# MeerKAT follow-up of enigmatic GLEAM 4-Jy (G4Jy) sources

Precious K. Sejake1, Sarah V. White1, Ian Heywood1,2,3, Kshitij Thorat4,
Hertzog L. Bester1,2, Sphesihle Makhathini1,2,5, and Bernie Fanaroff2
1Department of Physics and Electronics, Rhodes University, PO Box 94,
Grahamstown, 6140, South Africa
2South African Radio Astronomy Observatory (SARAO), 2 Fir Street, Observatory,
Cape Town, 7925, South Africa
3Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road,
Oxford, OX1 3RH
4Department of Physics, University of Pretoria, Hatfield, Pretoria, 0028,
South Africa
5School of Physics, University of the Witwatersrand, Johannesburg,
Braamfontein, 2000, South Africa E-mail<EMAIL_ADDRESS>
(Accepted 2022 November 16. Received 2022 November 16; in original form 2022
August 20)

###### Abstract

We present the results from studying 140 radio sources in the GLEAM (GaLactic
and Extragalactic All-sky MWA [Murchison Widefield Array]) 4-Jy (G4Jy) Sample.
These sources were followed-up with MeerKAT to assess their radio morphology
and enable host-galaxy identification, as existing radio images of 25 to
45-arcsec resolution do not provide sufficient information. We refer to these
sources as the MeerKAT-2019 subset. The aim is to identify the host galaxy of
these sources by visually inspecting the overlays comprising radio data from
four surveys (at 150, 200, 843/1400, and 1300 MHz). Our morphological
classification and host-galaxy identification relies upon the $\sim$7-arcsec
resolution images from MeerKAT (1300 MHz). Through the visual inspection of
the overlays, 14 radio sources in the MeerKAT-2019 subset have wide-angle tail
(WAT) morphology, 10 are head-tail, and 5 have X-, S-/Z-shaped morphology.
Most of the remaining sources have the radio morphology of typical symmetric
lobes. Of 140 sources, we find host galaxies for 98 sources, leaving 42 with
no identified host galaxy. These 42 sources still have ambiguous
identification even with higher resolution images from MeerKAT.

###### keywords:

galaxies: formation – galaxies: evolution – galaxies: active – galaxies: jets

††pubyear: 2023††pagerange: MeerKAT follow-up of enigmatic GLEAM 4-Jy (G4Jy)
sources–MeerKAT follow-up of enigmatic GLEAM 4-Jy (G4Jy) sources

##  1 Introduction

Two main galaxy populations are known to radiate vast amounts of radio
emission: star-forming galaxies and galaxies with an active galactic nucleus
(AGN), with AGNs being the dominant emitter. Both populations play a
tremendous role in galaxy evolution. The impact of star formation on galaxy
evolution is through the collapse of the molecular clouds to form stars,
thereby building stellar mass. However, stellar mass growth may be halted
(usually in galaxies with low mass) if, for instance, the supernova is
powerful enough to expel gas from the system (Efstathiou, 2000) or the gas is
stripped away due to the interaction with another galaxy (Mihos et al., 1991).

Meanwhile, the process of material falling onto the supermassive black hole
(SMBH) at the galaxy’s centre may sometimes produce relativistic radio jets
(Urry & Padovani, 1995; Netzer, 2015). AGN activity has been shown to
significantly impact the host galaxy by suppressing or promoting star
formation; however, this is still a point of contention. Morganti et al.
(2013) demonstrated that star formation might be suppressed whereby the radio
jets expel gas from the system. Meanwhile, Croft et al. (2006) showed that the
jets might trigger the collapse of the molecular gas and promote star
formation.

In order to gain a better acumen of which of these scenarios is more
significant, a large sample of AGN with powerful relativistic radio jets is
needed to provide robust statistics on the features of these AGN, taking into
account attributes such as jet inclination with respect to the plane of the
galaxy (García-Burillo et al., 2014), the size of the molecular gas reservoirs
(Emonts et al., 2011), and jet power (Mukherjee et al., 2016). Additionally,
we need a large sample to unravel the effects of jet power, age, and
environmental density on radio luminosity and investigate these properties as
a function of redshift. Moreover, the mechanism of how jets are launched is
still poorly understood, as well as how these sources evolve with redshift.
This is because most studies are constrained to the 173 ’radio loud’ AGN in
the revised Third Cambridge Catalogue of Radio Sources (3CRR,
$\text{S}_{178~{}\text{MHz}}>10.9~{}\text{Jy}$; Laing et al. 1983). Therefore,
this work is significant in that it will help to better populate the high-
luminosity end.

AGN activity occurs in at least two modes, leading to two intrinsically
different AGN populations. This dichotomy is most likely caused by the
difference in the efficiency of the mechanism governing accretion onto the
central SMBH (e.g. see review by Heckman & Best 2014). The first AGN activity
mode is ‘quasar mode’, also called ‘cold mode’, ‘radiative mode’ or ‘high-
excitation’. The material is accreted onto the central SMBH through a
radiatively efficient, optically thick and geometrically thin disc (Shakura &
Sunyaev, 1973). These radiatively efficient AGN are typically connected to the
most luminous AGN, radiating across a broad range of the electromagnetic
spectrum. A fraction of these AGN are radio-loud AGN with powerful radio jets
extending for tens or hundreds of kpc. The second AGN activity mode is ‘radio
mode’, also called ‘jet mode’ or ‘low-excitation’. In this mode, the accretion
of material onto the central SMBH produces little radiated energy but can
result in the production of highly energetic jets. These radiatively
inefficient AGN are connected to low to intermediate luminosity AGN. The
evolution of the low-luminosity AGN has been probed by several studies using
deeper, narrow-area surveys such as VLA-COSMOS 3GHz Large project (Smolčić et
al., 2017) and LoTSS Deep Field (Sabater et al., 2019; Kondapally et al.,
2022).

White et al. (2020a, b) have created a large sample of AGNs with the purpose
of identifying how the properties of such sources vary as a function of
redshift and/or environment. This sample is known as the GLEAM (GaLactic and
Extragalactic All-sky MWA [Murchison Widefield Array; Tingay et al. 2013];
Hurley-Walker et al. 2017) 4-Jy (G4Jy) Sample and is ten times larger than the
3CRR, due to its lower flux density limit and larger survey area
($24,731\,\text{deg}^{2}$). The 3CRR sample, selected at low radio frequency
and complete with optical data, has made it possible to conduct groundbreaking
research such as the link between radio jet power and optical luminosity
(Rawlings & Saunders, 1991), which indicates that extragalactic radio sources
share a central engine mechanism responsible for their emission. Moreover, the
3CRR sample was used in the study of Barthel (1989) to show that a unification
model based on the orientation of the AGN can explain the observed properties
of quasars and radio galaxies.

The G4Jy Sample, which is a complete sample above 4-Jy at 151 MHz, is made up
of 1,863 of the brightest radio sources selected from the GLEAM survey using
the extragalactic catalogue (Galactic latitude, |b| > $10^{\circ}$). The GLEAM
survey uses observations from MWA, the precursor telescope for the low-
frequency component of the SKA (Square Kilometre Array). MWA surveyed the
entire southern sky (Declination, Dec < $30^{\circ}$) at low radio frequencies
in the range 72 - 231 MHz. The resolution of the GLEAM survey is declination
dependant and is approximated by $2.5\times
2.2\,\text{arcmin}^{2}/\text{cos}(\delta+26.7^{\circ})$ at the central
frequency of 154 MHz, which corresponds to a synthesised beam of
$\sim$2-arcmin at 200 MHz. The G4Jy Sample does not suffer from orientation
bias as these sources are selected at low frequencies. Lower frequency
observations do not suffer from Doppler-boosting effects, a phenomenon
inherent in surveys at higher frequencies.

White et al. (2020a, b) performed the host-galaxy identification for 1,606
radio sources in the G4Jy Sample through visual inspection of overlays
comprising a set of radio contours overlaid onto mid-infrared WISE (Wide-field
Infrared Survey Explorer; Wright et al. 2010) W1 band images. The sets of
radio contours used are from; GLEAM ($\sim$2-arcmin resolution), NVSS (NRAO
VLA Sky Survey [45-arcsec resolution]; Condon et al. 1998) or SUMSS (Sydney
University Molonglo Sky Survey [45-arcsec resolution]; Mauch et al. 2003;
Murphy et al. 2007) and TGSS ADR1 (TIFR GMRT Sky Survey first alternative data
release [25-arcsec resolution]; Intema et al. 2017). The host galaxy
identification of the remaining sources was constrained by the poor resolution
provided by NVSS/SUMSS and TGSS. Moreover, there were some discrepancies in
the literature concerning the host galaxy for some of the sources. As a
result, higher-resolution observations are needed to perform host galaxy
identification of these sources. The main objective of this study is to
identify the host galaxies of a subset of 140 G4Jy radio sources, which we
regard as the MeerKAT-2019 subset. The MeerKAT-2019 subset sources showed
complex ‘enigmatic’ radio morphologies at 45 and 25-arcsec resolution images.
These sources were observed with MeerKAT (1.3 GHz and $\sim 7-\text{arcsec}$
resolution), providing better sensitivity and angular resolution than TGSS and
NVSS/SUMSS.

###  1.1 Paper outline

This paper is outlined as follows. Section 2 gives a brief overview of the
radio and infrared data we used to construct the overlays. The MeerKAT
observations and data reduction are briefly described. Section 3 explains how
the MeerKAT-2019 subset was constructed, the overlays creation, and our labels
for host-galaxy identification. The results, of which we give comments on the
most interesting sources, are given in section 4. The discussion and
conclusion are given in sections 5 and 6, respectively. Throughout this paper,
a flat Lambda-CDM model was assumed: $\mathrm{H_{0}}=70~{}\mathrm{km/s/Mpc}$,
$\Omega_{m}=0.3$ and $\Omega_{\Lambda}=0.7$ (Planck Collaboration et al.,
2016), and we use the sign convention $S\propto\nu^{\alpha}$, where $\alpha$
is the spectral index and $S$ is the integrated flux density at frequency
$\nu$.

