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.