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

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., ${\alpha }_{\nu 12}$ ($1000\sim 2000\mathring{\mathrm{A}}$) and ${\alpha }_{\nu 24}$ ($2000\sim 4000\mathring{\mathrm{A}}$) derived from power law fitting. Generally, the UV spectra slope becomes harder (higher ${\alpha }_{\nu }$) towards higher bolometric luminosity. On the other hand, when quasars are further grouped into luminosity bins, we find both ${\alpha }_{\nu 12}$ and ${\alpha }_{\nu 24}$ 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 ${\alpha }_{\nu }-z$ relation. We build a simple cosmic dust extinction model to quantify the observed reddening tendency and find an effective dust density $n\sigma_v \sim 10^{-5}h~\rm Mpc^{-1}$at$z<1.5$. The other possibilities that could produce such a reddening effect have also been discussed.