GTC spectra ofz≈ 2.3 quasars: comparison with local luminosity analogs

Abstract

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 M BH or L/L Edd 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/L Edd . 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 M BH or L/L Edd with redshift. The two samples appear representative of a slowly evolving quasar population that is most likely present at all redshifts.

Low- and High-zHighly Accreting Quasars in the 4D Eigenvector 1 Context

“…We have here reviewed recent results obtained especially in [61,62,117,111,116]. Highly accreting quasars can be identified and studied most easily in the 4DE1 context originally proposed by Sulentic et al [112].…”

Spectral Evolution in High Redshift Quasars From the Final Baryon Oscillation Spectroscopic Survey Sample

“…In any flux-limited sample, the minimum luminosity increases with increasing redshift, so the range of allowed black hole masses and Eddington ratios able to account for a given luminosity at a higher redshift is then narrowed relative to a lower redshift (e.g., Figure 2 of Sulentic et al 2014). To inspect spectral variations, it is important to choose parameters that can be derived from both the low and high redshift ranges of the sample in identical fashion.…”

Highly accreting quasars: The SDSS low-redshift catalog

“…Thus, spectra belonging to the same bin are expected to have very similar characteristics (e.g., line profiles and UV line ratios). The MS organizes the diverse quasar properties and makes it possible to identify quasars in different accretion states (Sulentic et al 2014a). In this work we present in particular those which are characterized by having R FeII > 1 (i.e., belonging to the A3 and A4 bins), most likely associated with high accretion rates (e.g., Sulentic et al 2014b;Du et al 2016).…”