 8, there
is no clear trend between H i deficiency, projected distance from the cluster
centre, and $A_{mod}$ in the galaxies in A2626. Interestingly, the most H i
deficient galaxy in this plot is also the most H i asymmetric galaxy with a
high $A_{mod}$ value ($A_{mod}$ = 0.6).

Considering all panels of Fig. 8, apart from the clear trend between H i
deficiency and the projected distance from the cluster centre, we conclude
that there are no further obvious correlations between H i deficiency, H i
morphology, and location of the galaxy in the cluster, except that VClass 3
galaxies are not H i deficient.

##  4 Star formation rates and depletion times in A2626 and the Swarm galaxies

Figure 9: Top left panel: SFMS for the non-substructure (filled circles) and
substructure (open circles) galaxies in A2626. The SFMS relation and quenching
threshold are taken from Cluver et al. (2020) which is calibrated using the
WISE data and the same methodologies that is used for stellar mass and SFR
calculations for the galaxies in our survey. The colour scale (shown in the
right) represents H i deficiency for the galaxies. The down-arrows are
2$\sigma$ upper limits on the SFR from WISE observations. Both the non-
substructure and substructure galaxies seem to have slight lower SFR than the
SFR expected for normal galaxies. In the top panels we show H i maps overlaid
on DECaLS colour images for some outlier galaxies. Top right panel: Similar to
the plot on the top left, except it presents galaxies in the Swarm. Bottom
left panel: H i depletion time vs stellar mass of the non-substructure (filled
circles) and substructure (open circles) galaxies in A2626. The colour scale
presents the H i deficiency of the galaxies. Bottom right panel: Similar to
the bottom left plot, except it presents the galaxies in the Swarm.

###  4.1 Galaxies on the SFMS

We have combined the information on star formation rates (SFR), stellar masses
with the H i properties of the galaxies in A2626 and the Swarm to investigate
the extent of the effect of different environments on the star formation
properties of the H i detected galaxies. The star formation activity and the
stellar mass of a galaxy are related in a systematic way, described by a well-
established scaling relation, the ‘star formation main sequence’ (SFMS, e.g.
Brinchmann et al. 2004; Noeske et al. 2007; Elbaz et al. 2007; Speagle et al.
2014; Tomczak et al. 2016). We investigated the location of the galaxies in
different environments (non-substructure and sub-structure galaxies in A2626,
and the Swarm galaxies) with respect to the SFMS. We have adopted the SFMS and
the star formation quenching threshold relation from Cluver et al. (2020),
which are calibrated using WISE data. For consistency, we have also adopted
stellar masses and SFRs based on WISE data derived with the same methodology
as developed by Cluver et al. (2014) (see Sect. 1.3).

We have also investigated the relation between H i depletion time and stellar
mass for galaxies in different environments as a function of H i deficiency.
Depletion time is the time required for a galaxy to deplete its H i gas due to
star formation and is defined as $t_{dep}$ = MHI/ SFR. The depletion timescale
of normal star-forming galaxies spans a range of $\sim$2-10 Gyr (Kennicutt,
1989, 1998; Bigiel et al., 2008).

###  4.2 HI deficiency versus SF deficiency

In Fig. 9, we have plotted SFMS in the top two panels and the depletion time
vs stellar mass in the bottom two panels for the galaxies in A2626 and the
Swarm. To represent the two different environments in A2626, we have used
filled and open circles respectively for the non-substructure and sub-
structure galaxies in A2626 in the top and bottom left panel of Fig. 9. In all
the panels, we have colour coded the H i deficiencies. The orange and black
dotted lines are the SFMS and the quenching threshold from Cluver et al.
(2020) respectively. The downward pointing arrows are the 2$\sigma$ upper
limits on the SFR. Interestingly, galaxies in all three environments – the
non-substructure and sub-structure galaxies in A2626, and the Swarm galaxies
are mostly below the SFMS from Cluver et al. (2020), thus show an overall SF
deficiency.

Figure 10: Offset from SFMS i.e. SF deficiency in A2626 and the Swarm
galaxies. Galaxies above the horizontal dashed line lie below the SFMS and can
therefore be considered to have a positive SF deficiency. The colour coding is
similar to the colour coding in Fig. 6. Figure 11: Similar to Fig. 9, with
additional information regarding H i morphologies of galaxies in A2626 and the
Swarm.

To examine if there is any difference in the distribution of galaxies in the
three environments with respect to the SFMS, in Fig. 10, we have plotted the
offset of the galaxies from the SFMS, i.e. the SF deficiency, in those three
environments (green: non-substructure galaxies in A2626, magenta: sub-
structure galaxies in A2626, orange: the Swarm galaxies). A positive SF
deficiency (galaxies above the horizontal dotted line) indicates that these
galaxies are below the SFMS and a negative SF deficiency (galaxies below the
horizontal dotted line) indicates that these galaxies are above the SFMS.
Along the horizontal axis we plotted MHI/M⋆ to investigate if the SF
deficiency relates to the relative H i content of those galaxies. We do not
observe any differences for the galaxies in different environments. This means
that the star formation activity of galaxies with a similar stellar mass has
no dependence on environment.

###  4.3 H i depletion time versus H i morphology

In the bottom panels of Fig. 9, we do not observe any clear trend between the
depletion time and stellar mass for the galaxies, regardless of their
environment. The downward pointing arrows are 2$\sigma$ upper limits on the
depletion times. The figures show that there are no galaxies with large
stellar masses and large depletion times. Massive galaxies tend to have a high
SFR as expected from the SFMS and thus exhaust their H i fuel relatively fast.
There is no strong correlation of depletion time with H i deficiency. However,
we note that the most H i deficient galaxies in A2626 have a short depletion
time while the three galaxies in the Swarm with the longest depletion time
(purple symbols) have a negative H i deficiency.

###  4.4 H i morphology versus SF deficiency

In Fig. 11, we present similar SFMS plots in six different panels for three
different environments, using the same symbols as Fig. 9. The only differences
are that in the two top panels of Fig. 11 we have used different symbols for
the three visual classifications, in the middle panels we have colour coded
with H i offset, and in the bottom panels we have colour coded with $A_{mod}$.
Here again, we do not observe any meaningful correlation between the SFR and H
i offset or $A_{mod}$, as a function of environment or stellar mass.

##  5 Interesting galaxies

Before we discuss our results in detail, we first present the detailed H i
maps of several interesting galaxies to illustrate the diversity of the H i
morphologies. These galaxies are resolved in our MeerKAT H i observations and
we compare them with the existing DECaLS optical colour images, star formation
rate from WISE observations, and their location in phase-space to infer the
ongoing physical mechanisms acting on those galaxies.

Figure 12: Interesting galaxies with diverse H i morphologies in and around
A2626.

###  5.1 Potential ram-pressure stripped galaxies

In Fig. 12 (a), we present several galaxies that are potentially experiencing
ram-pressure stripping or other effects like thermal evaporation or
starvation. Galaxies 110 and 128 are non-substructure galaxies in A2626,
located close to the cluster core (within R200) and fully exposed to the ICM.
Moreover, they are highly H i deficient, have small H i discs with yellowish
optical discs hinting at a quenched star formation, confirmed by their low
star formation rates inferred from the WISE fluxes. Hence, they probably have
experienced ram-pressure stripping or thermal evaporation for a long time
which has resulted in the removal of a significant amount of H i gas. Galaxy
178 is located in a substructure in A2626. Although it is not very close to
the cluster core ($\sim$ 1.5 R200), the H i morphology hints at ongoing ram-
pressure stripping. Its H i contours are compressed on the northern side of
the H i disc and the outer H i contour seems to suggest that the H i gas is
pushed to the south. Interestingly, galaxy 178 is moving at a velocity similar
to the mean velocity of the cluster and has a low star formation rate.
Therefore one may conclude that galaxy 178 is a backsplash galaxy, but the H i
content of the galaxy suggests it is still on its first infall trajectory.

