Implications from the optical to UV flux ratio of FeII emission in quasars

TL;DR

This study analyzes FeII emission in quasars to understand the physical conditions of the Broad Line Region, revealing that radiation pressure influences gas dynamics and challenging classical photoionization models.

Contribution

It provides new insights into FeII emission mechanisms, the role of radiation pressure, and the physical interpretation of Eigenvector 1 in quasars, based on a large SDSS quasar sample.

Findings

FeII(4570)/FeII(UV) correlates with Eddington ratio.

Classical photoionization models underestimate FeII emission ratios.

Most FeII-emitting clouds are under super-Eddington conditions.

Abstract

We investigate FeII emission in Broad Line Region (BLR) of AGNs by analyzing the FeII(UV), FeII(4570) and MgII emission lines in 884 quasars in the Sloan Digital Sky Survey (SDSS) Quasar catalog in a redshift range of 0.727 < z < 0.804. FeII(4570)/FeII(UV) is used to infer the column density of FeII-emitting clouds and explore the excitation mechanism of FeII emission lines. As suggested before in various works, the classical photoionization models fail to account for FeII(4570)/FeII(UV) by a factor of 10, which may suggest anisotropy of UV FeII emission; otherwise, an alternative heating mechanism like shock is working. The column density distribution derived from FeII(4570)/FeII(UV) indicates that radiation pressure plays an important role in BLR gas dynamics. We find a positive correlation between FeII(4570)/FeII(UV) and the Eddington ratio. We also find that almost all FeII-emitting clouds are to be under super-Eddington conditions unless ionizing photon fraction is much smaller than that previously suggested. Finally we propose a physical interpretation of a striking set of correlations between various emission-line properties, known as ``Eigenvector 1''.