Far-UV Fe Emission as Proxy of Eddington Ratios

TL;DR

This study investigates the potential of far-UV Fe emission lines as indicators of Eddington ratios in quasars, revealing significant correlations and distinct line patterns linked to accretion properties.

Contribution

It demonstrates that far-UV Fe II emission can serve as a new proxy for Eddington ratios, expanding diagnostic tools for quasar accretion analysis.

Findings

Far-UV Fe II emission correlates with optical Fe II strength.

High Eddington ratio quasars show stronger low-ionization UV lines.

High-ionization lines are broader and weaker in high Eddington ratio quasars.

Abstract

The Eddington ratio is a key parameter that governs the diversity of quasar properties. It can be scaled with a strong anti-correlation between optical Fe II and [O III] emission. In search of such indicators in the far-UV band, the HST far-UV spectra of 150 low-redshift quasars are analyzed in combination with their optical SDSS counterparts. The strength of Fe II+Fe III 1123 emission is significantly correlated with that of optical Fe II. A moderate correlation may also exist between Fe II 1071 and optical Fe II. The finding opens the possibility that far-UV Fe II emission may serve as a new gauge of the Eddington ratios. The high- and low-ionization lines in the far-UV band display different patterns: for the quasars with higher Eddington ratios, the low-ionization UV lines are stronger, and the high-ionization lines are broader and weaker.