###  5.2 Galaxies with unsettled H i discs

In Fig. 12 (b), we present some well-resolved galaxies with unsettled H i
discs (Vclass 3) and high $A_{mod}$ values ($A_{mod}$ $>$0.5). Galaxy 87 is a
face-on system in the Swarm with a regular optical morphology, but an extended
unsettled H i disc. We note a nearby companion $\sim$ 70” to the south-west.
Galaxy 99 is in a substructure in A2626, not only does it have an unsettled H
i disc, but optically it is also very disturbed. It is likely that it has
experienced a tidal interaction with another galaxy in the substructure.
Galaxy 130 is a foreground galaxy at a redshift of zHI=0.0382, and therefore
is excluded from our overall analysis. Its optical morphology is that of an
early-type, barred ring galaxy. H i emission is not only found in the bright
central regions of the galaxy, but clumps of H i gas are also found in the
faint outer stellar ring surrounding the bar, similar to NGC 4736 (Bosma et
al., 1977).

###  5.3 Interacting galaxies

In Fig. 12 (c), we present examples of interacting galaxies in our sample that
are confirmed to have similar velocities based on the MMT as well as H i
redshifts (Healy et al., 2021b). Among the interacting galaxies shown in Fig.
12 (c), galaxies 15, 122, 135, 138, 163 and 164 are in A2626 itself. Galaxies
52 and 57 are in the Swarm while galaxies 93, 96, 108, 109, and 174 are in the
background cluster A2637. The H i emission of galaxies 15, 122, and 174 is
blended with that of their companion galaxies. We conclude that the
gravitational interactions are prevalent in both the group and the cluster
environment in and around A2626.

##  6 Discussion

Healy et al. (2021a) presented the first results of our MeerKAT observations
of A2626, including online atlas pages of the H i detected galaxies. In this
work, we characterised H i asymmetries in various ways and calculated H i
deficiencies. We then related these to local and global environment and
specific galaxy properties such as SFR, stellar mass etc. As discussed in the
review paper by Cortese et al. (2021), during the last few Gyrs in today’s
passive galaxies, it is inevitable that most of the primary quenching
mechanisms must have played a role in shaping their properties (e.g. McGee et
al. 2009; Han et al. 2018). Since multiple processes can be at play
simultaneously, it is difficult to identify the dominant process
observationally. Still, as mentioned before, by exploring various gas and star
formation properties of galaxies, we can investigate which plausible physical
mechanisms are acting on their H i gas reservoirs. Mechanical gas removal
processes like ram-pressure stripping and tidal stripping are likely to push
or pull the H i gas away from the stellar body, causing offset and asymmetric
H i discs, or uni-directional tails. Sometimes, however, when exposed to ram-
pressure for a long time, H i discs lose most of the gas from the outer disc
and become truncated. Processes like starvation, harassment or thermal
evaporation mainly occur in dense cluster environments and also give rise to
truncated or small H i discs, rather than offset H i discs. So, if the H i
deficient galaxies have settled, small H i discs, a gas removal process like
ram-pressure stripping probably has happened a while ago and / or other more
subtle processes may be at play such as starvation or thermal evaporation. In
this section, we address some of these observational findings and questions
raised by our analysis.

###  6.1 Trends with projected distance

First of all, the clearest result we found while examining the H i properties
of the galaxies in A2626 is the increase of H i deficient galaxies towards the
cluster core in Fig. 7. This trend probably indicates an increase of gas
removal efficiencies in the cluster core, similar to what previous
observations have found (e.g. Solanes et al. 2001; Chung et al. 2009).
Moreover, H i detected galaxies in the substructures are mainly found in the
cluster outskirts. Interestingly, beyond R200, both non-substructure and
substructure galaxies in A2626 as well as the galaxies in the Swarm have a
similar range of H i deficiencies. This hints at pre-processing in some of the
substructure galaxies in A2626 and in the Swarm galaxies, before they are
fully exposed to the intra-cluster medium.

Next, we have investigated whether the shapes of the outer H i discs are also
affected by the different physical mechanisms driving H i deficiencies in the
various environments in and around A2626. In Fig. 8, we have explored how H i
deficiency vs. projected distance from the cluster centre depends on visual
classes (top panel), H i offset (middle panel), and $A_{mod}$ (bottom panel).
In the top panel of Fig. 8, the VClass 3 or unsettled galaxies (stars) seem to
have H i deficiencies similar to field galaxies and even tend to be slightly
more gas rich on average. Thus, these unsettled galaxies are still in an
active stage of evolution and their H i discs still contain a large gas
reservoir for star formation. The occurrence of more H i asymmetric galaxies
in substructures of A2626 is possibly due to more effective tidal interactions
among the neighbouring galaxies in the substructure environment than the non-
substructure galaxies in A2626. But these interactions do not remove the H i
gas efficiently, making the galaxies in the substructures only moderately H i
deficient. This hints at ongoing pre-processing in the substructure galaxies
in A2626 before they fall into the cluster core. Considering the most H i
deficient galaxies (HIdef$>$0.3) within $\sim$ 1.5 R200, we find that 12 out
of the 17 galaxies have disturbed H i discs (VClass 2), indicating that they
may be still under the influence of ram-pressure stripping. The other 5
galaxies have settled H i discs (VClass 1), but their H i discs seem to be
small with respect to their stellar discs, indicating that they are in an
advanced stage of ram-pressure stripping or under the influence of processes
like starvation or thermal evaporation. In the middle panel of Fig. 8, we do
not observe any clear correlation between H i deficiency and the amount of H i
offset. This can be understood by realizing that the direction of ram-pressure
stripping is not always perpendicular to the line-of-sight and the small
offsets induced by harassment would not have a preferred direction either. In
the bottom panel of Fig. 8, we find no clear relation of H i deficiency with
$A_{mod}$ although the most H i deficient galaxy is highly asymmetric.

###  6.2 Trends with respect to the SFMS

In Fig. 9 and 11, we have explored the location of the A2626 and the Swarm
galaxies with respect to the SFMS as well as the relation between their H i
depletion times and stellar masses. The fact that the galaxies in all three
environments (both the substructure and non-substructure galaxies in A2626 and
the Swarm galaxies) tend to be below the SFMS from Cluver et al. (2020) means
that both the cluster and group environments are inducing slightly lower SFR
than the expected SFR for normal galaxies. There is no clear relation between
H i deficiency and offset from the SFMS, i.e. SF deficiency, for both the
cluster and the Swarm galaxies. This implies that the mechanisms causing H i
deficiencies are probably not immediately impacting the SFR and vice versa,
irrespective of the environment.

