Estimating Black Hole Masses in Obscured AGN from X-ray and Optical Emission Line Luminosities
We test a novel method for estimating black hole masses ($M_{\rm BH}$) in obscured active galactic nuclei (AGN) that uses proxies to measure the full-width half maximum of broad H$α$ (FWHM$_{\rm bHα}$) and the accretion disk luminosity at 5100 Angstrom ($λL_{\rm 5100 Angstrom}$). Using a published correlation, we estimate FWHM$_{\rm bHα}$ from the narrow optical emission line ratio $L_{\rm [O\,III]}/L_{\rm nHβ}$. Using a sample of 99 local obscured AGN from the Swift-BAT AGN Spectroscopic Survey, we assess the agreement between estimating $λL_{\rm 5100 Angstrom}$ from the intrinsic 2-10 keV X-ray luminosity and from narrow optical emission lines. We find a mean offset of $0.32 \pm 0.68$ dex between these methods, which propagates to a factor of $\sim$2 uncertainty when estimating $M_{\rm BH}$ using a virial mass formula where $L_{\rm [O\,III]}/L_{\rm nHβ}$ serves as a proxy of FWHM$_{\rm bHα}$ ($M_{\rm BH,[O\,III]/nHβ}$). We compare $M_{\rm BH,[O\,III]/nHβ}$ with virial $M_{\rm BH}$ measurements from broad Paschen emission lines. For the 14 (12) BASS AGN with broad Pa$α$ (Pa$β$) detections, we find $M_{\rm BH,[O\,III]/nHβ}$ to be systematically higher than $M_{\rm BH,Paα}$ ($M_{\rm BH,Paβ}$) by a factor of 0.39 $\pm$ 0.44 dex (0.48 $\pm$ 0.51 dex). Since these offsets are within the scatter, more data are needed to assess whether $M_{\rm BH,[O\,III]/nHβ}$ is biased high. For 151 BASS AGN with measured stellar velocity dispersions ($σ_{\rm *}$), we find that the $σ_{\rm *}$-derived $M_{\rm BH}$ agrees with $M_{\rm BH,[O\,III]/nHβ}$ to within 0.08 dex, albeit with wide scatter (0.74 dex). The method tested here can provide estimates of $M_{\rm BH}$ in thousands of obscured AGN in spectroscopic surveys when other diagnostics are not available, though with an uncertainty of $\sim$3-5.