The luminosity dependence of quasar UV continuum slope: dust extinction scenario

TL;DR

This study analyzes the UV continuum slopes of quasars from SDSS DR7, revealing a luminosity dependence and suggesting dust extinction as the cause of redder spectra, consistent with the standard thin disk model.

Contribution

It demonstrates that dust reddening explains the observed UV continuum slope variations, supporting the standard thin disk model as the intrinsic quasar spectrum.

Findings

Fainter quasars have redder UV spectra.

A common upper limit of ~1/3 for the continuum slope is luminosity-independent.

Dust extinction with E(B-V)~0.1-0.3 mag explains the observed slope discrepancy.

Abstract

We investigate the UV continuum slope $\alpha$ of a large quasar sample from SDSS DR7. By using specific continuum windows, we build two samples at lower ($0.71<z<1.19$) and higher ($1.90<z<3.15$) redshifts, which correspond to the continuum slopes at longer (NUV) and shorter (FUV) rest wavelength ranges respectively. Overall, the average continuum slopes are $-0.36$ and $-0.51$ for $\alpha_{\rm NUV}$ and $\alpha_{\rm FUV}$ with similar dispersions $\sigma_{\alpha} \sim 0.5$. For both samples, we confirm the luminosity dependence of the continuum slope, i.e., fainter quasars have redder spectra. We further find that both $\alpha_{\rm NUV}$ and $\alpha_{\rm FUV}$ have a common upper limit ($\sim 1/3$) which is almost independent of the quasar luminosity $L_{\rm bol}$. This finding implies that the intrinsic quasar continuum (or the bluest quasar), at any luminosity, obey the standard thin disk model. We propose that the other quasars with redder $\alpha$ are caused by the reddening from the dust {\it locally}. With this assumption, we employ the dust extinction scenario to model the observed $L_{\rm bol}-\alpha$ relation. We find that, a typical value of $E(B-V)\sim0.1$ to $0.3$ mag (depending on the types of extinction curve) of the quasar {\it local} dust is enough to explain the discrepancy of $\alpha$ between the observation ($\sim-0.5$) and the standard accretion disk model prediction ($\sim 1/3$).