# MIGHTEE-HI: Discovery of an H i-rich galaxy group at $z=0.044$ with MeerKAT

Shilpa Ranchod1,2, Roger P. Deane,1,2, Anastasia A. Ponomareva3, Tariq
Blecher4, Bradley S. Frank5,6,7, Matt J. Jarvis3,8, Natasha Maddox9, Wanga
Mulaudzi5, Marcin Glowacki8,10, Kelley M. Hess11,12, Madalina Tudorache3,
Lourdes Verdes-Montenegro13, Nathan J. Adams3, Rebecca A. A. Bowler3, Jordan.
D. Collier7,14, Russ Taylor7,10
1Department of Physics, University of Pretoria, Private Bag X20, Pretoria
0028, South Africa
2Wits Centre for Astrophysics, School of Physics, University of the
Witwatersrand, 1 Jan Smuts Avenue, 2000, South Africa
3Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road,
Oxford OX1 3RH, UK
4Department of Physics and Electronics, Rhodes University, PO Box 94,
Grahamstown 6140, Eastern Cape, South Africa
5Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch
7701, South Africa
6 South African Radio Astronomy Observatory, 2 Fir Street, Observatory, 7925,
South Africa
7 The Inter-University Institute for Data Intensive Astronomy (IDIA),
Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch,
7701, South Africa
8 Department of Physics and Astronomy, University of the Western Cape, Robert
Sobukwe Road, Bellville 7535, South Africa
9 Faculty of Physics, Ludwig-Maximilians-Universität, Scheinerstr. 1, 81679
Munich, Germany
10 The Inter-University Institute for Data Intensive Astronomy (IDIA),
Department of Physics and Astronomy, University of the Western Cape, Bellville
7535, South Africa
11 ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA,
Dwingeloo, The Netherlands
12 Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700
AV Groningen, The Netherlands
13 Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la
Astronomía, 18008 Granada, Spain
14 School of Science, Western Sydney University, Locked Bag 1797, Penrith, NSW
2751, Australia
E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present the serendipitous discovery of a galaxy group in the XMM-LSS field
with MIGHTEE Early Science observations. Twenty galaxies are detected in H i
in this $z\sim 0.044$ group, with a $3\sigma$ column density sensitivity of
$N_{\mathrm{HI}}=1.6\times 10^{20}\,\mathrm{cm}^{-2}$. This group has not been
previously identified, despite residing in a well-studied extragalactic legacy
field. We present spatially-resolved H i total intensity and velocity maps for
each of the objects, which reveal environmental influence through disturbed
morphologies. The group has a dynamical mass of
$\log_{10}(M_{\mathrm{dyn}}/\mathrm{M}_{\odot})=12.32$, and is unusually gas-
rich, with an H i-to-stellar mass ratio of
$\log_{10}(f_{\mathrm{HI}}^{\mathrm{*}})=-0.2$, which is 0.7 dex greater than
expected. The group’s high H i content, spatial, velocity, and identified
galaxy type distributions strongly suggest that it is in the early stages of
its assembly. The discovery of this galaxy group is an example of the
importance of mapping spatially-resolved H i in a wide range of environments,
including galaxy groups. This scientific goal has been dramatically enhanced
by the high sensitivity, large field-of-view, and wide instantaneous bandwidth
of the MeerKAT telescope.

###### keywords:

galaxies: evolution – galaxies: groups: individual – instrumentation:
interferometer – radio lines: galaxies

††pubyear: 2021††pagerange: MIGHTEE-HI: Discovery of an H i-rich galaxy group
at $z=0.044$ with MeerKAT–MIGHTEE-HI: Discovery of an H i-rich galaxy group at
$z=0.044$ with MeerKAT

##  1 Introduction

The current understanding of structure formation in the Universe is the
$\Lambda$CDM hierarchical merging paradigm, where larger structures are formed
by the mergers of smaller dark matter haloes (e.g. Springel et al., 2017).
Galaxy groups, with typical virial masses of order $M_{\mathrm{vir}}\sim
10^{12-14}\,$M${}_{\odot}\,$(Feldmann et al., 2011), occupy an interesting
intermediary region within the dark matter halo mass spectrum to study the
physical processes of galaxy evolution. In the local Universe, up to 50% of
galaxies reside in groups, most of which have only a handful of members
($<10$; Huchra & Geller, 1982; Berlind et al., 2006; Crook et al., 2007; Lim
et al., 2017). These tend to be spiral-galaxy dominated, and unlike clusters,
do not host easily detectable X-ray emitting cores of hot intergalactic
material, at least with current X-ray telescope sensitivity limits. Massive
groups ($\sim 20$ members) are comparatively rare (e.g. Hess & Wilcots, 2013),
but make up an important part of the continuum between low-mass groups and
massive clusters.

The halo mass, galaxy number density, and intergalactic medium density make
groups an important laboratory of environmental transformation effects, and
their context within a hierarchical galaxy formation and cosmological
framework. Given their lower dark matter halo masses, the velocity dispersion
for groups ranges between $\sigma_{\rm v}\sim$30–450 km s-1, significantly
lower than that of clusters $\sigma_{\rm v}\sim$ 1000 km s-1. This means that
morphological transformation effects such as ram-pressure stripping (Gunn &
Gott, 1972) and tidal stripping (Moore et al., 1998), caused by high velocity
encounters with other galaxies or interactions with the IGM, are more likely
to occur in dense clusters (e.g. Joshi et al., 2020; Ramatsoku et al., 2020),
but have also been observed in groups (e.g. Sulentic et al., 2001; Oosterloo
et al., 2018). Galaxy-galaxy interactions in the form of major and minor
mergers are expected to be more likely in the group environment (e.g. Serra et
al., 2019) compared to clusters. The morphology-density relation (Dressler et
al., 1997) captures how ‘early-type’ galaxies (i.e. elliptical and lenticular)
are preferably found in high-density environments, such as clusters, and
‘late-type’ galaxies (i.e. spiral and irregular) are found in low-density
environments, such as the field, which indicates that a galaxy’s environment
can be a primary determinant of its gas content and evolution.

