A Systematic Analysis of Fe II Emission in Quasars: Evidence for Inflow to the Central Black Hole

TL;DR

This study analyzes Fe II emission in quasars, revealing that Fe II is often redshifted and narrower than Hbeta, suggesting it originates from an infalling region outside the main broad-line region.

Contribution

It provides a systematic analysis of Fe II emission in a large quasar sample, developing a detailed line-fitting method and revealing evidence for inflow dynamics.

Findings

Fe II emission is often redshifted by ~400 km/s to 2000 km/s.

Fe II lines are narrower than Hbeta broad component.

Redshift of Fe II correlates inversely with Eddington ratio.

Abstract

Broad Fe II emission is a prominent feature of the optical and ultraviolet spectra of quasars. We report on a systematical investigation of optical Fe II emission in a large sample of 4037 z < 0.8 quasars selected from the Sloan Digital Sky Survey. We have developed and tested a detailed line-fitting technique, taking into account the complex continuum and narrow and broad emission-line spectrum. Our primary goal is to quantify the velocity broadening and velocity shift of the Fe II spectrum in order to constrain the location of the Fe II-emitting region and its relation to the broad-line region. We find that the majority of quasars show Fe II emission that is redshifted, typically by ~ 400 km/s but up to 2000 km/s, with respect to the systemic velocity of the narrow-line region or of the conventional broad-line region as traced by the Hbeta line. Moreover, the line width of Fe II is significantly narrower than that of the broad component of Hbeta. We show that the magnitude of the Fe II redshift correlates inversely with the Eddington ratio, and that there is a tendency for sources with redshifted Fe II emission to show red asymmetry in the Hbeta line. These characteristics strongly suggest that Fe II originates from a location different from, and most likely exterior to, the region that produces most of Hbeta. The Fe II-emitting zone traces a portion of the broad-line region of intermediate velocities whose dynamics may be dominated by infall.