However, some galaxies (e.g. substructure galaxy 19 and non-substructure
galaxy 88 in A2626 in Fig. 9) have higher SFR than the SFMS. Their H i
morphologies appear to be fairly regular (insets of the top left panel in Fig.
9) while they are moderately H i deficient and have short H i depletion times.
Galaxy 88 was identified as a jellyfish candidate galaxy from the B-band
optical image by Poggianti et al. (2016). However, since its H i morphology is
regular and it is only moderately H i deficient, this galaxy was later
identified as a non-jellyfish galaxy in Deb et al. (2022). We have further
inspected H i maps of some other outlier galaxies from the SFMS. Galaxy 150 is
below the quenching threshold, yet it is not particularly H i deficient. The H
i disc of this galaxy probably has a low column density, below the star
formation limit which results in a large depletion time because of the lower
SFR.

We also investigated the H i maps of the most H i deficient galaxies in A2626
(galaxy 16, 65, 110, 128, see Fig. 9 and 7), all four of them located below
the SFMS. Though the H i disc of galaxy 65 seems small and regular, the other
three galaxies have asymmetric H i morphologies. As already shown with H i
contours overlaid on the optical colour images in Fig. 7, all these galaxies
have small, offset, or truncated H i discs with yellowish optical colours. As
mentioned before, these three galaxies are below the SFMS, signifying low
relative SFRs, although they are still above the star formation quenching
threshold. However, because of their high H i deficiencies, they have very
short depletion times. Thus, all these four galaxies are plausibly in the last
stages of exhausting their gas and on their way to drop below the star
formation quenching threshold.

Focusing on the H i morphologies of the most H i deficient galaxies in the
Swarm (galaxy 25, 60, 70, 105 in Fig. 9), we note that all of them have nearby
companions, and two of these galaxies (galaxy 60, 105) have asymmetric or
offset H i discs. From the optical colour, galaxy 105 seems dusty, that might
have caused some internal reddening. Galaxy 60 is tidally interacting, with a
visible stellar stream to the west, similar to the tidally interacting system
Arp 282 (Zaragoza-Cardiel et al., 2018). Though we do not have optical or H i
redshift information for the companions of galaxies 25 and 70, these galaxies
are clearly experiencing tidal interactions, given their H i morphologies.

Finally, we have investigated the relation between SF deficiency and H i
morphology using the three different morphological definitions (visual
classes, H i offset, $A_{mod}$, see Fig. 11). The fact that there is no
correlation between SF deficiency and H i offset or $A_{mod}$ implies that an
enhancement or quenching of their star formation rates are not directly
related to their H i morphologies or the environment in which they reside.

We conclude this discussion by noting that the H i content and H i
morphologies of galaxies are useful tracers of various environmental effects.
Although the galaxies are H i deficient close to the cluster centre, those H i
deficient galaxies do not always have disturbed or unsettled H i discs.
Consequently, apart from processes like ram-pressure stripping and tidal
interactions, more subtle processes like harassment and thermal evaporation
may be at play in A2626. The presence of more H i asymmetric galaxies in the
substructures of A2626 indicates that tidal interactions between neighbouring
galaxies are already pre-processing the substructure galaxies before they fall
into the cluster core. The fact that SF deficiency is not related to the H i
deficiency or H i morphology of galaxies in all three environments suggests
that the physical mechanisms causing disturbed H i morphologies take some time
to affect the star formation activity. Although investigating the H i
deficiencies and morphologies of galaxies is important, the interpretation is
difficult without ancillary data to better understand the effect of nurture on
galaxy evolution.

##  7 Summary

In this work, we have presented H i deficiencies, H i morphologies, and star
formation deficiencies of the H i detected galaxies in and around the A2626
cluster observed with MeerKAT. There are three main environments in our
surveyed volume: non-substructure or isolated galaxies in A2626 (cluster
environment), substructure galaxies in A2626 (groups influenced by the cluster
environment) and the Swarm galaxies (group environment). We are interested in
understanding if there is any correlation between the H i deficiency, H i
morphology, and the star formation deficiency of the H i detected galaxies and
the environment in which the galaxies are residing.

To characterise the H i morphology of the outer H i distribution of individual
galaxies, we have used three approaches. First, we have used three visual
classifications based on the outermost reliable H i contour (settled, one-
sided asymmetric, unsettled galaxies). Second, we have measured the offset of
the H i distribution with respect to the optical centre of the galaxy. Third,
we have calculated the modified asymmetry parameter $A_{mod}$ (as introduced
by Lelli et al. 2014), which depends on the choice of the galaxy centre, the H
i column density above which it is measured, and how well the galaxy is
spatially resolved. We chose an H i column density threshold of 25 $\times
10^{19}$ cm-2 to calculate the $A_{mod}$ value for galaxies with at least
three beams in size and with a peak signal-to-noise$\geq$ 5.

First, we have explored the relationship between our three different types of
classifications of the H i morphologies. We have plotted $A_{mod}$ vs. H i
offset from the optical centre and found a very strong correlation between
them as expected. We have found that high $A_{mod}$ ($A_{mod}$ $\geq$ 0.4),
low H i offset galaxies are strongly H i asymmetric galaxies. In case of
galaxies with both high $A_{mod}$ and high H i offsets, the $A_{mod}$ value is
driven by the large offset of the H i disc compared to the stellar disc.
Galaxies with a low $A_{mod}$ value and a small H i offset are mostly H i
symmetric galaxies with settled H i discs. There is a strong correspondence
between $A_{mod}$ and our visual classifications; indicating that all three
characterisations of H i morphology are useful tracers of H i asymmetries.

In Fig. 8 we have investigated whether an environmental dependence exists on
the H i asymmetry and H i offset. We find that substructures contain a higher
fraction of asymmetric galaxies compared to the population of non-substructure
galaxies in A2626. This suggests that tidal interactions are more efficient in
substructures than outside substructures. This might be expected because the
relative velocity differences of the neighbouring galaxies in the
substructures tend to be lower compared to the cluster galaxies outside the
substructures.

There is a strong correlation between H i deficiency and projected distance
from the cluster centre. Outside R200, non-substructure and substructure
galaxies in A2626, and galaxies in the Swarm have a similar range of H i
deficiencies. This signifies that the mechanisms that cause H i deficiencies
are possibly pre-processing the galaxies prior to their infall into the
cluster. Focusing on the most H i deficient and most H i rich galaxies in
A2626, we observe that the H i deficient galaxies have yellowish optical
colours and have small and truncated H i discs while the H i rich galaxies
have bluish optical colours and extended H i discs. This clearly shows that
the H i deficient galaxies are in a later stage of their evolution while H i
rich galaxies are actively star forming with sufficient amounts of H i gas.

Galaxies with an offset or asymmetric H i disc are found in all three
environments that we considered, implying that pre-processing of the H i discs
plays an important role. However, though the substructure galaxies in A2626
are more H i asymmetric than the non-substructure galaxies, the asymmetric
galaxies are not necessarily H i deficient. This suggests that, although the
substructure environment instigates more interactions among galaxies, those
interactions do not deplete the H i gas out of those galaxies completely.

The galaxy population in all three different environments (substructure, non-
substructure, and the Swarm) are mostly below the SFMS from Cluver et al.
(2020). This means that the gas removal mechanisms in the cluster environment
as well as pre-processing in the substructures and the Swarm are causing
slightly lower SFRs than the usual SFR for normal galaxies. There is no clear
relation between H i deficiency, H i asymmetry or H i offset with star
formation deficiency for the galaxies in all the three environments. This
signifies that the environmental mechanisms causing H i deficiencies or
asymmetries are probably not immediately impacting the star formation activity
and vice versa.

