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) over dot 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 similar or equal to 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 H alpha, H beta, Mg II lambda 2798 and C IV lambda 1549. 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 sigma(line) and associated M-BH(sigma(line)) 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 M II 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 H alpha and H beta show very similar line widths, the mean FWHM(MgII) is about 30 per cent narrower than FWHM(H beta). 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.