High Metal Content of Highly Accreting Quasars

We present an analysis of UV spectra of 13 quasars believed to belong to extreme Population A (xA) quasars, aimed at the estimation of the chemical abundances of the broad-line-emitting gas. Metallicity estimates for the broad-line-emitting gas of quasars are subject to a number of caveats; xA sources with the strongest Fe ii emission offer several advantages with respect to the quasar general population, as their optical and UV emission lines can be interpreted as the sum of a low-ionization component roughly at quasar rest frame (from virialized gas), plus a blueshifted excess (a disk wind), in different physical conditions. Capitalizing on these results, we analyze the component at rest frame and the blueshifted one, exploiting the dependence of several intensity line ratios on metallicity Z. We find that the validity of intensity line ratios as metallicity indicators depends on the physical conditions. We apply the measured diagnostic ratios to estimate the physical properties of sources such as density, ionization, and metallicity of the gas. Our results confirm that the two regions (the low-ionization component and the blueshifted excess) of different dynamical conditions also show different physical conditions and suggest metallicity values that are high, and probably the highest along the quasar main sequence, with Z ∼ 20-50 Z o˙, if the solar abundance ratios can be assumed constant. We found some evidence of an overabundance of aluminum with respect to carbon, possibly due to selective enrichment of the broad-line-emitting gas by supernova ejecta.