AGN and Starburst Radio Emission from Optically Selected QSOs
  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 AGNs. The space density of the
low-redshift QSOs evolves as rho proportional to (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) ~ 0.3 and 0.05 in the low- and high-redshift samples,
corresponding to 1.4 GHz spectral luminosities log[L(W/Hz)] ~ 22.7$ and 24.1,
respectively. We suggest that the faint radio sources are powered by star
formation at rates ~20 M_sun per year in the moderate luminosity (median M_i ~
-23.4) low-redshift QSOs and ~500 M_sun per year in the very luminous (M_i} ~
-27.5) high-redshift QSOs. Such luminous starbursts [ log(L / L_sun) ~ 11.2 and
12.6, respectively] are consistent with "quasar mode" accretion in which cold
gas flows fuel both AGN and starburst.