Despite the clear correlation between galaxy morphology and environment,
several directly related questions remain, including how galaxies evolve in
the group environment, specifically their gas content, SFRs and morphologies,
and how this differs as a function of group mass. By studying the depletion
and removal of cold gas we can understand the transformation of galaxy
morphology in dense environments (e.g. Sancisi et al., 2008; Schawinski et
al., 2014). Within galaxies, the distribution of neutral hydrogen (H i) is
diffuse and extends well beyond the stellar component of the galaxy (e.g.
Broeils & Rhee, 1997; Leroy et al., 2008). It is therefore a sensitive
dynamical tracer and can be used to observe such environmental processes (e.g.
Oosterloo & van Gorkom, 2005; Serra et al., 2012; Saponara et al., 2017). By
observing H i in galaxies in the group environment, much work has been done to
understand the evolution of H i within this environment (e.g. Yun et al.,
1994; Hess & Wilcots, 2013; Jones et al., 2019). Spiral-dominated groups are H
i rich and have a higher galaxy-galaxy interaction rate than elliptical-
dominated groups. Elliptical-dominated groups have a higher velocity
dispersion (hence, fewer mergers), lower H i mass, and can have a diffuse,
X-ray emitting halo (Freeland et al., 2009). H i deficiency in compact groups
is linked to a proposed evolutionary sequence where galaxies become
increasingly H i deficient following multiple tidal interactions (Verdes-
Montenegro et al., 2001; Borthakur et al., 2015). The H i mass function (HIMF)
varies significantly with environment (e.g. Rosenberg & Schneider, 2002;
Springob et al., 2005; Jones et al., 2020), and is flatter for galaxy groups
at $M_{\mathrm{HI}}<M^{*}_{\mathrm{HI}}$ relative to the global HIMF. This
implies a relative deficit of low H i-mass galaxies and indicates that denser
environments, such as groups, are responsible for removing the H i from low
mass galaxies (e.g. Verheijen, 2004; Freeland et al., 2009; Busekool et al.,
2020). Using an Arecibo Legacy Fast ALFA (ALFALFA; Giovanelli et al., 2005)
sample, consisting of 1613 H i galaxies in 620 groups in the redshift range of
$0.020<z<0.042$, sensitivity limited to an H i mass of $M_{\mathrm{HI}}\geq
10^{8.9}\,$M${}_{\odot}\,$, Hess & Wilcots (2013) conclude that as the
optically-detected membership of a group increases, the H i-detected galaxies
reside increasingly towards the outskirts of the group. As halo mass
increases, the fraction of H i galaxies decreases, and galaxies at the centre
of the group are stripped of their gas due to transformation effects. This
should be evident in the H i morphologies, but the H i detections from ALFALFA
are spatially unresolved. In comparison with less dense environments, galaxies
located in group-sized dark matter haloes contain $\sim 0.4$ dex lower H i.
Haloes with virial masses $M_{\rm vir}>10^{13}\,$M${}_{\odot}\,$have gas
fractions of $\log(M_{\mathrm{HI}}/M_{*})\approx-1.3$, while haloes with
masses $M_{\rm vir}<10^{13}\,$M${}_{\odot}\,$have gas fractions of
$\log(M_{\mathrm{HI}}/M_{*})\approx-0.9$. However, it should be noted that
this result is biased to high-mass galaxies with
$\log(M_{*}/\mathrm{M}_{\odot})>10$ (Catinella et al., 2013), and so higher
sensitivity surveys, to probe lower H i masses, will provide a more complete
view.

Even though groups are excellent laboratories for studying the environmental
effects on galaxy evolution, they are challenging to identify in 2D images.
Without reliable spectroscopic redshifts, groups can be difficult to identify
and study, because their dark matter haloes are poorly populated (Hess &
Wilcots, 2013). This, along with their loose composition make groups difficult
to identify via traditional clustering algorithms, especially for low-
redshift, low mass systems (Papastergis et al., 2013). Unlike clusters, their
IGM haloes are rarely sufficiently bright to be observed via X-ray emission
(Robson & Davé, 2020) or inverse-Compton scattering via the Sunyaev–Zel’dovich
effect (Birkinshaw, 1999), so identification through these standard techniques
for clusters is not possible at lower mass, except at a statistical level.
Identification and selection via H i emission at cosmological distances are
not readily available as long integration times or very high sensitivities are
needed to directly detect the intrinsically faint H i emission (e.g. Rhee et
al., 2017). The study of H i in a range of environments requires wide-area
surveys at $\sim$few arcsecond angular resolution to spatially resolve
galaxies and trace their H i morphological and dynamical characteristics.
This, along with deep H i surveys, which are key for the statistical study of
H i, will deeply enrich our view of the group assembly and transformation
history, particularly for H i-rich, low-mass galaxy groups. With the
sensitivity and frequency coverage of the South African Square Kilometre Array
(SKA) precursor, MeerKAT (Jonas, 2009), this will be possible with shorter
integration times and over a large redshift range. The MeerKAT International
GHz Tiered Extragalactic Exploration (MIGHTEE) Survey (Jarvis et al., 2016) is
an ongoing Large Survey Project (LSP) conducted over four deep extragalactic
legacy fields, namely COSMOS, XMM-LSS, ECDFS and ELAIS-S1, totalling an area
of $\sim$20 deg2. The spectral sensitivity and frequency coverage of this
survey allows the detection of H i in a statistically significant number of
galaxies at $z\sim 0.2$, as well as rare, massive galaxies out to $z\gtrsim
0.3$. The angular resolution of $\sim 8\,\arcsec$ enables us to study the
morphology of individual galaxies at low to intermediate redshifts, adding a
powerful diagnostic of environmental effects.

In this paper, we present the discovery of a galaxy group in the XMM-LSS field
from the MIGHTEE survey Early Science data. This serendipitous discovery is
directly linked to the MIGHTEE-H i key science goal of studying H i as a
function of environment and demonstrates MeerKAT’s suitability for these
investigations. This massive group contains 20 H i galaxies (19 distinct H i
detections) and has not been previously identified despite residing in the
well-studied XMM-LSS field. We analyse and discuss the properties of the
detected H i galaxies in the group and compare with their multi-wavelength
properties. This paper is organized as follows. In Section 2 we summarise the
specifications of MeerKAT, describe the MIGHTEE observations and summarise the
ancillary data. In Section 3, we present the total H i intensity (moment 0)
and velocity field (moment 1) maps of the detected H i sources, a well as
their integrated H i spectra. Using simple models, we also derive the
dynamical masses and neutral gas fractions of the H i sources themselves as
well as the group as a whole. Section 4 is a discussion of the morphology and
kinematics of the sources and a summary follows in Section 5.

Throughout this work we assume cosmological values of
$\Omega_{M}=0.315,\Omega_{\Lambda}=0.685$ and $H_{0}=67.4\,$ km
s-1$\mathrm{Mpc}^{-1}$ (Planck Collaboration et al., 2018), resulting in a
spatial scale of 866 pc arcsec-1 at $z=0.044$.

