Quasar Evolution and the Baldwin Effect in the Large Bright Quasar Survey

Abstract

From a large homogeneous sample of optical/UV emission line measurements for 993 quasars from the Large Bright Quasar Survey (LBQS), we study correlations between emission-line equivalent width and both rest-frame ultraviolet luminosity (i.e., the Baldwin effect) and redshift. Our semiautomated spectral fitting accounts for absorption lines, fits blended iron emission, and provides upper limits to weak emission lines. Use of a single large, well-defined sample and consistent emission-line measurements allows us to sensitively detect many correlations, most of which have been previously noted. A new finding is a significant Baldwin effect in UV iron emission. Further analysis reveals that the primary correlation of iron emission strength is probably with redshift, implying an evolutionary rather than a luminosity effect. We show that for most emission lines with a significant Baldwin effect, and for some without, evolution dominates over luminosity effects. This may reflect evolution in abundances, in cloud covering factors, or overall cloud conditions such as density and ionization. We find that in our sample, a putative correlation between Baldwin effect slope and the ionization potential is not significant. Uniform measurements of other large quasar samples will extend the luminosity and redshift range of such spectral studies and provide even stronger tests of spectral evolution.