The Quasar Mass-Luminosity Plane I: A Sub-Eddington Limit for Quasars
  We use 62185 quasars from the Sloan Digital Sky Survey Data Release 5 sample
to explore the relationship between black hole mass and luminosity. Black hole
masses were estimated based on the widths of their H{\beta}, MgII and CIV lines
and adjacent continuum luminosities using standard virial mass estimate scaling
laws. We find that, over the range 0.2 < z < 4.0, the most luminous low-mass
quasars are at their Eddington luminosity, but the most luminous high-mass
quasars in each redshift bin fall short of their Eddington luminosities, with
the shortfall of the order of 10 or more at 0.2 < z < 0.6. We examine several
potential sources of measurement uncertainty or bias and show that none of them
can account for this effect. We also show the statistical uncertainty in virial
mass estimation to have an upper bound of ~0.15 dex, smaller than the 0.4 dex
previously reported. We also examine the highest mass quasars in every redshift
bin in an effort to learn more about quasars that are about to cease their
luminous accretion. We conclude that the quasar mass-luminosity locus contains
a number of new puzzles that must be explained theoretically.
