New Estimators of Black Hole Mass in Active Galactic Nuclei with Hydrogen Paschen Lines

More than 50% of Active Galactic Nuclei (AGNs) are suspected to be red and affected by dust-obscuration. Meanwhile, popular spectral diagnostics of AGNs are based on optical or ultraviolet light, making the dust obscuration as a primary concern for understanding the general nature of AGNs and supermassive black holes residing in them. To provide with a method of investigating properties of the dusty AGNs, we derive new black hole (BH) mass estimators based on velocity widths and luminosities of Near Infrared (NIR) hydrogen emission lines such as P$α$ and P$β$, and also investigate the line ratios of these Hydrogen lines. To derive the BH mass ($M_{\rm BH}$) estimators, we used a sample of 37 unobscured Type-1 AGNs with a $M_{\rm BH}$ range of $10^{6.8}$-$10^{9.4} M_{\odot}$, where $M_{\rm BH}$ come from either reverberation mapping method or single-epoch measurement method using Balmer lines. Our work shows that $M_{\rm BH}$ can be estimated from the Paschen line luminosities and the velocity widths to the accuracy of 0.18 - 0.24 dex (rms scatter). We also show that the mean line ratios of the Paschen lines and the Balmer lines are $\mathrm{\frac{Hα}{Pα}} \simeq 9.00$, $\mathrm{\frac{Hβ}{Pα}} \simeq 2.70$, which are consistent with a Case B recombination under a typical AGN broad line region environment. These ratios can be used as reference points when estimating the amount of dust extinction over the broad line region (BLR) for red AGNs. We expect the future application of the new BH mass estimators on red, dusty AGNs to provide a fresh view of obscured AGNs.

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