VLT/ISAAC Spectra of the Hbeta Region in Intermediate-Redshift Quasars III. Hbeta Broad Line Profile Analysis and Inferences about BLR Structure

TL;DR

This study analyzes the Hbeta spectral region in 53 intermediate-redshift quasars, revealing trends in line widths and equivalent widths related to luminosity and accretion properties, and distinguishes between different quasar populations based on spectral features.

Contribution

It provides new spectral data and insights into the structure of the broad line region in quasars, highlighting differences between populations and the impact of luminosity and Eddington ratio.

Findings

Minimum FWHM Hbeta increases with luminosity.

Equivalent width of [OIII] decreases with luminosity.

Distinct spectral profiles for Population A and B quasars.

Abstract

[Abridged] 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 Hbeta spectral region of these very luminous quasars. We find two major trends: (1) a systematic increase of minimum FWHM Hbeta with luminosity (discussed in a previous paper). This lower FWHM envelope is best fit 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 of equivalent width of [OIII] (from W ~ 15 to ~ 1 A) with increasing source bolometric luminosity (from log L ~ 43 to log L ~ 49). Further identified trends required discrimination between so-called Population A and B sources. We generate median composite spectra in six luminosity bins Pop. A sources show reasonably symmetric Lorentzian Hbeta profiles at all luminosities while Pop. B sources require two component fits involving an unshifted broad and a redshifted very broad component. Very broad Hbeta increases in strength with increasing L while the broad component remains constant resulting in an apparent "Baldwin effect" with equivalent width decreasing from W ~ 80 to ~ 20 A over our sample luminosity range. The roughly constant equivalent width shown by the Hbeta 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 Hbeta ~ 4000 km/s which we relate to a critical Eddington ratio (~ 0.2 +/- 0.1).