Volume 570, October 2014
|Number of page(s)||24|
|Published online||29 October 2014|
Instituto de Astrofisíca de Andalucía, IAA-CSIC,
Glorieta de la Astronomia s/n,
e-mail: firstname.lastname@example.org; email@example.com; firstname.lastname@example.org
2 INAF, Osservatorio Astronomico di Padova, vicolo dell’ Osservatorio 5, 35122 Padova, Italy
3 Instituto de Astronomía, Universidad Nacional Autónoma de México, 04510 Mexico D.F., Mexico
4 Instituto Nacional de Astrofísica, Óptica y Electrónica, Tonantzintla, Puebla, Mexico
Accepted: 16 June 2014
Context. The advent of 8−10 m class telescopes for the first time makes it possible to compare in detail quasars with similar luminosity and very different redshifts.
Aims. We conducted 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 [erg s-1]), 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.
Methods. Moderate signal-to-noise ratio spectra for 22 high-redshift sources were obtained with the 10.4 m 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.
Results. Civλ1549, the strongest and most reliable diagnostic line, shows very similar properties at both redshifts, which confirms the Civλ1549 profile differences at high redshift between populations A and B, which are well established in local quasars. The Civλ1549 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 Civλ1549 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 MBH or L/LEdd is found.
Conclusions. Spectroscopic differences established at low z are also present in much higher redshift quasars. Our results on the Civλ1549 blueshift suggest that it depends both on source luminosity and L/LEdd. 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 MBH or L/LEdd with redshift. The two samples appear representative of a slowly evolving quasar population that is most likely present at all redshifts.
Key words: quasars: emission lines / quasars: supermassive black holes / ISM: abundances / line: profiles / cosmology: observations
Full Figs. 4 and 5 and Appendix A are available in electronic form at http://www.aanda.org
Reduced spectra as FITS files are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (184.108.40.206) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/570/A96
© ESO, 2014
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