Phenomenology of broad emission lines in active galactic nuclei

Broad emission lines hold fundamental clues about the kinematics and struct ure of the central regions in AGN. In this article we review the most robus t line profile properties and correlations emerging from the best data avai lable. We identify fundamental differences between the profiles of radio-qu iet and radio-loud sources as well as differences between the high- and low -ionization lines, especially in the radio-quiet majority of AGN. An Eigenv ector 1 correlation space involving FWHM H beta, W(FeIIopt)/W(H beta), and the soft X-ray spectral index provides optimal discrimination between all p rincipal AGN types (from narrow-line Seyfert 1 to radio galaxies). Both opt ical and radio continuum luminosities appear to be uncorrelated with the E1 parameters. We identify two populations of radio-quiet AGN: population A s ources (with FWHM(H beta) less than or similar to 4000 km s(-1), generally strong pen emission and a soft X-ray excess) show almost no parameter space overlap with radio-loud sources. Population B shows optical properties lar gely indistinguishable from radio-loud sources, including usually weak Pen emission, FWHM(H beta) greater than or similar to 4000 km s(-1) and lack of a soft X-ray excess. There is growing evidence that a fundamental paramete r underlying Eigenvector 1 may be the luminosity-to-mass ratio of the activ e nucleus (L/M), with source orientation playing a concomitant role.