Active galactic nuclei at z~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 ($M_{\mathrm{BH}}$), accretion rate ($\dot{M}$) and spin ($a_{\ast }$) 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 $z\simeq 1.55$ observed by VLT/X-shooter, selected to map a large range in $M_{\mathrm{BH}}$ and $L/L_{\mathrm{edd}}$ and covers the most prominent UV-optical (broad) emission lines, including H$\hat{I}\pm$, H${\hat{I}}^2$, MgII, and CIV. This paper focuses on single-epoch, "virial" $M_{\mathrm{BH}}$ 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 $M_{\mathrm{BH}}\left(\mathrm{FWHM}\right)$. However, the line dispersion $\ddot{I}{ƒ}_{\mathrm{line}}$ and associated $M_{\mathrm{BH}}\left(\ddot{I}{ƒ}_{\mathrm{line}}\right)$ are strongly affected by the continuum placement and provides less reliable mass estimates than FWHM-based methods. Our analysis shows that H$\hat{I}\pm$, H${\hat{I}}^2$ and MgII can be safely used for virial $M_{\mathrm{BH}}$ 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$\hat{I}\pm$ and H${\hat{I}}^2$ show very similar line widths, the mean $\mathrm{FWHM}\left(\mathrm{MgII}\right)$ is about 30% narrower than $\mathrm{FWHM}\left(\mathrm{H}{\hat{\mathrm{I}}}^2\right)$. 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.

26 Pages, 15 figures, Accepted for publication in MNRAS