##  2 MeerKAT Observations and Data Reduction

###  2.1 MeerKAT

MeerKAT is a radio interferometer consisting of 64 dishes, located in the
Northern Cape, South Africa. Of these 64, a subset of 48 is located within the
central core with a diameter of $\sim$1 km, and the remaining 16 dishes spread
out up to maximum baselines of $\sim$8 km. Each dish has offset-Gregorian
optics, consisting of a 13.5 m diameter main reflector and a 3.8 m sub-
reflector. The receiver indexer is located just below the sub-reflector
ensuring a completely unobstructed aperture, which improves the imaging
dynamic range by lowering the sensitivity of the primary beam sidelobes to
strong sources and radio frequency interference (RFI) outside the main lobe.
The large number of MeerKAT antennas, along with the relatively small dish
size, results in a high sensitivity array while retaining a large field of
view. This makes MeerKAT an excellent facility for wide-field, yet deep
imaging surveys. For more detail on the MeerKAT receptors, receivers, primary
beam, and correlator see Camilo et al. (2018); Mauch et al. (2020).

The MIGHTEE observations in this work use the MeerKAT L-band receiver, which
has a usable frequency range from 900 to 1670 MHz. These Early Science
observations use MeerKAT’s 4k correlator mode, which has 4096 channels, and a
channel width of 209 kHz, corresponding to 44 km s-1at $z=0$.

###  2.2 The MIGHTEE Survey

MIGHTEE (Jarvis et al., 2016) is a deep MeerKAT survey of four legacy multi-
wavelength fields. It is one of the LSP’s being undertaken by MeerKAT. Given
that the MeerKAT correlator has simultaneous continuum and spectral line
modes, a significant element within the MIGHTEE survey is an H i emission
survey, referred to as MIGHTEE-HI. MeerKAT’s high sensitivity and wide field-
of-view enable deep interferometric H i surveys over a significant number of
extragalactic legacy fields. For more detail on the MIGHTEE-HI component of
the survey, see Maddox et al. (2021) and Frank et al. (in prep.). Within Early
Science, a galaxy group at $z\sim 0.044$ was discovered in the XMM-LSS field.
The H i-detected galaxies in the group are shown in Fig. 1, and the details of
the observation are summarised in Table 1.

Figure 1: MeerKAT H i contours (red) of the 19 detected sources overlaid on the HSC $R$-band image of the XMM-LSS field (greyscale). The H i contour levels correspond to the column densities of [1.25, 3] $\times 10^{20}\mathrm{cm}^{-2}$. A spatial scale bar is shown in the bottom right. Table 1: Summary of MeerKAT MIGHTEE data products used in this work (i.e. limited to the study of this galaxy group). More detail on the observations themselves, as well as the data processing is described in Frank et al. (in prep.). Channel width | 209 kHz
---|---
| 46 km s-1for H i at z = 0.044
Pixel size | 2 arcsec
H i cube weighting | Briggs robust = 0.5
Median H i channel rms noise | 85 $\mu$Jy beam-1
Restoring beam FWHM | 12 $\times$ 9 arcsec2
| PA = -30 deg
On-source integration time | 12 hours
$N_{\mathrm{HI}}$ sensitivity ($3\sigma$) | 1.6 $\times\,10^{20}\,\mathrm{cm}^{-2}$ (per channel)

###  2.3 Calibration and Imaging

MIGHTEE-HI data products were produced using the ProcessMeerKAT calibration
and imaging pipeline, described in detail in Frank et al. (in prep.). Very
briefly, this is a parallelised Casa-based111http://casa.nrao.edu pipeline run
within Singularity222https://singularity.lbl.gov containers on the ILIFU Cloud
facility333http://www.ilifu.ac.za. The pipeline does the continuum and
polarimetric calibration, following standard calibration routines and
strategies, including flagging, delay, bandpass, and complex gain calibration,
with automated self-calibration in development. The continuum cross-
calibration was done at a lower frequency resolution (averaging by a factor of
4), and was followed by self-calibration. The resultant cross and self-
calibration gains were applied to the full resolution data, followed by a
final bandpass calibration.

Wide-field, spectral-line imaging was performed using Casa’s TCLEAN task with
a Briggs (robust=0.5) weighting scheme. Continuum subtraction was done in the
visibility and the image domain. The continuum model was subtracted using
UVSUB and was followed by subtracting a fit to the residual visibilities using
UVCONTSUB. Once the cubes were produced, per-pixel median filtering was
implemented on the cubes to reduce the impact of direction-dependent
artefacts. The final, continuum-subtracted H i spectral line sub-cube has an
angular extent of 2.3$\times$2.3 deg2 and spans 1330.198 – 1379.936 MHz. The
average Point Spread Function FWHM for the cubes is 9$\times$12 arcsec,
corresponding to a spatial scale of 7.8$\times$10.4 kpc for the galaxy group
members at $z=0.044$.

###  2.4 Ancillary Data

One of the primary motivations for targeting the legacy fields is the wealth
of multi-wavelength measurements and derived properties that are available for
a large number of galaxies. Amongst the primary data products in the XMM-LSS
field are from the Hyper SuprimeCam Subaru Strategic Program (HSC-SSP, Aihara
et al., 2017) and Canada-France-Hawaii Telescope Legacy Survey (CFHTLS;
Cuillandre et al., 2012) in optical wavelengths, and the VISTA VIDEO (Jarvis
et al., 2013) survey in the near-infrared. A detailed description of these is
provided in Maddox et al. (2021).

The MIGHTEE-HI team extracted fluxes in the $ugrizYJHK_{s}$-bands from the HSC
and VIDEO images (image reduction is described in Bowler et al. 2020).
Together, this provides a wavelength coverage of $\sim 0.4\text{-}2\,\mu$m.
Since our sources are highly extended and relatively few in number, we
manually fit elliptical apertures to each source, from which flux is
extracted. The SED-fitting code LePhare (Arnouts et al., 1999; Ilbert et al.,
2006) was used to determine the stellar masses and star-formation rates for
each of the detected objects following the same process used in Adams et al.
(2021). To summarise, we begin by fixing the redshifts of each source to those
determined from the HI line $z_{\rm HI}$ (see Section 3.1 and Table 2). We
then fit synthetic templates of galaxy spectra derived from Bruzual & Charlot
(2003) using minimisation of $\chi^{2}$ with LePhare. These synthetic spectra
are generated with star formation histories following either a constant star
formation rate or exponentially decaying star formation rates with timescales
$\tau=[0.1,0.3,1,2,3,5,10,15,30]$ Gyr. These template spectra are further
modified by the effects of redshifting and various levels of dust extinction
(Calzetti et al., 2000). The result of this process provides the best fitting
galaxy SED along with the associated stellar mass and star formation rate for
that template (Table 2).

