EDDINGTON RATIO GOVERNS THE EQUIVALENT WIDTH OF Mg II EMISSION LINE IN ACTIVE GALACTIC NUCLEI

We have investigated the ensemble regularities of the equivalent widths (EWs) of Mg II λ2800 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 the Sloan Digital Sky Survey Fourth Data Release. We find a strong correlation between the EW of Mg II and the AGN Eddington ratio (L/LEdd): EW(Mg II) ∝ (L/L Edd)-0.4. Furthermore, for AGNs with the same L/LEdd, their EWs of Mg II show no correlation with luminosity, black hole mass, or line width, and the Mg II line luminosity is proportional to continuum luminosity, as expected by photoionization theory. Our result shows that Mg II EW is not dependent on luminosity, but is solely governed by L/LEdd.