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 the. 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 similar to 2000 angstrom) and alpha(nu 24) (2000 similar to 4000 angstrom) derived from a. power-law fitting. Generally, the UV spectra slope becomes harder (higher a.) toward higher bolometric luminosity. On the other hand, when quasars are further grouped into luminosity bins, we find that both alpha(nu 12) and alpha(nu 24) show significant anti-correlations with redshift (i.e., the quasar continuum becomes redder toward 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(nu) similar to 10(-5)h Mpc(-1) at z < 1.5. The other possibilities that could produce such a reddening effect have also been discussed.