Selection of highly-accreting quasars: Spectral properties of FeII
  emitters not belonging to extreme Population A
  The quasar class of extreme Population A (xA) (also known as super-Eddington
accreting massive black holes, SEAMBHs) has been hailed as potential distance
indicators for cosmology. The aim of this paper is to define tight criteria for
their proper identification starting from the main selection criterion
$R_{FeII} > 1 $, and to identify potential intruders not meeting the selection
criteria, but nonetheless selected as xA because of the coarseness of automatic
searches. Inclusion of the spurious xA sources may dramatically increase the
dispersion in the Hubble diagram of quasars obtained from virial luminosity
estimates. We studied a sample of 32 low-$z$ quasars originally selected from
the SDSS DR7 as xA. All of them show moderate-to-strong FeII emission and the
wide majority strong absorption features in their spectra are typical of fairly
evolved stellar populations. We performed a simultaneous fit of a host galaxy
spectrum, AGN continuum, FeII template and emission lines to spectra, using the
fitting technique based on ULySS, full spectrum fitting package. For sources in
our sample (of spectral types corresponding to relatively low Eddington ratio),
we found an overall consistency between narrow components of H$\beta$ and
[OIII]$\lambda\lambda$4959, 5007 line shifts and the mean stellar velocity
obtained from the host galaxy fit (within $\lesssim |60|$ km/s). Only one
source in our sample qualify as xA source. We found high fraction of host
galaxy spectrum (in half of the sample even higher then 40\%). When absorption
lines are prominent, and the fraction of the host galaxy is high, SSP is
mimicking FeII, and that may result in a mistaken identification of FeII
spectral features. We have identified several stellar absorption lines that,
along with the continuum shape, may lead to an overestimate of $R_{FeII}$, and
therefore to the misclassification of sources as xA sources.
