MIGHTEE/COSMOS-3D: the discovery of three spectroscopically confirmed radio-selected star-forming galaxies at z = 4.9–5.6

Abstract

Radio observations offer a dust-independent probe of star formation and active galactic nucleus (AGN) activity, but sufficiently deep data are required to access the cross-over luminosity between these processes at high redshift ((Formula presented)). We present three spectroscopically confirmed high-redshift radio sources (HzRSs) detected at 1.3 GHz at (Formula presented) –5.6, with radio luminosities spanning (Formula presented) –(Formula presented). These sources were first identified as high-redshift candidates through spectral energy distribution (SED) fitting of archival Hubble, James Webb Space Telescope (JWST) NIRCam + MIRI, and ground-based photometry, and then spectroscopically confirmed via the (Formula presented) emission line using wide-field slitless spectroscopy from JWST COSMOS-3D. The star formation rates (SFRs) measured from SED fitting, the (Formula presented) flux, and the 1.3 GHz luminosity, span (Formula presented) –(Formula presented), demonstrating broad agreement between these SFR tracers. We find that these three sources lie either on or 0.5–1.0 dex above the star-forming main sequence at (Formula presented) –6 and have undergone a recent burst of star formation. The sources have extended rest-ultraviolet (UV)/optical morphologies with no evidence for a dominant point source component, indicating that an AGN is unlikely to dominate their rest-UV and optical emission. Two of the sources have complex, multicomponent rest-frame UV/optical morphologies, suggesting that their starbursts may be triggered by merging activity. These HzRSs open up a new window towards probing radio emission powered by star formation alone at (Formula presented), representing a remarkable opportunity to begin tracing star formation, independent of dust, in the early Universe.