An Ultradeep Multiband Very Large Array Survey of the Faint Radio Sky (COSMOS-XS): New Constraints on the Optically Dark Population

Abstract

Attempts to trace star formation with rest-frame UV/optical observations at redshifts z > 2 are affected by the presence of potentially substantial, yet uncertain, dust attenuation. Recent studies have demonstrated the existence of a population of galaxies that are virtually invisible in the observed optical/near-infrared (NIR) due to dust obscuration, but which could contribute substantially to the star formation history at 2 4). Therefore, we conservatively assume these sources lie between z = 2 and z = 5. We derive the radio luminosity function (LF) for the sample with and without “optically dark” sources by fixing the faint and bright end shape of the radio LF to the local values and allowing for luminosity evolution. By integrating both LFs, we estimate the contribution of the “optically dark” galaxies to the radio SFRD to be $\sim {15}_{-7}^{+7} \% $ at z ∼ 5. This is consistent with constraints from NIR-dark and UV-dark sources, while being in disagreement with some estimates using H -dropouts. This result implies that “optically dark” sources play a nonnegligible role at high redshift.

Keywords

• Galaxy evolution
• Star formation
• Radio continuum emission