2009A&A...495...83M

We present new VLT ISAAC spectra for 30 quasars, which we combine with previous data to yield a sample of 53 intermediate-redshift (z≃0.9-3.0) sources. The sample is used to explore properties of prominent lines in the hβ spectral region of these very luminous quasars. We compare this data with two large low-redshift (z<0.8) samples in a search for trends over almost 6dex in source luminosity. We find two major trends: (1) a systematic increase in minimum FWHM hβ with luminosity (discussed in a previous paper). This lower FWHM envelope is best fit by assuming that the narrowest sources radiate near the Eddington limit, show line emission from a virialized cloud distribution, and obey a well-defined broad line region size vs. luminosity relation. (2) A systematic decrease in equivalent width of [oIII]λλ4959,5007 (from W≃15 to ∼1Å) with increasing source bolometric luminosity (from logL_bol≃43 to logL_bol≃49). Other identified trends require differntiating between so-called Population A and Bsources. We generate median composite spectra in six luminosity bins to maximize S/N. Population A sources show reasonably symmetric Lorentzian hβ profiles at all luminosities, while Pop. B sources require two component fits involving an unshifted broad and a redshifted very broad component. Very broad hβ increases in strength with increasing logL_bol, while the broad component remains constant, resulting in an apparent ``Baldwin effect'' with equivalent width decreasing from W∼80 to ∼20Å over our sample luminosity range. The roughly constant equivalent width shown by the hβ very broad component implies production in optically-thick, photoionized gas. The onset of the redshifted very broad component appears to be a critical change that occurs near the Pop. A-B boundary at FWHM hβ≃4000km/s, which we relate to a critical Eddington ratio (≃0.2±0.1).