THE TWO-COMPONENT RADIO LUMINOSITY FUNCTION OF QUASI-STELLAR OBJECTS: STAR FORMATION AND ACTIVE GALACTIC NUCLEUS

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 new6GHzExpandedVery 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 M-i = -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 mu Jy, or log[L-6GHz(W Hz(-1))] approximate to 21.5 at z = 0.25, well below the radio luminosity, log[L-6(W Hz(-1))] approximate to 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 6 GHz observations for log[L6(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.