Optical and Radio Properties of Extragalactic Sources Observed by the FIRST Survey and the Sloan Digital Sky Survey

We discuss the optical and radio properties of 30,000 FIRST sourcespositionally associated with an SDSS source in 1230 deg$^2$ of sky. Themajority (83%) of the FIRST sources identified with an SDSS source brighterthan r=21 are optically resolved. We estimate an upper limit of 5% for thefraction of quasars with broad-band optical colors indistinguishable from thoseof stars. The distribution of quasars in the radio flux -- optical flux planesupports the existence of the "quasar radio-dichotomy"; 8% of all quasars withi<18.5 are radio-loud and this fraction seems independent of redshift andoptical luminosity. The radio-loud quasars have a redder median color by 0.08mag, and a 3 times larger fraction of objects with red colors. FIRST galaxiesrepresent 5% of all SDSS galaxies with r<17.5, and 1% for r<20, and aredominated by red galaxies. Magnitude and redshift limited samples show thatradio galaxies have a different optical luminosity distribution than non-radiogalaxies selected by the same criteria; when galaxies are further separated bytheir colors, this result remains valid for both blue and red galaxies. Thedistributions of radio-to-optical flux ratio are similar for blue and redgalaxies in redshift-limited samples; this similarity implies that thedifference in their luminosity functions, and resulting selection effects, arethe dominant cause for the preponderance of red radio galaxies in flux-limitedsamples. We confirm that the AGN-to-starburst galaxy number ratio increaseswith radio flux, and find that radio emission from AGNs is more concentratedthan radio emission from starburst galaxies (abridged).Comment: submitted to AJ, color gif figures, PS figures available from ivezic@astro.princeton.ed