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 dust obscuration the primary concern for understanding the general nature of AGNs and the supermassive black holes residing in them. To provide a method for investigating properties of 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 Pα and Pβ, and also investigate the line ratios of these hydrogen lines. To derive the BH mass (M_BH) estimators, we used a sample of 37 unobscured type 1 AGNs with an M_BH range of 10^6.8-10 ^9.4 M_⊙ where M_BH comes from either the reverberation mapping method or single-epoch measurement method using Balmer lines. Our work shows that M_BH can be estimated from the Paschen line luminosities and velocity widths to an 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 Hα/Pα ≃ 9.00, Hβ/Pα ≃ 2.70, which are consistent with case B recombination under a typical AGN broad-line region (BLR) environment. These ratios can be used as reference points when estimating the amount of dust extinction over the 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.