##  3 Results

###  3.1 HI Detections

Source finding was done visually and unguided by members of the MIGHTEE-HI
Working Group (WG), as described in Maddox et al. (2021). We find 19 H i
detections in the region we associate with the galaxy group, both spatially
and spectrally. The 19 detections included 20 galaxies, two of which are a
closely interacting pair. All of the detected H i sources are shown in Fig. 1,
where the $1.25\times 10^{20}\,\mathrm{cm}^{-2}$ column density contour is
overlaid on the $R$-band HSC image. The galaxies are labelled as listed in
Table 2. The moment 0 maps, moment 1 maps, and the integrated H i spectra of
these sources are shown in Fig. 2. The RGB images in these figures are
comprised of the HSC $G$, $R$, and $I$ bands. The sources reside in the
frequency range of 1330.198 – 1379.936 MHz, which corresponds to a recession
velocity range of 12,679 – 13,093 km s-1, and a redshift range of 0.043 –
0.045. All H i detections have a peak channel flux density signal-to-noise
ratio above $4\sigma$, and are optically visible, with aperture-defined
$R$-band magnitudes ranging from 20.0 > magr > 13.9. The moment 0 maps were
constructed according to the following procedure.

  1. 1.

Each cubelet centred on a detection was smoothed to a circular beam of 20" and
clipped at $3\sigma$ level.

  2. 2.

The resulting mask was applied to the original resolution cubelet to account
for the low column density extended H i emission.

  3. 3.

Every moment 0 map was examined by eye and isolated by masking out the noise
peaks if any were present within the field of view.

The spectra were extracted from the unsmoothed cube in the regions defined by
the mask. We note that the continuum subtraction might result in a slight
suppression of the integrated flux around the emission region of large
sources, this effect was not accounted for in our work.

Figure 2: From left to right, the spectra, moment 0 contours plotted over RGB
cutouts from the HSC survey, and moment 1 maps of source ID’s 0–6 of the 19
detected H i sources in the group. The Gaussian restoring beam in the top left
(black) has a mean FWHM of $12\times 9\,\mathrm{arcsec}^{2}$ and the spatial
scale bar in the bottom right indicates 20 kpc. The contours shown correspond
to H i column densities of [1, 3, 5, 10] $\times 10^{20}\mathrm{cm}^{-2}$.
Figure 3 (cont.): As per Fig. 2, but for source ID’s 7–13. Figure 4 (cont.):
As per Fig. 2, but for source ID’s 14–18.

The stellar masses $M_{*}$ and SFR’s of the detected sources were measured
from the SED fitting of the multi-wavelength photometry, as discussed in
Section 2.4. The stellar masses have a conservative uncertainty estimate of
$\pm$0.1 dex. The objects were visually cross-matched with spatial coordinates
by the MIGHTEE-HI WG.

Table 2: Catalogue of H i-detected sources.

ID | RA (J2000) | Dec (J2000) | $z_{\mathrm{HI}}$ | $D_{L}$ | log | $w_{50}$ | $d_{\mathrm{HI}}$ | i | log | log(SFR/ | $f_{\mathrm{HI}}^{*}$ | log | $f_{\mathrm{HI}}^{\mathrm{dyn}}$
---|---|---|---|---|---|---|---|---|---|---|---|---|---
| h:m:s |  |  | [Mpc] | ($M_{\mathrm{HI}}/\mathrm{M}_{\odot}$) | [km s-1] | [kpc] | [deg] | ($M_{*}/\mathrm{M}_{\odot}$) | M⊙ yr-1) |  | ($M_{\mathrm{dyn}}/\mathrm{M}_{\odot}$) |
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9) | (10) | (11) | (12) | (13) | (14)
4 | 02:24:55.35 | -04:17:35.25 | 0.0440 | 201.4 | 10.076 $\pm$ 0.004 | 462 | 91 | 39.41 | 10.76 | -0.16 | -0.68 | 11.81 | -1.73
7 | 02:24:38.69 | -04:34:18.06 | 0.0437 | 199.8 | 9.905 $\pm$ 0.002 | 225 | 51 | 67.70 | 9.47 | -0.13 | 0.43 | 10.99 | -1.08
5 | 02:24:29.62 | -04:40:36.72 | 0.0440 | 201.4 | 9.504 $\pm$ 0.005 | 181 | 19 | 78.76 | 8.55 | -0.84 | 0.95 | 10.39 | -0.89
10 | 02:24:17.02 | -04:56:42.19 | 0.0436 | 199.1 | 8.476 $\pm$ 0.036 | 41 | - | 42.65 | 7.71 | -0.58 | 0.77 | - | -
2 | 02:24:11.49 | -04:47:44.84 | 0.0445 | 203.6 | 8.600 $\pm$ 0.033 | 124 | 10 | 50.64 | 9.42 | -0.11 | -0.82 | 9.85 | -1.25
18 | 02:24:04.97 | -04:46:47.45 | 0.0442 | 202.1 | 8.421 $\pm$ 0.045 | 201 | 11 | 56.65 | 8.85 | -0.90 | -0.43 | 10.22 | -1.80
8 | 02:24:04.13 | -04:48:12.50 | 0.0436 | 199.1 | 10.664 $\pm$ 0.000 | 248 | 115 | 32.76 | 10.28 | 0.71 | 0.38 | 11.41 | -0.75
1 | 02:24:03.77 | -04:33:05.07 | 0.0442 | 202.1 | 10.221 $\pm$ 0.001 | 276 | 63 | 54.51 | 10.18 | 0.73 | 0.04 | 11.24 | -1.02
9a | 02:24:02.64 | -04:41:36.30 | 0.0434 | 198.3 | 10.089 $\pm$ 0.001 | 66 | 74 | 40.37 | 11.00 | 0.57 | -0.84 | 10.30 | -0.14
17 | 02:24:00.94 | -04:49:43.42 | 0.0440 | 201.4 | 8.951 $\pm$ 0.013 | 218 | - | 70.21 | 8.71 | 0.25 | 0.24 | - | -
9b | 02:23:59.20 | -04:41:48.15 | 0.0434 | 198.3 | 9.344 $\pm$ 0.004 | 380 | 33 | 42.81 | 10.20 | 0.22 | -0.86 | - | -
3 | 02:23:57.07 | -04:43:57.22 | 0.0439 | 200.6 | 9.494 $\pm$ 0.004 | 15 | - | 78.39 | 10.65 | 0.32 | -1.15 | - | -
6 | 02:23:50.63 | -04:50:45.02 | 0.0440 | 201.4 | 10.117 $\pm$ 0.001 | 98 | 52 | 71.54 | 8.57 | -0.53 | 1.55 | 10.40 | -0.28
15 | 02:23:40.35 | -05:00:45.18 | 0.0447 | 204.4 | 9.917 $\pm$ 0.001 | 157 | 57 | 41.93 | 9.47 | 0.03 | 0.45 | 10.77 | -0.85
0 | 02:23:32.21 | -04:49:27.54 | 0.0447 | 204.4 | 9.191 $\pm$ 0.007 | 101 | 25 | 43.70 | 8.70 | -1.27 | 0.49 | 10.11 | -0.92
11 | 02:23:25.54 | -04:17:52.68 | 0.0432 | 197.6 | 9.801 $\pm$ 0.006 | 243 | - | 80.77 | 10.21 | 0.07 | -0.40 | - | -
16 | 02:22:56.00 | -05:13:59.47 | 0.0447 | 204.4 | 9.452 $\pm$ 0.006 | 85 | 30 | 62.64 | 7.96 | 0.96 | 1.49 | 10.06 | -0.61
12 | 02:22:12.31 | -04:42:51.79 | 0.0442 | 202.1 | 8.716 $\pm$ 0.026 | 154 | 11 | 60.33 | 9.09 | -0.57 | -0.38 | 10.05 | -1.33
13 | 02:21:17.84 | -04:50:40.48 | 0.0436 | 199.1 | 8.726 $\pm$ 0.041 | 58 | 11 | 52.19 | 7.71 | -1.85 | 1.02 | 9.41 | -0.68
14 | 02:20:51.46 | -04:57:03.72 | 0.0440 | 201.4 | 10.279 $\pm$ 0.002 | 331 | 97 | 44.59 | 9.92 | -0.18 | 0.36 | 11.57 | -1.29
Note: Cols (2)–(3): RA and Dec (J2000) coordinates, based on the optical
counterparts of the H i detections. Cols (4)–(5): H i redshifts and luminosity
distances. Cols (6)–(9)
H i mass, corrected FWHM of integrated H i line profile, H i disk diameter,
and inclination, as described in Section 3.1. Cols (10)–(11): Stellar mass and
star-formation rates
determined from SED fitting. Col (12): H i-to-stellar mass ratio. Col (13):
Dynamical mass. Col (14): H i-to-dynamical mass ratio. Uncertainties on d,
$M_{\mathrm{HI}}$ and W50 are
discussed in Section 3.