We conclude that H i deficiency and H i morphology are good tracers of
different environmental mechanisms acting on galaxies and their H i
characteristics and star formation deficiencies are affected by pre-processing
before the galaxies enter the cluster environment.

Encouraged by the high number of direct H i detections, thanks to the
unprecedented sensitivity of the MeerKAT telescope, the H i study of galaxies
in and around A2626 will be expanded by four additional MeerKAT pointings in
32k-mode covering the ENE-SSW sector of A2626. The primary motivation of those
observations is to enable a detailed investigation of so called pre-processing
of H i discs of galaxies before they have enter the cluster environment. The
higher spectral resolution of these observations will supplement the analysis
of H i morphologies presented in this paper and, additionally, will allow for
detailed investigations of the kinematical asymmetries of the detected
galaxies.

###### Acknowledgements.

This paper makes use of the MeerKAT data (Project ID: SCI-20190418-JH-01). The
MeerKAT telescope is operated by the South African Radio Astronomy
Observatory, which is a facility of the National Research Foundation, an
agency of the Department of Science and Innovation. MV acknowledges the
Netherlands Foundation for Scientific Research support through VICI grant
016.130.338 and the Leids Kerkhoven-Bosscha Fonds (LKBF) for travel support.
JMvdH acknowledges support from the European Research Council under the
European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Grant
Agreement nr. 291531 (HiStoryNU)

## References

  * Abramson et al. (2011) Abramson, A., Kenney, J. D., Crowl, H. H., et al. 2011, Astronomical Journal, 141
  * Aguado et al. (2019) Aguado, D. S., Ahumada, R., Almeida, A., et al. 2019, The Astrophysical Journal Supplement Series, 240, 23
  * Alpaslan et al. (2016) Alpaslan, M., Grootes, M., Marcum, P. M., et al. 2016, Monthly Notices of the Royal Astronomical Society, 457, 2287
  * Baldry et al. (2004) Baldry, I. K., Glazebrook, K., Brinkmann, J., et al. 2004, The Astrophysical Journal, 600, 681
  * Balogh et al. (2009) Balogh, M. L., McGee, S. L., Wilman, D., et al. 2009, Monthly Notices of the Royal Astronomical Society, 398, 754
  * Balogh et al. (2000) Balogh, M. L., Navarro, J. F., & Morris, S. L. 2000, ApJ, 540, 113
  * Batuski & Burns (1985) Batuski, D. J. & Burns, J. O. 1985, Astrophysical Journal, 299, 5
  * Bigiel et al. (2008) Bigiel, F., Leroy, A., Walter, F., et al. 2008, aj, 136, 2846
  * Bilimogga et al. (2022) Bilimogga, P. V., Oman, K. A., Verheijen, M. A. W., & van der Hulst, T. 2022, MNRAS, 513, 5310
  * Bosma et al. (1977) Bosma, A., van der Hulst, J. M., & Sullivan, W. T., I. 1977, aap, 57, 373
  * Brinchmann et al. (2004) Brinchmann, J., Charlot, S., White, S. D. M., et al. 2004, MNRAS, 351, 1151
  * Byrd & Valtonen (1990) Byrd, G. & Valtonen, M. 1990, ApJ, 350, 89
  * Chamaraux et al. (1986) Chamaraux, P., Balkowski, C., & Fontanelli, P. 1986, aap, 165, 15
  * 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. (2014) Cluver, M. E., Jarrett, T. H., Hopkins, A. M., et al. 2014, ApJ, 782, 90
  * Cluver et al. (2020) Cluver, M. E., Jarrett, T. H., Taylor, E. N., et al. 2020, ApJ, 898, 20
  * Conselice et al. (2003) Conselice, C. J., Bershady, M. A., Dickinson, M., & Papovich, C. 2003, AJ, 126, 1183
  * Cortese et al. (2021) Cortese, L., Catinella, B., & Smith, R. 2021, pasa, 38, e035
  * Cowie & Songaila (1977) Cowie, L. L. & Songaila, A. 1977, Nature, 266, 501
  * Crain et al. (2015) Crain, R. A., Schaye, J., Bower, R. G., et al. 2015, MNRAS, 450, 1937
  * Davies & Lewis (1973) Davies, R. D. & Lewis, B. M. 1973, Monthly Notices of the Royal Astronomical Society, 165, 231
  * Deb et al. (2020) Deb, T., Verheijen, M. A. W., Gullieuszik, M., et al. 2020, MNRAS, 494, 5029
  * Deb et al. (2022) Deb, T., Verheijen, M. A. W., Poggianti, B. M., et al. 2022, arXiv e-prints, arXiv:2208.12950
  * Dénes et al. (2014) Dénes, H., Kilborn, V. A., & Koribalski, B. S. 2014, mnras, 444, 667
  * Dey et al. (2019) Dey, A., Schlegel, D. J., Lang, D., et al. 2019, AJ, 157, 168
  * Dressler (1980) Dressler, A. 1980, The Astrophysical Journal, 236, 351
  * Elbaz et al. (2007) Elbaz, D., Daddi, E., Le Borgne, D., et al. 2007, A&A, 468, 33
  * Giese et al. (2016) Giese, N., van der Hulst, T., Serra, P., & Oosterloo, T. 2016, MNRAS, 461, 1656
  * Gunn & Gott (1972) Gunn, J. E. & Gott, III, J. R. 1972, ApJ, 176, 1
  * Han et al. (2018) Han, S., Smith, R., Choi, H., et al. 2018, apj, 866, 78
