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Description
The advent of 8-10m class telescopes for the first time makes it possible to compare in detail quasars with similar luminosity and very different redshifts. We conducted a search for z-dependent gradients in line-emission diagnostics and derived physical properties by comparing, in a narrow bolometric luminosity range (logL~46.1+/-0.4erg/s), 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. Moderate signal-to-noise ratio spectra for 22 high-redshift sources were obtained with the 10.4m Gran Telescopio Canarias (GTC), for which the HST (largely the Faint Object Spectrograph) archive provides a low-redshift control sample. We compared the spectra in the context of the 4D Eigenvector 1 formalism, meaning that we divided both source samples into highly accreting population A and population B sources accreting at a lower rate. CIV {lambda}1549, the strongest and most reliable diagnostic line, shows very similar properties at both redshifts, which confirms the CIV {lambda}1549 profile differences at high redshift between populations A and B, which are well established in local quasars. The CIV {lambda}1549 blueshift that appears quasi-ubiquitous in higher L sources is found in only half (population A) of the quasars observed in the two samples. A CIV {lambda}1549 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. Spectroscopic differences established at low z are also present in much higher redshift quasars. Our results on the CIV {lambda}1549 blueshift suggest that it depends both on source luminosity and Eddington ratio. 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 are not large enough to effectively constrain possible changes of black hole mass or Eddington ratio with redshift. The two samples appear representative of a slowly evolving quasar population that is most likely present at all redshifts.
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