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., $\alpha_{\nu12}$
($1000\sim 2000 \rm\AA$) and $\alpha_{\nu24}$ ($2000 \sim 4000 \rm\AA$) 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 $\alpha_{\nu12}$
and $\alpha_{\nu24}$ 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 $\alpha_\nu-z$
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.
