The two-component radio luminosity function of quasi-stellar objects: Star formation and active galactic nucleus

Abstract

Despite decades of study, it remains unclear whether there are distinct radio-loud and radio-quiet populations of quasi-stellar objects (QSOs). Early studies were limited by inhomogeneous QSO samples, inadequate sensitivity to probe the radio-quiet population, and degeneracy between redshift and luminosity for flux-density-limited samples. Our new 6GHz Expanded Very Large Array (EVLA) observations allow us for the first time to obtain nearly complete (97%) radio detections in a volume-limited color-selected sample of 179 QSOs more luminous than Mi = -23 from the Sloan Digital Sky Survey (SDSS) Data Release Seven in the narrow redshift range 0.2 < z < 0.3. The dramatic improvement in radio continuum sensitivity made possible with the new EVLA allows us, in 35 minutes of integration, to detect sources as faint as 20 μJy, or log [L 6 GHz(W Hz-1)] 21.5 at z = 0.25, well below the radio luminosity, log [L 6(W Hz-1)] 22.5, that separates star-forming galaxies from radio-loud active galactic nuclei (AGNs) driven by accretion onto a supermassive black hole. We calculate the radio luminosity function (RLF) for these QSOs using three constraints: (1) EVLA 6GHz observations for log [L 6(W Hz-1)] < 23.5, (2) NRAO-VLA Sky Survey observations for log [L 6(W Hz-1)] > 23.5, and (3) the total number of SDSS QSOs in our volume-limited sample. We show that the RLF can be explained as a superposition of two populations, dominated by AGNs at the bright end and star formation in the QSO host galaxies at the faint end. © 2011. The American Astronomical Society. All rights reserved.