The UV Continuum of Quasars: Models and SDSS Spectral Slopes

TL;DR

This study measures UV spectral slopes of quasars from SDSS, revealing luminosity-dependent trends and comparing observations with accretion disk models, highlighting potential dust reddening effects.

Contribution

It provides the first large-scale analysis of quasar UV spectral slopes across different luminosities and compares these with theoretical accretion disk models.

Findings

Luminosity correlates with bluer near UV slopes.

Far UV slopes are generally redder and show no luminosity dependence.

Model predictions differ from observations, possibly due to dust reddening.

Abstract

We measure long (2200-4000 ang) and short (1450-2200 ang) wavelength spectral slopes \alpha (F_\nu proportional to \nu^\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 z=1.67-2.07 ranges, respectively. We calculate mean slopes after binning the data by monochromatic luminosity at 2200 ang and virial mass estimates based on measurements of the MgII line width and 3000 ang 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, though 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.