Amy Kimball (CASS)

Amy Kimball Colloquium -- The Radio Luminosity Function of Optically Selected QSOs: Star Formation and AGNs

Abstract

Despite decades of study, it has remained unclear whether there are distinct radio-loud and radio-quiet populations of quasi-stellar objects (QSOs). The upgraded Karl G. Jansky Very Large Array has allowed us for the first time to obtain nearly complete radio detections in a volume-limited, optically selected sample of QSOs in the redshift range 0.2 < z < 0.3. We were able to detect sources as faint as log(L_6GHz) ~ 10^21.5 L_sun, well below the radio luminosity that separates star-forming galaxies from radio-loud active galactic nuclei (AGNs) driven by accretion onto a super-massive black hole. Extrapolation of the radio-AGN luminosity function to low luminosities predicts that most QSOs should be extremely radio quiet, but our results show that they are not. The resulting radio luminosity function of QSOs can be explained by thecombination of two radio emission components, with AGN emission dominating at the bright end and starbursting host galaxies dominating at the faint end. The hypothesis that faint radio sources in most QSOs are powered by starbursts can be tested with ALMA sub-millimeter observations, to determine whether they follow the radio/far-infrared correlation obeyed by almost all star-forming galaxies, or with high resolution VLBI imaging to look for AGN-jet structure.