  * Haynes et al. (1984) Haynes, M. P., Giovanelli, R., & Chincarini, G. L. 1984, Annual Review of Astronomy and Astrophysics, 22, 445
  * Healy et al. (2021a) Healy, J., Deb, T., Verheijen, M. A. W., et al. 2021a, arXiv e-prints, arXiv:2106.13018
  * Healy et al. (2021b) Healy, J., Willner, S. P., Verheijen, M. A. W., & Blyth, S. L. 2021b, arXiv e-prints, arXiv:2106.08806
  * Holwerda et al. (2013) Holwerda, B. W., Pirzkal, N., de Blok, W. J. G., & Blyth, S. L. 2013, mnras, 435, 1020
  * Holwerda et al. (2011) Holwerda, B. W., Pirzkal, N., de Blok, W. J. G., et al. 2011, MNRAS, 416, 2415
  * Jaffé et al. (2015) Jaffé, Y. L., Smith, R., Candlish, G. N., et al. 2015, Monthly Notices of the Royal Astronomical Society, 448, 1715
  * Jaffé et al. (2016) Jaffé, Y. L., Verheijen, M. A. W., Haines, C. P., et al. 2016, MNRAS, 461, 1202
  * Kennicutt (1989) Kennicutt, Robert C., J. 1989, apj, 344, 685
  * Kennicutt (1998) Kennicutt, Jr., R. C. 1998, araa, 36, 189
  * Kleiner et al. (2017) Kleiner, D., Pimbblet, K. A., Jones, D. H., Koribalsk, B. S., & Serra, P. 2017
  * Kraljic et al. (2018) Kraljic, K., Arnouts, S., Pichon, C., et al. 2018, Monthly Notices of the Royal Astronomical Society, 474, 547
  * Laigle et al. (2018) Laigle, C., Pichon, C., Arnouts, S., et al. 2018, Monthly Notices of the Royal Astronomical Society, 474, 5437
  * Larson et al. (1980) Larson, R. B., Tinsley, B. M., & Caldwell, C. N. 1980, ApJ, 237, 692
  * Lelli et al. (2014) Lelli, F., Verheijen, M., & Fraternali, F. 2014, mnras, 445, 1694
  * Loni et al. (2021) Loni, A., Serra, P., Kleiner, D., et al. 2021, A&A, 648, A31
  * Lotz et al. (2004) Lotz, J. M., Primack, J., & Madau, P. 2004, AJ, 128, 163
  * McGee et al. (2009) McGee, S. L., Balogh, M. L., Bower, R. G., Font, A. S., & McCarthy, I. G. 2009, mnras, 400, 937
  * Merritt (1983) Merritt, D. 1983, ApJ, 264, 24
  * Molnar et al. (2021) Molnar, D. C., Serra, P., van der Hulst, T., et al. 2021, arXiv e-prints, arXiv:2112.12244
  * Moore et al. (1996) Moore, B., Katz, N., Lake, G., Dressler, A., & Oemler, A. 1996, Nature, 379, 613
  * Nelson (1982) Nelson, A. H. 1982, Nature, 295, 263
  * Noeske et al. (2007) Noeske, K. G., Weiner, B. J., Faber, S. M., et al. 2007, ApJ, 660, L43
  * Poggianti et al. (2016) Poggianti, B. M., Fasano, G., Omizzolo, A., et al. 2016, AJ, 151, 78
  * Poggianti et al. (2017) Poggianti, B. M., Moretti, A., Gullieuszik, M., et al. 2017, ApJ, 844, 48
  * Ramatsoku et al. (2020) Ramatsoku, M., Serra, P., Poggianti, B. M., et al. 2020, A&A, 640, A22
  * Ramatsoku et al. (2019) Ramatsoku, M., Serra, P., Poggianti, B. M., et al. 2019, MNRAS, 487, 4580
  * Sarron et al. (2018) Sarron, F., Martinet, N., Durret, F., & Adami, C. 2018, Astronomy and Astrophysics, 613, 67
  * Schaye et al. (2015) Schaye, J., Crain, R. A., Bower, R. G., et al. 2015, MNRAS, 446, 521
  * Serra et al. (2013) Serra, P., Koribalski, B., Duc, P. A., et al. 2013, Monthly Notices of the Royal Astronomical Society, 428, 370
  * Serra et al. (2015) Serra, P., Westmeier, T., Giese, N., et al. 2015, Monthly Notices of the Royal Astronomical Society, 448, 1922
  * 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, The Astrophysical Journal, 548, 97
  * Speagle et al. (2014) Speagle, J. S., Steinhardt, C. L., Capak, P. L., & Silverman, J. D. 2014, ApJS, 214, 15
  * Spitzer & Baade (1951) Spitzer, Jr., L. & Baade, W. 1951, ApJ, 113, 413
  * Springel (2000) Springel, V. 2000, MNRAS, 312, 859
  * Tinsley & Larson (1979) Tinsley, B. M. & Larson, R. B. 1979, MNRAS, 186, 503
  * Tomczak et al. (2016) Tomczak, A. R., Quadri, R. F., Tran, K.-V. H., et al. 2016, ApJ, 817, 118
  * Tonnesen et al. (2007) Tonnesen, S., Bryan, G. L., & van Gorkom, J. H. 2007, The Astrophysical Journal, 671, 1434
  * Toomre & Toomre (1972) Toomre, A. & Toomre, J. 1972, The Astrophysical Journal, 178, 623
  * Valluri (1993) Valluri, M. 1993, ApJ, 408, 57
  * Vulcani et al. (2019) Vulcani, B., Poggianti, B. M., Moretti, A., et al. 2019, Monthly Notices of the Royal Astronomical Society, 487, 2278
  * Williams & Rood (1987) Williams, B. A. & Rood, H. J. 1987, The Astrophysical Journal Supplement Series, 63, 265
  * Yoon et al. (2017) Yoon, H., Chung, A., Smith, R., & Jaffé, Y. L. 2017, The Astrophysical Journal, 838, 81
  * York et al. (2000) York, D. G., Adelman, J., John E. Anderson, J., et al. 2000, The Astronomical Journal, 120, 1579
  * Zaragoza-Cardiel et al. (2018) Zaragoza-Cardiel, J., Smith, B. J., Rosado, M., et al. 2018, apjs, 234, 35