The H i redshift $z_{\mathrm{HI}}$ was extracted from the central channel of
the emission profile of each source, and the H i mass was calculated according
to Meyer et al. (2017):

$\left(\frac{M_{\mathrm{HI}}}{M_{\odot}}\right)\simeq\frac{2.356\times
10^{5}}{(1+z)}\left(\frac{D_{L}}{\mathrm{Mpc}}\right)^{2}\left(\frac{S}{\mathrm{Jy}\,\mathrm{km\,s}^{-1}}\right)$
(1)

where $D_{L}$ is the cosmological luminosity distance to the source, and $S$
is the integrated H i flux density, determined from the moment-0 maps. The
luminosity distance was calculated for each source, based on its H i redshift,
and assuming the cosmological values stated in Section 1. At the distance of
these sources ($\sim 200$ Mpc), peculiar motions are negligible compared to
the Hubble flow, so the redshift can be converted directly to a distance. The
error on the integrated flux $S$ was measured by applying the 3D source mask
to the four emission-free regions around the detection and by calculating the
mean rms (Table 2). Inclination angles of each source were defined as
$\cos^{2}(i)=(b^{2}-\theta_{b}^{2})/(a^{2}-\theta_{a}^{2})$, where $a$ and $b$
are the major and minor axis of an ellipse, fitted to the outer contour of the
H i moment 0 map, $\theta_{a}$ and $\theta_{b}$ are the sizes of the
synthesised beam in the direction of the major and minor axis of the H i disk,
used to correct for the beam-smearing effect (Verheijen & Sancisi, 2001). For
simplicity we assume an infinitely thin H i disk. The H i diameter ($d_{\rm
HI}$) of each galaxy was measured by fitting an ellipse to the contour of the
inclination-corrected H i column density map corresponding to $\rm
1[M_{\odot}\,pc^{-2}]=1.249\times 10^{20}[cm^{-2}]$ (Ponomareva et al., 2016).
A half beam size of 5 kpc was assigned as an uncertainty of the dHI.

The full-width half maximum of the integrated H i line profile ($\rm W_{50}$)
of each galaxy was attained by fitting Busy Functions (BF) (Westmeier et al.,
2013) using PyMultiNest (Buchner et al., 2014), a Monte Carlo library that
uses Bayesian analysis for parameter estimation. Considering that the Early
Science data has coarse channel width, a bootstrap technique was implemented
to solve for and fix the BF parameters during different PyMultiNest runs. The
maximum likelihood method was then used to identify the best-fit model
(Mulaudzi et al. in prep.). The resulting $\rm W_{50}$ values were corrected
for projection effects ($\mathrm{W_{50}}/\sin(i)$), instrumental broadening
and turbulent motions following the standard prescriptions (Verheijen &
Sancisi, 2001). Specifically, the instrumental broadening correction for our
data is $\sim 26$ km s-1, while $5$ km s-1was adopted as a standard correction
for the turbulent motions when $\rm W_{50}$ is matched to $\rm V_{max}$
(Ponomareva et al., 2016). The resulting measurements are summarised in Table
2. The channel width was assigned as the typical error on the $\rm W_{50}$
measurement.

###  3.2 Properties of individual sources

Given the coarse frequency resolution of these Early Science data, combined
with the relatively few spatial resolution elements across each source, we
restricted ourselves to a simple dynamical mass modelling approach. Following
de Blok & Walter (2014), the dynamical mass is estimated using

$M_{\mathrm{dyn}}=\frac{R}{\mathrm{G}}\left(\frac{\rm W_{50}}{2\sin
i}\right)^{2},$ (2)

where $R=d_{\mathrm{HI}}/2$, and W50 and $i$ are determined as described in
Section 3.1. These dynamical masses are listed in Table 2 and have typical
uncertainties of ${}^{+0.14}_{-0.19}$dex, which are merely intended to be
indicative and are estimated by channel velocity width and the positional
uncertainty related to the FWHM of the restoring beam.

###  3.3 Properties of the entire galaxy group

The non-Gaussian velocity distribution of the HI-selected galaxies show a
group that is clearly not yet virialised (see Fig. 6). i.e. the group is still
in the process of assembly. However, the close proximity in space and redshift
suggest that the vast majority, if not all, of these galaxies are bona fide
group members. For brevity, we refer to all galaxies presented here as group
members, mindful of the caveat that a handful may possibly still be in the
process of joining the system.

