The chemical properties of low-redshift QSOs

TL;DR

This study examines the chemical properties of low-redshift QSOs using UV spectra, revealing that broad-line region metallicity correlates with Eddington ratio and suggesting a link between galactic metal enrichment and black hole activity.

Contribution

It provides new insights into the relationship between BLR metallicity and AGN properties at low redshift, highlighting differences from high-redshift QSOs.

Findings

BLR metallicity correlates with Eddington ratio.

Weak dependence of metallicity on black hole mass.

Metal enrichment linked to black hole accretion activity.

Abstract

We investigate the chemical properties of low-z QSOs, using archival UV spectra obtained with the HST and IUE for a sample of 70 Palomar-Green QSOs at z < 0.5. By utilizing the flux ratio of UV emission lines (i.e., NV /CIV, (SiIV+OIV])/CIV, and NV/HeII) as metallicity indicators, we compare broad-line region (BLR) gas metallicity with AGN properties, i.e., black hole mass, luminosity, and Eddington ratio. We find that BLR metallicity correlates with Eddington ratio while the dependency on black hole mass is much weaker. Although these trends of low-z AGNs appear to be different from those of high-z QSOs, the difference between low-z and high-z samples is partly caused by the limited dynamical range of the samples. We find that metal enrichment at the center of galaxies is closely connected to the accretion activity of black holes and that the scatter of metallicity correlations with black hole mass increases over cosmic time.