# WALLABY Pre-Pilot Survey: Radio Continuum Properties of the Eridanus
Supergroup

J. A. Grundy  International Centre for Radio Astronomy Research, Curtin
University, Bentley, WA6102, Australia  [ O. I. Wong  CSIRO Space and
Astronomy, PO Box 1130, Bentley, WA6102, Australia  K. Lee-Waddell
International Centre for Radio Astronomy Research, University of Western
Australia, 35 Stirling Hwy, Crawley, WA6009, Australia  N. Seymour
International Centre for Radio Astronomy Research, Curtin University, Bentley,
WA6102, Australia  B.-Q. For  International Centre for Radio Astronomy
Research, University of Western Australia, 35 Stirling Hwy, Crawley, WA6009,
Australia  C. Murugeshan  CSIRO Space and Astronomy, PO Box 1130, Bentley,
WA6102, Australia  B. S. Koribalski  CSIRO Astronomy and Space Science,
Australia Telescope National Facility, P.O. Box 76, NSW1710, Australia  J. P.
Madrid  The University of Texas Rio Grande Valley, One West University Blvd,
Brownsville, TX78520 USA  J. Rhee  International Centre for Radio Astronomy
Research, University of Western Australia, 35 Stirling Hwy, Crawley, WA6009,
Australia  T. Westmeier  International Centre for Radio Astronomy Research,
University of Western Australia, 35 Stirling Hwy, Crawley, WA6009, Australia

(20 Sep 2022; 24 Jan 2023; 23 Feb 2023)

###### Abstract

We present the highest resolution and sensitivity $\sim 1.4\,$GHz continuum
observations of the Eridanus supergroup obtained as a part of the Widefield
ASKAP _L_ -band Legacy All-sky Blind surveY (WALLABY) pre-pilot observations
using the Australian Square Kilometer Array Pathfinder (ASKAP). We detect 9461
sources at 1.37 GHz down to a flux density limit of $\sim 0.1$ mJy at
$6.1^{\prime\prime}\times 7.9^{\prime\prime}$ resolution with a mean root-
mean-square (RMS) of 0.05 mJy/beam. We find that the flux scale is accurate to
within 5 % (compared to NVSS at 1.4 GHz). We then determine the global
properties of eight Eridanus supergroup members, which are detected in both
radio continuum and neutral hydrogen (HI) emission, and find that the radio-
derived star formation rates (SFRs) agree well with previous literature. Using
our global and resolved radio continuum properties of the nearby Eridanus
galaxies, we measure and extend the infrared-radio correlation (IRRC) to lower
stellar masses and inferred star formation rates than before. We find the
resolved IRRC to be useful for: 1) discriminating between AGN and star-forming
galaxies (SFGs); 2) identifying background radio sources; and 3) tracing the
effects of group environment pre-processing in NGC 1385. We find evidence for
tidal interactions and ram-pressure stripping in the HI, resolved spectral
index and IRRC morphologies of NGC 1385. There appears to be a spatial
coincidence (in projection) of double-lobed radio jets with the central HI
hole of NGC 1367. The destruction of polycyclic aromatic hydrocarbons (PAHs)
by merger-induced shocks may be driving the observed WISE W3 deficit observed
in NGC 1359. Our results suggest that resolved radio continuum and IRRC
studies are excellent tracers of the physical processes that drive galaxy
evolution and will be possible on larger sample of sources with upcoming ASKAP
radio continuum surveys.

###### doi:

xx

###### keywords:

radio continuum: galaxies – galaxies: star formation – infrared: galaxies –
galaxies: groups: general – galaxies: interactions

CSIRO Space and Astronomy, PO Box 1130, Bentley, WA6102, Australia J. A.
<EMAIL_ADDRESS>International Centre for Radio
Astronomy Research, University of Western Australia, 35 Stirling Hwy, Crawley,
WA6009, Australia ARC Centre of Excellence for Astrophysics in Three
Dimensions (ASTRO 3D), Australia CSIRO Space and Astronomy, PO Box 1130,
Bentley, WA6102, Australia International Centre for Radio Astronomy Research,
Curtin University, Bentley, WA6102, Australia ARC Centre of Excellence for
Astrophysics in Three Dimensions (ASTRO 3D), Australia ARC Centre of
Excellence for Astrophysics in Three Dimensions (ASTRO 3D), Australia School
of Science, Western Sydney University, Locked Bag 1797, Penrith, NSW2751,
Australia ARC Centre of Excellence for Astrophysics in Three Dimensions (ASTRO
3D), Australia ARC Centre of Excellence for Astrophysics in Three Dimensions
(ASTRO 3D), Australia dd Mmm YYYY

##  1 Introduction

Star formation (SF) is one of the dominant physical processes in the formation
and evolution of galaxies. The majority of the star formation rate (SFR)
indicators in galaxies are either indirect indicators of young stars and/or
affected by dust extinction due to being in the ultraviolet and the optical
bands of the electromagnetic spectrum. Radio continuum emission is optically
thin at GHz frequencies and has been shown to be a dust unbiased tracer of
recent SF within galaxies (Davies et al., 2017). Synchrotron emission is
produced primarily by the acceleration of cosmic ray (CR) electrons by shocks
in a galaxy’s magnetic field after young massive stars ($\geq 8$ M⊙) undergo
core-collapse supernova. These CR electrons undergo a number of different
cooling processes as they propagate throughout the galaxy including inverse
Compton (IC), free-free and ionization losses (Murphy et al., 2009). The
ultraviolet (UV) light from young fairly massive stars ($\geq 5$ M⊙) also
ionises the surrounding hydrogen gas cloud producing thermal (free-free) radio
emission and heating the nearby dust which re-radiates in the infrared (IR)
regime. Synchrotron emission, which composes a majority of the radio emission
at low frequencies ($\lesssim$30 GHz; Condon, 1992; Yun et al., 2001; Bell,
2003), is dominated by active galactic nuclei (AGN) for most bright radio
sources. However, as the sensitivity and resolution of large scale radio
surveys has improved, we find that they are uncovering a large population of
low-redshift SFGs. In order to utilise the strengths of radio continuum
emission as a SF tracer it has been calibrated against previously established
SF tracers such as $H\alpha$, UV and IR emission down to 1 mJy using the Faint
Images of the Radio Sky at Twenty centimeters (FIRST) survey in the northern
hemisphere and Galaxy and Mass Assembly (GAMA) fields (Davies et al., 2017;
Molnár et al., 2021). Radio-SFR relations are found have a tight spread
comparable to widely used SF tracers ($\sim$0.15 dex; Tabatabaei et al., 2017)
and a (mostly) linear correlation to the total IR emission ($\sim$0.16 dex;
Molnár et al., 2021).

The correlation between the far or total IR radiation and the radio continuum
emission is known as the ”infrared-radio correlation” (IRRC) and has been
studied extensively (Helou et al., 1985; Condon, 1992; Yun et al., 2001; Bell,
2003; Delhaize et al., 2017; Molnár et al., 2021; Delvecchio et al., 2021) on
the low-z IRRC; however, the physical processes that govern this relationship
are still poorly understood. The IRRC, which is usually parameterised by the
$q$ value (Eq. 5; the ratio of IR to radio luminosities), has been examined
against the variation of other galaxy properties such as stellar mass
(Magnelli et al., 2015; Delvecchio et al., 2021) and galaxy type (Morić et
al., 2010; Nyland et al., 2017) and appears to hold over at least three orders
of magnitude in both radio and IR luminosity (Helou et al., 1985; Condon,
1992; Delvecchio et al., 2021; Molnár et al., 2021) even in merging galaxies
(Condon et al., 2002; Murphy, 2013). Another area of interest is the evolution
of the IRRC with redshift, which has been a hotly debated topic (Jarvis et
al., 2010; Sargent et al., 2010; Magnelli et al., 2015; Delhaize et al., 2017)
with recent studies finding a slight decrease in the $q$ value as redshift
increases Delhaize et al. (2017); Molnár et al. (2021); Delvecchio et al.
(2021). This tight, linear correlation is upheld by a number of physical
processes and has been investigated in terms of both calorimetric and non-
calorimetric models. Calorimetric models assume that the galaxies are
optically thick to UV such that their UV emission is fully re-radiated in the
IR and that the CR electrons radiate away their total energy as synchrotron
emission before escaping the galaxy (Voelk, 1989) however this implies that
the IRRC is likely to break down towards lower mass galaxies (Bourne et al.,
2012). Whilst, non-Calorimetric or optically thin models (Helou & Bicay, 1993;
Bell, 2003; Lacki & Thompson, 2010; Vollmer et al., 2022) argue a sort of
“conspiracy” between physical mechanisms which acts to maintain the perhaps
unexpected linearity of the IRRC. However, studies by Murphy et al. (2009)
have also found the $q$ value to be systematically lower in cluster
environments in comparison to field galaxies suggesting that environmental
effects may affect the physical processes which give rise to the IRRC. Thus
understanding the physical mechanisms which cause these discrepancies between
observations and model predictions is crucial to our use of the IRRC and radio
emission as a SFR tracer.

While there are many corroborating studies on the existence of the IRRC in
SFGs there has been limited research on how the IRRC varies within galaxies,
primarily due to the limited resolution and sensitivity of past IR and radio
surveys Murphy et al. (2008, 2009); Tabatabaei et al. (2013); Vollmer et al.
(2020). We expect variations in interstellar medium (ISM) properties and the
effects of environmental interactions to lead to observed variations of the
IRRC within individual galaxies Murphy et al. (2009); Tabatabaei et al.
(2013); Vollmer et al. (2020). As synchrotron emission is dependent on both
the energy density of CR electrons and magnetic field strength, physical
mechanisms which modify these properties will impact the IRRC. Furthermore the
significant difference in mean free path of CR electrons (1-2 kpc) relative to
that dust heating photons ($\sim 100$ pc) suggests that variations in the IRRC
can be expected Bicay & Helou (1990). Hughes et al. (2006) finds that the
synchrotron haloes around individual star-forming regions extend further than
their corresponding FIR emission within the Large Magellanic Cloud. Murphy et
al. (2009); Vollmer et al. (2020) show that Virgo cluster galaxies
experiencing intra-cluster medium (ICM)-ISM interactions often display radio-
deficit regions at the interacting edge of the galaxy and synchrotron tails
caused by the re-acceleration of the CR electrons in the ICM driven shocks. A
correlation between SFR density and magnetic field strength is observed in
Tabatabaei et al. (2013) and Heesen et al. (2022a, b) suggesting that
environmental effects which can enhance or quench SF in different parts of
galaxies will also cause discrepancies in the IRRC within galaxies.

###  1.1 Eridanus Supergroup

The Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY; Koribalski et
al. (2020)) is conducted with the Australian Square Kilometre Array Pathfinder
(ASKAP; Hotan et al. (2021)) and will utilise the telescope’s large field of
view to image hundreds of thousands of HI galaxies out to a redshift of $z\sim
0.26$ Koribalski et al. (2020). The WALLABY team has performed a pre-pilot
survey of the Eridanus region that contains three distinct galaxy groups
namely the NGC 1407, NGC 1332 and Eridanus (or NGC 1395) groups, which form a
‘supergroup’ or a group of groups that may eventually merge to form a cluster
(Willmer et al., 1989; Omar & Dwarakanath, 2005; Brough et al., 2006; For et
al., 2021). Brough et al. (2006) find the NGC 1407 group to be the most
evolved and contains a larger fraction of early-type galaxies compared to the
NGC 1332 and Eridanus groups. Studies of the disturbed HI morphologies from
recent WALLABY observations of the Eridanus galaxies suggest that these
galaxies are currently undergoing different stages of tidal interactions and
ram-pressure stripping (For et al., 2021; Murugeshan et al., 2021; Wang et
al., 2022).

An additional product of the Eridanus pre-pilot HI observations are high-
resolution radio continuum images centred at $\sim 1.37$ GHz. Using these
observations we aim to determine if the environmental effects and pre-
processing, which have been shown to be affecting the HI gas and impacting the
SF properties of galaxies in the Eridanus supergroup, manifest in the radio
continuum emission. Utilising NASA’s Wide-field Infrared Survey Explorer
(WISE; Wright et al., 2010) observations we also examine the effect that
environmental pre-processing has on the IRRC both globally and within
individual galaxies. Full WALLABY as well as other upcoming radio continuum
surveys such as the Evolutionary Map of the Universe (EMU; Norris et al.
(2011)) will allow a more thorough investigation of the radio continuum and
hence SF properties within individual galaxies. These future observations will
also allow us to examine the resolved IRRC of individual galaxies and provide
the means to understand the physical processes that give rise to this
relationship and what may cause it to break down.

This paper complements the other WALLABY Pre-Pilot Survey papers by exploring
the impact of the environmental interactions on the SF and ISM properties
probed using the IRRC and improving our understanding of the physical origin
of this correlation. In section 2, we provide details about the observations,
data processing, and source extraction for the WALLABY continuum (WC)
catalogue and the Eridanus supergroup members. Section 3 describes the
ancillary data used. Section 4 presents our results including a quality
assessment of the WC catalogue. Section 4 also presents the global and
resolved SFR, IRRC, and spectral index properties of eight Eridanus supergroup
galaxies. In section 5, we discuss our results including possible
interpretations and limitations. Section 6 concludes our findings. Throughout
the paper, we adopt a $\Lambda$ cold dark matter cosmology model
($\Lambda$CDM) with $\Omega_{\rm M}$ = 0.27, $\Omega_{\rm K}$ = 0,
$\Omega_{\Lambda}$ = 0.73 and $H_{0}$ = 73 ${\rm kms^{-1}Mpc^{-1}}$. Following
For et al. (2021) we assume a distance to the Eridanus supergroup of 21 Mpc.

##  2 WALLABY Data

###  2.1 Observations and Data Processing

The Murchison Radio astronomy Observatory (MRO) – located in the remote
outback of Western Australia – hosts ASKAP, a radio interferometer consisting
of 36 12-m antennas. Each antenna is equipped with a second generation MK II
phased array feed from which 36 dual-polarisation beams are formed. These
beams are designed to provide an instantaneous field of view of
$5.5^{\circ}\times 5.5^{\circ}$ (at $\sim$1.3 GHz) with a bandwidth of 288 MHz
(McConnell et al., 2016; Hotan et al., 2021). WALLABY carried out a pre-pilot
survey in March 2019 targeting the Eridanus galaxy supergroup field (hereafter
the Eridanus field) which utilised the full array for two interleaving
footprints (footprints A and B) to obtain uniform sensitivity.

Each footprint has a $6\times 6$ beam pattern as shown in Fig. 1. The Eridanus
field central coordinates for footprints A and B are given in Table 9 along
with other observing parameters. The observations for footprint A were carried
out during the day and those for footprint B were obtained mostly at night.
The primary calibrator PKS 1934-638 was observed for 2-3 hours between the two
sets of science observations. Each observation has a scheduling block
identification number (SBID) that can be used to access the corresponding
public data set in the CSIRO ASKAP Science Data
Archive111https://research.csiro.au/casda/ (CASDA).

The observations were processed using the ASKAPSOFT pipeline version
0.24.7222https://www.atnf.csiro.au/computing/software/askapsoft/sdp/docs/
current/general/releaseNotes.html#october-2019 (Whiting, 2020) with WALLABY-
specific processing parameters. Only the upper half of the 288 MHz bandwidth
(1295.5-1439.5 MHz, centred at 1367.5 MHz) was processed due to satellite
radio frequency interference (RFI) in the lower half. Footprints A and B (Fig.
1) were processed separately. For a detailed description of the processing
steps refer to For et al. (2021). The calibrated visibilities for all
baselines were used for the radio continuum, whereas imaging for the WALLABY
HI cubes only used baselines up to two kilometres. The continuum visibilities
were CLEANed, after applying a Wiener filter, with robustness value of -0.5
and no applied taper. A single loop of automated self-calibration in included
in the ASKAPSOFT continuum pipeline. After imaging, the two individual
footprints were linearly mosaicked to produce the final images and data cubes.
These images are not convolved to a common resolution which is part of the
current pipeline for processing of radio continuum images with ASKAP and has
been shown to produce more accurate flux density measurements Norris et al.
(2021); Hale et al. (2021).

