Searching for high-z radio galaxies with the MGCLS
  We present results from a search for high-redshift radio galaxy (H$z$RG)
candidates using 1.28 GHz data in the Abell 2751 field drawn from the MeerKAT
Galaxy Cluster Legacy Survey (MGCLS). We use the H$z$RG criteria that a radio
source is undetected in all-sky optical and infrared catalogues, and has a very
steep radio spectrum. We cross-match the radio catalogue against
multi-wavelength galaxy catalogues from DECaLS and AllWISE. For those radio
sources with no multi-wavelength counterpart, we further implement a radio
spectral index criterium of $\alpha < -1$, using in-band spectral index
measurements from the wide-band MeerKAT data. Using a 5$\sigma$ signal-to-noise
cut on the radio flux densities, we find a total of 274 HzRG candidates: 179
ultra-steep spectrum sources, and 95 potential candidates which cannot be ruled
out as they have no spectral information available. The spectral index
assignments in this work are complete above a flux density of 0.3 mJy, at least
an order of magnitude lower than existing studies in this frequency range or
when extrapolating from lower frequency limits. Our faintest HzRG candidates
with and without an in-band spectral index measurement have a 1.28\,GHz flux
density of 57 $\pm$ 8 $\mu$Jy and 68 $\pm$ 13 $\mu$Jy, respectively. Although
our study is not complete down to these flux densities, our results indicate
that the sensitivity and bandwidth of the MGCLS data makes them a powerful
radio resource to search for H$z$RG candidates in the Southern sky, with 20 of
the MGCLS pointings having similar image quality as the Abell~2751 field and
full coverage in both DECaLS and AllWISE. Data at additional radio frequencies
will be needed for the faintest source populations, which could be provided in
the near future by the MeerKAT UHF band (580 -- 1015 MHz) at a similar
resolution ($\sim$ 8-10 arcsec).
