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^BH), accretion rate (?M) and spin (a^*) of supermassive 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 ⋍ 1.55 observed by Very Large Telescope/X-Shooter, selected to map a large range in M^BH and L/L^Edd and covers the most prominent UV-optical (broad) emission lines, including Hα, Hβ, MgII λ2798 and CIV λ1549. This paper focuses on single-epoch, 'virial' M^BH determinations from broad emission lines and examines the implications of different continuum modelling approaches in line width measurements. We find that using a local power-law continuum instead of a physically motivated thin disc continuum leads to only slight underestimation of the full width at half-maximum (FWHM) of the lines and the associated M^BH(FWHM). However, the line dispersion σline and associated M^BH(σ^line) are strongly affected by the continuum placement and provides less reliable mass estimates than FWHM-based methods. Our analysis shows that Hα, Hβ and MgII can be safely used for virial M^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 Hα and Hβ show very similar line widths, the mean FWHM(MgII) is about 30 per cent narrower than FWHM(Hβ). We confirm several recent suggestions to improve the accuracy in C IV-based mass estimates, relying on other UV emission lines. Such improvements do not reduce the scatter between C IV-based and Balmer-line-based mass estimates.