Estimating Black Hole Masses in Obscured AGN from X-ray and Optical Emission Line Luminosities

Abstract

We test a novel method for estimating black hole masses ($M_{\mathrm{BH}}$) in obscured active galactic nuclei (AGN) that uses proxies to measure the full-width half maximum of broad H$\hat{I}\pm$ (FWHM${}_{\mathrm{bH}\hat{\mathrm{I}}\pm }$) and the accretion disk luminosity at 5100 Angstrom ($\hat{I}»L_{5100Angstrom}$). Using a published correlation, we estimate FWHM${}_{\mathrm{bH}\hat{\mathrm{I}}\pm }$ from the narrow optical emission line ratio $L_{[\mathrm{O}\mathrm{III}]}/L_{\mathrm{nH}{\hat{\mathrm{I}}}^2}$. Using a sample of 99 local obscured AGN from the Swift-BAT AGN Spectroscopic Survey, we assess the agreement between estimating $\hat{I}»L_{5100Angstrom}$ 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 $M_{\mathrm{BH}}$ using a virial mass formula where $L_{[\mathrm{O}\mathrm{III}]}/L_{\mathrm{nH}{\hat{\mathrm{I}}}^2}$ serves as a proxy of FWHM${}_{\mathrm{bH}\hat{\mathrm{I}}\pm }$ ($M_{\mathrm{BH},[\mathrm{O}\mathrm{III}]/nH{\hat{\mathrm{I}}}^2}$). We compare $M_{\mathrm{BH},[\mathrm{O}\mathrm{III}]/nH{\hat{\mathrm{I}}}^2}$ with virial $M_{\mathrm{BH}}$ measurements from broad Paschen emission lines. For the 14 (12) BASS AGN with broad Pa$\hat{I}\pm$ (Pa${\hat{I}}^2$) detections, we find $M_{\mathrm{BH},[\mathrm{O}\mathrm{III}]/nH{\hat{\mathrm{I}}}^2}$ to be systematically higher than $M_{\mathrm{BH},\mathrm{Pa}\hat{\mathrm{I}}\pm }$ ($M_{\mathrm{BH},\mathrm{Pa}{\hat{\mathrm{I}}}^2}$) 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 $M_{\mathrm{BH},[\mathrm{O}\mathrm{III}]/nH{\hat{\mathrm{I}}}^2}$ is biased high. For 151 BASS AGN with measured stellar velocity dispersions ($\ddot{I}{ƒ}_{\ast }$), we find that the $\ddot{I}{ƒ}_{\ast }$-derived $M_{\mathrm{BH}}$ agrees with $M_{\mathrm{BH},[\mathrm{O}\mathrm{III}]/nH{\hat{\mathrm{I}}}^2}$ to within 0.08 dex, albeit with wide scatter (0.74 dex). The method tested here can provide estimates of $M_{\mathrm{BH}}$ in thousands of obscured AGN in spectroscopic surveys when other diagnostics are not available, though with an uncertainty of $\sim$3-5.

Accepted for publication in ApJ; 19 pages, 6 figures