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

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 < z < 6. Here, we make use of ultradeep 3 GHz Karl G. Jansky Very Large Array observations from the COSMOS-XS survey to investigate the contribution 20of radio-selected "optically dark" galaxies (undetected to a depth of K_S ∼ 25.9 mag) to the cosmic star formation rate density (SFRD). We identify 19 such "optically dark" sources and utilize recent deblended far-infrared photometry to determine photometric redshifts based on IR and radio information for 11 of them. Through stacking, we infer that the remaining eight sources reside predominantly at high redshift (z > 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.