These continuum images produced by WALLABY are the highest resolution and
sensitivity $\sim 1.4\,$GHz observations available for the Eridanus field,
boasting an angular resolution of 7.9′′ $\times$ 6.1′′ with integrated flux
densities between 0.14 and 831.6 mJy, a dynamic range of $\sim 1.2\times
10^{4}$, and a median RMS noise value of 50 $\mu$Jy/beam.

Figure 1: WALLABY continuum image of the entire Eridanus field including
locations of the Eridanus (NGC 1395) group members marked in blue (or red for
the AGN) with the group central galaxy NGC 1395 marked in yellow. Dark cloud
2, an HI cloud with no detected stellar component (Wong et al., 2021) is also
marked in black. The detected NGC 1407 group member NGC 1359 is marked in
orange. The cyan and orange circles represent footprints A and B respectively
for the ASKAP observations listed in Table 9 and each beam has a FWHM of
$1^{\circ}$. The black circles mark the maximum radial extent of the groups
belonging to the Eridanus supergroup as identified in Brough et al. (2006).

###  2.2 Source Extraction

Source extraction and flux measurements for WALLABY radio continuum sources
were performed using the PROFOUND function within ProFound (Robotham et al.,
2018), which has been shown to be an excellent pixel-based source extraction
tool for high resolution radio surveys that exhibit both complex and simple
sources (Hale et al., 2019). The full details of how ProFound works are given
in Robotham et al. (2018) and we refer the reader to Hale et al. (2019,
section 2.1) for a brief description of its application to radio data.
Parameters were left at default values outside of those shown in Table A.11,
which were selected in order to identify a large number of initial seeds that
are then dilated to allow the merging of extended/diffuse segments (islands
which encapsulate the flux of a radio source). Modification of these
parameters largely affects the number of faint sources detected (box, skycut,
pixcut) or whether extended/diffuse sources are correctly combined into one
segment (tolerance, reltol, ext). Final values were chosen based on test
fields from different regions of the Eridanus field containing a number of
different radio features and noise levels to achieve a balance between
completeness and reliability.

Initial source extraction from ProFound finds 9948 components that are then
cut at a 5$\sigma$ threshold. This 5$\sigma$ threshold is defined by the final
peak flux value divided by the mean segment RMS noise for each component since
the initial skycut of 3$\sigma$ is performed before accurate backgrounds and
RMS maps are measured. We note that significant sidelobes from the dirty beam
occur around bright components ($S_{\rm Peak}\gtrsim 50\,$mJy) in a spiral
residual pattern which are often detected as individual components by
ProFound. In order to remove these sidelobes from the component list we remove
segments within 180′′ of bright components that have SNR¡10$\sigma$. Upon
visual inspection, this criteria correctly removes sidelobes around all but
one bright component that exists in the outer edge of the observed field,
which has no footprint overlap. For this component, the three significant
segments that are consistent with the spiral residual pattern were manually
removed from the component catalogue. Real faint sources that are within this
residual pattern may also be removed using this criteria. Visual inspection or
machine learning methods are often required to correctly identify complex
radio sources which have multiple associated components. No attempt was made
to classify components as radio objects (doubles/triples) as this is beyond
the scope of this work. These issues will be better addressed in the WALLABY
pilot survey and WALLABY full survey as better primary beam models from
holography measurements are used (Norris et al., 2021; Hale et al., 2021).
Overall the final WALLABY continuum (WC) catalogue contains 9461 components
above a $5\sigma$ threshold with a resolution of 7.9′′ $\times$ 6.1′′. In the
top panel of Fig. 2, we show the uniformity of the noise across the field with
localised increases due to low-level calibration errors around bright sources
and at the edges of the field where the beam sensitivities drop off. This
noise at the edge of the field causes the extended RMS tail as seen in the
bottom panel of Fig. 2.

Figure 2: Top: map of the RMS noise in the Eridanus field (with the colour bar
in units of mJy/beam). Bottom: histogram of RMS values with a median value of
0.05 mJy/beam shown by the dashed line.

###  2.3 Identifying Unresolved Sources

We select unresolved sources for comparisons between surveys using the method
employed for the Rapid ASKAP Continuum Survey (RACS; McConnell et al., 2020;
Hale et al., 2021) which follows from Bondi et al. (2008), Smolčić et al.
(2017) and Shimwell et al. (2019) and involves defining an envelope used to
distinguish between resolved and unresolved sources. We construct this
envelope by selecting sources with SNR $\geq 5$ where SNR is defined by the
peak flux of a segment divided by the estimated mean background RMS noise of
that segment. We then measure the ratio of integrated flux ($S_{\rm int}$ to
peak flux ($S_{\rm peak}$) as a function of SNR. For unresolved sources the
ratio of integrated to peak flux ($S_{\rm int}/S_{\rm peak}$) should equal one
by construction if the synthesised beam size is an accurate representation of
image resolution. Typically we see a scatter around unity especially at low
SNR where noise has a greater impact on flux measurements. We find that for
our data $S_{\rm int}/S_{\rm peak}$ tends towards 1.16 (as measured by the
highest SNR unresolved point source) likely due to unmodelled source smearing
possibly due to tracking errors or inappropriate primary beam modelling. The
discrepancy of this value does not impact our analysis and construction of the
envelope. Following Hale et al. (2021) we expect unresolved sources to have
values of $S_{\rm int}/S_{\rm peak}$ as a function of SNR which lie between
the envelopes as described by:

$\frac{S_{\rm int}}{S_{\rm peak\pm}}=1.16\pm A\times{\rm SNR}^{-B}.$ (1)

We determine values of A and B from the lower envelope shown in Fig. 3,
$S_{\rm int}/S_{\rm peak-}$, and declare sources with $S_{\rm int}/S_{\rm
peak}$ ¿ $S_{\rm int}/S_{\rm peak+}$ to be resolved as they will have elevated
$S_{\rm int}/S_{\rm peak}$ values. We generate this fit using equally spaced
bins in log(SNR) (excluding the lowest SNR bin) which each contain at least 50
sources. Then the $S_{\rm int}/S_{\rm peak}$ ratio that contains 95$\%$ of the
sources with $S_{\rm int}/S_{\rm peak}$ ¡ 1.16 is determined. These points are
then fit to the lower envelope of Eq. 1 which yields
$1.16-1.78\times\mbox{SNR}^{-0.66}$. The lower envelope is reflected around
$S_{\rm int}/S_{\rm peak}$ = 1.16 to create the upper envelope. Sources which
lie between these two envelopes are defined as unresolved and are shown in
Fig. 3. We find that $\sim$76.5% of WC sources are unresolved at a resolution
of $7.9^{\prime\prime}\times 6.1^{\prime\prime}$ and hence should also be
unresolved in the National Radio Astronomy Observatory (NRAO) Very Large Array
(VLA) Sky Survey (NVSS; Condon et al., 1998) and RACS.

Figure 3: The ratio of integrated to peak flux as a function of SNR for
sources with SNR ¿ 5 and the envelope used to define unresolved sources. Blue
points indicate sources which are unresolved by our criteria and grey points
indicate resolved sources as defined by the envelope. The red crosses
indicates the $S_{\rm int}/S_{\rm peak}$ values in which 95$\%$ of the sources
below $S_{\rm int}/S_{\rm peak}=1.16$ are included within the envelope for
bins of width 0.1 dex and including greater than 50 sources. The black dashed
line indicates $S_{\rm int}/S_{\rm peak}=1.16$.

Sources are then selected from WC and matched to comparison surveys using the
same four criteria as described in Hale et al. (2021):

  1. 1.

Sources are isolated to within an angular separation of twice the FWHM of the
comparison survey with poorer angular resolution i.e. $50^{\prime\prime}$ and
$90^{\prime\prime}$ angular separations for comparisons with RACS and NVSS
respectively.

  2. 2.

Sources have a peak SNR $\geq 10$ in WC.

  3. 3.

Sources are unresolved as described by the envelope criterion above.

  4. 4.

Sources match positions within an angular separation of $10^{\prime\prime}$,
which allows for variation in the positions of measured sources given the
different angular resolutions of the comparison surveys.

After which, we find 1418 matches between WC and RACS, and 384 matches common
to all three catalogues.

###  2.4 Eridanus Supergroup

In order to examine whether the environmental effects within supergroups are
reflected in the radio continuum emission, we select galaxies that are known
to be interacting with their local environment. The WALLABY HI observations in
the Eridanus supergroup find that almost all HI-detected group member galaxies
down to an HI mass of $10^{7.7}\,M_{\odot}$ have disturbed HI morphologies
(For et al., 2021). Most group members are observed to be HI deficient
compared to their stellar masses and integrated disk stability suggesting that
gas has been removed from the galaxies due to tidal interactions, mergers and
ram pressure stripping (For et al., 2021; Murugeshan et al., 2021; Wang et
al., 2022). We look to investigate the radio continuum emission and derived
physical properties to determine whether we can measure the impact that
environment has on these SFGs.

####  2.4.1 Sample Selection

The catalogue of 43 Eridanus supergroup HI sources detected in the WALLABY
pre-pilot survey (For et al., 2021) are positionally matched – within 10′′ –
to sources in the WC catalogue. We detect eight galaxies with corresponding
radio continuum emission which are shown in Fig. 11. There was also one
background star-forming galaxy with radio continuum emission that was
incorrectly cross-matched to LEDA 13743 and not included in our sample. The
measured integrated flux densities at multiple wavelengths are presented for
each galaxy along with their position and morphology in Table 1. NVSS (Condon
et al., 1998), RACS (Hale et al., 2021) and, GMRT (Omar & Dwarakanath, 2005)
integrated flux values are obtained for the eight WC-selected galaxies whilst
the WC flux densities are measured using ProFound. To ensure we encapsulate
all the flux from the observed isolated radio continuum components in the
star-forming disks of IC 1952, IC 1953 and NGC 1359 WISE W1 segment maps are
passed to ProFound. The remaining Eridanus supergroup members use the measured
WC catalogue flux densities as their less complex radio morphology allows
correct assignment and measurement of their total flux density.

Table 1: Radio continuum properties of Eridanus supergroup members.

Designation | Common ID | R.A | Dec | Morphology | $S_{\rm int}^{\rm WC}$ | $S_{\rm int}^{\rm NVSS}$ | $S_{\rm int}^{\rm RACS}$ | $S_{\rm int}^{\rm O05}$
---|---|---|---|---|---|---|---|---
|  | (∘) | (∘) |  | (mJy) | (mJy) | (mJy) | (mJy)
WC J033326-234229 | IC 1952 | 53.359759 | -23.708036 | SB(s)bc? | $1.0\pm 0.3^{a,b}$ | $4.5\pm 0.6$ | $8\pm 1$ | $7\pm 1$
WC J033328-213353 | ESO 548-G036 | 53.365284 | -21.565734 | Sc | $7.9\pm 1.1$ | $7.6\pm 0.5$ | $13\pm 2$ | $9\pm 1$
WC J033342-212843 | IC 1953 | 53.424245 | -21.478690 | SB(rs)d | $6.9\pm 1.4^{b}$ | $13.0\pm 2.2$ | $5\pm 1$ | $9\pm 2$
WC J033348-192935 | NGC 1359 | 53.448790 | -19.492060 | SB(s)m? | $18.9\pm 3.6^{b}$ | $31.1\pm 2.4$ | $40\pm 3$ | $32\pm 3$
WC J033501-245558 | NGC 1367 | 53.754798 | -24.932908 | SAB(rs)a | $13.2\pm 1.8$ | $15.4\pm 1.8$ | $25\pm 3$ | $20\pm 2$
WC J033729-243001 | NGC 1385 | 54.369089 | -24.500355 | SB(s)cd | $142.2\pm 18.7$ | $178.5\pm 6.1$ | $246\pm 18$ | $180\pm 20$
WC J034057-223353 | NGC 1415 | 55.236565 | -22.564758 | (R)SAB0/a(s) | $23.4\pm 3.1$ | $25.3\pm 1.2$ | $36\pm 3$ | $27\pm 3$
WC J034132-214049 | NGC 1422 | 55.382081 | -21.680283 | SBab | $0.9\pm 0.2$ |  |  |

  * Note: Column (1): WC designation. Column (2): Galaxy common ID. Column (3): R.A of the WC flux weighted centre. Column (4): Declination of the WC flux weighted centre. Column (5): Optical morphology from NED. Column (6): 1.37 GHz WC total integrated flux density. Column (7): 1.4 GHz NVSS total integrated flux density. Column (8): 0.88 GHz RACS total integrated flux density. Column (9): 1.4 GHz Omar & Dwarakanath (2005) total integrated flux density.

  * a

The background contribution is removed.

  * b

We measure WC integrated flux density using an aperture which encapsulates the
disk and core continuum emission.

##  3 Ancillary Data

Table 2: Basic survey information.

Survey | $\nu(\lambda)$ | Bandwidth | Resolution | Sensitivity | Source
---|---|---|---|---|---
| GHz ($\mu m$) | GHz ($\mu m$) | ′′ | mJy |
WC | 1.3675 | 0.144 | $6.1\times 7.9$ | $\geq 0.14$ | This paper
RACS | 0.8875 | 0.288 | $25\times 25^{a}$ | $\geq 1$ | Hale et al. (2021)
NVSS | 1.4 | 0.042 | $45\times 45$ | $\geq 2.2$ | Condon et al. (1998)
WISE W1 | 3.4* | 1.1* | $6.1\times 6.1$ | $\geq 0.06$ | Wright et al. (2010)
WISE W2 | 4.6* | 1.4* | $6.4\times 6.4$ | $\geq 0.12$ | Wright et al. (2010)
WISE W3 | 12* | 9.8* | $6.5\times 6.5$ | $\geq 0.4$ | Wright et al. (2010)

  * *

Note: Entries are given in terms of wavelength rather than frequency.

  * a

The initial resolution of RACS observations in this field (CASDA SBID: 12468)
are $14^{\prime\prime}\times 14.6^{\prime\prime}$ which are used to perform
any resolved spectral index studies.

###  3.1 Radio Data

The Eridanus field has been included in multiple all-sky radio surveys
including NVSS and RACS. The details of the ancillary data used from these
observations are presented in Table 2.

###  3.2 WISE

The Eridanus field was also covered by observations from the WISE survey. The
data is publicly available and obtained from the NASA/IPAC Infrared Science
Archive333https://irsa.ipac.caltech.edu/applications/WISE/ (IRSA). We chose to
use WISE observations due to its high angular resolution, allowing the
analysis of resolved properties within galaxies, as well as its coverage and
data availability for the entire Eridanus field.

####  3.2.1 Flux Measurement

Integrated WISE $W1$, $W2$, and $W3$ band flux densities are measured for our
Eridanus supergroup galaxy sample using ProFound and shown in Table. 3. To
ensure we are not missing flux from the longer wavelength WISE bands, we
determine the segments used to measure the integrated W2 and W3 flux from the
W1 segment map which tend to cover a larger area. IC 1952 contains a
background source behind the galactic disk and we are unable to remove its
contribution to the total WISE flux density however the background source
accounts for $<5\%$ of the total flux density in each WISE band as measured by
ProFound. Errors in the flux density are measured by ProFound and added in
quadrature to the flux calibration errors of $2.4\%$, $2.8\%$ and $4.5\%$ in
W1, W2 and W3 respectively.

Table 3: Measured WISE fluxes using ProFound.

Common ID | $S_{\rm int}^{\rm W1}$ | $S_{\rm int}^{\rm W2}$ | $S_{\rm int}^{\rm W3}$
---|---|---|---
| (mJy) | (mJy) | (mJy)
IC 1952a | $47.3\pm 1.4$ | $28.5\pm 0.9$ | $85\pm 5$
ESO 548-G036 | $28.3\pm 0.9$ | $18.8\pm 0.7$ | $109\pm 6$
IC 1953 | $76.5\pm 2.2$ | $46.3\pm 2.0$ | $218\pm 15$
NGC 1359 | $26.9\pm 1.2$ | $15.8\pm 0.7$ | $56\pm 4$
NGC 1367 | $279.0\pm 13.0$ | $159.0\pm 8.8$ | $88\pm 18$
NGC 1385 | $190.1\pm 8.0$ | $130.0\pm 4.6$ | $845\pm 50$
NGC 1415 | $179.6\pm 4.7$ | $109.8\pm 3.7$ | $282\pm 15$
NGC 1422 | $22.6\pm 0.7$ | $13.1\pm 0.4$ | $29\pm 2$

  * Note. Column (1): Galaxy common ID. Column (2): WISE W1 integrated flux density. Column (3): WISE W2 integrated flux density. Column (4): WISE W3 integrated flux density.

