The ultraviolet continuum of quasars -I. The shape of the continuum, continuum reddening and intervening absorption.

Abstract

We present ultraviolet data for a sample of 81 quasars covering a wide redshift and luminosity range. After allowing for the effect of absorption due to intervening clouds, the intrinsic optical and ultraviolet continuum for quasars is found to steepen towards shorter wavelengths from ~ 1 micron to ~800 A by ~1 in spectral index. The mean ratio of the predicted flux at 2 keV, calculated by extrapolating the continuum shortward of Lyα to that observed = 5+/-3. The mean shape of the ultraviolet continuum is very similar for the radio and non-radio selected quasars. The ultraviolet continuum above Lyα is found to get slightly harder with increasing redshift, suggesting possible evolution. No conclusive evidence is found for any intrinsic continuum reddening in the majority of known quasars. The increase with increasing redshift of the observed continuum depression between the Lyα and Lyβ/OVI emission lines relative to the continuum above Lyα implies evolution in the comoving number density of the intervening absorbing clouds.