GTC Spectra of z ~ 2.3 Quasars: Comparison with Local Luminosity
  Analogues
  [Abridged] Context: The advent of 8-10m class telescopes makes possible for
the first time detailed comparison of quasars with similar luminosity and very
different redshifts. Aims: A search for z-dependent gradients in line emission
diagnostics and derived physical properties by comparing, in a narrow
bolometric luminosity range (log L ~ 46.1 +/- 0.4 [\ergss]), some of the most
luminous local (z < 0.6) quasars with some of the lowest luminosity sources yet
found at redshift z = 2.1 ~ 2.5. Method: Spectra for 22 high z sources were
obtained with the 10.4m Gran Telescopio Canarias (GTC) while the HST (largely
FOS) archive provides a low redshift control sample. Comparison is made in the
context of the 4D Eigenvector 1 formalism meaning that we divide both source
samples into high accreting Population A and low accreting Population B
sources. Results: CIV 1549 shows very similar properties at both redshifts
confirming at high redshift the CIV profile differences between Pop. A and B
that are well established in local quasars. The CIV blueshift that appears
quasi- ubiquitous in higher L sources is found in only half (Pop. A) of quasars
observed in both of our samples. A CIV evolutionary Baldwin effect is certainly
disfavored. We find evidence for lower metallicity in the GTC sample that may
point toward a gradient with z. No evidence for a gradient in black hole mass
or Eddington ratio is found. Conclusions: Spectroscopic differences established
at low redshift are also present in much higher redshift quasars. Given that
our samples involve sources with very similar luminosity the evidence for a
systematic metallicity decrease, if real, points toward an evolutionary effect.
Our samples appear representative of a slow evolving quasar population likely
present at all redshifts.