  * a

The contribution from the background source is not removed but accounts for
$<5\%$ of the total flux density in each WISE band.

We also calculate the Vega magnitude in each WISE band using:

$W_{\rm x}=-2.5\log(\frac{S_{\rm int}^{\rm Wx}}{S_{0}})\bigg{.}$ (2)

where $S_{\rm int}$W is the WISE W1, W2 or W3 integrated flux density in Jy
and $S_{0}$ is the WISE Vega zero magnitude flux density Wright et al. (2010);
Jarrett et al. (2011) with values of 309.540, 171.787 and 31.674 Jy for bands
$W1$, $W2$ and $W3$ respectively.

####  3.2.2 WISE W3PAH-SFR

The $W3$ band is an excellent tracer of ISM emission with contributions from
PAH emission, nebular emission, and dust heating; however, it also has a
contribution from evolved stellar populations. This component of the $W3$ band
emission is estimated and removed using the method described in Cluver et al.
(2017) whereby the W1 band emission is used as a proxy to determine the
stellar emission of which 15.8 percent is determined to be in the W3 band,
following Silva et al. (1998), Helou et al. (2004) and Jarrett et al. (2011).
We subtract 15.8 percent of the W1 band flux from the W3 band flux for both
the global and pixel mapped flux densities to determine the contribution from
primarily the ISM. We denote these modified W3 flux densities and any other
measurements derived from them by using the W3PAH subscript.

The W3PAH luminosity is calculated using:

$\bigg{(}\frac{\rm L_{W3PAH}}{\rm WHz^{-1}}\bigg{)}=(9.52\times
10^{15})4\pi\,\bigg{(}\frac{\rm R}{\rm Mpc}\bigg{)}^{2}\bigg{(}\frac{\rm
S_{\rm int}^{\rm W3PAH}}{\rm mJy}\bigg{)}$ (3)

with $R$ being the distance to the Eridanus supergroup, $S_{\rm int}^{\rm
W3PAH}$ being the integrated W3PAH flux.

The W3PAH-SFR is estimated using the monochromatic W3PAH luminosity ($\nu
L_{\rm W3PAH}$) and equation 5 from (Cluver et al., 2014):

$\log\bigg{(}\frac{\rm SFR_{W3PAH}}{\rm
M_{\odot}\,yr^{-1}}\bigg{)}=1.13\log\bigg{(}\frac{\nu L_{\rm W3PAH}}{\rm
L_{\odot}}\bigg{)}-10.24$ (4)

where $\nu$ is the WISE W3 band central frequency (2.498 $\times 10^{13}$ Hz)
and $L_{\odot}$ = 3.828 $\times 10^{26}$ W. This relationship has been
calibrated against SFR derived from H$\alpha$ observations, which has been
shown to be a reliable SFR tracer. Lastly, estimates of the SFR as determined
using far-UV (or near-UV, if unavailable) plus WISE Band 4 24 $\mu$m IR
emission from For et al. (2021) are utilised; see Wang et al. (2017) for a
detailed description of how these SFRs are calculated.

The use of W3PAH as a SFR tracer (to replace the TIR or FIR measurements
traditionally used to measure the IRRC) is primarily due to the availability
of high resolution WISE observations which are used for the resolved IRRC
analysis in S 4.5 and lack of FIR/TIR data in this region. The WISE W3 band
encompasses a wide range of features within its broad $7-16.5\mu$m bandwidth
most notably the $11.3\mu$m PAH emission feature as well as nebular lines and
continuum emission from warm large grains which in general follows the
behaviour of the TIR emission (Li & Draine, 2002). This emission is thought to
be mainly powered by the diffuse interstellar radiation fields produced in
star-forming regions (Popescu et al., 2011) with PAH fractions being high in
regions of active SF suggesting they are produced in molecular clouds
(Sandstrom et al., 2010). However PAH’s are known to be destroyed by the hard
interstellar radiation fields produced by AGNs and massive stars (Smith et
al., 2007) which is somewhat counteracted by the increase in continuum
emission as a result of warm, large grains. Shocks caused by supernovae as
well as galactic interactions and mergers are known to destroy PAH’s via
either thermal or inertial sputtering (Draine & Li, 2007; Micelotta et al.,
2010; Murata et al., 2017). Jarrett et al. (2013), Cluver et al. (2014) and
Cluver et al. (2017) find strong correlation between SFRs derived using
multiple different well established SFR tracers ($H\alpha$, mid-IR+UV and TIR)
and the W3PAH luminosity suggesting its viability as a SFR tracer. Cluver et
al. (2014) notes however that the W3PAH-SFR relationship is not expected to
hold for low-metallicity environments due to the lack of PAH emission which
accounts for $\sim 34\%$ of the W3 band emission on average (in their sample).
The relationship between W3PAH and TIR luminosities is also not expected to
not hold for subregions of galaxies where dust composition, PAH features and
dust temperature vary greatly (Cluver et al., 2017). Our sample of galaxies
are not significantly starbursting nor do they contain AGN (aside from NGC
1367) and lie within the W3PAH luminosity range where the relationship holds
in (Cluver et al., 2017). While we do not have measurements of the metallicity
within most of this sample, NGC 1385 was previously found to have a global
$(12+\log(\rm O/H))>8.3$ (Williams et al., 2021) consistent with abundances
used in the Cluver et al. (2017) calibrations. Overall this suggests that use
of W3PAH emission as a SFR tracer (and IRRC proxy) in lieu of TIR or FIR
emission does not significantly change any conclusions drawn from our global
and resolved results.

####  3.2.3 Infrared-Radio Correlation

The IRRC is a relationship that has been observed to be very consistent over
wide a range of magnitudes in luminosity and redshift for SFGs (Yun et al.,
2001; Bell, 2003; Delhaize et al., 2017; Molnár et al., 2021) due to its
suspected SF-based origin. The IRRC is typically measured using the
$q$-parameter:

$q_{\lambda}=\log\bigg{(}\frac{L_{\lambda}}{\rm 3.75\times
10^{12}~{}W}\bigg{)}-\log\bigg{(}\frac{L_{1.4}}{\rm W\,Hz^{-1}}\bigg{)}.$ (5)

where $L_{\lambda}$ is the IR luminosity at a wavelength ${lambda}$ and
$L_{1.4}$ is the total 1.4 GHz luminosity. We measure $q_{12}$ and $q_{\rm
TIR}$ using the W3PAH and TIR luminosities respectively.

While most analyses of the IRRC use the total or far IR emission to measure
$q$, information can still be gleaned from using W3PAH emission as it also
traces dust and the ISM, which are heated due to SF and is supported by the
tight relationship between W3PAH and TIR emission found in Cluver et al.
(2017):

$\log\bigg{(}\frac{\rm L_{\rm TIR}}{\rm L_{\odot}}\bigg{)}=(0.889\pm
0.018)\log\bigg{(}\frac{\nu L_{\rm W3PAH}}{\rm L_{\odot}}\bigg{)}+(2.21\pm
0.15).$ (6)

where $\nu L_{\rm W3PAH}$ is the monochromatic W3PAH luminosity.

To be able to observe how the continuum properties of galaxies vary across
their disks and whether the group environment in Eridanus affects their
physical properties, we measure the resolved IRRC for these eight galaxies. To
create the maps which measure the resolved $q_{\rm 12}$ parameter, we first
matched the angular resolution of the WALLABY and WISE W1 and W3 images by
convolving the images to a common beam size of 8′′. This beam corresponds to a
distance of 0.8 kpc and is chosen to maximise the angular resolution and allow
observations of the small scale differences within the galactic disks. After
removing the sky background component of both WISE (W1 and W3) images, the W3
images then had their old stellar contribution removed following Cluver et al.
(2017) by subtracting $15.8\%$ of the W1 emission on a per pixel basis
resulting in what we denote the WISE W3PAH image. The WISE W3PAH image was
then regridded to a pixel scale of 2′′ per pixel to match the WALLABY
observations. We determine the background and RMS of each WALLABY and W3PAH
image using ProFound and perform a background subtraction for each W3PAH
image. The $q_{12}$ maps are created using Eq. 5 on a per pixel basis where
the galaxies show $\geq 3\sigma$ detections in both WALLABY and W3PAH images.
These $q_{\rm 12}$ maps are then overlaid on WALLABY HI contours provided by
For et al. (2021) to determine whether any features observed relate to
properties in the HI observations. We present these images in Fig. 11 for each
of the eight galaxies in our sample.

###  3.3 _g-Band_ Optical

We also make use of co-added _g-band_ optical images from the DESI Legacy
Imaging Surveys DR8 (Dey et al., 2019), which are based on the Dark Energy
Camera Legacy Survey (DECaLS) southern observations, for our sample of
Eridanus group galaxies in order to show the extent of radio and IR emission
compared to the stellar disks.

##  4 Results

###  4.1 Radio Continuum Catalogue

Table 4: First five rows of the simplified WC catalogue.

Designation | RAcen | Deccen | S_peak | S_int | S_int | S_int | S_int | sky_RMS | SNR | N100 | R100
---|---|---|---|---|---|---|---|---|---|---|---
|  |  |  |  | _toterr | _err | _err_cal | _mean |  |  |
| (∘) | (∘) | (mJy/beam) | (mJy) | (mJy) | (mJy) | (mJy) | (mJy) |  | (pixels) | (arcsec)
WC J031926-222623 | 49.859961 | -22.439857 | 8.152 | 11.705 | 1.668 | 0.142 | 1.527 | 0.018 | 32.695 | 59 | 4.683
WC J031928-222815 | 49.868565 | -22.470727 | 1.628 | 1.516 | 0.321 | 0.123 | 0.198 | 0.018 | 6.761 | 48 | 4.576
WC J031930-223602 | 49.876814 | -22.600451 | 1.667 | 1.837 | 0.361 | 0.121 | 0.240 | 0.016 | 7.547 | 55 | 5.365
WC J031931-223941 | 49.877981 | -22.661258 | 1.284 | 1.642 | 0.331 | 0.116 | 0.214 | 0.017 | 5.584 | 47 | 4.949
WC J031939-223000 | 49.913872 | -22.500039 | 1.043 | 1.140 | 0.239 | 0.090 | 0.149 | 0.015 | 5.114 | 36 | 4.280

  * Note: The columns are described in C.

The first five rows of the simplified WC catalogue are shown in Table 4 with
the full catalogue, also containing additional statistics measured by
ProFound, provided in the online supplementary material. A description for
each column included in the extended WC catalogue is provided in C and
described in detail in Robotham et al. (2018). Fig. 4 shows the component
counts of each survey in the Eridanus field.

Figure 4: Component counts for WALLABY, RACS, and NVSS in the Eridanus field.
Flux bins are of width 0.1 dex and vertical dotted lines show the median
integrated flux density values of 0.6, 4.8 and 6.0 mJy for WALLABY, RACS and
NVSS observations respectively.

####  4.1.1 Astrometry

We assess the astrometry of WC compared to both RACS and NVSS using matched
unresolved sources that satisfy the selection criteria in section 2.3. The RA
offset is defined as $\Delta_{\rm RA}=RA_{\rm WC}-RA_{\rm comp}$ where comp
refers to the comparison survey and the Dec offset is defined in the same way.
We find small mean systematic offsets between WC and NVSS ($\Delta_{\rm
RA}=-0.7\pm 3.5^{\prime\prime}$ and $\Delta_{\rm Dec}=-0.1\pm
3.5^{\prime\prime}$), which are constrained to within a couple pixels in the
WC observations (2′′ per pixel) and less than the estimated position error of
NVSS sources. Comparisons to RACS also find small mean systematic offsets with
$\Delta_{\rm RA}=-0.1\pm 1.6^{\prime\prime}$ and $\Delta_{\rm Dec}=-0.6\pm
1.6^{\prime\prime}$ which are constrained to within one pixel in RACS (2.5′′
per pixel).

####  4.1.2 Flux Offsets

Flux density scale comparisons are made between WC and NVSS due to their
similar frequencies, which will minimise the impact of errors from the
uncertainty in the spectral index. We measure the ratio of integrated flux
densities:

$R^{\rm a}_{\rm b}=(S_{\rm a}/S_{\rm b})$ (7)

where $S_{a}$ and $S_{b}$ are the integrated flux densities from survey a and
b respectively.

We compare the flux densities determined in the WC catalogue to those in the
NVSS and RACS catalogues at 1.4 GHz and 0.888 GHz respectively. This is to
ensure we have a consistent flux scale and to investigate whether the measured
spectral index is in agreement with our previous studies of the radio source
population. Each of these surveys has a different angular resolution and
sensitivity. As such, to ensure that these differences do not impact our
comparisons, we restrict comparisons to unresolved, isolated, and high SNR
sources as defined in section 2.3.

Figure 5: Comparison of the integrated flux density ratios between WC and NVSS
for the 384 sources matched using the criteria from Section 2.3. This is shown
with (filled) and without (empty) a flux limit of $S_{\rm int}^{\rm NVSS}\geq
6$ mJy applied and assuming allowed spectral index values of $-2\leq\alpha\leq
0.4$. The blue dashed lines indicate the mean flux ratio for the flux limited
population and one standard deviation either side whilst the black dashed line
indicates the expected flux ratio of 0.98 assuming $\alpha=-0.8$.

After applying a flux cut of 6 mJy to NVSS and assuming allowed spectral index
values of $-2\leq\alpha\leq 0.4$ to remove significant outliers and encompass
the majority of $\alpha$ values observed Smolčić et al. (2017), we find the
mean flux ratio $R^{\rm NVSS}_{\rm WC}=1.03\pm 0.13$ for 122 sources. We
therefore conclude that the flux scale for our observation is accurate within
the estimated scatter. A $13\%$ error is added in quadrature to the measured
WC flux density in order to maintain the flux scale agreement between WC and
NVSS throughout this research. However we note that the expected flux ratio
between NVSS and WC is 0.98 assuming $\alpha=-0.8$ and so WC likely has a
systematic flux deficit of approximately $5\%$, which is also observed in For
et al. (2021). Our source catalogue as well as the RACS catalogue have shown
position-dependent flux density variations due to inaccurate primary beam
corrections. These issues will be addressed in future observations where
instrumental flux calibration will utilise holography beam measurements rather
than Gaussian beams. The results are summarised in Fig. 5.

####  4.1.3 Spectral Index Analysis

The spectral index $\alpha$ is used to define the broadband radio emission as
a power law of the form $S_{\nu}\sim\nu^{\alpha}$, where $S_{\nu}$ is the flux
density at a frequency, $\nu$. Hence, we define $\alpha$ as:

$\alpha^{\rm a}_{\rm b}=\frac{\log(S_{\rm a}/S_{\rm b})}{\log(\nu_{\rm
a}/\nu_{\rm b})}.$ (8)

where $\nu_{a}$ and $\nu_{b}$ are the frequencies of survey a and b
respectively. $\alpha$ is typically found to have values between $-0.7$ and
$-0.8$ for SFGs in the synchrotron dominated regime ($\nu\leq 3\,GHz$)
(Condon, 1992; Smolčić et al., 2017). We also make comparisons after
performing flux cuts that are chosen to prevent any bias in $\alpha$, which
would bias the spectral indices to higher or lower values based on sensitivity
limits and frequencies of the comparison surveys.

