Double-peaked low-ionization emission lines in active galactic nuclei

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 AGNs, 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 AGNs 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 ergs s ^-1) and FWHM on average 6 times broader than the AGNs in the parent sample. The double-peaked AGNs 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 nonaxisymmetric 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 of (200-800)R _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).