VLT/ISAAC spectra of the Hβ region in intermediate-redshift quasars III. Hβ broad-line profile analysis and inferences about BLR structure

Aims. We present new VLT ISAAC spectra for 30 quasars, which we combine with previous data to yield a sample of 53 intermediate-redshift (z approximate to 0.9-3.0) sources. The sample is used to explore properties of prominent lines in the H beta spectral region of these very luminous quasars. Methods. We compare this data with two large low-redshift (z < 0.8) samples in a search for trends over almost 6dex in source luminosity. Results. We find two major trends: (1) a systematic increase in minimum FWHM H beta 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]lambda lambda 4959, 5007 (from W approximate to 15 to similar to 1 angstrom) with increasing source bolometric luminosity (from log L(bol) approximate to 43 to log L(bol) approximate to 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 H beta profiles at all luminosities, while Pop. B sources require two component fits involving an unshifted broad and a redshifted very broad component. Very broad H beta increases in strength with increasing log L(bol), while the broad component remains constant, resulting in an apparent Baldwin effect with equivalent width decreasing from W similar to 80 to similar to 20 angstrom over our sample luminosity range. The roughly constant equivalent width shown by the H beta 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 H beta approximate to 4000 km s(-1), which we relate to a critical Eddington ratio (approximate to 0.2 +/- 0.1).