A New Method to Separate Star‐forming from AGN Galaxies at Intermediate Redshift: The Submillijansky Radio Population in the VLA‐COSMOS Survey

Abstract

We explore the properties of the submillijansky radio population at 20\ncm by applying a newly developed optical color-based method to separate\nstar-forming (SF) from active galactic nucleus (AGN) galaxies at\nintermediate redshifts (z<~1.3). Although optical rest-frame colors\nare used, our separation method is shown to be efficient and not biased\nagainst dusty starburst galaxies. This classification method has been\ncalibrated and tested on a local radio-selected optical sample. Given\naccurate multiband photometry and redshifts, it carries the potential to\nbe generally applicable to any galaxy sample where SF and AGN galaxies\nare the two dominant populations. In order to quantify the properties of\nthe submillijansky radio population, we have analyzed ~2,400 radio\nsources, detected at 20 cm in the VLA-COSMOS survey; 90% of these have\nsubmillijansky flux densities. We classify the objects into (1) star\ncandidates, (2) quasi-stellar objects, (3) AGN, (4) SF, and (5)\nhigh-redshift (z>1.3) galaxies. We find, for the composition of the\nsubmillijansky radio population, that SF galaxies are not the dominant\npopulation at submillijansky flux levels, as previously often assumed,\nbut that they make up an approximately constant fraction of 30%-40% in\nthe flux density range of ~50 μJy to 0.7 mJy. In summary, based on\nthe entire VLA-COSMOS radio population at 20 cm, we find that the radio\npopulation at these flux densities is a mixture of roughly 30%-40% of SF\nand 50%-60% of AGN galaxies, with a minor contribution (~10%) of QSOs.