Highly accreting quasars: The SDSS low-redshift catalog
  The most highly accreting quasars (xA) are of special interest in studies of
the physics of AGNs and host galaxy evolution. Quasars accreting at high rates
(L/LEdd $\sim$ 1) hold promise for use as 'standard candles': distance
indicators detectable at very high redshift. However, their observational
properties are largely unknown. A large sample of xA can clarify the main
properties of quasars radiating near L/LEdd $\sim$ 1 in the Hb spectral range
for redshift $\lt$ 0.8. We use selection criteria derived from 4DE1 studies to
identify and analyze spectra for a sample of 334 candidate sources identified
from the SDSS DR7 database. The source spectra were chosen to show a ratio
RFeII between the FeII emission blend at 4570 \AA\ and Hb, RFeII $\gt$ 1.
Composite spectra were analyzed for systematic trends as a function of FeII
strength, line width, and [OIII] strength. We introduced tighter constraints on
the signal-to-noise ratio (S/N) and RFeII values that allowed us to isolate
sources most likely to be extreme accretors. We provide a database of detailed
measurements. Analysis of the data allows us to confirm that Hb shows a
Lorentzian function with a FWHM of Hb $\lt$ 4000 km s-1. Systematic [OIII] blue
shifts, as well as a blueshifted component in Hb are revealed. We interpret the
blueshifts as related to the signature of outflowing gas from the quasar
central engine. The FWHM of Hb is still affected by the blueshifted emission,
however, the effect is non-negligible if the FWHM Hb is used as a 'virial
broadening estimator'. A effect of the viewing angle on Hb broadening, deriving
a correction for those sources that shows disagreement between virial and
concordance cosmology luminosity values. The scatter between concordance
cosmology and virial luminosity estimates can be reduced if a correction for
orientation effects is included in the FWHM Hb value.