##  2 Data

In this section, we briefly describe (1) the MeerKAT observations and data
reduction and (2) additionally the data sets used to perform the host galaxy
identification of our sample.

###  2.1 Radio data

####  2.1.1 MeerKAT observations (1.3 GHz)

We observed111Project code: SCI-20190418-SW-01, PI: White 140 objects from the
G4Jy Sample using the L-band (856 – 1712 MHz) receivers of MeerKAT (Jonas &
MeerKAT Team, 2016). A combination of the brightness of the G4Jy sources, and
the high instantaneous sensitivity and excellent ($u$,$v$) plane coverage of
MeerKAT meant that 5-minute snapshot observations per target were sufficient
to achieve the two primary goals of unambiguous host galaxy identification,
and morphological classification. Targets were observed in four blocks,
grouped by Right Ascension. The overhead encompassed scans of the primary
calibrator (either PKS B1934$-$638 or PKS 0408$-$65, depending on the RA), an
RA-dependent secondary calibrator that was typically observed after every five
target scans, the primary polarisation calibrator (either 3C 138 or 3C 286),
and slewing time. The telescope correlator was configured to deliver 8-second
integrations and 4,096 frequency channels, with the latter being averaged by a
factor of 4 prior to processing.

The data were processed in standard fashion, with the raw MeerKAT visibilities
for each of the four blocks being converted to Measurement Set format using
the KAT Data Access Library222https://github.com/ska-sa/katdal. Standard
flagging commands were applied to all targets to remove the low gain edges of
the telescope’s bandpass response, as well as to flag known regions of
persistent radio frequency interference (RFI) on interferometer spacings below
600 m. Autoflagging software was then used on the calibrator sources, after
which the calibrators were used to derive instrumental delay, gain, and
bandpass corrections. Application of these corrections to the calibrators
themselves was followed by another round of autoflagging on the residual
(corrected$-$model) visibilities, after which the instrumental corrections
were re-derived and applied to the targets. The calibrated target data were
then flagged and imaged. A multifrequency clean component model derived from
deconvolution within a masked region was used to perform phase and delay self-
calibration, and the corrected data were re-imaged and finally primary beam
corrected. A Briggs robust value (Briggs, 1995) of $-$0.3 was used, except for
a small number of targets where uniform (robust = $-$2.0) weighting was used
to increase angular resolution and reduce residual side lobe emission that
remained due to calibration deficiencies.

The self-calibrated data were also re-imaged in eight frequency chunks with
data on spacings below 164 wavelengths discarded from the inner region of the
($u,v$) plane. The resulting cube was used to produce a spectral-index image
for each source. This was achieved by masking all pixels where the Stokes I
brightness was below 1 mJy beam-1, and subsequently extracting the spectrum of
each sight-line through the cube and fitting for its slope in log-brightness
vs log-frequency space.

Flagging was performed using the casa (McMullin et al., 2007) and
tricolour333https://github.com/ska-sa/tricolour/ packages. Referenced
calibration made use of the casa package, with the cubical (Kenyon et al.,
2018) software used for self-calibration. All imaging made use of the wsclean
(Offringa et al., 2014) software. Primary beam correction was done in the
image plane, using an azimuthally averaged image of the Stokes I primary beam,
evaluated at the nominal band centre frequency of 1284 MHz using the eidos
(Asad et al., 2021) package. Our data processing scripts are available
online444https://github.com/IanHeywood/oxkat (Heywood, 2020), and can be
consulted for further details of the reduction process. These scripts were
also used to deploy the data processing jobs on the two high-performance
computing facilities used for this work, namely the
ilifu555http://www.ilifu.ac.za cloud computing facility, and the Centre for
High-Performance Computing (CHPC666https://www.chpc.ac.za/) _Lengau_ cluster.

####  2.1.2 GLEAM catalogue and images (72-231 MHz)

The GLEAM survey constitutes observations from MWA, which observed the entire
southern sky Dec. < $30^{\circ}$ at low radio frequencies in the range of
72-231 MHz. We use 200 MHz images with a resolution of $\sim$2 arcmin of the
GLEAM survey for visual inspection in this work. GLEAM survey covers 24,831
square degrees for Dec. < $30^{\circ}$ and |b| > $10^{\circ}$. The survey is
99.97% reliable above a $5\sigma$ ($\sim$50 mJy) threshold, 90% complete at
170 mJy, and 50% complete at 55 mJy.

####  2.1.3 TGSS ADR1 catalogue and images (150 MHz)

TGSS is made up of observations from the Giant Metrewave Radio Telescope
(GMRT; Swarup 1991), which surveyed the sky above Dec. = $-55^{\circ}$.
Observations at Dec. > $-53^{\circ}$ were retained for the ADR1 due to the
poor data quality at low elevations. Moreover, TGSS has incomplete coverage in
the region 6.5h < R.A. < 9.5h, $25^{\circ}$ < Dec. < $39^{\circ}$ (Intema et
al., 2017). This survey, observed at 150 MHz frequency with a resolution of
$25\times 25\text{arcsec}^{2}$ (or $25^{"}\times
25^{"}/\text{cos}(\text{Dec}-19^{\circ}$) for sources at Dec < $19^{\circ}$,
complements the broad frequency range and surface-brightness sensitivity of
the MWA. The survey has a sensitivity limit below 5 mJy/beam ($7\sigma$
threshold) over the majority of its coverage and an astrometric accuracy of <
2 arcsec in R.A. and Dec.

####  2.1.4 SUMSS catalogue and images (843 MHz)

The Molonglo Observatory Synthesis Telescope (MOST; Mills 1981; Robertson
1991) previously surveyed the southern sky (Dec. < $-30^{\circ}$, |b| >
$10^{\circ}$) at 843 MHz creating SUMSS with a resolution of $45^{"}\times
45^{"}$. At Dec. < $-50^{\circ}$ SUMSS reaches a sensitivity limit of $\sim
5\sigma$ (6 mJy/beam) threshold and 10 mJy/beam at Dec. > $-50^{\circ}$. For
sources brighter than 200 mJy at 843 MHz, the observed astrometric accuracy is
$\sim$ 1-2 arcsec, while the largest positional error observed for the survey
is $\sim$ 30 arcsec. This survey has a similar resolution and sensitivity to
NVSS. For this work, SUMSS is used for GLEAM components at Dec
$<-39.5^{\circ}$ and NVSS for GLEAM components at Dec $\geq-39.5^{\circ}$.

####  2.1.5 NVSS catalogue and images (1.4 GHz)

NVSS constitutes observations from the Very Large Array (VLA; Thompson et al.
1980) which surveyed the northern sky (Dec. = $-40^{\circ}$) at 1.4 GHz
frequency. The survey has a resolution of 45 arcsec and a $5\sigma$ threshold
in peak source brightness of $\sim$2.5 mJy/beam. For sources brighter than 15
mJy, the observed astrometric accuracy is $\leq$ 1 arcsec.

###  2.2 Mid-infrared

####  2.2.1 AllWISE catalogue

WISE, launched in 2009, is an infrared space telescope that has mapped the
entire sky at 4 wavelength bands 3.4, 4.6, 12 and 22 $\mu m$ known as W1, W2,
W3 and W4, respectively. The 4 bands correspond to 6.1, 6.4, 6.5, and 12.0
arcsec resolution. In this work, we use the 6.1 arcsec resolution images (W1
band, 3.4 $\mu m$) as our grayscale and the AllWISE (Cutri et al., 2013)
positions for host galaxy identification, complemented by the radio contours
from the surveys mentioned in section 2.1. At the $5\sigma$ threshold, AllWISE
has an improved sensitivity of 0.054, 0.071, 0.73, and 5.0 mJy in the four
WISE bands, respectively, and positional error of < 1 arcsec, as compared to
the WISE All-Sky data release (Cutri et al., 2012).

###  2.3 Optical redshift surveys

####  2.3.1 6dFGS

The 6-degree Field Galaxy Survey (6dFGS; Jones et al. 2004) obtained optical
spectroscopy for the southern sky (Dec < $0^{\circ}$, |b| > $0^{\circ}$) using
the 6dF multifibre spectroscopy on the United Kingdom Schmidt Telescope (UKST;
Tritton 1978). For sources brighter than $K$ = 12.65 in the 2MASS Extended
Source Catalog (Jarrett et al., 2000), the final data release of 6dFGS (DR3;
Jones et al. 2009) provides redshifts, which we use for our work. 6dFGS has a
resulting median redshift of 0.053.

####  2.3.2 2MRS

The 2 Micron All Sky Survey (2MASS; Skrutskie et al. 2006) Redshift Survey
(2MRS; Huchra et al. 2012) constitute spectroscopic observations from various
telescopes (see Table 1). The survey initially observed galaxies with $K_{s}$
< 11.25 mag and later observed galaxies below the Galactic latitude limit of
6dFGS (|b| > $10^{\circ}$). We use redshifts from this survey for G4Jy sources
not detected in the 6dFGS. For sources which had no available redshift in
6dFGS and 2MRS, we further searched their redshifts from the NASA/IPAC
Extragalactic Database, Kilo-Degree Survey (KiDS; de Jong et al. 2017, Sloan
Digital Sky Survey (SDSS; Alam et al. 2015, and Half Million Quasars (HMQ)
Catalogue (Flesch, 2015).

Table 1: Telescopes and instruments used to compile 2MRS. "Res." in column five is the spectral resolution. Table taken from Huchra et al. (2012).  Observatory/telescope |  Camera |  Coverage |  Res. |  No. of galaxies with $K_{s}$
---|---|---|---|---
|  |  (Å) |  (Å) |  <11.75 >11.75
Fred L. Whipple 1.5 m |  FAST |  3500-7400 |  5 |  7590 2596
Cerro Tololo 1.5 m |  RCSpec |  3700–7200 |  7 |  3245 238
McDonald 2.1 m |  es2 |  3700–6400 |  4 |  114 50
Cerro Tololo 4 m |  RCSpec |  3700–7400 |  3 |  48
Hobby–Eberly 9.2 m |  LRS |  4300–10800 |  9 |  3

##  3 Method

###  3.1 Construction of the MeerKAT-2019 subset

Our sample, defined as the MeerKAT-2019 subset, comprises 140 radio sources
from the G4Jy Sample. Poor resolution data at 25 and 45 arcsec and complex
morphologies limited host-galaxy identification. The MeerKAT-2019 subset
includes; 13 sources (Table 2) whose host galaxy was provided (White et al.,
2020a, b), one G4Jy source (G4Jy 1523) whose host galaxy is affected by nearby
bright, mid-infrared emission, and 126 sources with uncertain identification.
For the 13 sources, there were some discrepancies in the literature concerning
their host galaxy; therefore, higher-resolution images were required to
confirm the provided host galaxy.