We measure the spectral indices between WC and RACS as well as NVSS and RACS
(for sources common to WC) with results summarised in Table 5. The spectral
index values between WC and RACS imply a steeper than normal spectral index;
however, given the significant scatter in measured values, the expected value
of -0.8 is recovered. Interestingly, $\alpha^{\rm NVSS}_{\rm RACS}$ is
measured to be flatter than $\alpha^{\rm WC}_{\rm RACS}$ without taking into
account the large error budget. The steeper than normal spectral index between
WC and RACS can be explained by the compounding systematic flux offsets where
WC has an approximately $5\%$ flux deficit and RACS has a slight excess, as
suggested by the spectral index analysis of RACS (see Section 5.2.5 of Hale et
al., 2021). The agreement between RACS and NVSS hints that the primary cause
for discrepancies in the comparisons is the slight WC flux deficit, measured
to be $\alpha^{\rm WC}_{\rm RACS}=-0.91\pm 0.41$ as compared to $\alpha^{\rm
NVSS}_{\rm RACS}=-0.80\pm 0.39$ for the same 122 sources. The spectral index
values between WC and RACS are measured to be steeper when looking at the
entire sample compared to after applying a flux cut suggesting that either WC
is measuring less flux or RACS is measuring excess flux for faint sources. We
believe this is likely due to imaging differences between WC and RACS
including different weightings, and RACS convolution to a 25′′ FWHM beam. We
note that these discrepancies are not statistically significant due to the
magnitude of uncertainty in spectral indices between WC, RACS, and NVSS. These
findings suggest that future comparisons to the WC and RACS catalogues will
exhibit systematic flux discrepancies; however, future improvements to ASKAP
processing will likely minimise such discrepancies.

Table 5: Flux offsets and spectral indices between catalogues.

Matched Catalogues | Sample | No. of Sources | Median $\alpha^{a}_{b}$ | Mean $\alpha^{a}_{b}$
---|---|---|---|---
|  |  | 16th and 84th Percentiles | $\pm~{}\mbox{Std(SE)}$
RACS-WC | All | 1418 | $-1.24^{+0.71}_{-0.65}$ | $-1.26\pm~{}0.77$
| 3 mJy RACS cut | 697 | $-1.11^{+0.46}_{-0.48}$ | $-1.09\pm~{}0.47$
RACS-NVSS (WC) | All | 384 | $-0.92^{+0.64}_{-0.45}$ | $-0.82\pm~{}0.67$
| 6 mJy NVSS cut | 122 | $-0.87^{+0.45}_{-0.30}$ | $-0.80\pm~{}0.39$
RACS-WC (NVSS) | All | 384 | $-1.00^{+0.42}_{-0.50}$ | $-0.94\pm~{}0.55$
| 6 mJy NVSS cut | 122 | $-0.96^{+0.31}_{-0.39}$ | $-0.91\pm~{}0.41$
RACS-WC | Eridanus | $5^{c}$ |  | $-1.34\pm~{}(0.12)$
RACS-NVSS | Eridanus | $6^{c}$ |  | $-0.87\pm~{}(0.11)$

  * Note: Column (1): Catalogues compared in a-b format. Column (2): Sample used. All flux cuts also include cuts between $-2\leq\alpha^{a}_{b}\leq 0.4$. Column (3): Number of sources in the sample from catalogue $a$ (top) and catalogue $b$ (bottom). Column (4): Median spectral index for sample with 16th and 84th percentiles. Column (5): Mean spectral index for sample with the standard deviation (standard error) about the mean.

  * c

Outliers have been removed.

###  4.2 Radio Continuum Properties of Eridanus Supergroup

Table 1 showed the measured radio continuum fluxes for Eridanus supergroup
members. Overall we see agreement between the WC, NVSS and Omar & Dwarakanath
(2005) $\sim 1.4\,GHz$ integrated flux densities for ESO 548-G036 and NGC
1415, which exhibit compact radio emission. The higher resolution afforded by
WC observations allows us to discern that, for IC 1952, a majority of the
radio emission is due to a background radio source behind the disk of IC 1952.
After subtracting this background contribution, the measured flux density of
IC 1952 is $\sim$6 times lower than previous surveys that were unable to
separate and remove this background contribution. WC measures a flux deficit
for IC 1953 and NGC 1385, which both have extended diffuse emission that is
likely resolved out. We also measure a flux deficit for NGC 1359 due to loss
of sensitivity at the edge of the field. Our flux for NGC 1367 agrees with
NVSS; however, Omar & Dwarakanath (2005) measures $\sim 50\%$ more. WC has
higher sensitivity to point sources than previous surveys allowing us to
measure the faint radio continuum emission from the nucleus of NGC 1422 which
was previously undetected. Column 1 of Fig. 11, shows the WALLABY continuum
emission contours overlaid on SDSS _g-_ band optical images for each of these
eight galaxies.

####  4.2.1 Global Spectral Indices

The spectral index values for each individual galaxy can be seen in Table 6.
Measuring the global spectral index values for these galaxies between WC and
RACS as well as RACS and NVSS, we find the mean of the derived values of
$\alpha^{\rm WC}_{\rm RACS}=-1.34\pm 0.12$ and $\alpha^{\rm NVSS}_{\rm
RACS}=-0.87\pm 0.11$ respectively, where errors are the standard error of the
mean. We remove outliers IC 1953 and IC 1952, the latter being contaminated by
a background radio source. IC 1953 has flux deficits in WC and RACS, likely
due to resolving out extended radio flux, resulting in $\alpha>0.9$ in both
cases. The spectral index value we find for the Eridanus sample is steeper
than the $\sim-0.8$ generally found for SFGs (Condon, 1992; An et al., 2021)
whilst $\alpha^{\rm NVSS}_{\rm RACS}$ is in agreement with this value.
Considering the steeper spectral index between WC and RACS compared to RACS
and NVSS for our nearby galaxy sample we assume that WC is missing some
extended flux. Overall, due to the small population biases and systematic flux
offsets between WC and RACS, we are unable to observe any obvious impact that
environmental interactions have on the global radio spectral indices.

Table 6: Derived radio continuum properties of Eridanus supergroup members.

Common ID | LWC | $\alpha^{WC}_{RACS}$ | $\alpha^{NVSS}_{RACS}$
---|---|---|---
|  $\times 10^{20}$ (W Hz-1) |  |
IC 1952 | $0.51\pm 0.15$ | $-4.79\pm 0.80$ | $-1.16\pm 0.47$
ESO 548-G036 | $4.19\pm 0.57$ | $-1.20\pm 0.43$ | $-1.23\pm 0.31$
IC 1953 | $3.66\pm 0.75$ | $0.91\pm 0.78$ | $2.24\pm 0.69$
NGC 1359 | $9.98\pm 1.89$ | $-1.74\pm 0.47$ | $-0.56\pm 0.24$
NGC 1367 | $6.96\pm 0.95$ | $-1.51\pm 0.39$ | $-1.09\pm 0.34$
NGC 1385 | $75.06\pm 9.88$ | $-1.26\pm 0.35$ | $-0.70\pm 0.18$
NGC 1415 | $12.36\pm 1.64$ | $-0.96\pm 0.37$ | $-0.75\pm 0.22$
NGC 1422 | $0.47\pm 0.09$ |  |

  * Note. Column (1): Galaxy common ID. Column (2): WC luminosity assuming a distance of 21 Mpc. Column (3): Spectral index between WC and RACS. Column (4): Spectral index between NVSS and RACS.

####  4.2.2 Resolved Spectral Index Properties of NGC 1385

NGC 1385 is the brightest galaxy in our sample and is almost face-on with
resolved radio continuum observations also available in RACS at 888 MHz (Hale
et al., 2021). This allows us to investigate the resolved spectral index
properties and investigate the environmental impact on CR electron transport.
As flux is missing in the WC observations of NGC 1385 we primarily perform
this as a qualitative analysis to observe whether a significant gradient is
observed across the galactic disk. To generate these maps we convolve the
WALLABY data to a matching $14.6\times 14^{\prime\prime}$ beam FWHM with RACS
observations of the Eridanus field. We then regrid to a common pixel size of
$2.5^{\prime\prime}$ and positionally match the images. The spectral index is
calculated using Eq. 8 for each pixel with a SNR above $10\sigma$ in both
images. We expect to find a flatter spectral index in SF regions and a steeper
spectral index in regions where CR electron transport has occurred.

The spectral index map between WC and RACS for NGC 1385 (Fig. 6) is quite
steep with a median $\alpha^{\rm WC}_{\rm RACS}=-1.10$ and mean $\alpha^{\rm
WC}_{\rm RACS}=-1.23$, which agree with the global value of $\alpha^{\rm
WC}_{\rm RACS}=-1.26\pm 0.35$. The lower values measured throughout the galaxy
can be partly explained by the missing flux in WC and flux excess in RACS
compounding their systematics. We see some structure in the spectral index map
with a flatter spectral index being measured in the nucleus and star forming
regions in the spiral arms where the injection spectral index is measured to
be $\alpha^{\rm WC}_{\rm RACS}\sim-0.7$ as compared to $\alpha_{\rm
synch}=-0.65\pm 0.1$ measured in Tabatabaei et al. (2013) for NGC 6946. There
appears to be a slight gradient of steepening spectral index across the
galactic disk from north-east to south-west (as denoted by the blue arrow)
coinciding with the HI and IRRC features suggesting that evidence of CR
electron streaming may be seen in the spectral index map of NGC 1385.

Figure 6: Top: Spectral index map of NGC 1385 between WALLABY at 1.37 GHz and
RACS at 0.88 GHz. Contours from $g$-band SDSS images are overlaid to show the
extent and brightest regions of the galaxy. The red bar indicates a scale
length of 5 kpc whilst the red circle indicate the RACS FWHM beam size of
$14.6\times 14^{\prime\prime}$. The red arrow indicates 1/50th of the distance
to the group central galaxy NGC 1395 and the green arrow indicates 1/50th of
the distance towards Dark cloud 2 (Wong et al., 2021). The blue arrow
indicates the likely direction of CR electron streaming and we note a slightly
flatter spectral index to the south-west of the galactic centre (ignoring the
outer edges of the galaxy). Bottom: The estimated error in the spectral index
map of NGC 1385.

###  4.3 Star-Formation Properties of the Eridanus Supergroup

Radio continuum emission has been shown to be an excellent tracer of recent
star-formation due to its dependence on massive short-lived stars and its dust
unbiased nature. To investigate whether the measured global radio SFRs for
these galaxies are consistent with SFRs measured using other multiwavelength
methods we use the relationship from Molnár et al. (2021):

$\log\bigg{(}\frac{\rm SFR_{1.4}^{\rm Molnar}}{\rm
M_{\odot}\,yr^{-1}}\bigg{)}=(0.823\pm 0.009)\log\bigg{(}\frac{L_{1.4}}{\rm
W\,Hz^{-1}}\bigg{)}-(17.5\pm 0.2)$ (9)

where $L_{1.4}$ is the 1.4 GHz radio luminosity in ${\rm W~{}Hz^{-1}}$ and is
calibrated against the $q_{\rm TIR}$ (see Eq. 5) derived SFR to estimate the
radio SFR. The WC flux density is calibrated against NVSS and as such the 1.4
GHz luminosity is calculated using:

$\bigg{(}\frac{\rm L_{\rm WC}}{\rm W\,Hz^{-1}}\bigg{)}=(9.52\times
10^{15})4\pi\,\bigg{(}\frac{\rm R}{\rm Mpc}\bigg{)}^{2}\bigg{(}\frac{S_{\rm
int}^{\rm WC}}{\rm mJy}\bigg{)}$ (10)

where R is the distance to the Eridanus supergroup in Mpc and $S_{\rm int}$ is
the integrated flux density. We do not apply K-corrections to the luminosity
due to the small distance of the Eridanus supergroup.

We compare the radio SF properties of Eridanus supergroup members to those
using other multiwavelength methods. In For et al. (2021) for both IC 1953 and
ESO 548-G036 only the W4-SFRs are calculated due to UV contamination and are
assumed to be lower limits. All radio and UV+W4 derived SFR measurements are
estimated using a Chabrier initial mass function (IMF) (Chabrier, 2003). The
W3PAH-SFR relationship from Cluver et al. (2014) relies on the BG03 IMF
(Baldry & Glazebrook, 2003) which has been shown to be accurate for low SFR
systems (Cluver et al., 2014). Conversion of the BG03 IMF into a Chabrier IMF
causes the SFR relationship in Cluver et al. (2014) to overestimate the SFRs
by a factor of $\sim 2$ by construction due to their calibration.

We see agreement between the SFR estimates in Fig. 7 and Table 7, especially
for the sources with higher SFRs, with all but IC 1952 and IC 1953 being
within the scatter of the radio-SFR relationship (Eq. 9). Fig. 7b reveals
discrepancies between the radio and IR emission for some of the Eridanus
galaxies, from which details about their physical properties can be inferred.
Most of the galaxies show an agreement within the scatter between their SFRs
as determined by their W3PAH and radio flux densities. NGC 1359 appears to
have a deficit of W3PAH flux in comparison to the radio flux. NGC 1367 also
has significant excess radio continuum emission relative to the W3PAH SFR but
is comparable to the UV+W4 SFR. IC 1952 lies below the 1:1 line for both
comparisons involving the radio SFR due to the removal of the strong
background radio source, which results in a loss of a majority of the measured
radio flux from IC 1952. This background source however does not contribute
significantly to the IR or UV emission. Lastly, we see excellent agreement
between the W3PAH and UV+W4 derived SFRs for all but NGC 1367 and NGC 1359
providing further evidence that we have a deficit of W3PAH emission in these
galaxies as well as a radio deficit for IC 1952.

Table 7: Derived SFR properties of Eridanus supergroup members.

Common ID | $SFR_{\rm WC}^{\rm Molnar}$ | $SFR_{\rm UV+W4}$ | $SFR_{W3PAH}$ | $\log(\Sigma(SFR_{\rm WC}))$ | $\log(\Sigma(gas))$ | Area
---|---|---|---|---|---|---
| ($M_{\odot}yr^{-1}$) | ($M_{\odot}yr^{-1}$) | ($M_{\odot}yr^{-1}$) | ($M_{\odot}yr^{-1}kpc^{-2}$) | ($M_{\odot}yr^{-1}pc^{-2}$) | ($kpc^{2}$)
IC 1952 | $0.05\pm 0.01$ | $0.18\pm 0.02$ | $0.19\pm 0.02$ | $-2.59\pm 0.05$ | $0.94\pm 0.01$ | 28
ESO 548-G036 | $0.30\pm 0.03$ | $>0.27\pm 0.07^{b}$ | $0.27\pm 0.02$ | $-2.03\pm 0.05$ | $0.38\pm 0.01$ | 32.6
IC 1953 | $0.27\pm 0.04$ | $>1.26\pm 0.07^{b}$ | $0.58\pm 0.06$ | $-1.92\pm 0.05^{c}$ | $0.25\pm 0.04$ | 8.7
NGC 1359 | $0.61\pm 0.09$ | $1.04\pm 0.07$ | $0.12\pm 0.01$ | $>-2.60\pm 0.05^{d}$ | $0.98\pm 0.002$ | 225.5
NGC 1367 | $0.45\pm 0.05^{a}$ | $0.35\pm 0.02$ | $0.10\pm 0.02$ |  |  |
NGC 1385 | $3.19\pm 0.35$ | $3.33\pm 0.28$ | $2.72\pm 0.28$ | $-2.04\pm 0.05$ | $0.89\pm 0.001$ | 358
NGC 1415 | $0.72\pm 0.08$ | $0.73\pm 0.07$ | $0.73\pm 0.06$ | $-1.56\pm 0.05$ | $0.46\pm 0.01$ | 28.1
NGC 1422 | $0.05\pm 0.01$ | $0.06\pm 0.01$ | $0.05\pm 0.005$ | $-2.20\pm 0.05$ | $0.78\pm 0.07$ | 5.2

  * Note. Column (1): Galaxy common ID. Column (2): Total WC SFR. Column (3): $UV+W4$ SFR determined in For et al. (2021). Column (4): Total W3PAH SFR. Column (5): WC SFR surface density of the WC selected star-forming regions of each galaxy. Column (6): Gas surface density of the WC selected star-forming regions of each galaxy. Column (7): Area of star-forming ellipse used to calculate surface densities.

  * a

WC emission is entirely AGN based however we provide a SFR based on the global
emission for comparison.

  * b

Considered as lower limits due to UV contamination.

  * c

We only consider the nuclear starburst region.

  * d

Due to the irregular shape of NGC 1359 WC emission being improperly measured
by an ellipse this value is considered as a lower limit.