##  Appendix A Catalogue of H i detections.

Table 4: A table that lists all of the relevant H i and stellar physical and morphological characteristics of the galaxies used in this study. The column entries of table LABEL:tab:catalogue are as follows: Column (1): The assigned H i identifier from Healy et al. 2021a. Column (2): The SDSS identifier for the optical counterpart of the H i detected galaxies, based on their Right Ascension and Declination (J2000.0). Column (3): Substructure identifier. SS=‘0’,‘1’: non-substructure and substructure galaxies in A2626, SS=‘2’: galaxies in the Swarm. Column (4): Redshift measured as the midpoint of the 20% line width of the H i global profile. Column (5,6): Log of H i mass and uncertainty as mentioned in Healy et al. 2021a. Column (7): H i deficiency using the scaling relation from Dénes et al. (2014). Column (8): Calculated modified asymmetry using the definition of Lelli et al. (2014). Column (9): Measured offset of the H i centre from the optical centre (in kpc). Column (10): Visual classifications of galaxies. ‘1’: settled sources, ‘2’: disturbed sources, ‘3’: unsettled sources. Column (11,12): Log of stellar mass and uncertainty derived from M/L from Cluver et al. (2014). Column (13,14): 12 $\mu$m star formation rates star formation rate and uncertainty from WISE observations (provided by T. Jarrett, private communication). Column (15): g-r magnitude from DECaLS (Dey et al. 2019) survey (corrected for extinction.) HI ID | Name | SS | z | Log(MHI ) | $\pm$ | HIdef | Amod |  H i offset | VClass | Log(M∗ ) | $\pm$ | SFR | $\pm$ | g-r
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---
|  |  |  |  | (kpc) |  |  | (M⊙/yr) | mag
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11) | (12) | (13) | (14) | (15)
10 | J233409.36+211641.9 | 1 | 0.0524 | 9.24 | 0.09 | 0.12 | 0.60 | 2.79 | 2 | 8.83 | 1.28 | 0.23 | -.– | 0.42
12 | J233413.05+212327.5 | 1 | 0.0515 | 9.60 | 0.05 | 0.07 | 0.48 | 2.75 | 2 | 9.52 | 0.13 | 0.55 | 0.22 | 0.55
13 | J233425.70+213122.9 | 1 | 0.0553 | 9.74 | 0.04 | -0.02 | 0.43 | 3.35 | 2 | 9.56 | 1.27 | 0.37 | -.– | 0.46
15 | J233438.15+211851.7 | 1 | 0.0538 | 9.93 | 0.03 | -0.07 | 0.45 | 2.06 | 2 | 10.36 | 0.15 | 3.33 | 1.15 | 0.73
16 | J233438.80+211721.0 | 1 | 0.0529 | 9.07 | 0.08 | 0.64 | 0.87 | 5.85 | 2 | 10.22 | 0.11 | 2.0 | 0.70 | 0.73
17 | J233440.31+203710.8 | 0 | 0.0573 | 9.95 | 0.04 | 0.01 | 0.64 | 6.18 | 2 | 10.46 | 0.10 | 3.08 | 1.07 | 0.70
19 | J233453.14+213344.9 | 1 | 0.0554 | 9.38 | 0.06 | 0.39 | 0.49 | 0.85 | 2 | 9.78 | 0.13 | 2.03 | 0.71 | 0.77
21 | J233455.82+212245.8 | 1 | 0.0525 | 9.50 | 0.04 | 0.21 | 0.54 | 3.38 | 2 | 9.63 | 1.27 | 0.36 | -.– | 0.52
23 | J233500.37+205908.6 | 0 | 0.0588 | 9.31 | 0.06 | 0.34 | 0.39 | 1.80 | 1 | 9.18 | 1.27 | 0.27 | -.– | 0.40
24 | J233510.54+212147.8 | 1 | 0.0527 | 9.35 | 0.05 | 0.42 | 0.37 | 0.93 | 2 | 9.99 | 0.26 | 0.74 | 0.28 | 0.63
25 | J233512.34+214628.1 | 2 | 0.0649 | 9.58 | 0.08 | -.– | 0.29 | 0.79 | 1 | 10.23 | 0.2 | 2.14 | 0.76 | 0.60
29 | J233525.93+204419.5 | 2 | 0.0617 | 9.26 | 0.09 | 0.07 | 0.34 | 2.16 | 1 | 8.81 | 1.27 | 0.31 | -.– | 0.37
31 | J233526.81+211638.3 | 2 | 0.0650 | 9.87 | 0.03 | -0.23 | 0.35 | 0.31 | 2 | 10.12 | 0.14 | 1.34 | 0.48 | 0.83
32 | J233527.64+204059.2 | 2 | 0.0662 | 9.54 | 0.09 | -0.36 | 0.87 | 37.91 | 1 | 8.85 | 1.29 | 0.33 | -.– | 0.36
33 | J233528.63+203803.2 | 2 | 0.0625 | 9.54 | 0.09 | -0.04 | 0.21 | 1.10 | 1 | 9.22 | 1.27 | 0.42 | -.– | 0.29
36 | J233532.73+211011.3 | 2 | 0.0659 | 10.21 | 0.01 | -0.51 | 0.35 | 1.31 | 1 | 9.91 | 0.31 | 1.01 | 0.39 | 0.46
37 | J233533.49+210252.1 | 0 | 0.0567 | 9.63 | 0.04 | 0.13 | 0.46 | 2.08 | 2 | 10.40 | 0.24 | 2.19 | 0.77 | 0.71
39 | J233535.77+204159.6 | 1 | 0.0613 | 9.99 | 0.04 | -0.10 | 0.50 | 2.56 | 3 | 10.72 | 0.10 | 3.88 | 1.35 | 0.78
40 | J233535.93+203807.1 | 2 | 0.0626 | 10.02 | 0.03 | -0.51 | 0.29 | 2.50 | 1 | 10.12 | 0.26 | 0.37 | 0.17 | 0.48
42 | J233536.98+210440.3 | 0 | 0.0545 | 9.23 | 0.04 | -0.04 | 0.33 | 1.93 | 2 | 9.44 | 1.27 | 0.35 | -.– | 0.54
43 | J233537.03+204639.6 | 0 | 0.0486 | 9.21 | 0.06 | 0.46 | 0.47 | 1.50 | 2 | 9.33 | 1.27 | 0.29 | -.– | 0.53
44 | J233537.44+211025.1 | 2 | 0.0654 | 9.12 | 0.08 | 0.17 | 0.86 | 1.75 | 2 | 8.99 | 1.29 | 0.39 | -.– | 0.29
45 | J233539.99+210844.6 | 0 | 0.0613 | 9.81 | 0.02 | -0.17 | 0.25 | 1.05 | 3 | 9.54 | 1.27 | 0.26 | -.– | 0.49
46 | J233540.43+205357.4 | 0 | 0.0578 | 8.90 | 0.11 | -.– | 0.97 | 1.94 | 1 | -.– | -.– | -.– | -.– | 0.29
52 | J233544.62+210228.7 | 2 | 0.0663 | 9.55 | 0.04 | 0.18 | 0.13 | 0.54 | 1 | 9.69 | 0.25 | 0.73 | 0.29 | 0.62
