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The main sequence offers a method for the systematization of quasar spectralproperties. Extreme FeII emitters (or extreme Population A, xA) are believed tobe sources accreting matter at very high rates. They are easily identifiablealong the quasar main sequence, in large spectroscopic surveys over a broadredshift range. The very high accretion rate makes it possible that massiveblack holes hosted in xA quasars radiate at a stable, extremeluminosity-to-mass ratio. After reviewing the basic interpretation of the mainsequence, we report on the possibility of identifying virial broadeningestimators from low-ionization line widths, and provide evidence of theconceptual validity of redshift-independent luminosities based on virialbroadening for a known luminosity-to-mass ratio.

The quasar main sequence and its potential for cosmology