  We present a method for obtaining accurate black hole (BH) mass estimates
from the MgII emission line in active galactic nuclei (AGNs). Employing the
large database of AGN measurements from the Sloan Digital Sky Survey (SDSS)
presented by Shen et al., we find that AGNs in the redshift range 0.3-0.9, for
which a given object can have both H-beta and MgII line widths measured,
display a modest but correctable discrepancy in MgII-based masses that
correlates with the Eddington ratio. We use the SDSS database to estimate the
probability distribution of the true (i.e., H-beta-based) mass given a measured
MgII line width. These probability distributions are then applied to the SDSS
measurements from Shen et al. across the entire MgII-accessible redshift range
(0.3-2.2). We find that accounting for this residual correlation generally
increases the dispersion of Eddington ratios by a small factor (~0.09 dex for
the redshift and luminosity bins we consider). We continue to find that the
intrinsic distribution of Eddington ratios for luminous AGNs is extremely
narrow, 0.3-0.4 dex, as demonstrated by Kollmeier et al. Using the method we
describe, MgII emission lines can be used with confidence to obtain BH mass
estimates.
