The UV Continuum of Quasars: Models and SDSS Spectral Slopes

• Published in: The Astrophysical journal Vol. 668; no. 2; pp. 682 - 698
• Main Authors: Davis, Shane W, Woo, Jong-Hak, Blaes, Omer M
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 20-10-2007; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Continuum; Active galaxies; Quasars; quasars: general; black hole physics; Black holes; accretion, accretion disks; galaxies: active; Spectral model; Cosmology; Accretion disks
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/521393

We measure long (2200-4000 AA) and short (1450-2200 AA) wavelength spectral slopes a (F sub(v) proportional to v alpha ) for quasar spectra from the Sloan Digital Sky Survey. The long- and short-wavelength slopes are computed from 3646 and 2706 quasars with redshifts in the z = 0.76 -1.26 and 1.67-2.07 ranges, respectively. We calculate mean slopes after binning the data by monochromatic luminosity at 2200 AA and virial mass estimates based on measurements of the Mg II line width and 3000 AA continuum luminosity. We find little evidence for mass-dependent variations in the mean slopes, but a significant luminosity-dependent trend in the near-UV spectral slopes is observed with larger (bluer) slopes at higher luminosities. The far-UV slopes show no clear variation with luminosity and are generally lower (redder) than the near-UV slopes at comparable luminosities, suggesting a slightly concave quasar continuum shape. We compare these results with Monte Carlo distributions of slopes computed from models of thin accretion disks, accounting for uncertainties in the mass estimates. The model slopes produce mass-dependent trends which are larger than observed, although this conclusion is sensitive to the assumed uncertainties in the mass estimates. The model slopes are also generally bluer than observed, and we argue that reddening by dust intrinsic to the source or host galaxy may account for much of the discrepancy.