Figure 7: Comparison between global SFRs measured for the eight Eridanus
supergroup galaxies. a) Comparison between radio (WC) and FUV(NUV)+$W4$
derived SFRs. b) Comparison between radio (WC) and W3PAH derived SFRs. c)
Comparison between W3PAH and FUV(NUV)+$W4$ derived SFRs. $SFR_{\rm WC}$ is the
1.4 GHz SFR as determined using eq. 9 (Molnár et al., 2021). Error bars result
from measured flux uncertainties in ProFound and calibration errors. Red
shaded region indicates the 1$\sigma$ scatter in the radio-SFR relationship
derived in Molnár et al. (2021) with their luminosity limit indicated by the
horizontal red line. Blue shaded region indicates 0.2 dex scatter. Both ESO
548-G036 and IC 1953 are assumed to be lower limits for FUV(NUV)+$W4$ derived
SFRs due to contamination allowing only W4-SFR to be determined.

We also examine where each of these galaxies lie on the Kennicutt-Schmidt law
(KS-law; Kennicutt, 1998):

$\Sigma_{\rm SFR}\propto\Sigma^{N}_{\rm gas}.$ (11)

where $\Sigma_{\rm SFR}$ is the SFR surface density and $\Sigma_{\rm gas}$ is
the cold gas mass surface density, which has a power-law index of N = 1.4
based on observations of samples of spiral and starburst galaxies. We measure
the integrated flux densities of WC and WALLABY HI in an ellipse chosen to
encapsulate the WC emission of each galaxy. This method ensures we accurately
estimate the instantaneous SFR surface density. We convert the WC integrated
flux density into a SFR using Eq. 9 and then calculate the SFR surface density
for each galaxy. The HI integrated flux density is converted into an HI gas
mass using equation 6 from For et al. (2021), and using a mass correction for
helium of 1.35 (Obreschkow et al., 2016; Murugeshan et al., 2021), the cold
gas mass surface density is calculated for each galaxy.

We find that most galaxies lie within the scatter of the KS-law with a few
notable outliers (see Fig.8). Most galaxies lie above the KS-law suggesting
that there is an overall gas deficiency as compared to their radio SFRs for
these group member galaxies which agrees with the findings of For et al.
(2021) and Murugeshan et al. (2021). None of these galaxies have
$log(\Sigma_{\rm SFR})\geq-1$ putting them below the typical threshold for
starburst galaxies. IC 1953, ESO 548-G036 and NGC 1415 sit well above the KS-
law indicating an enhanced SFR density likely due to their compact radio
continuum morphology or lack of sensitivity to faint extended radio continuum
emission. Both IC 1953 and NGC 1415 are also known to be undergoing nuclear
starbursts.

Figure 8: Kennicutt-Schmidt law for the sample of Eridanus galaxies. NGC 1359
is assumed to be at the lower limit of its SFR density due to its irregular
morphology. Grey points indicate the sample of normal disk and starburst
galaxies from Kennicutt (1998) with the dashed line indicating a power-law
index of 1.4.

WISE flux measurements allow us to see where these galaxies lie on the WISE
colour-colour diagram which can help separate highly efficient accreting AGN
and starburst galaxies from passively evolving ones (Jarrett et al., 2013;
Cluver et al., 2014).Fig. 9 shows that all of the galaxies in this sample lie
in the $W1-W2\leq 0.3$ colour space suggesting that these galaxies are
passively evolving and are not dominated by highly AGN ($>1\%$ Eddington). NGC
1367 occupies the lower left of the plot where low SFR early-type galaxies
usually lie. Most of the other galaxies lie within the region usually occupied
by low SFR spiral galaxies with NGC 1385 and ESO 548-G036 displaying slightly
higher $W2-W3$ colour suggesting higher levels of SF activity. Interestingly,
despite the observed post-merger starburst activity of NGC 1359, it lies in
the same region as the other lower SFR spirals. When compared to the KS-law
diagram (Fig. 8) we see agreement between the classifications for most
galaxies which are categorised as star-forming spirals. NGC 1385 and ESO
548-G036 are likely undergoing increased SF activity and NGC 1367’s radio
emission is a result of AGN processes.

Figure 9: WISE colour-colour diagram Jarrett et al. (2013) of the sample of
eight Eridanus galaxies. Early type galaxies with low SFR are expected to
occupy the bottom left of this diagram (W2 - W3 ¡ 2), starbursting disk
galaxies lie on the right side of this diagram (W2 - W3 ¿ 3.5) with normal
star-forming spirals lying between these regions. At above W1 - W2 = 0.8 mid
infrared emission is dominated by heating from dusty AGNs.

###  4.4 Global IRRC Properties of the Eridanus Supergroup

In terms of the $q_{12}$ values, we see an agreement with the general trends
shown in Molnár et al. (2021); An et al. (2021) whereby $q$ decreases with
increasing radio or TIR luminosity. We also observe that NGC 1367 lies below
the other galaxies due to the AGN mechanism being responsible for excess radio
emission in the form of radio jets. NGC 1359 lies in the same region as NGC
1367 despite being a SFG as a result of low W3PAH emission. The trend we see
here appears to be steeper than the trends generally observed for SFGs in
Molnár et al. (2021) eq. 16 & 18\. When comparing the $q_{\rm 12}$ values to
$q_{\rm FIR}$ values measured in Omar & Dwarakanath (2005) we find the values
follow the same trend for each galaxy with $q_{\rm FIR}$ being offset to
higher values by $\sim 0.6$ as expected due to the higher FIR luminosity. The
exception to this is IC 1952 in which we remove the background radio source
which isn’t individually resolved and removed in Omar & Dwarakanath (2005).

Discrepancies can partly be explained by IC 1952 and IC 1953 having a radio
flux deficit due to removal of a background AGN source and missing extended
flux respectively which would boost their $q_{\rm 12}$ values. We also note
that these Eridanus galaxies have low SFRs and generally lie on or slightly
below the luminosity limit where most IRRC relationships are measured and so
there is likely a larger amount of inherent scatter at these luminosities.
After converting the W3PAH emission into an estimated TIR emission, we
investigate whether the W3PAH emission is a good proxy for TIR emission in the
IRRC in the bottom row of Fig. 10. We see that most of the faint galaxies lie
above the relationship determined for SFGs in Molnár et al. (2021), however
some of these lie in the extrapolated regions (indicated by the dashed lines)
of the $q_{\rm TIR}$ relationships due to their low luminosities. NGC 1385
returns the expected $q_{\rm TIR}$ value in both plots and has a relatively
high SFR compared to the rest of the galaxies within the Eridanus supergroup,
suggesting that its possible radio continuum sensitivity limits have biased
the $q_{\rm TIR}$ luminosity relationship to flatter slopes; however, our
study is limited by the small sample size of seven SFGs. Future radio studies
will be useful for constraining our results of the IRRC at the low luminosity
end.

Table 8: Derived WISE quantities.

Common ID | $W1-W2$ | $W2-W3$ | $L_{W3PAH}$ | $L_{\rm TIR}$ | $q_{12}$ | $q_{\rm TIR}$
---|---|---|---|---|---|---
| (mag) | (mag) | ($log_{10}(L_{\odot})$) | ($log_{10}(L_{\odot})$) |  |
IC 1952 | $0.09\pm 0.03$ | $3.03\pm 0.04$ | $8.43\pm 0.04$ | $9.7\pm 0.3$ | $2.73\pm 0.13$ | $4.0\pm 0.5$
ESO 548-G036 | $0.20\pm 0.03$ | $3.74\pm 0.04$ | $8.56\pm 0.04$ | $9.8\pm 0.3$ | $1.94\pm 0.06$ | $3.2\pm 0.4$
IC 1953 | $0.09\pm 0.03$ | $3.52\pm 0.05$ | $8.85\pm 0.04$ | $10.1\pm 0.4$ | $2.30\pm 0.09$ | $3.5\pm 0.4$
NGC 1359 | $0.06\pm 0.04$ | $3.21\pm 0.05$ | $8.25\pm 0.05$ | $9.5\pm 0.3$ | $1.26\pm 0.08$ | $2.6\pm 0.4$
NGC 1367 | $0.03\pm 0.04$ | $1.19\pm 0.11$ | $8.18\pm 0.11$ | $9.5\pm 0.4$ | $1.34\pm 0.07$ | $2.6\pm 0.5$
NGC 1385 | $0.23\pm 0.03$ | $3.87\pm 0.04$ | $9.45\pm 0.04$ | $10.6\pm 0.4$ | $1.58\pm 0.06$ | $2.7\pm 0.4$
NGC 1415 | $0.10\pm 0.03$ | $2.86\pm 0.04$ | $8.94\pm 0.03$ | $10.2\pm 0.3$ | $1.86\pm 0.06$ | $3.1\pm 0.4$
NGC 1422 | $0.05\pm 0.03$ | $2.69\pm 0.04$ | $7.94\pm 0.04$ | $9.3\pm 0.3$ | $2.28\pm 0.09$ | $3.6\pm 0.4$

  * Note: Column (1): Galaxy common ID. Column (2): $W1-W2$ magnitude. Column (3): $W2-W3$ magnitude. Column (4): Total W3PAH luminosity. Column (5): Total estimated TIR luminosity. Column (6): global $q_{12}$ value. Column (7): global estimated $q_{\rm TIR}$ value.

Figure 10: Top: The global $q_{12}$ value measured for each galaxy compared to
their a) WC luminosity and b) W3PAH luminosity. The estimated $q_{\rm TIR}$
values determined using the relationship from Cluver et al. (2017) compared to
their c) WC luminosity and d) estimated TIR luminosity. The error bars are
determined using error propagation of the flux uncertainties as well as the
addition in quadrature of the scatter in the estimate for TIR luminosity in
Cluver et al. (2017). The black line with red scatter indicates the
relationships for $q_{\rm TIR}$ measured in Molnár et al. (2021) for SFGs. The
red vertical line and dashed portion of the (Molnár et al., 2021) relationship
indicates the luminosity below which data is extrapolated.

###  4.5 Resolved IRRC properties of the Eridanus Supergroup

In this section we examine the resolved radio continuum and IRRC properties of
the Eridanus galaxies, which are known to be experiencing environmental pre-
processing, using the WALLABY continuum and WISE W3PAH maps. We build upon the
analysis from the previous WALLABY pre-pilot survey papers to discuss the
radio properties of these specific galaxies in the context of their HI
distribution, HI deficiency, SFRs and disk stabilities (For et al., 2021;
Murugeshan et al., 2021), and tidal interaction strength (Wang et al., 2022)
to see whether we find any relationships between these properties and the
resolved radio continuum emission. The criteria for HI deficiency in For et
al. (2021) is defined as having less than half the measured HI mass compared
to the expected HI mass estimated using the R-band absolute magnitude. In
Murugeshan et al. (2021) HI deficiency is compared to the HI disk stability
and hence there may be disagreement between these classifications. The values
of stellar mass, HI masses and deficiency, and summed tidal parameter from For
et al. (2021), Murugeshan et al. (2021) and Wang et al. (2022) are provided in
Table 10 with a quantitave analysis available in previous WALLABY pre-pilot
articles. Galaxy morphology classifications are from
NED444https://ned.ipac.caltech.edu/.

We see a number of trends which are consistent among this sample of galaxies.
Firstly, radio continuum emission at this frequency is less extended than the
W3PAH, optical and HI emission. Most of the radio emission appears to be
produced in the core, due to higher SFRs, with certain galaxies exhibiting
compact star-forming sites in their disks. Kennicutt (1998) finds that for
normal SF spirals the mean total SFR density can be as low as $\sim
0.001\,M_{\odot}$yr-1kpc-2 with typical values of $\sim
0.01\,M_{\odot}$yr-1kpc-2 derived using H$\alpha$ observations. The minimum
3$\sigma$ surface brightness limit we measure is 0.11 mJy/beam, which
corresponds to a SFR density of $0.02\,$M⊙yr-1kpc-2 at 21 Mpc suggesting we
are limited by the surface brightness sensitivity to probe the low SFR density
regions for these galaxies. W3PAH emission occurs throughout the stellar disk
for all but NGC 1367. Across the sample most galaxies show little variation in
the IRRC in the central regions with $q_{12}$ values consistent with SF
($q_{\rm 12}\sim 1.20-1.65$) and the radial extent being limited by the radio
continuum sensitivity. The HI extent of these galaxies is larger than the
radio continuum, IR, and optical extent and shows disturbed asymmetric
morphology for all galaxies in the sample. A majority of the galaxies have
$q_{12}$ maps with a lower median than their global $q_{\rm 12}$, determined
in the previous section, due to the exclusion of extended IR flux from the
final $q_{\rm 12}$ maps as a result of the more compact radio emission.
Background radio sources and AGN activity are easily detected using the
$q_{12}$ maps due to their significant localised radio excess causing them to
be easily distinguishable from SF based IRRC measurements. This result may
provide another method for AGN and background source detection usable for
small, high-resolution samples of individual galaxies.

####  4.5.1 IC 1952

IC 1952 is loosely classified as a highly inclined S-shaped barred spiral
galaxy. Due to a contamination of the R-band image by a foreground star,
measurement of the expected HI mass is not possible hence For et al. (2021)
could not determine whether IC 1952 is HI deficient or not; however,
Murugeshan et al. (2021) do determine IC 1952 to be HI deficient as compared
to its integrated HI disk stability. There is evidence of extraplanar gas with
plumes of HI extending out of the disk as well as a slight extension towards
the south-east. Wang et al. (2022) determine this galaxy to be weakly
perturbed by tidal interactions.

Due to the high angular resolution of the WALLABY continuum observations, we
can easily distinguish the compact background radio source that lies behind IC
1952. We also observe localised SF occurring within the disk of IC 1952. The
W3PAH emission occurs throughout the disk tracing the stellar and higher
density HI emission. The ring of W3PAH emission to the south-east is caused by
a foreground star. The $q_{\rm 12}$ map for this galaxy shows the star-forming
regions as well as the radio excess caused by the background source. Combining
all these observations we have no visible evidence for environmental effects
outside of the HI distortion.

Figure 11: The left column shows the 1.37 GHz WC contours overlaid on the log-
scaled SDSS g-band image for each galaxy, with contour levels of
$3\sigma,10\sigma,25\sigma,50\sigma,100\sigma$ in blue, green, yellow orange
and red respectively. The right column shows the $q_{\rm 12}$ map where both
the radio and W3PAH emission are above $3\sigma$ with colours ranging from 0.6
(cyan - radio excess) to 1.9 (orange - radio deficient) overlaid on the WISE
W3PAH image which is log-scaled between $3\sigma$ and its maximum value. The
HI contours from For et al. (2021) are overlaid with contour levels of
$1,2,5,10,20\times 10^{20}\,cm^{-2}$ in blue, orange, red, purple and yellow
respectively. The red scale bar in the top left indicates 5 kpc. The red and
black filled circles in the bottom left indicate the beam size of WC/W3PAH
(8′′) and WALLABY HI (30′′) respectively. The red arrow indicates the distance
and direction to the Eridanus group central galaxy, NGC 1395, and is
comparable between each galaxy (not scaled to the background image across
galaxies). The orange arrow indicates the direction to NGC 1407, the central
galaxy in the NGC 1407 group.

####  4.5.2 NGC 1359

NGC 1359 is classified as a Magellanic S-shaped barred spiral galaxy in the
coalescence stage of a minor merger (Omar & Dwarakanath, 2005). Murugeshan et
al. (2021) measures NGC 1359 to have enhanced specific SFR (sSFR) while its
atomic gas fraction is higher than expected making it HI-normal for its
measured integrated disk stability as compared to other Eridanus group
members. This enhanced atomic gas fraction and sSFR is likely a result of the
accretion of fresh gas during the merger which leads to increased SF. Wang et
al. (2022) find this galaxy is the least affected by tidal interactions and
has the highest size ratio (between HI and stellar disk) in our sample;
however, as the tidal interaction measurement only takes into account
perturbations from galaxies outside this system, it does not take into account
the tidal impact of the past minor merger.

This galaxy exists at the outer edge of the WALLABY Eridanus field in an edge
beam containing no footprint overlap. As such the sensitivity of the WALLABY
observations is lower. The radio continuum emission does trace the starburst
sites extremely accurately. The W3PAH emission traces the g-band emission
however it does not extend beyond the stellar disk unlike most of the galaxies
in this sample. There is a significant flux deficit in measured W3PAH emission
suggesting that dust may have been removed and PAHs destroyed during the
merger and subsequent starbursts; however, we see no evidence of extended
W3PAH emission tracing the HI gas as we do in NGC 1367 or NGC 1415.

