A search for high-redshift radio galaxy candidates with the MeerKAT Galaxy Cluster Legacy Survey

Abstract

High-redshift radio galaxies (HzRGs) are among the earliest and most massive galaxies whose powerful radio emission allows us to probe the large-scale structure, galaxy evolution, and the epoch of reionisation. We present a new sample of HzRG candidates constructed using the 1.28 GHz data from the MeerKAT Galaxy Cluster Legacy Survey (MGCLS) Data Release 1 (DR1). We select a subsample of MGCLS fields with good dynamic range, astrometric accuracy, and full optical and near-infrared coverage from the Dark Energy Spectroscopic Instrument Legacy Imaging Surveys (LS) Data Release 10.1 and the All-sky Wide Infrared Survey Explorer (AllWISE), respectively. We use the likelihood ratio method to cross-match the unresolved radio sources against LS and AllWISE. For radio sources lacking optical and near-infrared counterparts, we apply an ultra-steep spectrum (USS) selection criterion of α < −1.0, and a 5σ signal-to-noise ratio cut, resulting in a total of 1,819 HzRG candidates, spread over a non-contiguous sky area of ~43 deg2, with 824 USS sources and 995 faint sources (0.036 mJy < S1.28 GHz < 0.278 mJy) lacking sufficient spectral data as potential candidates. We achieve complete spectral coverage above ~0.28 mJy and reliable in-band spectral index measurements down to 44 μJy, making this the faintest HzRG candidate search at GHz frequencies to date. Confirming the nature and redshifts of these candidates will require dedicated spectroscopic and deep near-infrared follow-up observations.