Broad Line Region Physical Conditions along the Quasar Eigenvector 1
  Sequence
  [Abridged] 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 UV spectra as well as the possibility to sample the
spectroscopic diversity in the 4D Eigenvector 1 context . In the latter sense
each source occupies a region (bin) in the FWHM(H-beta) vs. optical FeII
strength plane that is significantly different from the others. High S/N H-beta
emission line profiles are used as templates for modeling the other lines
(Ly-alpha, CIV 1549, HeII 1640, Al III 1860, Si III] 1892, and Mg II 2800). We
can adequately model all broad lines assuming the existence of three components
distinguished by blueshifted, unshifted and redshifted centroids (indicated as
blue, broad and very broad component respectively). The broad component (high
electron density, low ionization parameter; high column density) is present in
almost all type-1 quasars and therefore corresponds most closely to the
classical broad line emitting region (the reverberating component). The blue
component emission (lower electron density; high ionization; low column
density) arises in less optically thick gas; it is often thought to arise in an
accretion disk wind. The least understood component involves the very broad
component (high ionization and large column density). It is perhaps the most
distinguishing characteristic of quasars with FWHM H-beta > 4000 km/s that
belong to the so-called Population B of our 4DE1 space. Population A quasars
(FWHM H-beta < 4000 km/s) are dominated by broad component emission in H-beta
and blue component emission in CIV 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.