Due to a significant W3PAH flux deficit, we find very low $q_{12}$ values
indicative of AGN or background radio sources; however, due to their
distribution and coincidence with starburst sites, we rule out this
possibility. We do not observe evidence of environmental effects in the
$q_{\rm 12}$ map because of the decreased sensitivity of WC observations in
this region of the field. We do however see that NGC 1359 has an extremely
disturbed stellar and HI morphology with a massive debris field to the south
as a result of the minor merger that this galaxy has undergone.

####  4.5.3 IC 1953

IC 1953 is classified as a face-on barred spiral galaxy with a mixed inner
ring. It is determined to be HI deficient in comparison to both its expected
HI mass and integrated HI disk stability. As shown in Fig. 11 there is an HI
extension towards ESO 548-G036 that lies to the south-west. This is suggestive
of interaction between these galaxies, since Wang et al. (2022) measure this
galaxy is only weakly perturbed by the interaction.

The radio continuum emission is primarily produced in the core of the galaxy
but we do also see localised emission in the arms of the galaxy both of which
coincide with increased HI density suggesting SF is occurring in these areas.
It is likely that the extended radio emission is resolved out as we only
measure about half the total NVSS flux. Omar & Dwarakanath (2005) find that
there is diffuse emission throughout the disk and that there is evidence of a
nuclear starburst (Roussel et al., 2003). We see that the W3PAH emission
occurs throughout the stellar disk with localised increases in the spiral arms
corresponding to the star forming areas observed in the radio emission. An
extension towards the south-west is also seen in the W3PAH emission in the
direction of ESO 548-G036 tracing the HI extension.

The $q_{\rm 12}$ map for this galaxy does not show any evidence of
environmental effects or asymmetries. The $q_{\rm 12}$ map does however
provide evidence that the central emission is unlikely to be caused by AGN
activity because the values are roughly consistent with those related to SF in
the arms and not significantly radio excess such as in NGC 1367. Overall,
evidence for environmental effects are only seen in the HI and W3PAH maps for
this galaxy.

Figure 12: Continued, with the addition that the lime green arrows indicate
the distance and direction between IC 1953 and ESO 548-G036 and are scaled by
a factor of 3 with respect to the red arrow for visibility.

####  4.5.4 ESO 548-G036

ESO 548-G036 is classified as a face-on spiral galaxy and is measured to be HI
deficient with respect to its expected HI mass (see appendix Table. 10). ESO
548-G036 lies south-west of IC 1953 and shows a disturbed HI morphology with a
HI cloud detected north of the galaxy. This galaxy is also classified as
weakly perturbed by tidal interactions in Wang et al. (2022).

ESO 548-G036 has radio emission throughout the stellar disk suggesting SF is
occurring throughout the galaxy. Flux does not appear to be resolved out for
this galaxy due to its small angular size giving us comparable total flux
values to NVSS. The W3PAH emission extends beyond the stellar disk and roughly
coincides with the HI distribution.

The $q_{\rm 12}$ map has values consistent with SF and does not show a
statistically significant gradient across the disk thus providing no
additional evidence for interaction with IC 1953.

####  4.5.5 NGC 1367

Figure 13: Continued, with the addition that the yellow arrow indicates the
distance to Dark cloud 2 (Wong et al., 2021) and is directly comparable to the
red arrow. The blue arrow indicates the likely direction of CR electron
streaming or increasing radio continuum emission.

NGC 1367 is classified as a slightly inclined mixed-bar spiral galaxy with a
mixed inner ring and star forming outer ring. The HI deficiency parameter with
respect to its expected HI mass for this galaxy in not measured due to R-band
contamination as well as missing a large amount of extended HI flux.
Murugeshan et al. (2021) however do find this galaxy to be slightly HI
deficient with respect to its integrated HI disk stability. NGC 1367 is a
large galaxy with an extremely extended HI diameter of 22.3 arcmin as observed
by the $9.1^{\prime}$ observations by the Green Bank Telescope (Sorgho et al.,
2019), however WALLABY only recovers an HI diameter of $9.9^{\prime}$ and
$83\%$ of the total HI mass.

We observe double radio lobes and jets in this galaxy. The radio jets of this
galaxy have half the extent of the inner stellar disk, lie slightly offset
from the bar, and coincide with the HI hole. This structure suggests that NGC
1367 may be in the same class of galaxies as other nearby spirals with small
radio jets including Circinus (Harnett et al., 1990; Curran et al., 2008) and
NGC 3079 (Wong et al., 2016). Omar & Dwarakanath (2005) observe NGC 1367 has a
nuclear radio source coincident with the $H\alpha$ nuclear source observed by
Hameed & Devereux (1999); however, no $H\alpha$ is seen coincident with the
extended jet structure outside weak diffuse emission in the bulge. We also do
not observe significant amounts of radio or W3PAH emission in the bulge
suggesting that this galaxy is not strongly star-forming in this region. NGC
1367 displays UV and $H\alpha$ emission in an outer ring which is faintly
visible in the g-band image and roughly coincides with the W3PAH emission.
This observation suggests that SF is occurring in this region however either
due to surface brightness sensitivity limits or resolving out this extended
flux we do not see it in the WC. NGC 1367 also appears in the Chandra X-ray
survey and contains a nuclear source with surrounding diffuse emission which
is analysed in detail (see NGC 1371; Hughes et al., 2007) with the conclusion
that NGC 1367 contains a low-luminosity AGN similar to other nearby spiral
galaxies.

The $q_{\rm 12}$ map for this galaxy clearly shows the radio excess due to the
radio AGN in the central region and provides a good indicator for the
separation of AGN and SF activity based on its value. Overall due to the
existence of an AGN within this galaxy and its low SF, environmental effects
are not discernible.

####  4.5.6 NGC 1385

NGC 1385 is classified as an almost face-on ($i=44^{\circ},P.A=181^{\circ}$,
Leroy et al., 2021) barred spiral galaxy with S-shaped bars and is the highest
SFR galaxy in our sample (see Table. 7). NGC 1385 is located in the south-west
of the Eridanus group, is $0.6^{\circ}$ south of WALLABY J033723–235753 (C2 in
Wong et al. (2021)), an HI cloud with no detected stellar component. For et
al. (2021) measure this galaxy to be HI deficient as compared to its expected
HI mass as estimated by its stellar mass. The HI morphology of NGC 1385 is
extremely distorted with clear compression and extensions in the north-east
and south-west respectively. This galaxy also exhibits a southern tidal debris
field (which is partially cut off in Fig 11) with no optical counterpart
suggesting HI gas has been displaced likely due to tidal interactions with its
nearby companion WALLABY J033723–235753. Galaxies that are currently
interacting and accreting HI gas from their dwarf companions can have enhanced
SFR (Kennicutt et al., 1987; Ellison et al., 2011; Murugeshan et al., 2020).
Murugeshan et al. (2021) measures NGC 1385 to have enhanced sSFR while its
atomic gas fraction is higher than expected for the measured integrated disk
stability as compared to other Eridanus group members. This means that
compared to other galaxies within the Eridanus group this galaxy is one of two
that is actually HI-normal for their integrated disk stability, likely due to
the accretion of HI from its interacting companion. This accretion of fresh
gas for NGC 1385 leads to increased HI mass and SF which is consistent with
the findings of Ellison et al. (2018) and Murugeshan et al. (2020). This
galaxy is found to have extremely weak tidal interaction parameters due to its
mass and location within the Eridanus subgroup (Wang et al., 2022), however
this does not take into account the interaction with WALLABY J033723–235753
due to the lack of a stellar component.

NGC 1385’s disk is almost face on and the SFR is relatively high so this is a
good candidate for examining whether environmental effects (detected in the
HI) are also observed in the radio continuum. We see strong radio continuum
emission throughout the stellar disk except in the northernmost region likely
due faint extended emission being below our surface brightness sensitivity
limit. We observe radio emission in the south-western edge of NGC 1385 that
extends slightly beyond the stellar disk. The W3PAH emission does not show any
clear signs of disturbance and is observed throughout the galaxy.

The $q_{\rm 12}$ map of NGC 1385 shows slightly lower values due to enhanced
radio emission possibly caused by interactions with the environment. We can
see a clear gradient of decreasing $q_{\rm 12}$ moving from the north-east the
the south-west of the galaxy (as shown by the blue arrow in Fig. 11. This
roughly coincides with the direction of the group centre and WALLABY
J033723–235753 suggesting that ram-pressure and/or tidal interactions may be
leading to CR electron reacceleration and streaming which is forming a small
synchrotron tail. This trend agrees with the findings of that from Murphy et
al. (2009) and Vollmer et al. (2020), which suggests that CR electron
streaming results in a redistribution of CR electrons and hence radio
continuum emission. This bulk motion is not reflected in the ISM emission
measured by W3PAH due to different scale lengths of the mean free path of
ionising UV photons and CR electrons. The direction of the gradient in the
IRRC also coincides with the disturbance in HI morphology possibly hinting at
their common origin.

####  4.5.7 NGC 1415

NGC 1415 is classified as a moderately inclined spiral galaxy with S-shaped
mixed bars and an outer ring. It is located in the central region of the
Eridanus group and is the closest galaxy to the group centre in our sample.
NGC 1415 has been measured to be HI deficient as compared to its expected HI
mass and integrated disk stability. Wang et al. (2022) find NGC 1415 to be
weakly perturbed by tidal interactions; however, it has the second highest
summed tidal interaction strength for this sample.

The HI traces the stellar disk and outer ring whilst radio continuum emission
is produced primarily in the central region of the galaxy. W3PAH emission
occurs throughout and slightly beyond the stellar disk tracing the HI cloud.
It is likely that we are missing the extended radio emission in the outer disk
as we are missing flux compared to NVSS. Evidence of a nuclear geyser (bipolar
outflow) is apparent in the $H\alpha$ and measured kinematics of NGC 1415,
which is suggested to be caused by AGN Garcia-Barreto et al. (2019); however,
we do not identify any jet structure within NGC 1415. This galaxy shows a
truncated star forming disk compared to the entire stellar disk. Truncated
star forming disks are seen in Virgo spirals that are thought to have
experienced ram-pressure stripping Koopmann & Kenney (2004); Vollmer et al.
(2012).

The $q_{\rm 12}$ map shows no significant variation within the galaxy and
mostly traces the SF occurring in the central region. A slight but
statistically insignificant gradient can be seen with higher radio emission
observed to the south-west of the galactic centre and in the direction of the
group centre. However, we do not see any corresponding distortion in the HI
morphology in this direction. We also see a background AGN south-west of NGC
1415. Combining these features we do not have conclusive evidence of ram
pressure stripping or tidal interactions.

Figure 14: Continued.

####  4.5.8 NGC 1422

NGC 1422 is classified as an edge-on barred spiral galaxy and is the lowest
stellar mass and faintest galaxy in our sample. It is located in the north-
eastern region of the Eridanus group and is measured to be HI deficient. The
HI disk for NGC 1422 is truncated compared to the stellar disk, and extends
beyond the galactic plane suggesting ram-pressure stripping is occurring
within this galaxy (Murugeshan et al., 2021). Murugeshan et al. (2021) also
find this galaxy to be HI deficient with respect to its HI disk stability
parameter. Wang et al. (2022) find this galaxy to be strongly perturbed by
tidal interactions and is the only galaxy in our sample with this
classification.

NGC 1422 has the lowest measured SFR and no AGN activity leading to extremely
low radio continuum emission that only shows up at the $3\sigma$ detection
limit in the core of the galaxy and is undetected in NVSS. Similar to the HI
distribution, we see a truncation of the W3PAH emission at the edge of the
stellar disk and an extension beyond the galactic plane, which suggests the
heated dust and ISM is also being affected by ram-pressure stripping.

The $q_{\rm 12}$ map for this galaxy does not provide evidence for
environmental effects due to the extent of radio emission being limited to the
core of the galaxy, however we can likely rule out AGN related radio emission
due to the values being consistent with SF activity.

##  5 Discussion

###  5.1 Global Continuum Properties

We find (see Table. 6) that the global spectral indices of the eight Eridanus
supergroup members (after removal of outliers) are generally steeper than
expected of normal SFGs. A number of effects could be at play which are
causing this steepness. Firstly the previously mentioned flux scale issues
with RACS and WC will act to steepen the measured spectral index for these
galaxies however we can still compare them to each other. It has been shown
that SF flattens the spectral indices and that compact starbursts have a
flatter spectral index primarily thought to be caused by free-free absorption
(Murphy, 2013; An et al., 2021). As the Eridanus supergroup galaxies generally
have lower levels of SF this steeper spectral index may be somewhat expected.
Within our sample we generally observe the higher SFR galaxies to have a
flatter spectral index. We also see that the spectral index of NGC 1385 and
NGC 1359, the two systems with clear signs of past interactions in their HI
morphology, have spectral indices which agree with the results in Murphy et
al. (2009), Murphy (2013) and Donevski & Prodanović (2015). NGC 1385 has a
slightly steeper spectral index and is thought to be currently undergoing
tidal interactions which act to steepen the spectral index, whilst NGC 1359 is
post merger and contains compact starbursts which will flatten the spectral
index.

The SFRs for our sample of Eridanus galaxies are largely consistent with
previously derived luminosity-SFR relationships (Cluver et al., 2014; Davies
et al., 2017; Wang et al., 2017; Molnár et al., 2021) (Fig. 7) even whilst
probing their lower limits. Significant outliers can be explained by the
underestimation of radio flux (IC 1952, IC1953) or possible PAH destruction
(NGC 1367, NGC 1359). It also supports that the radio-SFR provides an accurate
estimate of the current SFR. Moorman et al. (2016); Murugeshan et al. (2020)
have shown that SFRs of galaxies undergoing interactions can be modified and
that the sSFR of these Eridanus group galaxies are generally lower (except for
NGC 1385 and NGC 1359 due to the accretion of fresh HI gas from their
companions) than those from the field Murugeshan et al. (2021).

We find a steeper dependence on W3PAH luminosity than radio luminosity in
contrast to the results for $q_{\rm TIR}$ (Morić et al., 2010; Molnár et al.,
2021). The estimated TIR luminosities are significantly higher than expected
resulting in discrepancies in $q_{\rm TIR}$ as compared to Molnár et al.
(2021). We suggest that the scaling relationship between W3PAH and TIR
luminosities determined for SINGS/KINGFISH galaxies (Eq. 6) may not be
appropriate for our sample due to their low sSFRs which may be less severely
impacted by PAH destruction. This discrepancy may be caused by the destruction
of PAHs in starbursts (Draine & Li, 2007; Cluver et al., 2017) which will
impact galaxies with higher SFRs more severely, causing the observed W3PAH
emission to increase more slowly than TIR emission and hence biasing $q_{12}$
towards lower values with increasing W3PAH luminosity.

NGC 1359 shows a significant deviation compared to the other SFGs due to its
low W3PAH emission, which is possibly due to merger-induced PAH destruction
(Appleton et al., 2006; Micelotta et al., 2010; Murata et al., 2017) and is
discussed in more detail in Section 6.3. When compared to the luminosity
trends observed in Molnár et al. (2021) we find that as we move towards lower
luminosities the galaxies begin to deviate from their expected properties.
This is likely because these galaxies have low radio luminosities (partially
due to missing flux problems) and probe the lower limits of current radio
surveys and so are not well sampled. It may also hint at the luminosity limit
of the expected breakdown of the IRRC due to UV and optical photons not being
fully reprocessed by dust (Bell, 2003; Lacki & Thompson, 2010), however this
would decrease observed $q_{\rm 12}$ values which are contradictory to our
findings. Previous studies by Miller & Owen (2001) and Reddy & Yun (2004) have
found the global $q$-value to be lower in cluster galaxies compared to those
in the field which is attributed to thermal compression and ram pressure based
enhancement of the synchrotron emission. Due to our approximate values for the
TIR luminosity and radio flux deficiency we do not see this. It is also worth
mentioning that these galaxies exist in a group environment which is less
dense and undergoing pre-processing before becoming a supergroup and hence
have not experienced the strong interactions that occur in cluster
environments. Overall we find that while W3PAH emission can be used to measure
the IRRC globally for galaxies because of the tight correlation between W3PAH
and TIR luminosities (Cluver et al., 2014, 2017), the impact of PAH
destruction and even initial PAH concentration due to differing metallicities
has to be taken into account. Future studies on the relationship between W3PAH
emission and $q_{12}$ will further verify our conclusions.

