Extreme quasars as distance indicators in cosmology
  Quasars accreting matter at very high rates (known as extreme Population A
[xA] quasars, possibly associated with super-Eddington accreting massive black
holes) may provide a new class of distance indicators covering cosmic epochs
from present day up to less than 1 Gyr from the Big Bang. At a more fundamental
level, xA quasars are of special interest in studies of the physics of AGNs and
host galaxy evolution. However, their observational properties are largely
unknown. xA quasars can be identified in relatively large numbers from major
optical surveys over a broad range of redshifts, thanks to selection criteria
defined from the systematic changes along the quasars main sequence. It has
been possible to build a sample of about 250 quasars at low and intermediate
redshift, and larger samples can be easily selected from the latest data
releases of the Sloan Digital Sky Survey. A large sample can clarify the main
properties of xA quasars which are expected - unlike the general population of
quasars - to radiate at an extreme, well defined Eddington ratio with small
scatter. As a result of the small scatter in Eddington ratio shown by xA
quasars, we propose a method to derive the main cosmological parameters based
on redshift-independent "virial luminosity" estimates from measurements of
emission line widths, roughly equivalent to the luminosity estimates based from
line width in early and late type galaxies. A major issue related to the
cosmological application of the xA quasar luminosity estimates from line widths
is the identification of proper emission lines whose broadening is
predominantly virial over a wide range of redshift and luminosity. We report on
preliminary developments using the AlIII 1860 intermediate ionization line and
the Hydrogen Balmer line H-beta as virial broadening estimators, and we briefly
discuss the perspective of the method based on xA quasars.
