THE LUMINOSITY DEPENDENCE OF QUASAR UV CONTINUUM SLOPE: DUST EXTINCTION SCENARIO

Abstract

ABSTRACT We investigate the UV continuum slope α of a large quasar sample from SDSS DR7. By using specific continuum windows, we build two samples at lower ( ) and higher ( ) redshifts, which correspond to the continuum slopes at longer (near-UV) and shorter (far-UV) rest wavelength ranges, respectively. Overall, the average continuum slopes are −0.36 and −0.51 for and with similar dispersions . For both samples, we confirm the luminosity dependence of the continuum slope, i.e., fainter quasars have redder spectra. We further find that both and have a common upper limit ( ), which is almost independent of the quasar luminosity . This finding implies that the intrinsic quasar continuum (or the bluest quasar), at any luminosity, obeys the standard thin-disk model. We propose that the other quasars with redder α are caused by the reddening from the dust locally. With this assumption, we employ the dust extinction scenario to model the observed relation. We find that a typical value of (depending on the types of extinction curve) of the quasar local dust is enough to explain the discrepancy of α between the observation ( ) and the standard accretion disk model prediction ( ).