Active galactic nuclei at z~1.5: II. Black Hole Mass estimation by means
  of broad emission lines
  This is the second in a series of papers aiming to test how the mass ($M_{\rm
BH}$), accretion rate ($\dot{M}$) and spin ($a_{*}$) of super massive black
holes (SMBHs) determine the observed properties of type-I active galactic
nuclei (AGN). Our project utilizes a sample of 39 unobscured AGN at
$z\simeq1.55$ observed by VLT/X-shooter, selected to map a large range in
$M_{\rm BH}$ and $L/L_{\rm edd}$ and covers the most prominent UV-optical
(broad) emission lines, including H$\alpha$, H$\beta$, MgII, and CIV. This
paper focuses on single-epoch, "virial" $M_{\rm BH}$ determinations from broad
emission lines and examines the implications of different continuum modeling
approaches in line width measurements. We find that using a "local" power-law
continuum instead of a physically-motivated thin disk continuum leads to only
slight underestimation of the FWHM of the lines and the associated $M_{\rm
BH}\left({\rm FWHM}\right)$. However, the line dispersion $\sigma_{\rm line}$
and associated $M_{\rm BH}\left(\sigma_{\rm line}\right)$ are strongly affected
by the continuum placement and provides less reliable mass estimates than
FWHM-based methods. Our analysis shows that H$\alpha$, H$\beta$ and MgII can be
safely used for virial $M_{\rm BH}$ estimation. The CIV line, on the other
hand, is not reliable in the majority of the cases, this may indicate that the
gas emitting this line is not virialized. While H$\alpha$ and H$\beta$ show
very similar line widths, the mean ${\rm FWHM\left(MgII\right)}$ is about 30%
narrower than ${\rm FWHM\left(H\beta\right)}$. We confirm several recent
suggestions to improve the accuracy in CIV-based mass estimates, relying on
other UV emission lines. Such improvements do not reduce the scatter between
CIV-based and Balmer-line-based mass estimates.
