2018A&A...618A.179M

Context. Quasars radiating at extreme Eddington ratios (hereafter xA quasars) are likely a prime mover of galactic evolution and have been hailed as potential distance indicators. Their properties are still scarcely known.
Aims. We aim to test the effectiveness of the selection criteria defined on the "4D Eigenvector 1" (4DE1) for identifying xA sources. We provide a quantitative description of their rest-frame UV spectra (1300-2200Å) in the redshift range 2≤z≤2.9, with a focus on major emission features.
Methods. Nineteen extreme quasar candidates were identified using 4DE1 selection criteria applied to SDSS spectra: AlIIIλ1860/SiIIIλ1892≥0.5 and CIIIλ1909/SiIIIλ1892≤1. The emission line spectra was studied using multicomponent fits of deep spectroscopic observations (S/N≥40-50; spectral resolution ~=250km/s) obtained with the OSIRIS at Gran Telescopio Canarias (GTC).
Results. GTC spectra confirm that almost all of these quasars are xA sources with very similar properties. We provide spectrophotometric and line profile measurements for the SiIVλ1397+OIV, CIVλ1549+HeIIλ1640, and the 1900Å blend. This last feature is found to be predominantly composed of AlIIIλ1860, SiIIIλ1892 and FeIII emission features, with weak CIIIλ1909. The spectra can be characterized as very low ionization (ionization parameter, logU~-3), a condition that explains the significant FeIII emission observed in the spectra. xA quasars show extreme properties in terms of CIVλ1549 equivalent width and blueshift amplitudes. CIVλ1549 shows low equivalent width, with a median value of 15Å≤30Å for the most sources), and high or extreme blueshift amplitudes (-5000≤c(1/2)≤-1000km/s). Weak-lined quasars appear as extreme xA quasars and not as an independent class. The CIVλ1549 high amplitude blueshifts coexists in all cases save one with symmetric and narrower AlIIIλ1860 and SiIIIλ1892 profiles. Estimates of the Eddington ratio using the AlIIIλ1860 FWHM as a virial broadening estimator are consistent with the ones of a previous xA sample.
Conclusions. xA quasars show distinguishing properties that make them easily identifiable in large surveys and potential "standard candles" for cosmological applications. It is now feasible to assemble large samples of xA quasars from the latest data releases of the SDSS. We provide evidence that AlIIIλ1860 could be associated with a low-ionization virialized subsystem, supporting previous suggestions that AlIII is a reliable virial broadening estimator.