Table 2: A list of sources in the MeerKAT-2019 subset that needed higher resolution images to confirm the host-galaxy identification provided by White et al. (2020a, b). G4Jy name | GLEAM component name(s)
---|---
G4Jy 40 | GLEAM J002056-190853 and GLEAM J002112-191041
G4Jy 285 | GLEAM J024103+084523 and GLEAM J024107+084452
G4Jy 333 | GLEAM J031152-312959
G4Jy 570 | GLEAM J054049-614233
G4Jy 717 | GLEAM J083710-195152
G4Jy 747 | GLEAM J090147-255516
G4Jy 917 | GLEAM J112554-352321
G4Jy 1094 | GLEAM J134855-252700
G4Jy 1205 | GLEAM J145509-365543
G4Jy 1260 | GLEAM J152659-135059
G4Jy 1537 | GLEAM J192606-573954
G4Jy 1638 | GLEAM J203444-354849
G4Jy 1741 | GLEAM J215415-455319 and GLEAM J215435-454954

###  3.2 Overlays

We use APLpy (Robitaille & Bressert, 2012) to create overlays that are
$\sim$10 arcmin across for all the sources in the MeerKAT-2019 subset. The
overlays consist of contours from four surveys (GLEAM, NVSS/SUMSS, TGSS and
MeerKAT) overlaid onto the mid-infrared WISE (W1, $3.4~{}\mu m$) images. The
W1 band is used as our grayscale image because it has better sensitivity and
resolution than W2, W3, and W4 bands. However, for radio sources with no
detected host in mid-infrared, we use the optical Pan-STARRS (Panoramic Survey
Telescope and Rapid Response System; Chambers et al. 2019) images instead.
Radio sources with a faint infrared host are most likely at high redshifts. So
we need deep surveys to probe them, hence the use of Pan-STARRS images in this
study. We set the lowest contour level to $3\sigma$ for the radio surveys in
the overlays. However, for sources with artefacts in the MeerKAT imaging, we
set the lowest contour to a higher value. We then cross-matched the
MeerKAT-2019 subset with the AllWISE catalogue, using a radius of 3 arcmin
from the centroid position via TOPCAT (Taylor, 2005), and overplot these
AllWISE positions in the overlays (indicated by green crosses, ’x’). Figure 1
is an example of a 10 by 10 arcmin overlay showing all of the datasets
mentioned in this section.

###  3.3 Radio morphology classification

Host galaxy identification is highly associated with understanding the radio
morphology of the radio source. In this paper, we determine the radio
morphology of the MeerKAT-2019 subset using the $\sim 7$ arcsec resolution
images from MeerKAT, and use the following four categories:

  * •

‘single’ - the radio source has a compact morphology in the MeerKAT image,

  * •

‘double’ - the radio source has two distinct lobes in the MeerKAT image but
there is no detection of the radio core, or the source has an extended,
elongated emission which is suggestive of radio lobes in the MeerKAT image,

  * •

‘triple’ - the radio source has a radio core and two distinct radio lobes in
the MeerKAT image,

  * •

‘complex’ - the radio source has a morphology that does not meet the above
three categories.

###  3.4 Host galaxy identification and flags

We aim to identify the host galaxy of the radio emission by assessing the
radio morphology and determining which AllWISE position is likely the host
galaxy. For sources with ‘single’ morphology, the host galaxy is likely to be
at the centre. The host galaxy is likely to be located between the two
distinct radio lobes for sources with ‘double’ morphology. However, if there
is more than one AllWISE position on-axis, we leave the source unidentified;
unless there is a definitive core position. For radio sources with ‘triple’
morphology, we expect the host galaxy to coincide with the position of the
radio core (see Figure 1). Lastly, we are unable to provide the host galaxy
for sources with ‘complex’ morphology. Following White et al. (2020a, b), we
then use the following labels to indicate the host flag for each source in the
MeerKAT-2019 subset;

  * •

‘i’ - the source has a corresponding host galaxy identified in the
AllWISE/2MASS catalogue,

  * •

‘u’ - it is unclear which AllWISE or 2MASS source is the host galaxy as (1)
the source has complex radio morphology or (2) there is no radio core detected
in the MeerKAT image that coincides with the AllWISE/2MASS position,

  * •

‘m’ - either the source is affected by nearby bright mid-infrared emission, or
there is no corresponding AllWISE position,

  * •

‘n’ - given the type of radio emission involved (e.g., cluster relic or radio
halo), no host-galaxy position should be indicated.

###  3.5 Flux densities at 1.3 GHz

We calculated the integrated flux density at 1.3 GHz
($S_{\rm{int},1.3~{}\rm{GHz}}$) for 138777Of the 140 sources, we were unable
to obtain images for two sources (G4Jy 453 and G4Jy 456) as the visibilities
were entirely flagged due to a burst of strong RFI. G4Jy sources by summing
the flux densities within the $3\sigma$ contour level, where $\sigma$ is the
local rms in the 1.3 GHz image from MeerKAT. This integrated flux density is
in Jy/beam, and to convert it to Jy; we divide the summation by the beam area.
We provide the core intensity ($S_{\rm{core}},1.3~{}\rm{GHz}$), in Jy/beam
units, for sources with an identified host galaxy by extracting the flux
density at the pixel value of the host galaxy position in the MeerKAT image.
Both integrated flux densities and core intensity are provided in Table 3.

###  3.6 Calculation of spectral indices

We calculated the two-point spectral index
($\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}$) between 151 and 1300 MHz for the
MeerKAT-2019 subset using equation 1;

$\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}=\frac{\rm{log}_{10}(S_{\nu_{2}})-\rm{log}_{10}(S_{\nu_{1}})}{\rm{log}_{10}(\nu_{2})-\rm{log}_{10}(\nu_{1})}$
(1)

where $S_{\nu_{1}}$ and $S_{\nu_{2}}$ are the integrated flux densities at
$\nu_{1}=151~{}\rm{MHz}$ and $\nu_{2}=1300~{}\rm{MHz}$, respectively. The
integrated flux densities at 151 MHz were obtained from the G4Jy catalogue
(White et al., 2020a, b). The spectral index values are presented in Table 3.

##  4 Results

The host-galaxy identification of radio emission of 140 G4Jy sources relies
upon the $\sim$7-arcsec resolution images from MeerKAT as existing radio
images from TGSS and NVSS/SUMSS with resolutions of 25 and 45 arcsec,
respectively, do not provide sufficient detail. We resolve the ’complex’
morphologies evident in the TGSS and/or NVSS/SUMSS images with these new
images from MeerKAT and enable host-galaxy identification for 98 sources in
the MeerKAT-2019 subset, through visual inspection of the overlays and
manually pinpointing the host galaxy. Of the 98 sources with an identified
host galaxy in the MeerKAT-2019 subset, the host galaxies of two sources (G4Jy
1371 and G4Jy 1540) were identified using the optical Pan-STARRS images as the
host galaxies of these sources are not detected in the mid-infrared AllWISE
images. In Table 3, we present our findings for the MeerKAT-2019 subset.

Figure 1: An overlay of G4Jy 40, centred at R.A. = 00:21:07.53, Dec. = -19:10:05.4. The radio contours are from GLEAM (170 - 231 MHz; red), NVSS (1.4 GHz; blue), TGSS (150 MHz; yellow) and MeerKAT (1.3 GHz; purple), overlaid on the inverted grayscale WISE (3.4 $\mu m$) image. Shown in the bottom left corner is the beam size for each survey; GLEAM (red), NVSS(blue), TGSS (yellow) and MeerKAT (purple). The cyan hexagon indicates the brightness-weighted centroid position, the green crosses ‘x’ signs are the AllWISE positions within 3 arcmin from the centroid position, and the white ‘+’ sign is the AllWISE position for the corresponding host galaxy for this source. The white diamond is the 6dFGS position, the red squares are the GLEAM components positions, and blue crosses are NVSS/SUMSS components positions. Table 3: Properties of sources in the MeerKAT-2019 subset. Columns 1-3 are; the G4Jy name, host flag, and the host galaxy name. Columns 4 is the redshift, column 5 is the redshift reference, and column 6 is the spectral index ($\alpha$) between 151 and 1300 MHz. Columns 7-9 are the core intensity at 1300 MHz (SC13) in mJy/beam, integrated flux density at 1300 MHz (S13) in Jy, and integrated flux density at 1400 MHz (S14) in Jy. Columns 10-11 are the radio luminosities at 1300 MHz (L13) and 1400 MHz (L14) in W/Hz. Column 12 is the linear size (LS), and column 13 is the radio morphology label based on MeerKAT $\sim 7-\text{arcsec}$ resolution images. The full table is available online. G4Jy name |  Host flag | Host name |  z |  z ref. |  $\alpha$ | $SC13$ | $S13$ |  $S14$ | $L13$ | $L14$ |  LS | Morphology
---|---|---|---|---|---|---|---|---|---|---|---|---
|  |  |  |  |  |  |  |  |  |  |  |
|  |  |  |  |  | [mJy/beam] | [Jy] |  [Jy] | [W/Hz] | [W/Hz] |  [Mpc] |
G4Jy 14 |  i | WISEA J000707.12$+$053609.6 |  0.216 |  (1) |  | $98.166$ | $0.441{\pm}0.085$ |  |  |  |  | Double
G4Jy 40 |  i | WISEA J002107.53$-$191005.4 |  0.096 |  (2) |  $-0.707$ | $0.230$ | $1.261{\pm}0.140$ |  | $2.85{\times}10^{25}$ |  |  | Triple
G4Jy 47 |  i | WISEA J002531.45$-$330246.2 |  0.05 |  (2) |  $-0.815$ | $0.424$ | $1.527{\pm}0.162$ |  1.494 | $8.95{\times}10^{24}$ | $8.75{\times}10^{24}$ |  0.091 | Double
G4Jy 77 |  u |  |  |  |  $-2.604$ |  | $0.046\pm 0.027$ |  0.035 |  |  |  | Complex
G4Jy 84 |  i | WISEA J004605.01$-$633319.2 |  0.075 |  (2) |  $-0.82$ | $0.269$ | $0.731{\pm}0.100$ |  | $9.94{\times}10^{24}$ |  |  0.215 | Triple

###  4.1 Radio sources with an identified host galaxy

####  4.1.1 Previous identifications

White et al. (2020a, b) provided the host-galaxy identification for 13 of 140
G4Jy radio sources (Figures 1, 2 and 4). However, as these were ‘borderline’
identifications (with some accompanied by discrepancies in the literature),
higher-resolution data was needed to confirm the host galaxy. With the MeerKAT
follow-up images, we confirm the host galaxy of the 13 sources provided by
White et al. (2020a, b).

