Eddington ratio governs the equivalent width of MgII emission line in active galactic nuclei

TL;DR

This study reveals that the equivalent width of the MgII emission line in active galactic nuclei is primarily controlled by the Eddington ratio, not by luminosity or black hole mass, based on analysis of SDSS data.

Contribution

It demonstrates a strong inverse correlation between MgII EW and Eddington ratio, highlighting a key factor influencing emission line properties in AGNs.

Findings

MgII EW is inversely proportional to Eddington ratio.

MgII EW shows no correlation with luminosity, black hole mass, or line width at fixed Eddington ratio.

MgII line luminosity scales with continuum luminosity, consistent with photoionization theory.

Abstract

We have investigated the ensemble regularities of the equivalent widths (EWs) of MgII 2800 emission line of active galactic nuclei (AGNs), using a uniformly selected sample of 2092 Seyfert 1 galaxies and quasars at 0.45 <= z <= 0.8 in the spectroscopic data set of Sloan Digital Sky Survey Fourth Data Release. We find a strong correlation between the EW of MgII and the AGN Eddington ratio (L/L_Edd): EW(MgII) \propto (L/L_Edd)^{-0.4}. Furthermore, for AGNs with the same L/L_Edd, their EWs of MgII show no correlation with luminosity, black hole mass or line width, and the MgII line luminosity is proportional to continuum luminosity, as expected by photoionization theory. Our result shows that MgII EW is not dependent on luminosity, but is solely governed by L/L_Edd.