High metal content of highly accreting quasars: Analysis of an extended sample

Abstract

Context. We present an analysis of UV spectra of quasars at intermediate redshifts believed to belong to the extreme Population A (xA), aimed to estimate the chemical abundances of the broad line emitting gas. We follow the approach described in a previous work, extending the sample to 42 sources. Aim. Our aim is to test the robustness of the analysis carried out previously, as well as to confirm the two most intriguing results of this investigation: evidence of very high solar metallicities and a deviation of the relative abundance of elements with respect to solar values. Methods. The basis of our analysis are multicomponent fits in three regions of the spectra centered at 1900, 1550, and 1400 Å in order to deblend the broad components of AlIIIλ1860, C III]λ1909, CIVλ1549, He IIλ1640, and SiIVλ1397+OIV]λ1402 and their blue excess. Results. By comparing the observed flux ratios of these components with the same ratios predicted by the photoionization code CLOUDY, we found that the virialized gas (broad components) presents a metallicity (Z) higher than 10 Z⊙. For nonvirialized clouds, we derived a lower limit to the metallicity around ∼5 Z⊙ under the assumption of chemical composition proportional to the solar one, confirming the previous results. We especially relied on the ratios between metal lines and He IIλ1640. This allowed us to confirm systematic differences in the solar-scaled metallicity derived from the lines of Aluminum and Silicon, and of Carbon, with the first being a factor ≈2 higher. Conclusions. For luminous quasars accreting at high rates, high Z values are likely, but Z-scaled values are affected by the possible pollution due to highly-enriched gas associated with the circumnuclear star formation. The high-Z values suggest a complex process involving nuclear and circumnuclear star formation and an interaction between nuclear compact objects and an accretion disk, possibly with the formation of accretion-modified stars.