G4Jy 40 (Figure 1): White et al. (2020a, b) inferred the host galaxy (AllWISE
J002107.53$-$191005.4) of this radio source (with radio lobes GLEAM
J002056$-$190853 and GLEAM J002112$-$191041) based on the interpretation of
Nilsson (1998) that G4Jy 40 has a ‘double’ morphology with the largest angular
size of 252 arcsec, using the radio map of Schilizzi & McAdam (1975). However,
the 4.7 GHz image from Reid et al. (1999) reveals a radio source with ‘triple’
morphology that spans $\sim$160 arcsec from lobe to lobe (cyan contours in
Figure 15 of White et al. 2020a). Due to the incorrect coordinates presented
in the Reid et al. (1999) image, a higher resolution image needs to be used to
confirm if GLEAM J002056$-$190853 is associated with G4Jy 40. The
$\sim$7-arcsec resolution image from MeerKAT reveals a radio source with
‘triple’ morphology. The tight concentric contours at the centre (R.A. =
00:21:05.54, Dec. = $-$19:10:05.34) verify that AllWISE J002107.53$-$191005.4
(g002107.53$-$191005.4, $z=0.096$) is the mid-infrared host galaxy of the
extended emission, and GLEAM J002056$-$190853 and GLEAM J002112$-$191041 are
the radio lobes, with flux densities of $0.919323~{}\mathrm{Jy}$ and
$4.862407~{}\mathrm{Jy}$ at 151 MHz, respectively.

G4Jy 285 (GLEAM J024103$+$084523 and GLEAM J024107$+$084452; Figure 2(a)): The
host galaxy of the extended radio emission of G4Jy 285, also known as NGC 1044
(4C $+08.11$), is AllWISE J024106.17$+$084416.9, also detected in 2MRS
(J02410618$+$0844167, $z=0.021$). This identification is in agreement with
White et al. (2020a, b) and van Velzen et al. (2012). The radio source traces
a zig-zag emission; however, it was unclear whether the low-frequency emission
towards the northeast (GLEAM J024133$+$084940) is associated with G4Jy 284
(White et al., 2020a, b). We cannot confirm whether the radio emission is
associated with G4Jy 284 as there is no detection of this emission in our 1.3
GHz image from MeerKAT. We only have a shallow ‘snapshot’ image. A longer
observation time might reveal something.

G4Jy 333 (GLEAM J031152$-$312959; Figure 2(b)): MeerKAT contours reveal
‘triple’ morphology with edge-brightened lobes. We confirm that AllWISE
J031154.01$-$313010.6 is the host galaxy. However, we find no redshift
information in the literature.

G4Jy 570 (GLEAM J054049$-$614233; Figure 2(c)): ‘Double’ extended morphology
is evident in SUMSS and MeerKAT contours. This radio source (PKS B0540$-$617)
is in cluster A3362. The host galaxy of this radio source is AllWISE
J054050.82$-$614237.2 ($z=0.081$) which agrees with White et al. (2020a, b)
and the optical identification of Jones & McAdam (1992).

G4Jy 717 (GLEAM J083710$-$195152; Figure 2(d)): This radio source, known as
PKS B0834$-$19, has ‘single’ morphology evident in all surveys. We confirm
that the host galaxy of this source is AllWISE J083711.18$-$195156.6
($z=1.032$), which agrees with the optical identification provided by Fugmann
et al. (1988) and di Serego Alighieri et al. (1994). We note the artefacts in
the MeerKAT image, which result in an overestimated integrated flux density at
1.3 GHz. We, therefore, present an overlay where the lowest contour level has
been adjusted to $48\sigma$. G4Jy 717 has the flattest spectral index
($\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}=-0.17$) in the MeerKAT-2019
subset.

G4Jy 747 (GLEAM J090147$-$255516; PKS 0859$-$25, Figure 2(e)): ‘Double’
extended radio morphology is evident in the MeerKAT contours. We confirm that
AllWISE J090147.26$-$255516.2 ($z=0.305$) is the host galaxy of the radio
emission. The MeerKAT contours start at $48\sigma$ as artefacts are present at
lower levels.

G4Jy 917 (GLEAM J112554$-$352321): PKS B1123$-$315 (Figure 3(a)) is in the
cluster AS 665. The higher resolution image from MeerKAT shows the distinct
inner structure of this radio source where knots are evident in the eastwards
jet (Figure 3(b)). Furthermore, this jet is bent forming a right angle while
the emission resulting from the westwards jet appears to be pushed towards
south. The radio core is evident in the 1.3 GHz image from MeerKAT, which
coincides with the position of the radio core from the 4.9 GHz image provided
by Ekers et al. (1989). The host galaxy of the radio emission is AllWISE
J112552.95$-$352340.3, which appears in 6dFGS (g1125529$-$352340, z = 0.033).
This identification is in agreement with van Velzen et al. (2012), and a
deeper MeerKAT image will be presented by Thorat et al. (in prep.).

(a) G4Jy 285

(b) G4Jy 333

(c) G4Jy 570

(d) G4Jy 717

(e) G4Jy 747

Figure 2: Overlays for 5 of 13 sources in the G4Jy Sample for which the
literature has disagreements regarding the host galaxy (see section 4.1.1).
The datasets, symbols, beams and contours are the same as those described in
Figure 1.

(a) An overlay of G4Jy 917

(b) MeerKAT intensity map.

Figure 3: Top panel: An overlay of G4Jy 917 (one of the 13 sources with
discrepancies in the literature concerning the host galaxy identification).
The datasets, symbols, beams and contours are the same as those described in
Figure 1. Bottom panel: The intensity map at 1.3 GHz of G4Jy 917 showing the
structure of this radio source. The morphology is very complex, and includes
the emission eastwards of the core forming a right-angled bend.

G4Jy 1094: The wide-angle tail (WAT) radio morphology is evident in the
MeerKAT image (Figure 4(a)). We confirm that the radio source is hosted by the
mid-infrared AllWISE source J134854.17$-$252724.5, detected in 6dFGS at
$z=0.126$. This radio source is in the cluster Abell 1791, located at the
centre (Slee et al., 1989).

G4Jy 1205: The radio source known as PKS 1452$-$367 (Figure 4(b)) has a
‘triple’ morphology with diffuse edges evident in MeerKAT contours. We confirm
that the mid-infrared AllWISE J145509.61$-$365507.4 (g1455096$-$365508,
$z=0.095$) is the host galaxy, which is in agreement with van Velzen et al.
(2012).

G4Jy 1260: The radio source known as PKS B1526$-$136 (Figure 4(c)) has a
‘single’ morphology evident in all surveys. We confirm that the host galaxy is
the mid-infrared AllWISE J152659.45$-$135100.0 at redshift $z=1.687$. This
radio-loud quasar has the highest redshift in the MeerKAT-2019 subset (so far)
and is one of the sources with a very flat spectral index
($\alpha_{1300~{}\rm{MHz}}^{151~{}\rm{MHz}}=-0.36$). The MeerKAT contours
start at $24\sigma$ as artefacts are present at lower levels.

G4Jy 1537 (PKS B1921$-$577 Figure 4(d)): ‘Triple’ morphology is evident in the
1.3 GHz image from MeerKAT. We confirm that the mid-infrared AllWISE
J192605.75$-$574016.4 (6dFGS g1926057$-$574017, $z=0.061$) is the host galaxy
of the WAT radio source. The MeerKAT contours start at $12\sigma$ as artefacts
are present at lower levels.

(a) G4Jy 1094

(b) G4Jy 1205

(c) G4Jy 1260

(d) G4Jy 1537

(e) G4Jy 1638

(f) G4Jy 1741

Figure 4: Overlays for 6 of 13 sources in the G4Jy Sample for which the
literature has disagreements regarding the host galaxy (see section 4.1.1).
The datasets, symbols, beams and contours are the same as those described in
Figure 1.

G4Jy 1638 (Figure 4(e)): The head-tail radio morphology is evident in the 1.3
GHz image from MeerKAT. The corresponding host galaxy of the radio emission is
AllWISE J203444.74$-$354901.7, appearing in 6dFGS as g2034447$-$354902
($z=0.089$).

G4Jy 1741 (Figure 4(f)): Edge-brightened radio lobes and a compact core are
evident in the 1.3 GHz image from MeerKAT. We confirm that AllWISE
J215422.96$-$455231.3 is the host galaxy of the ‘triple’ morphology radio
source, detected in 6dFGS (g2154230$-$455232, $z=0.145$).

