We discuss the optical and radio properties of ∼30,000 FIRST (radio, 20 cm, sensitive to 1 mJy) sources positionally associated within 1.5 arcsec with an SDSS (optical, sensitive to r * ∼22.2) source in 1230 deg 2 of sky. The matched sample represents ∼30% of the 108,000 FIRST sources and 0.1% of the 2.5 × 10 7 SDSS sources in the studied region. SDSS spectra are available for 4,300 galaxies and 1,154 quasars from the matched sample, and for a control sample of 140,000 galaxies and 20,000 quasars in 1030 deg 2 of sky. This large and unbiased catalog of optical identifications provides much firmer statistical footing for existing results and allows several new findings.The majority (83%) of the FIRST sources identified with an SDSS source brighter than r * =21 are optically resolved; the fraction of resolved objects among the matched sources is a function of the radio flux, increasing from ∼50% at the bright end to ∼90% at the FIRST faint limit. Nearly all optically unresolved radio sources have non-stellar colors indicative of quasars. We estimate an upper limit of ∼5% for the fraction of quasars with broad-band optical colors indistinguishable from those of stars. The distribution of quasars in the radio flux -optical flux plane supports the existence of the "quasar radio-dichotomy"; 8±1% of all quasars with i * <18.5 are radio-loud and this fraction seems independent of redshift and optical luminosity. The radio-loud quasars have a redder median color by 0.08±0.02 mag, and show a 3 times larger fraction of objects with extremely red colors.FIRST galaxies represent 5% of all SDSS galaxies with r * <17.5, and 1% for r * <20, and are dominated by red (u * − r * >2.22) galaxies, especially those with r * >17.5. Magnitude and redshift limited samples show that radio galaxies have a different optical luminosity distribution than non-radio galaxies selected by the same criteria; when galaxies are further separated by their colors, this result remains valid for both blue and red galaxies. For a given optical luminosity and redshift, the observed optical colors of radio-galaxies are indistinguishable from those of all SDSS galaxies selected by identical criteria. The distributions of radio-to-optical flux ratio are similar for blue and red galaxies in redshift-limited samples; this similarity implies that the difference in their luminosity functions, and resulting selection effects, are the dominant cause for the preponderance of red radio galaxies in flux-limited samples. The fraction of radio galaxies whose emission line ratios indicate an AGN (30%) rather than a starburst origin is 6 times larger than the corresponding fraction for all SDSS galaxies (r * <17.5). We confirm that the AGN-to-starburst galaxy number ratio increases with radio flux, and find that radio emission from AGNs is more concentrated than radio emission from starburst galaxies.

We present a new sample of 116 double-peaked Balmer line Active Galactic Nuclei (AGNs) selected from the Sloan Digital Sky Survey. Double-peaked emission lines are believed to originate in the accretion disks of AGN, a few hundred gravitational radii (R G ) from the supermassive black hole. We investigate the properties of the candidate disk emitters with respect to the full sample of AGN over the same redshifts, focusing on optical, radio and X-ray flux, broad line shapes and narrow line equivalent widths and line flux-ratios. We find that the disk-emitters have medium luminosities (∼10 44 erg s −1 ) and FWHM on average six times broader than the AGN in the parent sample. The double-peaked AGN are 1.6 times more likely to be radio-sources and are predominantly (76%) radio quiet, with about 12% of the objects classified as LINERs. Statistical comparison of the observed double-peaked line profiles with those produced by axisymmetric and non-axisymmetric accretion disk models allows us to impose constraints on accretion disk parameters. The observed Hα line profiles are consistent with accretion disks with inclinations smaller than 50 • , surface emissivity slopes of 1.0-2.5, outer radii larger than ∼2000R G , inner radii between 200-800R G , and local turbulent broadening of 780-1800 km s −1 . The comparison suggests that 60% of accretion disks require some form of asymmetry (e.g., elliptical disks, warps, spiral shocks or hot spots).

The first gamma-ray burst (GRB) confirmed to be bright enough to be seen with the naked eye, GRB 080319B at redshift z = 0.937, allowed for exquisite follow-up observations across the electromagnetic spectrum. We present our detailed optical and infrared observations of the afterglow, consisting of over 5000 images starting 51 s after the GRB trigger, in concert with our own analysis of