Quasars as cosmological probes:: The ionizing continuum, gas metallicity, and the Wλ-L relation

Using a realistic model for line emission from the broad emission line regions of quasars, we are able to reproduce the previously observed correlations of emission-line ratios with the shape of the spectral energy distribution (SED). In agreement with previous studies, we find that the primary driving force behind the Baldwin effect (W-lambda proportional to L-beta, beta < 0) is a global change in the SED with quasar luminosity, in that more luminous quasars must have characteristically softer ionizing continua. This is completely consistent with observations that show (1) a correlation between L-uv, alpha(ox), and alpha(uvx); (2) correlations of SED shape-sensitive line ratios with alpha(ox), alpha(uvx), and L-uv; and (3) correlations between line equivalent widths and alpha(ox), alpha(uvx), and L-uv. However, to explain the complete lack of a correlation in the W-lambda(N v)-L-uv diagram, we propose that the more luminous quasars have characteristically larger gas metallicities (Z). As a secondary element, nitrogen's rapidly increasing abundance with increasing Z compensates for the losses in W-lambda(N v) emitted by gas illuminated by softer continua in higher luminosity quasars. A characteristic relationship between Z and L has an impact on the W-lambda-L-uv relations for other lines as well. For a fixed SED, an increasing gas metallicity reduces the W-lambda of the stronger metal lines (the gas cools), as well as that of Ly alpha and especially He II (because of the increasing metal opacity), while the weaker lines (e.g., C III] lambda 1909) generally respond positively. The interplay between the effects of a changing SED and Z with L results in the observed luminosity-dependent spectral variations. All of the resulting dependences on L-uv are within the range of the observed slopes.