For all galaxies, we calculate the H i-to-stellar mass fraction, which is
defined as $f_{\mathrm{HI}}^{*}\equiv\log(M_{\mathrm{HI}}/M_{*})$. We also
calculate the H i-to-dynamical mass fraction, which is defined as
$f_{\mathrm{HI}}^{\mathrm{dyn}}\equiv\log(M_{\mathrm{HI}}/M_{\mathrm{dyn}})$.
These results are displayed in Table 2. The total H i mass in the group is
$\log(M_{\mathrm{HI}}/\mathrm{M}_{\odot})=11.20\pm 0.01$, assuming all H i
mass is detected. By summing the dynamical masses of the individual galaxies,
the total dynamical mass of galaxies in the group is
$\log(M_{\mathrm{dyn,gal}}^{\mathrm{HI}}/\mathrm{M}_{\odot})=12.32\pm 0.23$,
typical for an intermediate-mass galaxy group (Tully, 2015), assuming that the
group is solely comprised of the galaxies detected in H i. From this, the
total H i gas mass fraction of galaxies in the group is
$f_{\mathrm{HI,tot}}^{\mathrm{dyn}}=-1.26$. The ratio between the sum of the
stellar mass and H i mass to dynamical mass is
$f_{\mathrm{HI+*,tot}}^{\mathrm{dyn}}=-0.90$ (12.6%), which is broadly
consistent with the baryon fraction of intermediate halo mass groups, that is
$\sim 13.6\%$ for $M_{\mathrm{halo,vir}}=2.1\times
10^{13}\,\mathrm{M}_{\odot}$ (Giodini et al., 2009).

In the above discussion, we assume that the H i galaxies comprise all of the
galaxies in the group. However, H i-deficient galaxies obviously may be
missing with this selection. We therefore use the deep optical and near-
infrared imaging, along with photometric redshifts to determine whether there
may be a large number of H i-deficient group members. Fig. 5 shows the
potential galaxy group members based on their photometric redshift probability
density functions (Bowler et al., 2020; Adams et al., 2020, 2021), where we
assume that a galaxy is a potential group member if its photometric redshift
lies in the redshift range $0.035<z<0.055$ and $g-$band magnitude $<21.5$.
Within the primary beam of the MIGHTEE observations discussed here, we find 13
galaxies which meet this criteria. The photo-z uncertainty ($\Delta z=0.02$)
is too large to associate, with high probability, any of these optically-
selected sources with the HI galaxy group. Although we cannot rule out any of
these galaxies being members of the group, the density of such sources is
similar to that across the wider XMM-LSS field, as such we do not find strong
evidence for a large number of H i-deficient galaxies in the group. The
availability of spectroscopic redshifts for HI-deficient sources in the
vicinity of the group will provide a better assessment of group membership.

Figure 5: The spatial and redshift distribution of other known nearby sources
in the XMM-LSS field. There are 92 sources in total, of which 13 fall within
the beam FWHM of this pointing. The black circle shows the FWHM of the primary
beam. The stars represent the H i-detected group members. The colourbar shows
the photometric redshift of the non-detected sources (H i redshift for
detected sources), which have large uncertainties $\Delta z=0.02$. Their group
membership is therefore uncertain.

##  4 Discussion

The majority of the identified H i sources in Fig. 2 can be classified as
spiral or disk-like galaxies, based on the optical images, with the exception
of sources ID 0 and ID 6, which appear to be irregular galaxies. Unlike the
other sources, sources ID 0 and ID 6 do not have a sign of spiral structure in
the optical images. Sources ID’s 2, 10, 13, 17 and 18 have amongst the lowest
H i masses of the detected sources
($8.4<\log(M_{\mathrm{HI}}/\mathrm{M}_{\odot})<8.8$). The optical images do
not show structure within these galaxies. Object ID’s 1, 2, 3, 4, 5, 7, 9, 11,
and 15 exhibit the characteristic double-horned spectral profile of an
inclined spiral galaxy, with $\rm W_{50}$ parameters that range between
119–313 km s-1.

Figure 6: Histograms showing the distribution H i masses and velocities
relative to the group centre of the 19 detected sources. The colour of the
points corresponds to the stellar mass.

In most cases, the H i extent of the galaxy is significantly larger than the
optical. Most notably, sources ID 6 and ID 8 with H i radii approximately two
and four times that of their $R$-band radii respectively. Source ID 1 is faint
and diffuse in the optical, with clumpy star-formation in the arms, but with
high column density H i ($N_{\mathrm{HI}}\leq 2.5\times
10^{21}\,\mathrm{cm}^{-2}$). Clumpy star-formation regions can also be seen in
sources ID 7 and ID 8. All detected galaxies have a typical H i mass of
$8<\log(M_{\mathrm{HI}}/\mathrm{M}_{\odot})<11$ (as illustrated in Fig. 6).
The higher-mass outlier, source ID 6, has an H i mass of
$\log(M_{\mathrm{HI}}/\mathrm{M}_{\odot})=10.66$, which is on the high-mass
end of the H i-mass function
$\log(M_{\mathrm{HI}}^{*}/\mathrm{M}_{\odot})>9.94\pm 0.02\pm 0.05h_{70}^{-2}$
(Jones et al., 2018). This source also has the highest H i-to-stellar mass
ratio of the group of $f_{\mathrm{HI}}^{*}=1.55$, which is interesting, as the
most H i-massive objects tend to be stellar-dominated (Maddox et al., 2015).

(a)  (b)  (c)

Figure 7: Left: The group members (green), compared to the
$M_{\mathrm{HI}}-M_{*}$ relation from Maddox et al. (2015) (black). Centre:
The group members (red), compared to the $d-M_{\mathrm{HI}}$ relation (black)
from Wang et al. (2016). Right: The group members (pink), compared to the main
sequence of star-formation (black) from Parkash et al. (2018). The shaded
region indicates the 1 and 3$\sigma$ scatter of the respective relations.
Errors on all plots are relatively small compared to the size of the symbol.
Figure 8: The position-velocity distribution of the 19 detected sources,
classified by disturbances in their morphology. Sources without disturbed
morphologies are shown as filled circles, and sources with disturbed
morphologies are shown as unfilled circles. The size of the points represents
an exponential scaling in $f^{\mathrm{dyn}}_{\mathrm{HI}}$ mass fraction, and
sources without known dynamical masses are shown as squares. The colour of the
points correspond to the relative systemic velocity of the individual galaxies
($\rm V_{\mathrm{mean}}=12,630\,$ km s-1). The black cross represents the
unweighted average position of all 19 sources, which serves as an indicative
group centre.

