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

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 ($\sim 1/3$).