Estimating Black Hole Masses in Obscured Active Galactic Nuclei from X-Ray and Optical Emission Line Luminosities

Résumé fourni par la source

Abstract We test a novel method for estimating black hole masses (M BH) in obscured active galactic nuclei (AGN) that uses proxies to measure the FWHM of broad Hα (FWHMbHα ) and the accretion disk luminosity at 5100 Å (λL 5100 Å). Using a published correlation, we estimate FWHMbHα from the narrow optical emission line ratio L [O iii]/L nHβ . Using a sample of 99 local obscured AGN from the Swift Burst Alert Telescope AGN Spectroscopic Survey (BASS), we assess the agreement between estimating λL 5100 Å from the intrinsic 2 to 10 keV X-ray luminosity and from narrow optical emission lines. We find a mean offset of 0.32 ± 0.68 dex between these methods, which propagates to a factor of ∼2 uncertainty when estimating M BH using a virial mass formula where L [O iii]/L nHβ serves as a proxy of FWHMbHα (M BH,[O iii]/nHβ ). We compare M BH,[O iii]/nHβ with virial M BH measurements from broad Paschen emission lines. For the 14 (12) BASS AGN with broad Paα (Paβ) detections, we find M BH,[O iii]/nHβ to be systematically higher than M BH,Paα (M BH,Paβ ) by a factor of 0.39 ± 0.44 dex (0.48 ± 0.51 dex). Since these offsets are within the scatter, more data are needed to assess whether M BH,[O iii]/nHβ is biased high. For 151 BASS AGN with measured stellar velocity dispersions (σ *), we find that the σ *-derived M BH agrees with M 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 BH in thousands of obscured AGN in spectroscopic surveys when other diagnostics are not available, though with an uncertainty of ∼3–5.

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