###  5.2 Resolved Continuum Properties

We find that the $q_{\rm 12}$ values in star-forming regions are largely
consistent with each other across our sample providing evidence that the IRRC
derived using W3PAH is a result of SF processes. We also find that $q_{\rm
12}$ is lower in the centre of each galaxy. This suggests that, unlike $q_{\rm
TIR}$ which is enhanced in star-forming regions (Tabatabaei et al., 2013;
Vollmer et al., 2020), the interstellar radiation field becomes hard in these
areas destroying the PAHs and lowering the W3PAH emission (and hence $q_{\rm
12}$) which is not entirely offset by the increase in $W3$ continuum emission
produced by the large warm grains. This is possibly caused by the existence of
radio-quiet, weak AGN, but, due to the lack of X-ray and spectroscopic data in
this field, we are unable to assess this possibility for each galaxy.

The WISE color-color diagram suggests that these galaxies are unlikely to
harbor powerful AGN (see Fig. 9) as they exist well below the $W1-W2\geq 0.5$
region where AGN sources typically exist (Jarrett et al., 2013). Following the
findings of For et al. (2021), Murugeshan et al. (2021) and Wang et al.
(2022), we conclude that the environmental pre-processing for these galaxies
has removed HI gas and suppressed the collapse of HI gas into molecular clouds
and hence decreased the sSFR for these galaxies without a significant
enhancement due to tidal interactions. A major finding for these low SFR
galaxies is that the resolved $q_{\rm 12}$ maps also allow us to easily
distinguish background radio sources (IC 1952) and AGN activity due to a
significant decrease in measured $q_{\rm 12}$ value.

In the case of NGC 1359 however the $q_{\rm 12}$ map suggests that due to the
merger and enhanced sSFR, PAH destruction likely plays a significant role in
lowering the measured $q_{\rm 12}$ value. It is likely then that this deficit
in W3PAH emission is caused by PAH destruction either by shocks during the
merger or the subsequent starburst activity. Indeed it has been shown that
mergers especially during the late stages can destroy PAH’s via large scale
shocks (Murata et al., 2017), both low and high speed, which can cause thermal
and inertial sputtering (Micelotta et al., 2010). Mergers with high collision
speeds ($>500\,$km/s) such as those in Stephan’s quintet are also known to
destroy PAHs via high speed, highly dissociative shocks (Hollenbach & McKee,
1980, 1989; Appleton et al., 2006).

NGC 1367 is interesting due to its AGN jets with distinct radio lobes, very
little W3PAH emission and a $>10^{\prime}$ HI extent. We do not observe
evidence of any environmental effects in the $q_{12}$ map. The bulge does not
appear to be forming stars at a significant rate and has very little ISM
emission in the W3PAH band suggesting either a lack of UV photons being
produced or a lack of hot dust and PAHs. We see the strongest W3PAH emission
coincident with the location of the AGN where dust heating and PAH destruction
would be most prevalent. The absent W3PAH emission combined with the lack of
UV and $H\alpha$ emission in the bulge suggests that it is likely very little
SF is occurring, with most current SF occurring in the outer spiral arms as
traced by UV observations. The HI hole in the central region of this galaxy is
coincident with the jet structure and suggest the possibility that the AGN
activity may be related to the lack of HI gas in this central region. Jet-ISM
interactions have been posited to both suppress (via gas ejection, suppressing
halo gas accretion or induced turbulence) and enhance (via shock compression)
SF depending on a large number of factors (see Mukherjee et al., 2018; Mandal
et al., 2021; Mukherjee et al., 2021; Girdhar et al., 2022). However, an in
depth analysis of jet-ISM interactions is beyond the scope of this paper.

NGC 1385, the galaxy with the highest SFR in the Eridanus group, is understood
to be undergoing tidal interactions with a nearby HI cloud. We see slight
evidence of a small synchrotron tail in the south of NGC 1385 which suggests
CR electron streaming is occurring due to tidal interactions or interactions
between the ISM and intragroup medium (IGrM) wind. This IGrM wind is theorised
to cause shocks which travel quickly through the ISM transporting and
reaccelerating CR electrons leading to synchrotron tails and radio deficit
regions at the leading edge of the galaxy (Murphy et al., 2009). Murphy et al.
(2009) and Vollmer et al. (2020) show that this electron streaming as a result
of environmental effects leads to significant differences in the measured vs
expected radio emission across the galactic disk. These significant
differences are not typically observed in secularly evolving galaxies (Murphy
et al., 2008), however there can be a number of factors at play which affect
the synchrotron emission within individual galaxies such as magnetic field
strength, SFR, radiation fields and neutral gas (Tabatabaei et al., 2013).

The $q_{\rm 12}$ map shows a clear increase (radio deficit) in the north-east
of NGC 1385, exterior to the spiral arm, indicating either a lower CR electron
density, weakening of the turbulent magnetic fields (Murphy et al., 2009;
Vollmer et al., 2020) or conversely an excess of W3PAH emission. Due to a lack
of polarisation observations we can not measure the strength of the magnetic
fields within this galaxy and are unable rule out synchrotron enhancement by
compressed magnetic fields. An excess of W3PAH emission also seems unlikely as
it has been shown that W3PAH emission appears to have a weaker correlation
with SFR, particularly as the SFR density reaches a critical point whereby the
PAHs can be destroyed. Therefore we posit that IGrM-ISM and tidal interactions
which transport the CR electrons towards the south-west is likely to be the
primary cause of the radio deficit in this region. The south-west of the
galaxy then shows decreased $q_{\rm 12}$ values indicating a radio excess. We
also do not observe as prominent of a star forming spiral arm further
providing evidence that the CR electron density here is enhanced rather than
SF based magnetic field enhancement (Tabatabaei et al., 2013) or PAH
destruction causing a W3PAH deficit. Tidal interactions have also been shown
to cause tidal CR electron emission to dominate the global CR electron
emission and hence decrease $q_{\rm 12}$ locally. The $q_{\rm 12}$ gradient
almost perfectly aligns with the disturbances in HI morphology whereby we see
compression in the HI in the northern edge of NGC 1385 and extension with a
tidal debris field in the south. These features in turn all align with the
direction to the group centre as well as the interacting companion suggesting
that they are the cause of these variations in $q_{\rm 12}$ and HI morphology.

##  6 Summary and Conclusions

In this paper we present the radio continuum catalogue at 1.37 GHz from pre-
pilot WALLABY observations of the Eridanus field using the ASKAP telescope.

  1. 1.

We demonstrate that ASKAP produces excellent continuum image quality with a
sensitivity down to 0.1 mJy and mean RMS of 0.05 mJy/beam at an angular
resolution of $6.1\,^{\prime\prime}\times 7.9\,^{\prime\prime}$. We detect
9461 components in the field and find that the WC flux density measured using
ProFound is consistent within the scatter with NVSS at 1.4 GHz.

  2. 2.

Our flux offset and spectral index analysis suggests that WC slightly
underestimates the flux by $\sim 5\%$ for isolated unresolved sources and that
the RACS catalogue slightly overestimates flux despite both being accurate
within their estimated scatter.

  3. 3.

These pre-pilot Eridanus observations enable the study of the radio-SFR and
IRRC correlation down to lower SFRs and galaxies with lower stellar masses. We
find the 1.37 GHz radio continuum provides an accurate estimate of the total
SFRs within our sample of eight galaxies (aside from NGC 1367 due to AGN
contamination) compared to those determined using multi-wavelength methods and
W3PAH emission. We also probe the global IRRC for the Eridanus group galaxies
using W3PAH emission and find that the general trends from literature using
FIR or TIR emission are reproduced for SFGs.

  4. 4.

The sensitivity limit of WC observations is slightly too high to detect the
faint diffuse radio continuum emission due to SF throughout the disks for such
a low stellar mass and SFR sample. This prevents studies of the environmental
effects and their impact on the radio continuum emission and the IRRC for most
of our sample. However we do find that the resolved IRRC can be used to easily
identify background radio sources and discriminate between SF and AGN based
radio continuum emission despite the sensitivity limitations.

  5. 5.

The impact of environmental interactions are not as evident in the radio
continuum morphology as it primarily traces local star forming regions within
the galactic disks which are less susceptible than outer regions to weak
interactions with the environment.

  6. 6.

We confirm the existence of a double lobed radio jet within NGC 1367. The
radio jet is coincident with holes in the HI emission. SF appears to be
suppressed in the bulge of this galaxy as supported by the lack of W3PAH and
UV emission in this region.

  7. 7.

NGC 1359 is in the coalescence stage of a merger and has a significant deficit
of W3PAH emission. We suggest that merger induced shocks may have destroyed
the PAHs in this galaxy however MIR spectroscopy is needed to confirm whether
this is the case.

  8. 8.

NGC 1385 shows clear signs of environmental pre-processing in its radio
continuum and resolved IRRC maps. We observe a small synchrotron tail which
coincides with the direction of the HI tidal debris field. The resolved IRRC
shows a clear gradient from north-east to south-west suggesting CR electron
streaming is occurring in this direction. The resolved spectral index map
between RACS and WC for this galaxy has a slight gradient in the same
direction however poor angular resolution and flux scale errors limit the
conclusions drawn from the resolved spectral index map.

Overall we find that the radio continuum emission observations obtained during
the WALLABY pre-pilot observations of the Eridanus group allow the resolved
study of the SF properties of SFGs and AGNs. These SF properties can be
compared to their HI properties and other ancillary multiwavelength
observations to provide a clearer picture of the physical processes which
modify the IRRC within galaxies. Our understanding of these processes will
only improve with the completion of the full WALLABY survey and more sensitive
radio surveys such as EMU. Thus by understanding the physical processes which
cause deviations from the expected radio emission, spectral index and IRRC we
will be better able to characterise galaxies in larger samples of sources.

## Acknowledgements

The Australian SKA Pathfinder is part of the Australia Telescope National
Facility which is managed by CSIRO. Operation of ASKAP is funded by the
Australian Government with support from the National Collaborative Research
Infrastructure Strategy. ASKAP uses the resources of the Pawsey Supercomputing
Centre. Establishment of ASKAP, the Murchison Radio-astronomy Observatory and
the Pawsey Supercomputing Centre are initiatives of the Australian Government,
with support from the Government of Western Australia and the Science and
Industry Endowment Fund. We acknowledge the Wajarri Yamatji people as the
traditional owners of the Observatory site. This publication makes use of data
products from the Wide-field Infrared Survey Explorer, which is a joint
project of the University of California, Los Angeles, and the Jet Propulsion
Laboratory/California Institute of Technology, funded by the National
Aeronautics and Space Administration. This research has made use of the
NASA/IPAC Extragalactic Database (NED), which is operated by the Jet
Propulsion Laboratory, California Institute of Technology, under contract with
the National Aeronautics and Space Administration. Parts of this research were
supported by the Australian Research Council Centre of Excellence for All Sky
Astrophysics in 3 Dimensions (ASTRO 3D), through project number CE170100013.
We thank the referee for their in-depth review and comments on this
manuscript.

## Data Availability

The data underlying this article are available in the article and in the
online supplementary material. The processed ASKAP data can be retrieved via
CSIRO ASKAP Science Data Archive (CASDA) with a given scheduling block
identification number. The mosaicked continuum map of the Eridanus field
observations is available at the DOI: https://doi.org/10.25919/8ga8-0n09.

