The Shape of the Ultraviolet Continuum of Quasars and Intergalactic Dust

Abstract

We present measurements of the shape of the UV continuum of a large number of quasars. We investigate earlier claims that the near-UV spectral index α_UV_ gets flatter with increasing look-back time. We also investigate the dependence of α_UV_ on luminosity and radio type. The UV spectra of radio-quiet quasars are systematically slightly flatter than the spectra of radio-loud quasars. We find that α_UV_ is remarkably independent of luminosity over more than four orders of magnitude. A definite lack of correlation of Lyα equivalent width with α_UV_ strongly suggests that there is no scatter in the intrinsic UV spectral index. The observed scatter in α_UV_ is largely due to the measuring errors. We believe that the small scatter remaining after allowance for these can be explained by a small variation in the reddening [+/- 0.1 in E(B- V)]. We point out that this is in good agreement with the variation in intrinsic reddening predicted from emission lines and the optical continuum. We confirm that there is a slight dependence of α_UV_ on redshift, but we believe that this could be due mostly to an optical selection effect (and partly to the radio-loud/radio quiet difference). The slight redshift dependence we find is quite the opposite of that predicted by Wright's hypothesis of intergalactic reddening.