53 | J233545.43+205045.5 | 2 | 0.0639 | 9.10 | 0.09 | 0.13 | 0.64 | 2.15 | 2 | -.– | -.– | -.– | -.– | 0.47
54 | J233546.25+210031.0 | 1 | 0.0540 | 9.34 | 0.04 | -0.04 | 0.30 | 0.98 | 1 | 8.63 | 1.28 | 0.27 | -.– | 0.32
55 | J233546.32+203026.4 | 2 | 0.0655 | 10.16 | 0.04 | -0.48 | 0.57 | 0.75 | 3 | 10.34 | 0.14 | 2.07 | 0.73 | 0.59
57 | J233547.60+210207.0 | 2 | 0.0636 | 9.28 | 0.05 | 0.39 | 0.72 | 3.94 | 3 | 10.38 | 0.13 | 2.38 | 0.84 | 0.72
58 | J233547.85+204226.6 | 2 | 0.0619 | 10.02 | 0.03 | 0.24 | 0.29 | 3.37 | 1 | 11.23 | 0.10 | 5.04 | 1.76 | 0.87
59 | J233549.79+211356.4 | 2 | 0.0661 | 8.96 | 0.08 | 0.43 | 1.00 | 3.91 | 2 | 9.47 | 1.14 | 0.4 | 0.2 | 0.44
60 | J233550.08+202318.3 | 2 | 0.0639 | 10.09 | 0.05 | -.– | 0.74 | 7.28 | 3 | 10.54 | 0.12 | 2.75 | 0.97 | 0.82
62 | J233556.63+211333.0 | 2 | 0.0660 | 9.53 | 0.04 | 0.01 | 0.13 | 0.24 | 1 | 9.68 | 1.27 | 0.29 | -.– | 0.44
63 | J233557.05+211105.5 | 2 | 0.0649 | 9.94 | 0.03 | -0.06 | 0.33 | 1.10 | 1 | 10.42 | 0.13 | 3.80 | 1.33 | 1.17
64 | J233557.38+205307.1 | 0 | 0.0597 | 9.42 | 0.05 | 0.31 | 0.55 | 2.61 | 2 | 9.89 | 0.35 | 0.41 | 0.19 | 0.60
65 | J233557.68+211703.5 | 0 | 0.0501 | 8.99 | 0.06 | 0.71 | 0.22 | 0.92 | 1 | 9.44 | 0.38 | 0.26 | 0.14 | 0.53
70 | J233602.82+210821.1 | 2 | 0.0644 | 9.35 | 0.05 | -.– | 0.30 | 0.98 | 1 | 9.81 | 0.27 | 0.72 | 0.31 | 0.26
71 | J233604.52+210613.3 | 2 | 0.0661 | 9.65 | 0.03 | 0.25 | 0.90 | 4.83 | 3 | 10.77 | 0.10 | 0.24 | 0.13 | 0.92
73 | J233606.20+203113.8 | 2 | 0.0650 | 9.61 | 0.07 | -0.19 | 0.54 | 3.40 | 2 | 8.98 | 1.27 | 0.34 | -.– | 0.38
75 | J233607.00+210500.1 | 0 | 0.0576 | 9.44 | 0.04 | 0.00 | 0.27 | 0.56 | 1 | 8.89 | 1.27 | 0.30 | -.– | 0.30
77 | J233609.55+205428.4 | 0 | 0.0492 | 9.05 | 0.06 | -.– | 0.83 | 2.78 | 1 | -.– | -.– | -.– | -.– | 0.32
79 | J233611.37+205702.0 | 0 | 0.0536 | 9.22 | 0.06 | 0.10 | 0.56 | 4.16 | 2 | -.– | -.– | -.– | -.– | 0.28
81 | J233614.66+214434.3 | 1 | 0.0560 | 9.71 | 0.04 | -0.11 | 0.45 | 2.75 | 2 | 9.70 | 1.27 | 0.50 | -.– | 0.37
84 | J233615.59+205047.1 | 0 | 0.0492 | 9.27 | 0.05 | 0.19 | 0.52 | 1.97 | 2 | 9.32 | 0.28 | 0.52 | 0.20 | 0.38
86 | J233616.89+215412.1 | 1 | 0.0565 | 9.35 | 0.10 | -0.11 | 0.37 | 0.93 | 1 | 9.12 | 1.27 | 0.24 | -.– | 0.23
87 | J233618.06+203054.6 | 2 | 0.0649 | 10.15 | 0.03 | -0.25 | 0.49 | 2.45 | 3 | 10.37 | 0.12 | 1.90 | 0.68 | 0.94
88 | J233618.57+210402.6 | 0 | 0.0533 | 9.77 | 0.02 | 0.20 | 0.56 | 3.40 | 2 | 9.89 | 0.14 | 2.31 | 0.81 | 0.47
90 | J233619.79+204950.3 | 0 | 0.0514 | 8.84 | 0.12 | 0.60 | 1.00 | 4.29 | 2 | 9.53 | 1.27 | 0.36 | -.– | 0.48
95 | J233624.23+204355.5 | 2 | 0.0634 | 9.22 | 0.07 | 0.21 | 0.49 | 1.43 | 1 | 9.34 | 1.27 | 0.31 | -.– | 0.38
99 | J233626.71+215204.9 | 1 | 0.0570 | 10.26 | 0.03 | -0.38 | 0.69 | 6.67 | 3 | 9.90 | 0.14 | 1.10 | 0.40 | 0.06
102 | J233631.03+205750.0 | 0 | 0.0585 | 9.21 | 0.07 | 0.49 | 0.70 | 6.88 | 2 | 10.03 | 0.14 | 2.02 | 0.71 | 0.99
103 | J233631.53+205325.7 | 2 | 0.0624 | 9.10 | 0.07 | 0.59 | 1.00 | 5.30 | 2 | 10.05 | 0.14 | 1.66 | 0.59 | 0.61
105 | J233632.47+204110.8 | 2 | 0.0646 | 9.98 | 0.04 | -.– | 0.77 | 6.74 | 2 | 10.24 | 0.10 | 5.75 | 1.99 | 0.81
108 | J233632.86+203424.6 | 2 | 0.0661 | 9.13 | 0.09 | 0.57 | 0.88 | 4.97 | 2 | 10.18 | 0.11 | 5.29 | 1.84 | 0.81
109 | J233634.21+203441.1 | 2 | 0.0673 | 9.55 | 0.05 | 0.22 | 0.29 | 0.44 | 2 | 10.22 | 0.10 | 6.65 | 2.31 | 0.59
110 | J233639.74+210606.8 | 0 | 0.0589 | 9.46 | 0.04 | 0.65 | 0.57 | 3.86 | 2 | 10.98 | 0.11 | 6.35 | 2.20 | 0.74
111 | J233642.05+212810.6 | 0 | 0.0531 | 9.33 | 0.05 | 0.26 | 0.68 | 3.28 | 2 | 9.88 | 0.11 | 0.69 | 0.26 | 0.45
113 | J233642.70+212212.0 | 0 | 0.0550 | 9.58 | 0.03 | 0.16 | 0.39 | 2.17 | 1 | 10.08 | 0.13 | 2.52 | 0.88 | 0.63
115 | J233645.05+205108.7 | 0 | 0.0530 | 9.28 | 0.05 | -0.21 | 0.90 | 5.14 | 3 | 9.06 | 0.52 | 0.37 | 0.17 | 0.56
117 | J233647.54+212506.6 | 0 | 0.0563 | 9.02 | 0.10 | 0.29 | 1.00 | 5.84 | 2 | -.– | -.– | -.– | -.– | 0.40
118 | J233647.73+204136.1 | 0 | 0.0539 | 9.91 | 0.02 | 0.11 | 0.28 | 1.65 | 1 | 10.18 | 0.11 | 1.27 | 0.45 | 0.61
119 | J233648.56+205031.3 | 0 | 0.0521 | 8.77 | 0.12 | 0.56 | 0.41 | 1.47 | 1 | 8.98 | 1.28 | 0.34 | -.– | 0.25
122 | J233650.53+203534.6 | 0 | 0.0571 | 9.88 | 0.04 | -0.10 | 0.68 | 6.92 | 3 | 9.69 | 1.27 | 0.23 | -.– | 0.51
124 | J233652.31+204719.4 | 0 | 0.0592 | 9.39 | 0.06 | 0.29 | 0.30 | 0.86 | 1 | 9.70 | 1.27 | 0.47 | -.– | 0.56
126 | J233652.59+203726.2 | 0 | 0.0506 | 9.43 | 0.06 | 0.31 | 0.54 | 3.68 | 2 | 9.11 | 1.27 | 0.39 | -.– | 0.44
127 | J233654.75+203635.9 | 0 | 0.0517 | 9.23 | 0.08 | 0.24 | 0.49 | 3.24 | 2 | 9.05 | 1.27 | 0.4 | -.– | 0.28
128 | J233655.10+212708.9 | 0 | 0.0528 | 9.46 | 0.05 | 0.67 | 0.52 | 2.75 | 2 | 10.99 | 0.21 | 4.43 | 1.54 | 0.79
