Active galactic nuclei at<i>z</i>∼ 1.5 – II. Black hole mass estimation by means of broad emission lines

Abstract

This is the second in a series of papers aiming to test how the mass (M BH ), accretion rate ( ) 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 H, H, Mg II 2798 and C IV 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 line and associated M BH ( line ) are strongly affected by the continuum placement and provides less reliable mass estimates than FWHM-based methods. Our analysis shows that H, H and Mg II can be safely used for virial M BH estimation. The C IV 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 and H show very similar line widths, the mean FWHM(Mg II) is about 30 per cent narrower than FWHM(H). 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.