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

We compare broad emission-line profiles and estimate line ratios for all major emission lines between Ly alpha and H beta 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)(H beta) versus Fe ii(opt) strength plane that is significantly different from the others. High signal-to-noise ratio H beta emission-line profiles are used as templates for modelling the other lines (Ly alpha, C iv lambda 1549, He ii lambda 1640, Al iii lambda 1860, Si iii]lambda 1892 and Mg ii lambda 2800). 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 n(e) similar to 1012 cm-3, ionization parameter U similar to 10-2 and column density N-c greater than or similar to 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 ii lambda 2800 and Fe ii emission also arises in this region. The BLUE emission (log n(e) similar to 10, log U similar to -1 and log N-c < 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 (H beta) greater than or similar to 4000 km s-1 that belong to the so-called population B of our 4DE1 space. Population A quasars [FWHM (H beta) less than or similar to 4000 km s-1] are dominated by BC emission in H beta and BLUE component emission in C iv lambda 1549 and other high ionization lines. 4DE1 appears to be the most useful current context for revealing and unifying spectral diversity in type-1 quasars.