129 | J233655.43+204826.1 | 0 | 0.0579 | 9.94 | 0.02 | 0.09 | 0.30 | 1.05 | 1 | 10.02 | 0.16 | 1.35 | 0.48 | 0.52
131 | J233657.81+204254.2 | 2 | 0.0633 | 9.57 | 0.05 | -0.05 | 0.51 | 2.57 | 3 | 9.56 | 1.27 | 0.48 | -.– | 0.40
132 | J233658.57+203442.4 | 0 | 0.0517 | 9.72 | 0.04 | 0.08 | 0.83 | 9.59 | 3 | 9.49 | 0.16 | 0.38 | 0.17 | 0.31
134 | J233702.06+204756.0 | 0 | 0.0516 | 9.55 | 0.03 | -.– | 0.70 | 6.93 | 2 | -.– | -.– | -.– | -.– | 0.00
135 | J233703.24+205326.8 | 1 | 0.0576 | 9.04 | 0.06 | 0.39 | 1.00 | 1.73 | 2 | 9.43 | 0.84 | 0.43 | 0.21 | 0.26
138 | J233703.72+205304.1 | 1 | 0.0580 | 9.48 | 0.04 | 0.01 | 0.54 | 3.44 | 1 | 9.74 | 0.46 | 0.34 | 0.17 | 0.47
139 | J233705.27+210739.1 | 2 | 0.0638 | 9.38 | 0.05 | 0.05 | 0.36 | 1.30 | 2 | 9.27 | 1.27 | 0.32 | -.– | 0.50
142 | J233711.63+204243.3 | 2 | 0.0641 | 9.68 | 0.04 | -0.69 | 0.77 | 6.52 | 2 | 9.35 | 1.27 | 0.29 | -.– | 0.72
143 | J233712.13+204554.1 | 1 | 0.0581 | 9.15 | 0.11 | 0.02 | 0.88 | 2.86 | 2 | -.– | -.– | -.– | -.– | 0.34
144 | J233712.73+210406.5 | 0 | 0.0584 | 9.35 | 0.05 | 0.31 | 0.48 | 1.90 | 1 | 9.68 | 0.18 | 0.84 | 0.32 | 0.54
148 | J233720.02+204934.0 | 1 | 0.0583 | 9.17 | 0.07 | 0.12 | 0.88 | 4.31 | 2 | 9.45 | 0.22 | 0.86 | 0.32 | 0.70
149 | J233720.15+213258.3 | 0 | 0.0577 | 9.31 | 0.08 | 0.28 | 0.68 | 3.57 | 2 | 9.67 | 1.27 | 0.25 | -.– | 0.39
150 | J233721.12+205717.6 | 0 | 0.0563 | 9.84 | 0.02 | 0.20 | 0.27 | 1.86 | 1 | 10.81 | 0.10 | 1.17 | 0.42 | 1.04
153 | J233723.08+205719.7 | 0 | 0.0581 | 9.18 | 0.06 | 0.28 | 0.48 | 1.66 | 1 | -.– | -.– | -.– | -.– | 0.33
157 | J233726.14+210016.1 | 0 | 0.0525 | 8.78 | 0.10 | -.– | 0.98 | 2.70 | 1 | -.– | -.– | -.– | -.– | 0.42
158 | J233731.24+205616.2 | 1 | 0.0575 | 9.26 | 0.09 | 0.00 | 0.29 | 1.61 | 2 | 8.63 | 1.28 | 0.31 | -.– | 0.49
159 | J233733.15+203213.4 | 1 | 0.0593 | 9.52 | 0.08 | -0.21 | 0.56 | 3.52 | 2 | 9.12 | 0.48 | 0.37 | 0.19 | 0.47
160 | J233735.81+210101.8 | 0 | 0.0527 | 8.92 | 0.08 | 0.49 | 0.33 | 1.01 | 1 | 9.66 | 1.19 | 0.59 | 0.24 | 0.48
161 | J233739.94+203118.8 | 1 | 0.0573 | 9.73 | 0.07 | 0.10 | 0.72 | 1.26 | 2 | 10.16 | 0.15 | 2.08 | 0.73 | 0.60
162 | J233741.04+213051.6 | 0 | 0.0538 | 9.50 | 0.05 | 0.08 | 0.71 | 3.38 | 2 | 9.46 | 0.12 | 0.35 | 0.17 | 0.54
166 | J233745.24+210742.1 | 2 | 0.0640 | 9.50 | 0.04 | 0.10 | 0.53 | 2.72 | 1 | 9.43 | 1.27 | 0.38 | -.– | 0.39
169 | J233746.51+212126.6 | 0 | 0.0533 | 9.07 | 0.06 | 0.33 | 0.62 | 3.37 | 2 | -.– | -.– | -.– | -.– | 0.38
170 | J233748.75+204013.3 | 1 | 0.0561 | 9.64 | 0.04 | 0.15 | 0.38 | 2.82 | 1 | 10.12 | 0.12 | 2.27 | 0.79 | 0.53
172 | J233750.60+212454.3 | 0 | 0.0553 | 9.32 | 0.06 | 0.09 | 0.35 | 0.53 | 2 | -.– | -.– | -.– | -.– | 0.29
173 | J233751.32+211127.1 | 0 | 0.0553 | 9.58 | 0.04 | -0.04 | 0.36 | 1.49 | 2 | 9.11 | 1.27 | 0.24 | -.– | 0.39
176 | J233756.65+205436.8 | 1 | 0.0559 | 9.21 | 0.06 | 0.49 | 0.54 | 2.55 | 2 | 9.99 | 0.24 | 0.70 | 0.28 | 0.62
177 | J233756.99+212234.8 | 0 | 0.0556 | 9.16 | 0.06 | -.– | 0.97 | 6.03 | 2 | -.– | -.– | -.– | -.– | 0.26
178 | J233758.90+204000.6 | 1 | 0.0551 | 9.60 | 0.05 | 0.13 | 0.72 | 4.04 | 2 | 9.60 | 0.14 | 0.4 | 0.20 | 0.60
179 | J233800.62+205722.0 | 0 | 0.0559 | 9.52 | 0.05 | 0.55 | 0.58 | 2.00 | 2 | 10.72 | 0.10 | 5.07 | 1.77 | 0.79
182 | J233808.40+205755.0 | 0 | 0.0551 | 9.39 | 0.06 | 0.19 | 1.00 | 12.62 | 3 | 9.98 | 0.32 | 0.38 | 0.19 | 0.60
191 | J233825.56+204911.2 | 1 | 0.0564 | 9.86 | 0.03 | 0.05 | 0.56 | 3.09 | 1 | 9.81 | 0.16 | 0.54 | 0.21 | 0.52
192 | J233826.00+203517.6 | 1 | 0.0577 | 9.83 | 0.06 | -0.10 | 0.39 | 1.42 | 2 | 9.71 | 1.27 | 0.51 | -.– | 0.57
194 | J233828.47+212848.0 | 0 | 0.0533 | 9.41 | 0.10 | 0.20 | 0.84 | 6.03 | 2 | 9.89 | 0.10 | 0.91 | 0.34 | 0.61
196 | J233832.29+203644.9 | 1 | 0.0565 | 9.51 | 0.09 | 0.16 | 0.68 | 5.22 | 2 | 9.52 | 1.27 | 0.49 | -.– | 0.34
198 | J233841.11+212336.3 | 2 | 0.0633 | 9.57 | 0.07 | 0.16 | 1.00 | 17.94 | 3 | -.– | -.– | -.– | -.– | 0.86
200 | J233846.64+204855.3 | 1 | 0.0555 | 9.31 | 0.07 | 0.09 | 0.67 | 6.26 | 2 | 8.96 | 1.27 | 0.23 | -.– | 0.41
201 | J233847.26+204720.0 | 1 | 0.0568 | 9.50 | 0.06 | 0.26 | 0.65 | 2.95 | 2 | 10.07 | 0.24 | 0.57 | 0.23 | 0.43
204 | J233848.60+212311.0 | 2 | 0.0637 | 9.41 | 0.09 | -0.01 | 0.62 | 2.84 | 2 | 9.90 | 0.33 | 2.22 | 0.79 | 0.36
205 | J233848.61+205550.2 | 1 | 0.0562 | 9.73 | 0.05 | -0.08 | 0.60 | 3.16 | 2 | 9.99 | 0.14 | 2.43 | 0.85 | 0.50
210 | J233859.60+211723.6 | 1 | 0.0558 | 9.52 | 0.06 | 0.19 | 0.61 | 4.74 | 2 | 9.07 | 1.27 | 0.42 | -.– | 0.49
218 | J233948.55+205857.4 | 0 | 0.0565 | 9.88 | 0.09 | -0.10 | 2.00 | 1.42 | 1 | 9.90 | 0.16 | 1.26 | 0.45 | 0.44