Special Astrophysics Seminar: Amy Kimball, NRAO

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 new, sensitive 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 from the Sloan Digital Sky Survey (SDSS) in the narrow 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.  The resulting radio luminosity function of QSOs can be explained by the combination of two radio emission components, with AGN emission dominating at the bright end and starbursting host galaxies dominating at the faint end.

In a much larger flux-limited sample of z~2 QSOs, we obtained statistical radio detections from NRAO-VLA Sky Survey (NVSS) images to estimate the QSO luminosity function at the peak of AGN activity in the Universe.  Extrapolation of the radio-AGN luminosity function to low luminosities predicts that most QSOs should be extremely radio quiet, but the distribution of statistical detections indicates that they are not.  The average radio luminosity of the more radio-quiet QSOs at z~2 corresponds to a typical star-formation rate of ~500 M_sun/year, consistent with feedback models of galaxy evolution, in which cold gas flows power both an AGN and a starburst.

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.