####  4.1.2 Newly identified X- and S-/Z-shaped radio sources

X- and S-/Z-shaped radio galaxies form a subclass of radio galaxies with
intriguing radio morphology. These radio galaxies are characterised by an
additional pair of secondary radio lobes (generally referred to as wings) that
are more diffuse, have low surface brightness and are misaligned from the
radio core compared to the primary lobes (Ekers et al., 1978; Leahy & Parma,
1992). The orientation of the secondary lobes generally defines the X- or
S-/Z-shaped morphology. For S-/Z-shaped morphology, the low surface brightness
secondary lobes emerge from the edges of the high surface brightness secondary
lobes. For X-shaped morphology, the axis of the secondary lobes is nearly
perpendicular to the axis of the primary lobes. In the MeerKAT-2019 subset,
four radio sources (G4Jy 284, G4Jy 530, G4Jy 1377 and G4Jy 1798) have an
X-shaped morphology, and one radio source (G4Jy 1523) has an S-/Z-shaped
morphology (Figure 5). Below, we discuss only a few (11) that are interesting.

G4Jy 284 (GLEAM J023926$-$112806, Figure 5(a)): A (borderline) X-shaped
morphology, and the detection of the radio core, is evident in the MeerKAT
contours. The mid-infrared source that coincides with the radio core is
AllWISE J023926.84$-$112752.6. However, there is no redshift information in
the literature for the host galaxy.

G4Jy 530 (GLEAM J051250$-$482358, PKS 0511-48, Figure 5(b)): For this radio
source, Smith & Robertson (1985), using a radio map with a spatial resolution
of $53"\times 46"$, reported an optical identification that matches AllWISE
J051247.41$-$482416.5, classified as a Seyfert type 2 galaxy. However, White
et al. (2020a, b) asserted that the radio map of Smith & Robertson (1985) is
not of sufficient resolution to rule out the presence of another mid-infrared
source on the axis connecting the radio lobes. The X-shape morphology evident
in the MeerKAT contours support the optical identification of Smith &
Robertson (1985), and we, therefore, identify AllWISE J051247.41$-$482416.5 as
the host galaxy. The low surface brightness radio emission (secondary lobes)
might be due to plasma backflow in the primary lobes. The MeerKAT contours
start at $12\sigma$ as artefacts are present at lower levels.

G4Jy 1377 (GLEAM J165712$-$134911, Figure 5(c)): This radio source is known as
PKS B1654-137. An X-shaped morphology is evident in the MeerKAT contours, with
the primary lobes aligned northwest and southeast, while the secondary lobes
are aligned northeast and southwest. The mid-infrared counterpart of this
radio source is the AllWISE source, J165712.85$-$134909.5, at z = 0.124.

G4Jy 1798 (GLEAM J225641$-$461726, Figure 5(d)): There are two AllWISE
positions near the centroid, and it was not clear which AllWISE position is
the most-likely host galaxy. The MeerKAT image reveals an X-shaped galaxy with
the primary lobes aligned northwest and southeast while the secondary lobes
are aligned northeast and southwest. We then use a DSS image as our grayscale
and identify the mid-infrared host galaxy as 2MASS J22564196$-$4617345, at z =
0.080.

(a) G4Jy 284

(b) G4Jy 530

(c) G4Jy 1377

(d) G4Jy 1798

(e) G4Jy 1523

Figure 5: Four radio sources with X-shaped morphology and one G4Jy source
(G4Jy 1523) with S-/Z-shaped morphology in the MeerKAT-2019 subset. For G4Jy
1523, PanSTARRS image is used as grayscale base for the overlay and green
crosses are from 2MASS. The datasets, symbols, beams and contours are the same
as those described in Figure 1.

G4Jy 1523 (GLEAM J191757$-$243917, Figure 5(e)): MeerKAT contours reveal a
Z-shaped radio galaxy suggestive of precessing jets. The radio source was
previously assigned host flag ‘m’ as the host galaxy in the AllWISE image is
obscured by the nearby star (White et al., 2020a, b). As a result, the 2MASS
image was used as the grayscale base image for the overlay, and the identified
host galaxy is 2MASS 19175722$-$2439053\. We find no redshift information in
the literature, but follow-up SALT (Southern African Large Telescope; Buckley
et al. 2006) spectroscopy (PI: White) has been obtained for this source.

####  4.1.3 Newly identified head-tail and wide-angle tail (WAT) radio sources

Head-tail and WAT radio galaxies are a subclass of radio galaxies associated
with clusters. The WAT and head-tail morphology observed is explained by the
radio galaxy falling into a cluster. The ram pressure from the surrounding
medium pushes the radio jets and lobes backwards, forming a WAT or head-tail
morphology. In the MeerKAT-2019 subset, we find that 10 sources have head-tail
morphology and 14 have WAT morphology.

G4Jy 14 (GLEAM J000707$+$053607, Figure 6(a)): ‘Double’, extended radio
morphology, suggestive of radio lobes, is evident in NVSS and TGSS contours.
The host galaxy could not previously be provided for this source as there were
two mid-infrared positions at a similar distance from the centroid position,
and there was no detection of the radio core (White et al., 2020a, b).
However, the radio contours from MeerKAT reveal two resolved sources, with one
being a head-tail radio source towards the northeast. We, therefore, mark the
two sources as unrelated (see subsection 4.2.2) and identify the host galaxy
of the head-tail radio source as the mid-infrared AllWISE
J000707.12$+$053609.6 (z = 0.2161). The MeerKAT contours start at $12\sigma$
as artefacts are present at lower levels.

G4Jy 113 (GLEAM J010241$-$215227, Figure 6(b)): The radio source whose radio
morphology from NVSS and TGSS is challenging to understand is located in the
cluster Abell 133. White et al. (2020a, b) could not provide the host galaxy
due to the disagreements in the literature concerning the radio morphology of
G4Jy 113. Slee et al. (2001) classified the radio source as a cluster relic
generated via merger shocks, while Rizza et al. (2000) and Fujita et al.
(2002) classified this source as a remnant radio lobe. Our radio image from
MeerKAT indicates that G4Jy 113 is a head-tail radio source, with concentric
radio-contours nearby highlighting an unrelated source. The former has
spectral index, $\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}=-1.8$, and we
identify the host galaxy as the AllWISE J010241.76$-$215254.2, detected in
6dFGS as g0102418$-$215256 (z = 0.057). This galaxy is referred to as galaxy
’H’ by Slee et al. (2001).

(a) G4Jy 14

(b) G4Jy 113

(c) G4Jy 984 (towards NE) and G4Jy 983 (towards SW)

(d) G4Jy 1410

(e) G4Jy 1852

Figure 6: Radio sources with head-tail morphology in the MeerKAT-2019 subset.
The datasets, symbols, beams and contours are the same as those described in
Figure 1.

(a) G4Jy 47

(b) G4Jy 528

(c) G4Jy 637

(d) G4Jy 665

(e) G4Jy 693

(f) G4Jy 1067

Figure 7: Radio sources with WAT radio morphology in the MeerKAT-2019 subset.
The datasets, symbols, beams and contours are the same as those described in
Figure 1.

G4Jy 984 (GLEAM J121740$+$033940; Figure 6(c)): There is a tail of emission
evident in the TGSS contours for the northern source; however, it was unclear
where the host galaxy could be. The MeerKAT contours reveal a far more
detailed view of the structure of G4Jy 984, with the inner parts of the jet
appearing distinct before combining into a tail towards the southeast. We
identify the host galaxy as AllWISE J121738.77$+$033948.2 (z = 0.0778). The
southwest radio source (not part of the MeerKAT-2019 subset) appears in the
G4Jy catalogue as G4Jy 983 (4C +04.41), previously assigned host flag ’m’.
This radio source is in the cluster Z5029 (R.A. = 12:17:14, Dec. = 03:39:23).
Head-tail radio morphology is evident in the MeerKAT contours, and we identify
the host galaxy as the mid-infrared AllWISE source J121731.43$+$033656.4 (z =
0.773).

G4Jy 1410 (GLEAM J172437$-$024246; Figure 6(d)): Based on the compactness of
the TGSS contours, it was unclear whether G4Jy 1410 (the southern source in
the overlay) is a ‘double’ or a head-tail galaxy, and as such, the host galaxy
was not provided. MeerKAT contours reveal a head-tail radio galaxy (G4Jy 1410)
and a ‘double’ morphology radio galaxy (GLEAM J172436$-$024055) towards the
north, which has a flux density of 3.79 Jy at 151 MHz (White et al., 2020a,
b). Therefore, GLEAM J172436$-$024055 is not part of the G4Jy catalogue as it
does not meet the criterion of flux density being above 4-Jy at 151 MHz. We
identify the host galaxy of G4Jy 1410 as AllWISE J172437.79$-$024305.6
detected in 2MRS as J17243782$-$0243062 (z = 0.034). This identification is in
agreement with van Velzen et al. (2012).

G4Jy 1852 (GLEAM J234740$-$280839; Figure 6(e)): This radio source is in the
cluster Abell 4035. A head-tail radio morphology with diffuse tail emission is
evident in the MeerKAT contours. The mid-infrared source that coincides with
the radio core is AllWISE J234745.06$-$280826.2. This identification
corresponds to IC 5358, which is the brightest galaxy in a cluster.

G4Jy 47 (GLEAM J002530$-$330336; Figure 7(a)): Wide-angle tail radio
morphology is evident in the higher-resolution image from MeerKAT. This radio
source appears in the Molonglo Southern 4-Jy Sample (MS4; Burgess & Hunstead
2006) as MRC B0023-33. The host galaxy is the mid-infrared AllWISE
J002531.45-330246.2 at redshift z = 0.050, which agrees with the host galaxy
identification of Burgess & Hunstead (2006) and van Velzen et al. (2012). An
optical counterpart is ESO 350-G-15, the brightest galaxy in cluster AS 41
(ACO89).

G4Jy 528 (GLEAM J051104$-$131645; Figure 7(b)): WAT morphology with diffuse
tail emission is evident in MeerKAT contours. The host galaxy is AllWISE
J051104.82$-$131730.3 at $z=0.043$.