We investigate the $M_{\mathrm{HI}}-M_{*}$ relation by comparing the galaxies
in this group to the relation in Maddox et al. (2015), as shown in Fig. 7. All
19 sources fall within 3$\sigma$ of this relation, except for two low-H i mass
outliers. The outlier at $\log(M_{*}/\mathrm{M}_{\odot})\sim 9.0$ corresponds
to ID 18, which, as shown in Fig. 2, has a low column density of
$N_{\mathrm{HI}}\leq 2.9\times 10^{20}\mathrm{cm}^{-2}$. There is likely
fainter H i emission below the sensitivity limit of this survey. Similarly,
the outlier at $\log(M_{*}/\mathrm{M}_{\odot})\sim 8.5$ corresponds to source
ID 2.

We also compare our results to the diameter-H i mass relation from Wang et al.
(2016). This is shown in Fig. 7, where 15 of the 19 sources are plotted. The
$d_{\mathrm{HI}}$ values here have been corrected for inclination, after which
4 sources had too low column densities to determine reliable measurements
(Table 2). It should be noted that Wang et al. (2016) assume $H_{0}=70$ km
s-1Mpc-1. This introduces a 6% offset for diameter measurements, which is
almost negligible for the range of values shown in Fig. 7. We choose to remain
with the Planck 2018 cosmological parameters in order to keep consistency with
the remainder of the MIGHTEE survey papers. All sources fall within the
3$\sigma$ confidence intervals of this relation, with the exception of source
ID 5. This source has a long tidal tail. The disruption of the H i has likely
reduced the diameter of the disk.

To investigate whether the group galaxies are actively forming stars, we
compare them to the star-formation main sequence (SFMS), which is the relation
between stellar mass and SFR. Galaxies are considered to be quenched, or
undergoing quenching if they reside below the relation, and are undergoing
high star-formation if they reside above (Kauffmann et al., 2003). In
comparison to the SFMS from Parkash et al. (2018), we find that all H
i-detected galaxies, but two, lie within the $3\sigma$ scatter of the
relation. This implies that they are actively undergoing star-formation. Two
sources, ID’s 12 and 16 are undergoing a high rate of star-formation. These
sources have low masses in comparison to the rest of the sample, and both lie
on the outskirts of the group.

###  4.1 Morphological properties

The sample includes some unusual and disturbed H i morphologies. A special
case is source ID 9, which shows an interacting pair of spiral galaxies,
likely at the early stages of a potential merger. Comparable to the
interacting galaxies in the M$\,81$ group (Yun et al., 1994), in the optical,
there are no tidal tails or other evidence that the galaxies are interacting,
but there is a clear H i bridge, indicating that they are indeed interacting.
In the moment-1 map, there is a clear distinction between the two objects, a
view that will be enhanced by the improved velocity resolution of future
MIGHTEE observations. Interestingly, there is no obvious evidence of
interaction in the integrated H i spectrum of this source. This example
highlights the importance of spatially-resolved H i observations of
environmental effects in groups and clusters. In particular, the spatial
information provided by the MIGHTEE survey presents a view of a transformation
process not evident in the integrated spectrum alone.

Sources ID 1 and ID 5 also have somewhat distorted H i distributions,
suggestive of interactions with a putative intra-group medium. Source ID 5 is
an edge-on spiral, with an off-centre H i-disk. The disk extends further on
the eastern side, with a large tidal tail on the northern end of the galaxy,
in the opposite direction of the group’s centre. The moment-0 map for source
ID 1 shows a face-on disk and a clumpy, extended tail on the northern end.
Sources 1D 14 and ID 16 also have disturbed H i morphologies, but they lie
towards the outskirts of the group. Both of them have an off-centre H i disk.
The disk asymmetry in these galaxies is perpendicular to the group centre.
These sources are likely in the early stages of accreting into the galaxy
group halo, resulting in tidal stripping from the host halo.

A trend observed in most of the edge-on detected sources is an H i deficiency
in the centre of galaxies, as is frequently seen in spirals with large central
bulges or high SFR (Bigiel et al., 2008). This can be clearly seen in sources
ID 3, ID 4, and ID 11 (Fig. 2). There is a galaxy asymmetry or lopsidedness
throughout the sample, particularly in the edge-on spiral galaxies. An example
of this is the slightly disturbed morphology in source ID 11. On the southern
side of the H i disk, the H i distribution appears to be flaring out towards
the edges of the disk. Disturbed disks can also be seen in ID’s 1, 3, 4, 7 and
9. Besides the bridge between the interacting pair, we detect no H i residing
outside of the detected galaxies, or other evidence of interaction between
galaxies.

Following Catinella et al. (2013), we expect groups with dynamical masses of
$\log(M_{\mathrm{dyn}}/\mathrm{M}_{\odot})<13$ to have H i-to-stellar mass
fractions of $\log(f_{\mathrm{HI}}^{\mathrm{*}})\sim-0.9$. We find a total
$\log(f_{\mathrm{HI}}^{\mathrm{*}})=-0.2$, which is 0.7 dex greater than
expected. If no further stellar-mass dominant sources are present, then this
group has an unusually large number of H i-rich galaxies, which might suggest
that it is in the early stages of assembly, despite being a relatively high-
mass group. In addition to this, we do not see much evidence of galaxy-galaxy
interaction, with the exception of ID 9, and the beginning of disturbed H i
disks, suggesting that there has not been much time for these interactions to
take place. Furthermore, Fig. 6 shows no relation between stellar mass and
relative velocity, implying that the most massive galaxies are not residing in
the centre of the group, as expected for virialised groups. Finally, the
velocity dispersion of the sources, shown in Fig. 6, does not show the
characteristic Gaussian distribution of a virialised group. This further
suggests that the group is dynamically young.

To investigate the H i morphologies in relation to their location in the
group, we visually classified the galaxies into two broad classes based on the
moment-0 maps, those with and without an obviously disturbed H i morphology.
This is admittedly a somewhat subjective classification scheme, particularly
at this spatial and velocity resolution, and column density sensitivity.
However, it provides an indicative guide in exploring how these objects may or
may not be influenced by their environment, at least for the most obvious
cases. This classification and the 3D positions of the sources in the group
are plotted in Fig. 8. There also appears to be an asymmetry, showing some
hints of more disturbed sources (ID’s 1, 3, 5 and 9) just north of the
apparent centre of the group. ID 3 has the lowest H i-to-stellar mass ratio of
the sample, implying that H i has been removed from the system. This is
consistent with Hess & Wilcots (2013), who conclude that gas is preferentially
removed from galaxies in the centre of groups. However, deeper observations
and higher velocity resolution will be needed to make more definitive
statements as the classification of several sources is ambiguous or marginal
and potentially due to internal, secular processes. This ambiguity
notwithstanding, the power of large samples of spatially resolved sources at a
cosmologically significant distance is clearly evident in this serendipitously
discovered H i galaxy group.

