VLT/ISAAC spectra of the Hβ region in intermediate-redshift quasars^*

III. Hβ broad-line profile analysis and inferences about BLR structure

¹ INAF, Osservatorio Astronomico di Padova, Vicolo dell' Osservatorio 5, 35122 Padova, Italy e-mail: paola.marziani@oapd.inaf.it; massimo.calvani@oapd.inaf.it
² Department of Physics and Astronomy, University of Alabama, Tuscaloosa, AL 35487, USA e-mail: jsulenti@bama.ua.edu,zamfi001@bama.ua.edu
³ INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy e-mail: giovanna.stirpe@oabo.inaf.it

Received: 7 August 2008
Accepted: 18 November 2008

Abstract

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 ≈ 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.

Methods. We compare this data with two large low-redshift () 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 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]4959, 5007 (from W ≈ 15 to ~1 Å) with increasing source bolometric luminosity (from 43 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 . 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 , while the broad component remains constant, resulting in an apparent “Baldwin effect” with equivalent width decreasing from W ~ 80 to ~20 Å 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 ≈ 4000 km s^-1, which we relate to a critical Eddington ratio (≈ 0.2±0.1).