MeerKAT observations of Herschel protocluster candidates

ABSTRACT High-redshift protoclusters consisting of dusty starbursts are thought to play an important role in galaxy evolution. Their dusty nature makes them bright in the far-infrared (FIR)/submm but difficult to find in optical/near-infrared (NIR) surveys. Radio observations are an excellent way to study these dusty starbursts, as dust is transparent in the radio and there is a tight correlation between the FIR and radio emission of a galaxy. Here, we present MeerKAT 1.28 GHz radio imaging of three Herschel candidate protoclusters, with a synthesized beam size of $\sim 7.5\, \mathrm{ arcsec}\times 6.6$ arcsec and a central thermal noise down to $4.35~\mu$Jy beam−1. Our source counts are consistent with other radio counts with no evidence of overdensities. Around 95 per cent of the Herschel sources have 1.28 GHz IDs. Using the Herschel$250~\mu$m primary beam size as the searching radius, we find 54.2 per cent Herschel sources have multiple 1.28 GHz IDs. Our average FIR-radio correlation coefficient $q_{250\mu \text{m}}$ is $2.33\pm 0.26$. Adding $q_{250\mu \text{m}}$ as a new constraint, the probability of finding chance-aligned sources is reduced by a factor of $\sim 6$, but with the risk of discarding true identifications of radio-loud/quiet sources. With accurate MeerKAT positions, we cross-match our Herschel sources to optical/NIR data followed by photometric redshift estimations. By removing $z< 1$ sources, the density contrasts of two of the candidate protoclusters increase, suggestive of them being real protoclusters at $z> 1$. There is also potentially a $0.9< z< 1.2$ overdensity associated with one candidate protocluster. In summary, photometric redshifts from radio-optical cross-identifications have provided some tentative evidence of overdensities aligning with two of the candidate protoclusters.