G4Jy 637 (GLEAM J070130$+$231313; Figure 7(c)): A radio source known as 4C
$+23.18$ in the literature (Olsen, 1970). Based on the TGSS contours, White et
al. (2020a, b) identified the morphology as WAT. However, it was unclear which
AllWISE source was the host galaxy. With a higher resolution image from
MeerKAT, we confirm that this is indeed a WAT radio source. We identify the
host galaxy as AllWISE J070129.05$+$231325.6 at $z=0.092$.

G4Jy 665 (GLEAM J073104$-$523710; Figure 7(d)): The radio morphology of G4Jy
665 is not quite clear in both SUMSS and TGSS contours, and as a result, this
radio source was assigned a ‘complex’ morphology label, host flag ‘u’ and
confusion flag ‘0’. The MeerKAT image reveals two unrelated radio sources,
both having a WAT morphology and a detected host galaxy in the mid-infrared.
The host galaxy of the southern source (indicated with a white ‘+’ in the
overlay) is AllWISE J073104.92$-$523808.6 at z = 0.09035, and the host galaxy
of the northern source is AllWISE J073103.39$-$523546.8 at $z=0.07690$. Given
that the MeerKAT image reveals two unrelated sources with integrated flux
densities of 0.836 Jy (southern source) and 0.314 Jy (northern source) at 1.3
GHz, this implies the integrated flux density at 151 MHz (5.12469 Jy) of G4Jy
665 is the sum of the integrated flux densities of the two sources. With this
in mind, G4Jy 665 should be re-inspected in terms of the integrated flux
density at 151 MHz, as this parameter is the G4Jy Sample’s defining criterion
($S_{151~{}MHz}>4$ Jy).

G4Jy 693 (GLEAM J081630$-$703925; Figure 7(e)): WAT radio morphology is
evident in the MeerKAT contours, including detection of the radio core. An
optical identification provided by Jones & McAdam (1992) does not coincide
with the radio core detected in the MeerKAT image. The corresponding host
galaxy is the mid-infrared AllWISE J081611.74-703945.3 at redshift z = 0.033.

G4Jy 1067 (GLEAM J132606$-$272641 and GLEAM J132616$-$272632, Figure 7(f)):
The radio contours from TGSS, NVSS and MeerKAT indicate WAT radio morphology,
but the radio core is only detected in the MeerKAT image. The mid-infrared
source that coincides with the radio core is AllWISE J132610.59$-$272538.6 at
redshift $z=0.044$. This radio source, known as PKS B1323$-$271, is in the
cluster Abell 1736.

(a) G4Jy 1554

(b) Spectral index map

Figure 8: Top panel: Overlay of G4Jy 1554. The datasets, symbols, beams and
contours are the same as those described in Figure 1. Bottom panel: The
spectral index map with MeerKAT contours overlaid.

####  4.1.4 New identification via the spectral index map

G4Jy 1554 (GLEAM J193252$-$081739): White et al. (2020a, b) classified this
radio source as ‘double’ morphology based on NVSS and TGSS contours. ‘Complex’
morphology is evident in the MeerKAT contours (Figure 8(a)) with a slight
indication of a radio core. The spectral index map [see Section 2.1.1] (Figure
8(b)) indicates a radio core. We identify AllWISE J193252.79$-$081803.4
($z=0.101$) as the host galaxy.

####  4.1.5 Giant radio galaxies (GRGs)

GRGs are a subclass of radio galaxies with a projected linear size > 0.7 Mpc.
Their incredible Mpc-scale sizes make them one of the largest known objects in
the universe. We find three sources (Figures 4(f) and 9) with linear sizes
above 0.7 Mpc in the MeerKAT-2019 subset.

G4Jy 120 (GLEAM J010521$-$450527; Figure 9(a)): ‘Double’ morphology is evident
in the MeerKAT contours. We identify the host galaxy as AllWISE
J010522.21$-$450517.2 at $z=0.7$. This redshift value and the angular extent
of $139.717~{}$arcsec correspond to the linear size of 1.004 Mpc.

G4Jy 680 (GLEAM J080225$-$095823 and GLEAM J080253$-$095822; Figure 9(b)):
This radio source has several AllWISE positions that could be likely the host
galaxy (White et al., 2020a). The MeerKAT image reveals the radio core which
coincide with AllWISE J080236.28$-$095739.9, detected in 6dFGS as
g0802363$-$095740\. This identification is consistent with the optical
identification of Schilizzi (1975). The redshift of 0.070 and the angular
extent of 9.13 arcmin at 1.4 GHz correspond to the linear size of
$0.732~{}\mathrm{Mpc}$.

(a) G4Jy 120

(b) G4Jy 680

Figure 9: Radio galaxies with linear size above 0.7 Mpc in the MeerKAT-2019
subset. The datasets, symbols, beams and contours are the same as those
described in Figure 1.

####  4.1.6 Possible GRG

G4Jy 641 (GLEAM J070525$-$451328 and GLEAM J070546$-$451158; PKS B0703$-$451):
A possible GRG with radio emission that spans $\sim$8-arcmin from lobe to lobe
in the 1.3 GHz image from MeerKAT (Figure 10). ‘Triple’ morphology is evident
in the MeerKAT contours, and there is a detection of a hotspot in the
southwestern lobe. However, there is an AllWISE position at the center of this
hotspot emission, which makes it likely unrelated to G4Jy 641 An optical
identification provided by Jones & McAdam (1992) (marked with the orange ‘+’
sign in the overlay) appears to be incorrect. The corresponding host galaxy
that coincides with the radio core is AllWISE J070532.94$-$451308.8. We find
no redshift information in the literature and, therefore, cannot provide the
linear size of this radio galaxy.

Figure 10: G4Jy 641: A possible GRG in the MeerKAT-2019 subset. The orange ‘+’
sign indicate the optical identification provided by Jones & McAdam (1992).
The datasets, symbols, beams and contours are the same as those described in
Figure 1.

####  4.1.7 Quasi-stellar radio source

G4Jy 1843 (GLEAM J233511$-$663702; PKS B2332$-$62, Figure 11): This radio
source has a ‘complex’ morphology (likely artefacts) evident in the SUMSS
contours, and there is no coverage in TGSS. The diffraction spikes evident in
the mid-infrared image, and WISE colours, suggest that AllWISE
J233510.30$-$663655.7 (close to the centroid position) is a star. However, the
‘double’ morphology evident in the MeerKAT contours strongly suggest that this
AllWISE source is likely the host galaxy of the radio emission (cf. the quasar
3C 273, which appears in the G4Jy Sample as G4Jy 1003; White et al. 2020a).
We, therefore, regard AllWISE J233510.30$-$663655.7 as the host galaxy of this
radio emission.

Figure 11: An overlay of a quasi stellar object (G4Jy 1843). The datasets,
symbols, beams and contours are the same as those described in Figure 1.

###  4.2 Unidentified sources

The overlays for MeerKAT-2019 subset sources discussed in this section are
available as online supplementary material.

####  4.2.1 Amorphous morphology

G4Jy 77 (GLEAM J004130$-$092221) is B0038$-$096 in the cluster Abell 85. This
radio source is interpreted as a radio halo in the literature (Bagchi et al.,
1998). The unusual morphology in the NVSS and TGSS contours is also evident in
the MeerKAT image. We find that this radio source has the steepest spectral
index ($\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}=-2.604$) in the MeerKAT-2019
subset, followed by G4Jy 1117
($\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}=-2.018$).

G4Jy 513 (GLEAM J045826$-$300717, PKS B0456$-$30): This radio source in
cluster A3297 has a dense field of mid-infrared AllWISE sources. Jones &
McAdam (1992) interpreted the radio morphology as a cluster halo. The optical
identification they provided coincide with the mid-infrared AllWISE source
marked with the letter ‘A’ in our overlay. This galaxy is at z = 0.131, which
is consistent with the redshift of the cluster. We are unable to
provide/confirm whether the provided optical identification by Jones & McAdam
(1992) is the host galaxy for this radio emission due to the complex,
amorphous morphology evident even in the higher-resolution image from MeerKAT.
There is no detection of the radio core in neither the MeerKAT image nor the
spectral index map.

G4Jy 700 (GLEAM J082231$+$055626): A radio source with diffuse emission known
as 3C 198 in the 3C catalogue. Wyndham (1966) provided an optical
identification corresponding to AllWISE J082231.95$+$055706.8 marked with the
letter ‘A’ in our overlay. However, there is no core detection in the higher-
resolution image from MeerKAT nor in the spectral index map. The lack of core
detection could be that the MeerKAT images are generated from 5-minute
snapshot observations, and a longer integration time may be needed. The
spectral index map reveals old emission in the inner regions, indicating that
there is no ongoing ejection of electrons. This is the typical distribution of
spectral indices in radio galaxies. Often the plasma near the centre is a
result of backflow from the jets and is therefore older with a steeper
spectral index.

####  4.2.2 Unrelated sources

Radio sources whose radio emission is blended together by NVSS/SUMSS (45
arcsec resolution) and TGSS (25 arcsec resolution) but MeerKAT images ($\sim$7
arcsec resolution) reveal two resolved sources. We find six sources (G4Jy 14,
G4Jy 456, G4Jy 665, G4Jy 671, G4Jy 1491 and G4Jy 1815) in the MeerKAT-2019
subset where MeerKAT images (VLASS image for G4Jy 456) clearly reveal two
unrelated sources. For G4Jy 671 and G4Jy 1491, the morphology in both
NVSS/SUMSS and TGSS was uncertain, and the sources were therefore assigned
‘complex’ morphology (White et al., 2020a, b). Given that the MeerKAT contours
reveal two resolved, unrelated sources, we do not provide the properties (i.e,
$\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}$, $L_{1300~{}\rm{MHz}}$ and linear
size) of these sources since the integrated flux density at 151 MHz for these
sources is the sum of the two resolved, unrelated sources which were blended
by NVSS/SUMSS and TGSS.

G4Jy 456: The extended, ‘double’ morphology is evident in the NVSS and TGSS
contours. To resolve the ambiguity of the host galaxy for this source, we
obtained a higher-resolution radio image from archival VLASS (VLA Sky Survey
[3 GHz and 2.5 arcsec resolution]; Lacy et al. 2020) as the MeerKAT
visibilities for this particular source, and G4Jy 453 are entirely flagged.
The VLASS contours reveals two unrelated sources with ‘single‘ and ‘double’
morphologies, respectively. The radio source with ‘single’ morphology has a
faint mid-infrared host, while the mid-infrared counterpart of the ‘double’
morphology source is AllWISE J042536.76$+$083217.7 at $z=1.1$.

