An Apparent Redshift Dependence of Quasar Continuum: Implication for Cosmic Dust Extinction?

Abstract

We investigate the luminosity and redshift dependence of the quasar continuum by means of composite spectrum using a large non-BAL radio-quiet quasar sample drawn from the Sloan Digital Sky Survey. Quasar continuum slopes in the UV-Opt band are measured at two different wavelength ranges, i.e., $α_{\nu12}$ ($1000\sim 2000 \rmà $) and $α_{\nu24}$ ($2000 \sim 4000 \rmà $) derived from power law fitting. Generally, the UV spectra slope becomes harder (higher $α_ν$) towards higher bolometric luminosity. On the other hand, when quasars are further grouped into luminosity bins, we find both $α_{\nu12}$ and $α_{\nu24}$ show significant anti-correlation with redshift (i.e., quasar continuum becomes redder towards higher redshift). We suggest that the cosmic dust extinction is very likely the cause of this observed $α_ν-z$ relation. We build a simple cosmic dust extinction model to quantify the observed reddening tendency and find an effective dust density $nσ_v \sim 10^{-5}h~\rm Mpc^{-1}$ at $z<1.5$. The other possibilities that could produce such a reddening effect have also been discussed.

6 pages, 5 figures; published in ApJL