The GLEAMing of the first supermassive black holes: II. A new sample of
  high-redshift radio galaxy candidates
  While unobscured and radio-quiet active galactic nuclei are regularly being
found at redshifts $z > 6$, their obscured and radio-loud counterparts remain
elusive. We build upon our successful pilot study, presenting a new sample of
low-frequency-selected candidate high-redshift radio galaxies (HzRGs) over a
sky area twenty times larger. We have refined our selection technique, in which
we select sources with curved radio spectra between 72-231 MHz from the
GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. In
combination with the requirements that our GLEAM-selected HzRG candidates have
compact radio morphologies and be undetected in near-infrared $K_{\rm s}$-band
imaging from the Visible and Infrared Survey Telescope for Astronomy
Kilo-degree Infrared Galaxy (VIKING) survey, we find 51 new candidate HzRGs
over a sky area of approximately 1200 deg$^2$. Our sample also includes two
sources from the pilot study: the second-most distant radio galaxy currently
known, at $z=5.55$, with another source potentially at $z \sim 8$. We present
our refined selection technique and analyse the properties of the sample. We
model the broadband radio spectra between 74 MHz and 9 GHz by supplementing the
GLEAM data with both publicly available data and new observations from the
Australia Telescope Compact Array at 5.5 and 9 GHz. In addition, deep $K_{\rm
s}$-band imaging from the High-Acuity Widefield $K$-band Imager (HAWK-I) on the
Very Large Telescope and from the Southern Herschel Astrophysical Terahertz
Large Area Survey Regions $K_{\rm s}$-band Survey (SHARKS) is presented for
five sources. We discuss the prospects of finding very distant radio galaxies
in our sample, potentially within the epoch of reionisation at $z \gtrsim 6.5$.
