The UV continuum of quasars: Models and SDSS spectral slopes

We measure long ( 2200-4000 angstrom) and short ( 1450-2200 angstrom) wavelength spectral slopes alpha ( F-v alpha 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 angstrom and virial mass estimates based on measurements of the Mg II line width and 3000 8 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.