G4Jy 671: In both SUMSS and TGSS contours, the radio morphology of G4Jy 671 is
not clear. The radio morphology could either be a head-tail, core-jet or
‘double’ morphology with asymmetric jets. Hence this radio source was given
the morphology label ‘complex’, host flag ‘u’ and confusion flag ’0’. The 1.3
GHz radio map from MeerKAT shows two unrelated radio sources with triple
(northern source, $S_{1.3~{}\text{GHz}}=1.062~{}Jy$) and ‘single’ (southern
source; $S_{1.3~{}\text{GHz}}=0.237~{}$Jy) morphology, both having a detected
host galaxy in mid-infrared. The host galaxy of the triple morphology source
is AllWISE J074146.64$-$523415.9, and AllWISE J074151.46$-$523524.4 is the
host galaxy of the radio source with ‘single’ morphology. Given that there are
two unrelated sources resolved by MeerKAT, the confusion flag of G4Jy 671
needs to be updated to ‘1’.

G4Jy 1491 (GLEAM J183356$-$394023): Based on the SUMSS and TGSS contours, it
was unclear whether this radio source has a head-tail, ‘double’ or core-jet
morphology (White et al., 2020b, a), and as such, it was assigned a ’complex’
morphology label. The MeerKAT image indicates that there are two unrelated
sources having ‘single’ and ‘triple’ morphology, respectively. The radio
source with ‘single’ morphology towards the north has no detected host galaxy
in the mid-infrared, while the host galaxy of the southern source with
‘triple’ morphology is AllWISE J183359.36$-$394158.3. The integrated flux
density of G4Jy 1491 at 151 MHz is 4.24799 Jy. Given that the MeerKAT image
reveals two unrelated sources, this implies the integrated flux density at 151
MHz for G4Jy 1491 is the sum of the integrated flux densities of the two
sources resolved by MeerKAT. With this in mind, the integrated flux density at
151 MHz of G4Jy 1491 needs to be recalculated and the confusion flag updated
to ‘1’.

##  5 Discussion

The primary aim of this work is to confirm/determine the host galaxy of the
radio emission of 140 G4Jy sources (referred to as the MeerKAT-2019 subset)
via visual inspection of overlays. Identifying the host galaxy of the radio
source is required if we are to merge low-frequency radio data with other
datasets, allowing us to develop a comprehensive, multi-wavelength view of the
many processes occurring within these radio sources. Of the 140 sources making
up the MeerKAT-2019 subset, we have identified the host galaxy of 98 sources.
The host galaxy of G4Jy 1554 was identified through the inspection of the
spectral index map, as the host could not be resolved through the overlay. We,
therefore, assigned host flag ‘i’ to the sources with an identified host
galaxy. Of the remaining 42 sources with no identified host galaxy; 23 are
assigned host flag ‘u’, 18 have a faint mid-infrared host and are assigned
host flag ‘m’, and one source (G4Jy 77) is assigned host flag ‘n’ as this
radio source is identified as a radio halo. These MeerKAT-2019 subset sources
still have an ambiguous host galaxy, even with higher-resolution images from
MeerKAT.

###  5.1 Brightness-weighted centroid to host-galaxy separation

Figure 12 shows the separation between the brightness-weighted centroid
position and host galaxy position for all the sources with an identified host
galaxy, ncmp_GLEAM = 1 and confusion_flag = ‘0’ in the MeerKAT-2019 subset and
the G4Jy Sample. The brightness-weighted centroid position is the weighted
average of the NVSS/SUMSS components positions, where the flux densities of
these components are taken as weights. The NVSS/SUMSS components are
associated with the G4Jy sources if they are within the $3\sigma$ GLEAM
contour. The brightness-weighted centroid position is subject to error, if,
for instance, the G4Jy source has two components in NVSS/SUMSS, and one
component is two times brighter than the other component. Then the brightness-
weighted centroid position will be shifted to the brighter component. This
centroid position will then have a significant offset from the radio core and
the host galaxy position. The median offset in R.A. is
$0.108~{}\mathrm{arcsec}$, and the median offset in Dec. is
$2.322~{}\mathrm{arcsec}$ for the MeerKAT-2019 subset. The median offset in
R.A. and Dec. for the G4Jy Sample is $-0.144~{}\mathrm{arcsec}$ and
$-0.040~{}\mathrm{arcsec}$, respectively.

Overall, 1,215 sources in the G4Jy Sample have an identified host galaxy
(sources with host flag ‘i’), ncmp_GLEAM = ‘1’ and confusion_flag = ‘0’, while
the MeerKAT-2019 subset have 72 sources satisfying these criteria. About 10%
of the sources in the G4Jy Sample have a larger positional offset
($|\Delta\rm{R.A.}|>10"$, $|\Delta\rm{Dec.}|>10"$; sources outside the square
cyan region), while the majority of the sources (57%) in the MeerKAT-2019
subset populate this outer region (see Table 4). We expect to have this larger
positional offset for the MeerKAT-2019 subset because this subset is biased
towards G4Jy sources with ‘complex’ morphologies (i.e., head-tail, WAT and
morphology that is not compact, nor typical symmetric double lobed) evident in
NVSS/SUMSS and TGSS. For instance, sources with head-tail (Figure 6) and WAT
(Figure 7) morphologies have their centroid position at the centre of the
emission while their host galaxy is at the apex of the emission.

Figure 12: The brightness-weighted centroid position to host galaxy position separation plot for the G4Jy Sample (red unfilled triangles) and MeerKAT-2019 subset (blue filled circles) sources with an identified host galaxy, ncmp_GLEAM = ‘1’ and confusion_flag = ‘0’. The brightness-weighted centroid position is the weighted average of the NVSS/SUMSS components positions that are within the $3\sigma$ GLEAM contour of the G4Jy source. The ncmp_GLEAM indicates the number of GLEAM components associated with the G4Jy source. We selected sources with ncmp_GLEAM = ‘1’ because sources with more than one GLEAM component have the same brightness-weighted centroid position. The confusion_flag (either ‘0’ or ‘1’) indicates if the G4Jy source is affected by nearby unrelated emission detected above $6\sigma$ in NVSS/SUMSS and the position of the peak emission of this unrelated source is within the $3\sigma$ GLEAM contour level for the G4Jy source. We set the confusion_flag to ‘0’ to select sources that are not affected by nearby emission. The unusual square shape is because ‘symlog’ (a symmetric log) is used for the x and y scale. This means that it is linear in the region $|\Delta RA|<2"$ and $|\Delta Dec|<2"$ (black square) and a log scale outside this region. | Within the black outline | Between the black and cyan outlines | Outside of the cyan outline
---|---|---|---
| (Approximately $|\Delta\rm{R.A.}|<2^{\prime\prime}$, | (Approximately $2^{\prime\prime}<|\Delta\rm{R.A.}|<10^{\prime\prime}$, | (Approximately $|\Delta\rm{R.A.}|>10^{\prime\prime}$,
|  $|\Delta\rm{Dec.}|<2^{\prime\prime}$) |  $2^{\prime\prime}<|\Delta\rm{Dec.}|<10^{\prime\prime}$) |  $|\Delta\rm{Dec.}|>10^{\prime\prime}$)
G4Jy Sample | 41% (496) | 49% (597) | 10% (122)
MeerKAT-2019 subset | 4% (3) | 39% (28) | 57% (41)
Table 4: The fractions (and numbers) of G4Jy sources, within the full sample
and MeerKAT-2019 subset, with different degrees of offset between the
brightness-weighted centroid position and the host-galaxy position (see
Subsection 5.1). ‘Black outline’ and ‘cyan outline’ are referring to the
demarcations in Figure 12. Note that these G4Jy sources have had the following
criteria applied to them: host flag = ‘i’, confusion flag = ’0’, and
consisting of a single GLEAM component.

###  5.2 WISE colors

The WISE survey’s four bands make it an excellent instrument for investigating
galaxies’ stellar structure and interstellar processes. The two shorter
wavelength bands (W1 and W2) are used to map the stellar mass distribution in
galaxies, whilst the longer wavelength bands (W3 and W4) are used to map warm
dust emission and polycyclic aromatic hydrocarbon emission, which both trace
the present star formation activity and AGN heating. Additionally, the WISE
colours (W1 $-$ W2 and W2 $-$ W3) have been utilised to distinguish various
astronomical objects (e.g., Wright et al. 2010; Mateos et al. 2012; Mingo et
al. 2016), which occupy distinct areas in the WISE colour-colour plot. The
WISE colour-colour plot (Figure 13) is produced in this work to see where the
G4Jy Sample and MeerKAT-2019 subset sources lie. Overall, 1,606 sources from
the G4Jy Sample have host galaxy identified in AllWISE; however, 13 sources
form part of the MeerKAT-2019 subset (see section 3 on the subset
construction). The MeerKAT-2019 subset has 92 sources (including the 13) with
AllWISE identification. Most sources in both the G4Jy Sample (63%) and
MeerKAT-2019 subset (42%) occupy the AGN region (W1 $-$ W2 $>$ 0.5), with a
handful of sources in the AGN wedge (solid black box) defined by Mateos et al.
(2012) for the G4Jy Sample. The G4Jy Sample is dominated by powerful AGN
selected at low frequencies. There is a greater fraction of sources in the
elliptical region for the MeerKAT-2019 subset (34%) than in the G4Jy Sample
(10%).

The elliptical and spiral regions are known to be dominated by low-excitation
radio galaxies (LERGs) as these sources lack a dusty torus, while high-
excitation radio galaxies (HERGs) dominate the region above W1 $-$ W2 $>$ 0.5.
Additionally, LERGs are thought to be connected with FR-I, whereas HERGs are
thought to be related to FRII. Several investigations, however, demonstrate
that there is an overlap between HERG FR-Is and LERG FR-IIs (e.g., Hardcastle
et al. 2009; Best & Heckman 2012; Whittam et al. 2018; Mingo et al. 2019).

