Active Galactic Nucleus and Starburst Radio Emission from Optically Selected Quasi-stellar Objects

We used the 1.4 GHz NVSS to study radio sources in two color-selected QSO samples: a volume-limited sample of 1313 QSOs defined by M _i < -23 in the redshift range 0.2 < z < 0.45 and a magnitude-limited sample of 2471 QSOs with m _r <= 18.5 and 1.8 < z < 2.5. About 10% were detected above the 2.4 mJy NVSS catalog limit and are powered primarily by active galactic nuclei (AGNs). The space density of the low-redshift QSOs evolves as ρvprop(1 + z)⁶. In both redshift ranges the flux-density distributions and luminosity functions of QSOs stronger than 2.4 mJy are power laws, with no features to suggest more than one kind of radio source. Extrapolating the power laws to lower luminosities predicts the remaining QSOs should be extremely radio quiet, but they are not. Most were detected statistically on the NVSS images with median peak flux densities S _p(mJy beam^-1) ≈ 0.3 and 0.05 in the low- and high-redshift samples, corresponding to spectral luminosities log [L _{1.4 GHz}(W Hz^-1)] ≈ 22.7 and 24.1, respectively. We suggest that the faint radio sources are powered by star formation at rates \dot{M} \sim 20\ M_\odot \ yr^{-1} in the moderate luminosity (median langM _irang ≈ -23.4) low-redshift QSOs and \dot{M} \sim 500\ M_\odot \ yr^{-1} in the very luminous (langM _irang ≈ -27.5) high-redshift QSOs. Such luminous starbursts [langlog (L _IR/L _⊙)rang ~ 11.2 and 12.6, respectively] are consistent with "quasar mode" accretion in which cold gas flows fuel both AGN and starburst.