High metal content of highly accreting quasars

TL;DR

This study analyzes UV spectra of extreme Population A quasars to estimate their metallicity, revealing very high metal content, especially in the broad line emitting gas, and suggesting possible overabundance of certain elements due to supernova enrichment.

Contribution

It provides the first detailed metallicity estimates for the different components of extreme Population A quasars, highlighting their exceptionally high metal content and element overabundance.

Findings

Metallicity in the broad line region is very high, possibly exceeding 10 times solar.

Different physical conditions exist in the low-ionization and blueshifted components.

Evidence suggests overabundance of Aluminium relative to Carbon, indicating selective enrichment.

Abstract

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, although present data suggest the possibility of an increase along the quasar main sequence along with prominence of optical Fe II emission. Extreme Population A 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. Specifically, in terms of ionization parameter, cloud density, metallicity and column density. 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 blue-shifted 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 $Z \gtrsim 10 Z_{\odot}$. We found some evidence of an overabundance of Aluminium with respect to Carbon, possibly due to selective enrichment of the broad line emitting gas by supernova ejecta.