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

TL;DR

This study analyzes quasar spectra from SDSS, revealing a redshift-dependent reddening likely caused by cosmic dust extinction, with implications for understanding quasar properties and cosmic dust distribution.

Contribution

It demonstrates a significant redshift dependence of quasar continuum slopes and attributes this to cosmic dust extinction, introducing a simple model to quantify dust density at z<1.5.

Findings

Quasar UV spectra become redder at higher redshifts.

Continuum slopes correlate with luminosity and redshift.

Cosmic dust extinction likely causes the observed reddening.

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., $\alpha_{\nu12}$ ($1000\sim 2000 \rm\AA$) 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.