Figure 13: WISE colour-colour plot. W1, W2 and W3 correspond to wavelengths
$3.4~{}\mu m$, $4.6~{}\mu m$ and $12~{}\mu m$.

###  5.3 Radio properties

####  5.3.1 Spectral indices

The two-point spectral index ($\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}$)
between two radio frequencies (151 MHz and 1300 MHz) was calculated for
133888We have excluded sources where MeerKAT contours reveal two unrelated
sources (see Section 4.2.2). sources in the MeerKAT-2019 subset, assuming
$S\propto\nu^{\alpha}$, where $S$ is the integrated flux density at frequency
$\nu$. The top panel of Figure 14 shows the overall distribution of the two-
point spectral index between 151 MHz and 1300 MHz for the MeerKAT-2019 subset
compared with the distribution of the two-point spectral index between 151 MHz
and 1400 MHz for the G4Jy Sample (obtained from the G4Jy catalogue). Both
samples have a Gaussian-like distribution. The median spectral index of the
G4Jy Sample and the MeerKAT-2019 subset are $-0.781$ and $-0.819$,
respectively. One-hundred and twenty-seven (127) sources in the MeerKAT-2019
subset have a spectral index in the range
$-1.2<\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}<-0.5$, and the majority of
these sources (74) have a ‘double’ morphology (bottom panel of Figure 14). Six
MeerKAT-2019 subset sources have an ultra-steep spectral index
($\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}<-1.2$), where G4Jy 77 is
identified as a radio halo in the literature, G4Jy 1117 is likely a cluster
relic, G4Jy 113 and G4Jy 1852 have head-tail morphology, and G4Jy 708 has a
WAT morphology. The steep spectra of radio halos and cluster relics is due to
synchrotron and Inverse Compton (IC) losses.

Figure 14: Top panel: The distribution of the spectral index between 151 and
1300 MHz for the MeerKAT-2019 subset and the distribution of the spectral
index between 151 and 1400 MHz for the G4Jy Sample. Bottom panel: The
distribution of the spectral index between 151 and 1300 MHz for the full
MeerKAT-2019 subset and sources with ’single’, ’double’, ’triple’ and
’complex’ morphologies.

####  5.3.2 Radio luminosity

After identifying the host galaxy of the MeerKAT-2019 subset sources, the next
step was to obtain redshifts from existing catalogues/databases, so that we
can calculate the radio luminosity at 1.3 GHz and the linear size of the
MeerKAT-2019 subset. Of 98 radio sources with an identified host galaxy, 51
sources have redshift available from the literature. The 1.3 GHz radio
luminosity ($L_{1.3~{}GHz}$) was calculated for 478 of 51 sources using the
equation:

$L_{1.3~{}{\rm GHz}}=\frac{4\pi D_{L}^{2}S_{1.3~{}{\rm
GHz}}}{(1+z)^{1+\alpha}}$ (2)

where $D_{L}$ is the luminosity distance, $S_{1.3~{}GHz}$ is the integrated
flux density at 1.3 GHz, $z$ is the redshift, and $\alpha$ is the spectral
index between 151 MHz and 1300 MHz. The luminosity distance was calculated
using Ned Wright’s cosmology calculator999http://www.astro.ucla.edu/
wright/CosmoCalc.html, assuming a flat Lambda CDM model ($H_{0}$ = 70
km/s/Mpc, $\Omega_{m}$ = 0.3 and $\Omega_{\Lambda}=0.7$). Figure 15 shows the
distribution of the 1.3 GHz radio luminosity of the sources in the
MeerKAT-2019 subset where the redshift is available. Forty-three sources have
radio luminosities in the order of $10^{24}$ W/Hz to $10^{27}$ W/Hz. These
MeerKAT-2019 subset sources have a typical spectral index in the range
$-1.2<\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}<-0.5$. From Figure 16, we see
that the MeerKAT-2019 subset sources with radio luminosities of the order
$10^{28}$W/Hz have a flat spectral index (
$-0.5<\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}<0.5$). The high luminosity of
flat spectrum sources is due to the relativistic beaming effects. In contrast,
sources with ultra-steep spectral index
($\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}\leq-1.2$) have radio luminosities
of the order $10^{23}$ W/Hz to $10^{24}$ W/Hz.

The bottom panel of Figure 15 shows the distribution of the radio luminosities
at 178 MHz ($L_{178~{}\rm{MHz}}$) of the MeerKAT-2019 subset compared with the
radio luminosities 178 MHz ($L_{178~{}\rm{MHz}}$) of the 3CRR sample. For the
radio luminosity at 178 MHz of the MeerKAT-2019 subset, we obtained (1) the
integrated flux density at 181 MHz and (2) the G4Jy spectral index (calculated
within the GLEAM band) from the G4Jy catalogue and calculated the integrated
flux density at 178 MHz. The MeerKAT-2019 subset sources have typical radio
luminosities of the $10^{25}$ W/Hz to $10^{29}$ W/Hz, while 3CRR sources have
typical radio luminosities of the order $10^{24}$ to $10^{30}$ W/Hz.

Figure 15: Top panel: The distribution of the 1.3 GHz radio luminosity for the
MeerKAT-2019 subset. Bottom panel: For the purpose of comparison with the 3CRR
sample, we calculated the integrated flux density at 178 MHz to obtain the
radio luminosity at 178 MHz for the MeerKAT-2019 subset. Figure 16: Plot of
the two-point spectral index between 151 MHz and 1300 MHz vs. radio luminosity
at 1300 MHz for the MeerKAT-2019 subset.

##  6 Conclusions

This study presents the analysis from studying 140 radio sources in the G4Jy
Sample, referred to as the MeerKAT-2019 subset. Here, we summarise the work
done in this study.

  * •

We constructed $\sim$10’ by 10’ overlays for all the sources in the
MeerKAT-2019 subset. The overlays comprise radio data from GLEAM (200 MHz,
$\sim$2-arcmin resolution), NVSS (1400 MHz, 45-arcsec resolution) /SUMSS (843
MHz, 45-arcsec resolution), TGSS (150 MHz, 25-arcsec resolution), and MeerKAT
(1300 MHz, $\sim$7-arcsec resolution), overlaid onto mid-infrared WISE images
($3.4\mu m$).

  * •

We visually inspected the overlays and classified 11 sources as ‘single’
morphology, 78 as ‘double’, 33 as ‘triple’ and 16 as ‘complex’ morphology
radio sources. We note that most sources have radio morphology of typical
symmetric lobes, while 10 sources have head-tail morphology, 14 have WAT
morphology and 5 have X-, S-/Z-shaped morphology.

  * •

We have identified the host galaxy of 98 sources in the MeerKAT-2019 subset,
where the host galaxies were manually pinpointed through visual inspection.
The 98 sources are assigned host flag ‘i’, following White et al. (2020a, b)
Of the remaining sources with no host galaxy identified, 23 are assigned host
flag ‘u’ as they have an ambiguous host even with higher resolution images
from MeerKAT, G4Jy 77 is assigned host flag ’n’ as it is identified as a radio
halo in the literature, and 18 are assigned host flag ‘m’ as they have a faint
mid-infrared host.

  * •

Based on the brightness-weighted centroid position to host galaxy position
separation plot, the MeerKAT-2019 subset have larger positional offset
compared to the G4Jy Sample. The MeerKAT-2019 subset is biased towards the
G4Jy sources with ‘complex’ morphologies (including those with head-tail and
WAT morphologies). We, therefore, expect to have these larger positional
offsets. For instance, in head-tail sources, the host galaxy position will be
near the apex of the radio emission instead of at the centre, where the
brightness-weighted centroid position is.

  * •

We collected redshifts for 51 radio sources with an identified host galaxy in
the MeerKAT-2019 subset and calculated their radio luminosity at 1.3 GHz.
Radio sources with the steepest spectral index
($\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}<-1.2$) have low radio
luminosities. These MeerKAT-2019 subset sources have typical head-tail
morphology evident in MeerKAT contours. In contrast, sources with flat
spectral index ($-0.5<\alpha^{1300~{}\rm{MHz}}_{151~{}\rm{MHz}}<0.5$) have
higher radio luminosities and redshifts above 1. The higher luminosity of
compact, flat spectrum sources could be the result of relativistic beaming.

This study has demonstrated the significance of a telescope’s angular
resolution and sensitivity for morphological classification and host-galaxy
cross-identification of radio sources at low frequencies. The sensitivity and
angular resolution ($\sim$7 arcsec) of MeerKAT has allowed us to resolve
ambiguous host-galaxy identifications and enigmatic radio morphologies of the
MeerKAT-2019 subset, evident in NVSS/SUMSS (45 arcsec) and/or TGSS (25 arcsec)
images.

These new identifications are crucial for gathering multi-wavelength data,
with 42 of these MeerKAT-2019 subset sources having already been observed
(2020-1-MLT-008; PI: White) with SALT.

## Acknowledgements

The MeerKAT telescope is operated by the South African Radio Astronomy
Observatory, which is a facility of the National Research Foundation, an
agency of the Department of Science and Innovation. We 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. We thank Chris Schollar, Masechaba Sydil
Kupa, and Fernando Camilo for SARAO archive and DOI support. PKS and SVW
acknowledge the financial assistance of the South African Radio Astronomy
Observatory (SARAO; https://www.sarao.ac.za). IH also acknowledges support
from SARAO, which is a facility of the National Research Foundation (NRF), an
agency of the Department of Science and Innovation.

## Data Availability

For updates on the G4Jy Sample, please see https://github.com/svw26/G4Jy. The
MeerKAT images and overlays (DOI: 10.48479/wyab-t838) can be accessed at
https://zenodo.org/communities/g4jy, as well as via the SARAO archive
https://archive-
gw-1.kat.ac.za/public/repository/10.48479/wyab-t838/index.html). The full
Table 3 and overlays for sources discussed in section 4.2 are available as
online supplementary material.

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