##  5 Conclusion

We present an H i-discovered galaxy group in the XMM-LSS field obtained from
the MIGHTEE survey with the MeerKAT telescope. We detect 20 H i galaxies (19
distinct H i detections) down to a $3\sigma$ column density sensitivity of
$N_{\mathrm{HI}}=1.6\times 10^{20}\,\mathrm{cm}^{-2}$, within an area of
$2.3\deg^{2}$ and a recession velocity range of 12,679 – 13,093 km s-1($z\sim
0.044$), and a spectral resolution of 46 km s-1. The 20 galaxies amount to a
total group H i mass of $\log(M_{\mathrm{HI}}/\mathrm{M}_{\odot})=11.20$.
Using a simple model, we estimate a total group dynamical mass of
$\log(M_{\mathrm{dyn}}/\mathrm{M}_{\odot})=12.32$.

The serendipitous discovery of this H i group, in a well-studied field,
highlights the importance of blind H i surveys. H i observations are a useful
way to study and identify groups, and other loosely composed gravitationally
bound systems at low redshift, that do not have detectable IGM haloes. The
spatially resolved H i emission has revealed a pair of interacting galaxies
and numerous tidal tails that are not visible in optical images.

Overall, these results demonstrate the capability of MeerKAT with regard to H
i science, showing the advantages of its wide field-of-view coupled with high-
fidelity, spatially-resolved imaging of H i galaxies at cosmologically-
significant distances. This galaxy group is an example of what is to come with
the MIGHTEE survey and similar depth MeerKAT L-band observations: the
discovery of several hundred H i sources in a wide range of environments
across the extragalactic legacy fields. The large cosmological volume probed
by the MIGHTEE survey, and sensitivity of MeerKAT, will allow for the study of
low-mass ($M_{\mathrm{HI}}\sim 10^{7}\mathrm{M}_{\odot}$) galaxy populations
in groups and other environments. This data will be significantly improved,
with the currently ongoing finer velocity MIGHTEE observations ($\sim 5$ km
s-1), allowing for more accurate kinematic modelling of the resolved galaxies.

## Acknowledgements

We thank the anonymous referee for their quick and helpful comments. We thank
Thijs van der Hulst for helpful comments which improved this paper. The
research of SR and RPD is supported by the South African Research Chairs
Initiative (grant ID 77948) of the Department of Science and Innovation and
National Research Foundation. SR, RPD and MJJ acknowledge the financial
assistance of the South African Radio Astronomy Observatory (SARAO) towards
this research (www.ska.ac.za). The MeerKAT telescope is operated by the South
African Radio Astronomy Observatory, which is a facility of the National
Research Foundation, an agency of the Department of Science and Innovation. We
acknowledge use of the Inter-University Institute for Data Intensive Astronomy
(IDIA) data intensive research cloud for data processing. IDIA is a South
African university partnership involving the University of Cape Town, the
University of Pretoria and the University of the Western Cape. The authors
acknowledge the Centre for High Performance Computing (CHPC), South Africa,
for providing computational resources to this research project. This work has
made use of the Cube Analysis and Rendering Tool for Astronomy (CARTA Comrie
et al., 2021). This work is based on data products from observations made with
ESO Telescopes at the La Silla Paranal Observatory under ESO programme ID
179.A-2005 (Ultra-VISTA) and ID 179.A- 2006(VIDEO) and on data products
produced by CALET and the Cambridge Astronomy Survey Unit on behalf of the
Ultra-VISTA and VIDEO consortia. Based on observations obtained with
MegaPrime/MegaCam, a joint project of CFHT and CEA/IRFU, at the Canada-France-
Hawaii Telescope (CFHT) which is operated by the National Research Council
(NRC) of Canada, the Institut National des Science de l’Univers of the Centre
National de la Recherche Scientifique (CNRS) of France, and the University of
Hawaii. This work is based in part on data products produced at Terapix
available at the Canadian Astronomy Data Centre as part of the Canada-France-
Hawaii Telescope Legacy Survey, a collaborative project of NRC and CNRS.The
Hyper Suprime-Cam (HSC) collaboration includes the astronomical communities of
Japan and Taiwan, and Princeton University. The HSC instrumentation and
software were developed by the National Astronomical Observatory of Japan
(NAOJ), the Kavli Institute for the Physics and Mathematics of the Universe
(Kavli IPMU), the University of Tokyo, the High Energy Accelerator Research
Organization (KEK), the Academia Sinica Institute for Astronomy and
Astrophysics in Taiwan (ASIAA), and Princeton University. Funding was
contributed by the FIRST program from Japanese Cabinet Office, the Ministry of
Education, Culture, Sports, Science and Technology (MEXT), the Japan Society
for the Promotion of Science (JSPS), Japan Science and Technology Agency
(JST), the Toray Science Foundation, NAOJ, Kavli IPMU, KEK, ASIAA, and
Princeton University. AAP acknowledges the support of the STFC consolidated
grant ST/S000488/1. WM is supported by the South African Research Chairs
Initiative of the Department of Science and Technology and National Research
Foundation. RB acknowledges support from the Glasstone Foundation and from an
STFC Ernest Rutherford Fellowship [grant number ST/T003596/1]. NM acknowledges
support from the Bundesministerium für Bildung und Forschung (BMBF) award
05A20WM4. MJJ acknowledges support from the UK Science and Technology
Facilities Council [ST/S000488/1 and ST/N000919/1]. MT and MJJ acknowledge
support from the Oxford Hintze Centre for Astrophysical Surveys which is
funded through generous support from the Hintze Family Charitable Foundation.
LVM acknowledges financial support from the grants AYA2015-65973-C3-1-R and
RTI2018-096228-B-C31 (MINECO/FEDER, UE), as well as from the State Agency for
Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa”
award to the Instituto de Astrofísica de Andalucía (SEV-2017-0709). This
research made use of Astropy,444http://www.astropy.org a community-developed
core Python package for Astronomy. MG and JC acknowledge financial support
from the Inter-University Institute for Data Intensive Astronomy (IDIA).

## Data Availability

The data underlying this article are subject to the proprietary period and
data distribution policies of the MIGHTEE Large Survey Project. The MIGHTEE-H
i spectral cubes will be released as part of the first data release of the
MIGHTEE survey, which will include cubelets of the sources discussed in this
paper. The derived quantities from the multi-wavelength ancillary data will be
released with the final data release of the VIDEO survey in mid-2021.
Alternative products are already available from the Herschel Extragalactic
Legacy Project (HELP; Shirley et al., 2021) and also soon from the Deep
Extragalactic VIsible Legacy Survey (DEVILS; Davies et al., 2021).

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