Broad-line region physical conditions along the quasar eigenvector 1 sequence

Abstract

We compare broad emission-line profiles and estimate line ratios for all major emission lines between Lyα and Hβ in a sample of six quasars. The sources were chosen with two criteria in mind: the existence of high-quality optical and ultraviolet spectra and the possibility of sampling the spectroscopic diversity in the 4D eigenvector 1 (4DE1) context. In the latter sense, each source occupies a region (bin) in the full width at half-maximum (FWHM)(Hβ) versus Fe <scp>ii</scp><inf>opt</inf> strength plane that is significantly different from the others. High signal-to-noise ratio Hβ emission-line profiles are used as templates for modelling the other lines (Lyα, C <scp>iv</scp>λ1549, He <scp>ii</scp>λ1640, Al <scp>iii</scp>λ1860, Si <scp>iii</scp>]λ1892 and Mg <scp>ii</scp>λ2800). We can adequately model all broad lines assuming the existence of three components distinguished by blueshifted, unshifted and redshifted centroids [indicated as a blue component (BLUE), broad component (BC) and very broad component (VBC), respectively]. BC (electron density <it>n</it><inf>e</inf>∼ 1012 cm−3, ionization parameter <it>U</it>∼ 10−2 and column density <it>N</it><inf>c</inf>≳ 1023 cm−2) is present in almost all type-1 quasars and therefore corresponds most closely to the classical broad-line emitting region (the reverberating component). The bulk of Mg <scp>ii</scp>λ2800 and Fe <scp>ii</scp> emission also arises in this region. The BLUE emission (log <it>n</it><inf>e</inf>∼ 10, log <it>U</it>∼−1 and log <it>N</it><inf>c</inf> < 23) arises in less optically thick gas; it is often thought to arise in an accretion disc wind. The least understood component involves the VBC (high ionization and large column density), which is found in no more than half (but almost all radio-loud) type-1 quasars and luminous Seyfert nuclei. It is perhaps the most distinguishing characteristic of quasars with FWHM (Hβ) ≳ 4000 km s−1 that belong to the so-called population B of our 4DE1 space. Population A quasars [FWHM (Hβ) ≲ 4000 km s−1] are dominated by BC emission in Hβ and BLUE component emission in C <scp>iv</scp>λ1549 and other high ionization lines. 4DE1 appears to be the most useful current context for revealing and unifying spectral diversity in type-1 quasars.