Far-UV Fe emission as proxy of Eddington ratios

Abstract

The Eddington ratio is a key parameter that governs the diversity of quasar properties. It can be scaled with a strong anticorrelation between optical Fe II_opt and [O III] emission. In search of such indicators in the far-UV band, the Hubble Space Telescope far-UV spectra of 150 low-redshift quasars are analysed in combination with their optical SDSS counterparts. The strength of Fe II+Fe III λ1123 emission is significantly correlated with that of Fe II_opt. A moderate correlation may also exist between Fe II λ1071 and Fe II_opt. 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.