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

Xiao-Bo Dong, Ting-Gui Wang, Jian-Guo Wang, Xiaohui Fan, Huiyuan Wang, Hongyan Zhou, Weimin Yuan

Published 2009-07-28, updated 2009-10-15Version 2

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.