## References

  * An et al. (2021) An, F., Vaccari, M., Smail, I., et al. 2021, MNRAS, 507, 2643
  * Appleton et al. (2006) Appleton, P. N., Xu, K. C., Reach, W., et al. 2006, ApJ, 639, L51
  * Baldry & Glazebrook (2003) Baldry, I. K., & Glazebrook, K. 2003, ApJ, 593, 258
  * Bell (2003) Bell, E. F. 2003, ApJ, 586, 794
  * Bicay & Helou (1990) Bicay, M. D., & Helou, G. 1990, ApJ, 362, 59
  * Bondi et al. (2008) Bondi, M., Ciliegi, P., Schinnerer, E., et al. 2008, ApJ, 681, 1129
  * Bourne et al. (2012) Bourne, N., Maddox, S. J., Dunne, L., et al. 2012, MNRAS, 421, 3027
  * Brough et al. (2006) Brough, S., Forbes, D. A., Kilborn, V. A., Couch, W., & Colless, M. 2006, MNRAS, 369, 1351
  * Chabrier (2003) Chabrier, G. 2003, PASP, 115, 763
  * Cluver et al. (2017) Cluver, M. E., Jarrett, T. H., Dale, D. A., et al. 2017, ApJ, 850, 68
  * Cluver et al. (2014) Cluver, M. E., Jarrett, T. H., Hopkins, A. M., et al. 2014, ApJ, 782, 90
  * Condon (1992) Condon, J. J. 1992, ARA&A, 30, 575
  * Condon et al. (2002) Condon, J. J., Cotton, W. D., & Broderick, J. J. 2002, AJ, 124, 675
  * Condon et al. (1998) Condon, J. J., Cotton, W. D., Greisen, E. W., et al. 1998, AJ, 115, 1693
  * Curran et al. (2008) Curran, S. J., Koribalski, B. S., & Bains, I. 2008, MNRAS, 389, 63
  * Davies et al. (2017) Davies, L. J. M., Huynh, M. T., Hopkins, A. M., et al. 2017, MNRAS, 466, 2312
  * Delhaize et al. (2017) Delhaize, J., Smolčić, V., Delvecchio, I., et al. 2017, A&A, 602, A4
  * Delvecchio et al. (2021) Delvecchio, I., Daddi, E., Sargent, M. T., et al. 2021, A&A, 647, A123
  * Dey et al. (2019) Dey, A., Schlegel, D. J., Lang, D., et al. 2019, AJ, 157, 168
  * Donevski & Prodanović (2015) Donevski, D., & Prodanović, T. 2015, MNRAS, 453, 638
  * Draine & Li (2007) Draine, B. T., & Li, A. 2007, ApJ, 657, 810
  * Ellison et al. (2018) Ellison, S. L., Catinella, B., & Cortese, L. 2018, MNRAS, 478, 3447
  * Ellison et al. (2011) Ellison, S. L., Nair, P., Patton, D. R., et al. 2011, MNRAS, 416, 2182
  * For et al. (2021) For, B. Q., Wang, J., Westmeier, T., et al. 2021, MNRAS, 507, 2300
  * Garcia-Barreto et al. (2019) Garcia-Barreto, J. A., Divakara Mayya, Y., & Guichard, J. 2019, PASP, 131, 094101
  * Girdhar et al. (2022) Girdhar, A., Harrison, C. M., Mainieri, V., et al. 2022, MNRAS, arXiv:2201.02208
  * Hale et al. (2019) Hale, C. L., Robotham, A. S. G., Davies, L. J. M., et al. 2019, MNRAS, 487, 3971
  * Hale et al. (2021) Hale, C. L., McConnell, D., Thomson, A. J. M., et al. 2021, arXiv e-prints, arXiv:2109.00956
  * Hameed & Devereux (1999) Hameed, S., & Devereux, N. 1999, AJ, 118, 730
  * Harnett et al. (1990) Harnett, J. I., Whiteoak, J. B., Reynolds, J. E., Gardnere, F. F., & Tzioumis, A. 1990, MNRAS, 244, 130
  * Heesen et al. (2022a) Heesen, V., Staffehl, M., Basu, A., et al. 2022a, A&A, 664, A83
  * Heesen et al. (2022b) Heesen, V., Klocke, T. L., Brüggen, M., et al. 2022b, arXiv e-prints, arXiv:2208.11068
  * Helou & Bicay (1993) Helou, G., & Bicay, M. D. 1993, ApJ, 415, 93
  * Helou et al. (1985) Helou, G., Soifer, B. T., & Rowan-Robinson, M. 1985, ApJ, 298, L7
  * Helou et al. (2004) Helou, G., Roussel, H., Appleton, P., et al. 2004, ApJS, 154, 253
  * Hollenbach & McKee (1980) Hollenbach, D., & McKee, C. F. 1980, ApJ, 241, L47
  * Hollenbach & McKee (1989) —. 1989, ApJ, 342, 306
  * Hotan et al. (2021) Hotan, A. W., Bunton, J. D., Chippendale, A. P., et al. 2021, PASA, 38, e009
  * Hughes et al. (2006) Hughes, A., Wong, T., Ekers, R., et al. 2006, MNRAS, 370, 363
  * Hughes et al. (2007) Hughes, J. P., Chugai, N., Chevalier, R., Lundqvist, P., & Schlegel, E. 2007, ApJ, 670, 1260
  * Jarrett et al. (2011) Jarrett, T. H., Cohen, M., Masci, F., et al. 2011, ApJ, 735, 112
  * Jarrett et al. (2013) Jarrett, T. H., Masci, F., Tsai, C. W., et al. 2013, AJ, 145, 6
  * Jarvis et al. (2010) Jarvis, M. J., Smith, D. J. B., Bonfield, D. G., et al. 2010, MNRAS, 409, 92
  * Kennicutt (1998) Kennicutt, R. C., J. 1998, ARA&A, 36, 189
  * Kennicutt et al. (1987) Kennicutt, R. C., J., Keel, W. C., van der Hulst, J. M., Hummel, E., & Roettiger, K. A. 1987, AJ, 93, 1011
  * Koopmann & Kenney (2004) Koopmann, R. A., & Kenney, J. D. P. 2004, ApJ, 613, 866
  * Koribalski et al. (2020) Koribalski, B. S., Staveley-Smith, L., Westmeier, T., et al. 2020, Ap&SS, 365, 118
  * Lacki & Thompson (2010) Lacki, B. C., & Thompson, T. A. 2010, ApJ, 717, 196
  * Leroy et al. (2021) Leroy, A. K., Schinnerer, E., Hughes, A., et al. 2021, ApJS, 257, 43
  * Li & Draine (2002) Li, A., & Draine, B. T. 2002, ApJ, 572, 232
  * Magnelli et al. (2015) Magnelli, B., Ivison, R. J., Lutz, D., et al. 2015, A&A, 573, A45
  * Mandal et al. (2021) Mandal, A., Mukherjee, D., Federrath, C., et al. 2021, MNRAS, 508, 4738
  * McConnell et al. (2016) McConnell, D., Allison, J. R., Bannister, K., et al. 2016, PASA, 33, e042
  * McConnell et al. (2020) McConnell, D., Hale, C. L., Lenc, E., et al. 2020, PASA, 37, e048
  * Micelotta et al. (2010) Micelotta, E. R., Jones, A. P., & Tielens, A. G. G. M. 2010, A&A, 510, A36
  * Miller & Owen (2001) Miller, N. A., & Owen, F. N. 2001, AJ, 121, 1903
  * Molnár et al. (2021) Molnár, D. C., Sargent, M. T., Leslie, S., et al. 2021, MNRAS, 504, 118
  * Moorman et al. (2016) Moorman, C. M., Moreno, J., White, A., et al. 2016, ApJ, 831, 118
  * Morić et al. (2010) Morić, I., Smolčić, V., Kimball, A., et al. 2010, ApJ, 724, 779
  * Mukherjee et al. (2021) Mukherjee, D., Bicknell, G. V., & Wagner, A. Y. 2021, Astronomische Nachrichten, 342, 1140
  * Mukherjee et al. (2018) Mukherjee, D., Bicknell, G. V., Wagner, A. Y., Sutherland, R. S., & Silk, J. 2018, MNRAS, 479, 5544
  * Murata et al. (2017) Murata, K. L., Yamada, R., Oyabu, S., et al. 2017, MNRAS, 472, 39
  * Murphy (2013) Murphy, E. J. 2013, ApJ, 777, 58
  * Murphy et al. (2008) Murphy, E. J., Helou, G., Kenney, J. D. P., Armus, L., & Braun, R. 2008, ApJ, 678, 828
  * Murphy et al. (2009) Murphy, E. J., Kenney, J. D. P., Helou, G., Chung, A., & Howell, J. H. 2009, ApJ, 694, 1435
  * Murugeshan et al. (2020) Murugeshan, C., Kilborn, V., Jarrett, T., et al. 2020, MNRAS, 496, 2516
  * Murugeshan et al. (2021) Murugeshan, C., Kilborn, V. A., For, B. Q., et al. 2021, MNRAS, 507, 2949
  * Norris et al. (2011) Norris, R. P., Hopkins, A. M., Afonso, J., et al. 2011, PASA, 28, 215
  * Norris et al. (2021) Norris, R. P., Marvil, J., Collier, J. D., et al. 2021, PASA, 38, e046
  * Nyland et al. (2017) Nyland, K., Young, L. M., Wrobel, J. M., et al. 2017, MNRAS, 464, 1029
  * Obreschkow et al. (2016) Obreschkow, D., Glazebrook, K., Kilborn, V., & Lutz, K. 2016, ApJ, 824, L26
  * Omar & Dwarakanath (2005) Omar, A., & Dwarakanath, K. S. 2005, Journal of Astrophysics and Astronomy, 26, 89
  * Popescu et al. (2011) Popescu, C. C., Tuffs, R. J., Dopita, M. A., et al. 2011, A&A, 527, A109
  * Reddy & Yun (2004) Reddy, N. A., & Yun, M. S. 2004, ApJ, 600, 695
  * Robotham et al. (2018) Robotham, A. S. G., Davies, L. J. M., Driver, S. P., et al. 2018, MNRAS, 476, 3137
  * Roussel et al. (2003) Roussel, H., Helou, G., Beck, R., et al. 2003, ApJ, 593, 733
  * Sandstrom et al. (2010) Sandstrom, K. M., Bolatto, A. D., Draine, B. T., Bot, C., & Stanimirović, S. 2010, ApJ, 715, 701
  * Sargent et al. (2010) Sargent, M. T., Schinnerer, E., Murphy, E., et al. 2010, ApJ, 714, L190
  * Shimwell et al. (2019) Shimwell, T. W., Tasse, C., Hardcastle, M. J., et al. 2019, A&A, 622, A1
  * Silva et al. (1998) Silva, L., Granato, G. L., Bressan, A., & Danese, L. 1998, ApJ, 509, 103
  * Smith et al. (2007) Smith, J. D. T., Draine, B. T., Dale, D. A., et al. 2007, ApJ, 656, 770
  * Smolčić et al. (2017) Smolčić, V., Delvecchio, I., Zamorani, G., et al. 2017, A&A, 602, A2
  * Sorgho et al. (2019) Sorgho, A., Carignan, C., Pisano, D. J., et al. 2019, MNRAS, 482, 1248
  * Tabatabaei et al. (2013) Tabatabaei, F. S., Schinnerer, E., Murphy, E. J., et al. 2013, A&A, 552, A19
  * Tabatabaei et al. (2017) Tabatabaei, F. S., Schinnerer, E., Krause, M., et al. 2017, ApJ, 836, 185
  * Voelk (1989) Voelk, H. J. 1989, A&A, 218, 67
  * Vollmer et al. (2020) Vollmer, B., Soida, M., Beck, R., & Powalka, M. 2020, A&A, 633, A144
  * Vollmer et al. (2022) Vollmer, B., Soida, M., & Dallant, J. 2022, arXiv e-prints, arXiv:2207.06173
  * Vollmer et al. (2012) Vollmer, B., Soida, M., Braine, J., et al. 2012, A&A, 537, A143
  * Wang et al. (2017) Wang, J., Koribalski, B. S., Jarrett, T. H., et al. 2017, MNRAS, 472, 3029
  * Wang et al. (2022) Wang, S., Wang, J., For, B.-Q., et al. 2022, ApJ, 927, 66
  * Whiting (2020) Whiting, M. T. 2020, in Astronomical Society of the Pacific Conference Series, Vol. 522, Astronomical Data Analysis Software and Systems XXVII, ed. P. Ballester, J. Ibsen, M. Solar, & K. Shortridge, 469
  * Williams et al. (2021) Williams, T. G., Kreckel, K., Belfiore, F., et al. 2021, arXiv e-prints, arXiv:2110.10697
  * Willmer et al. (1989) Willmer, C. N. A., Focardi, P., da Costa, L. N., & Pellegrini, P. S. 1989, AJ, 98, 1531
  * Wong et al. (2016) Wong, O. I., Koss, M. J., Schawinski, K., et al. 2016, MNRAS, 460, 1588
  * Wong et al. (2021) Wong, O. I., Stevens, A. R. H., For, B. Q., et al. 2021, MNRAS, 507, 2905
  * Wright et al. (2010) Wright, E. L., Eisenhardt, P. R. M., Mainzer, A. K., et al. 2010, AJ, 140, 1868
  * Yun et al. (2001) Yun, M. S., Reddy, N. A., & Condon, J. J. 2001, ApJ, 554, 803

##  Appendix A ASKAP Observation Details

Table 9: ASKAP Observation Details.

UT date | Footprint | RA | Dec. | Calibrator SBID | Science SBID | Integration time | Bandwidth | Central frequency
---|---|---|---|---|---|---|---|---
(yyyy-mm-dd) |  | (${}^{h}:^{m}:^{s}$) | (${}^{\circ}:^{\prime}:^{\prime\prime}$) |  |  | (h) | (MHz) | (MHz)
2019-03-13 | A | 3:39:30 | -22:30:00 | 8169 | 8168 | 5.8 | 288 | 1295.5
2019-03-13 | B | 3:36:44.52 | -22:37:54.69 | 8169 | 8170 | 5.0 | 288 | 1295.5

  * Note: Column (1): universal time date. Column (2): footprint designation. Column (3): J2000 right ascension. Column (4): J2000 declination. Column (5-6): calibrator and science SBIDs. Column (7): total integration time. Column (8-9): bandwidth and central frequency.

##  Appendix B HI Properties of Eridanus Supergroup Members

Table 10: HI properties of Eridanus supergroup members

Common ID | Log($M_{*}^{M21}$) | Log($M_{HI}^{For21}$) | $DEF_{HI}^{For21}$ | Log($M_{HI}^{M21}$) | $\Delta\,f_{qHI}^{M21}$ | $S_{SUM}$
---|---|---|---|---|---|---
| ($M_{\odot}$) | ($M_{\odot}$) |  | ($M_{\odot}$) |  |
IC 1952 | $9.65\pm 0.07$ | $8.72\pm 0.20$ | b | $8.80\pm 0.21$ | 0.35 | $-2.35\pm 0.11$
ESO 548-G036 | $9.43\pm 0.23^{a}$ | $8.01\pm 0.23$ | $0.95$ | c | c | $-1.87\pm 0.38$
IC 1953 | $9.93\pm 0.06$ | $8.97\pm 0.20$ | $0.46$ | $9.07\pm 0.02$ | $0.21$ | $-1.99\pm 0.23$
NGC 1359 | $9.42\pm 0.06$ | $9.41\pm 0.20$ | b | $9.56\pm 0.20$ | $-0.07$ | $-2.62\pm 0.15$
NGC 1367 | $10.30\pm 0.06$ | $9.76\pm 0.20$ | b | $9.64\pm 0.20$ | $0.10$ | d
NGC 1385 | $10.08\pm 0.06$ | $9.34\pm 0.20$ | $0.34$ | $9.24\pm 0.20$ | $-0.12$ | $-2.47\pm 0.14$
NGC 1415 | $10.20\pm 0.08$ | $8.93\pm 0.20$ | $0.63$ | $8.87\pm 0.21$ | $0.30$ | $-1.78\pm 0.12$
NGC 1422 | $9.22\pm 0.09$ | $8.33\pm 0.21$ | $0.66$ | $8.32\pm 0.23$ | $0.38$ | $-1.61\pm 0.08$

  * Note: Column (1): Galaxy common ID. Column (2): Stellar mass from Murugeshan et al. (2021). Column (2): Stellar mass from Murugeshan et al. (2021). Column (3): HI mass from For et al. (2021). Column (4): HI deficiency parameter as compared to stellar mass from For et al. (2021), galaxies above 0.3 are considered HI deficient. Column (5): HI mass from Murugeshan et al. (2021). (6): HI deficiency parameter as compared to disk stability from Murugeshan et al. (2021), galaxies above 0.2 are considered HI deficient. Column (7): Summed tidal parameter from Wang et al. (2022) values less than -1.69 are considered to be weakly perturbed by tidal interactions.

  * a

Stellar mass from For et al. (2021) as this galaxy did not meet selection
criteria in Murugeshan et al. (2021).

  * b

Stellar mass is not determined in For et al. (2021) thus a HI deficiency is
not measured.

  * c

This galaxy did not meet selection criteria in Murugeshan et al. (2021).

  * d

This galaxy did not meet selection criteria in Wang et al. (2022).

##  Appendix C WC Catalogue Profound Input Parameters and Extended Catalogue
Column Descriptions

Table 11: Modified ProFound input parameters.

box | skycut | pixcut | tolerance | reltol | ext
---|---|---|---|---|---
100 | 3 | 3 | 10 | 1 | 4

  * Note: Descriptions for each input parameter can be found in Robotham et al. (2018) or at https://rdrr.io/cran/ProFound/man/profoundProFound.html.

We use a subset of the final column information generated by the ProFound to
generate the final simplified and extended WC catalogues. These columns
provide information on: IDs; astrometry; flux densities; shape information and
other important source information. The first five lines of the simplified WC
source catalogue are presented in Table. 4 sorted by R.A which includes a
subset of the columns described below.

  * •

Designation \- The designation of the source given in IAU convention
JHHMMSS$\pm$DDMMSS with the prefix WC.

  * •

segID \- The final segmentation ID number of the source.

  * •

segID2 \- The initial segmentation ID number of the source before performing
$\sigma$ cuts.

  * •

uniqueID \- Unique ID of the source which is fairly static and based on xmax
and ymax.

  * •

xcen and ycen \- Flux weighted position of the source in image coordinates.

  * •

xmax and ymax \- Flux maximum position of the source in image coordinates.

  * •

RAcen and Deccen \- Flux weighted position of the source in sky coordinates.

  * •

RAmax and Decmax \- Flux maximum position of the source in sky coordinates.

  * •

sep \- Radial offset between central and maximum flux positions in units of
arcsec.

  * •

S_peak \- The peak flux density in the brightest pixel of the source in units
of mJy/beam calculated by multiplying cenfrac, S_int and the beamsize (13.50).

  * •

S_int \- The integrated flux density of the source in units of mJy.

  * •

cenfrac \- The fraction of flux in the brightest pixel.

  * •

N50, N90, N100 \- The number of brightest pixels containing 50, 90 and 100% of
the flux respectively.

  * •

R50, R90, R100 \- The approximate elliptical semi-major axis containing 50, 90
and 100% of the flux respectively in units of arcsec.

  * •

con \- The concentration of flux given by R50/R90.

  * •

semimaj and semimin \- The weighted standard deviations along the major and
minor axes i.e. the semi-major/minor first moment in units of pixels.

  * •

axrat \- The axial ratio as given by semimin/semimax.

  * •

ang \- The orientation of the semi-major axis in degrees with the convention
that 0 is vertical and rotates anticlockwise.

  * •

SNR \- The SNR of the source calculated by
S_peak/sky_RMS_mean used to perform the $5\sigma$ cuts.

  * •

signif \- The approximate significance of the detection using the Chi-Square
distribution.

  * •

S_int_toterr \- The total error on the flux density of the source including
flux calibration errors.

  * •

S_int_err \- The total measurement error on the flux density of the source.

  * •

S_int_err_sky \- The sky subtraction component of the total error on the flux
density of the source.

  * •

S_int_err_skyRMS \- The sky RMS component of the total error on the flux
density of the source.

  * •

S_int_err_cal \- The flux calibration component of the total error on the flux
density of the source.

  * •

sky_mean \- The mean flux of the sky over all segment pixels.

  * •

sky_sum \- The total flux of the sky over all segment pixels.

  * •

sky_RMS_mean \- The mean value of the sky RMS over all segment pixels.

  * •

iter \- The iteration number when the source was flagged as having convergent
flux.

  * •

origfrac \- The ratio between the final converged flux and the initial iso-
contour estimate.

  * •

Norig \- The number of pixels in the original non-dilated segment and is
$\geq$pixcut by construction.

  * •

Separation \- The radial distance between a source and the nearest bright
(S_peak$\geq$50 mJy) source in arcsec if it is within 180 arcsec of a bright
source.