MIGHTEE: the evolving radio luminosity functions of star-forming galaxies to z ∼ 4.5 and the cosmic history of star formation

Abstract

A key question in extragalactic astronomy is how the star formation rate density (SFRD) evolves over cosmic time. A powerful way of addressing this question is using radio-continuum observations, where the radio waves are unaffected by dust and are able to reach sufficient resolution to resolve individual galaxies. We present an investigation of the 1.4 GHz radio luminosity functions (RLFs) of star-forming galaxies (SFGs) and active galactic nuclei (AGNs) using deep radio continuum observations in the COSMOS and XMM–LSS fields, covering a combined area of (Formula presented). These data enable the most accurate measurement of the evolution in the SFRD from mid-frequency radio continuum observations. We model the total RLF as the sum of evolving SFG and AGN components, negating the need for individual source classification. We find that the SFGs have systematically higher space densities at fixed luminosity than found in previous radio studies, but consistent with more recent studies with MeerKAT. We attribute this to the excellent low-surface brightness sensitivity of MeerKAT. We then determine the evolution of the SFRD. Adopting the far-infrared–radio correlation results in a significantly higher SFRD at (Formula presented), compared to combined UV and far-infrared measurements. However, using more recent relations for the correlation between star formation rate and radio luminosity, based on full spectral energy distribution modelling, can resolve this apparent discrepancy. Thus, radio observations provide a powerful method of determining the total SFRD in the absence of dust-sensitive far-infrared data.