Coexistence of Gravitationally Bound and Radiation Driven CIV Emission Line Regions in Active Galactic Nuclei

TL;DR

This study shows that in active galactic nuclei, CIV emission regions are driven by outflows at high Eddington ratios and gravitationally bound at low ratios, explaining previous contradictory models.

Contribution

It provides observational evidence that CIV line regions in AGNs coexist as outflows and gravitationally bound regions, depending on the Eddington ratio, reconciling conflicting models.

Findings

CIV kinematics differ from MgII, indicating different controlling mechanisms.

Strong correlation between CIV blueshift/asymmetry and Eddington ratio.

CIV emission is dominated by outflow at high Eddington ratios and by gravity at low ratios.

Abstract

There are mutually contradictory views in the literature of the kinematics and structure of high-ionization line (e.g. CIV) emitting regions in active galactic nuclei (AGNs). Two kinds of broad emission line region (BELR) models have been proposed, outflow and gravitationally bound BELR, which are supported respectively by blueshift of the CIV line and reverberation mapping observations. To reconcile these two apparently different models, we present a detailed comparison study between the CIV and MgII lines using a sample of AGNs selected from the Sloan Digital Sky Survey. We find that the kinematics of the CIV region is different from that of MgII, which is thought to be controlled by gravity. A strong correlation is found between the blueshift and asymmetry of the CIV profile and the Eddington ratio. This provides strong observational support for the postulation that the outflow is driven by radiation pressure. In particular, we find robust evidence that the CIV line region is largely dominated by outflow at high Eddington ratios, while it is primarily gravitationally bounded at low Eddington ratios. Our results indicate that these two emitting regions coexist in most of AGNs. The emission strength from these two gases varies smoothly with Eddington ratio in opposite ways. This explanation naturally reconciles the apparently contradictory views proposed in previous studies. Finally, candidate models are discussed which can account for both, the enhancement of outflow emission and suppression of normal BEL, in AGN with high Eddington ratios.