A comparative analysis of virial black-hole mass estimates of
  moderate-luminosity active galactic nuclei using Subaru/FMOS
  We present an analysis of broad emission lines observed in
moderate-luminosity active galactic nuclei (AGNs), typical of those found in
X-ray surveys of deep fields, with the aim to test the validity of single-epoch
virial black hole mass estimates. We have acquired near-infrared (NIR) spectra
of AGNs up to z ~ 1.8 in the COSMOS and Extended Chandra Deep Field-South
Survey, with the Fiber Multi-Object Spectrograph (FMOS) mounted on the Subaru
Telescope. These low-resolution NIR spectra provide a significant detection of
the broad Halpha line that has been shown to be a reliable probe of black hole
mass at low redshift. Our sample has existing optical spectroscopy which
provides a detection of MgII, a broad emission line typically used for black
hole mass estimation at z > 1. We carry out a spectral-line fitting procedure
using both Halpha and MgII to determine the virial velocity of gas in the broad
line region, the monochromatic continuum luminosity at 3000 A, and the total
Halpha line luminosity. With a sample of 43 AGNs spanning a range of two
decades in luminosity (i.e., L ~ 10^44-46 ergs/s), we find a tight correlation
between the continuum and line luminosity with a distribution characterized by
<log(L_3000/L_Halpha)> = 1.52 and a dispersion sigma = 0.16. There is also a
close one-to-one relationship between the FWHM of Halpha and of MgII up to
10000 km/s with a dispersion of 0.14 in the distribution of the logarithm of
their ratios. Both of these then lead to there being very good agreement
between Halpha- and MgII-based masses over a wide range in black hole mass
(i.e., M_BH ~ 10^7-9 M_sun). We do find a small offset in MgII-based masses,
relative to those based on Halpha, of +0.17 dex and a dispersion sigma = 0.32.
In general, these results demonstrate that local scaling relations, using MgII
or Halpha, are applicable for AGN at moderate luminosities and up to z ~ 2.
