BASS XXIX: The near-infrared view of the BLR: the effects of obscuration
  in BLR characterisation
  Virial black hole mass ($M_{BH}$) determination directly involves knowing the
broad line region (BLR) clouds velocity distribution, their distance from the
central supermassive black hole ($R_{BLR}$) and the virial factor ($f$).
Understanding whether biases arise in $M_{BH}$ estimation with increasing
obscuration is possible only by studying a large (N$>$100) statistical sample
of obscuration unbiased (hard) X-ray selected active galactic nuclei (AGN) in
the rest-frame near-infrared (0.8-2.5$\mu$m) since it penetrates deeper into
the BLR than the optical. We present a detailed analysis of 65 local
BAT-selected Seyfert galaxies observed with Magellan/FIRE. Adding these to the
near-infrared BAT AGN spectroscopic survey (BASS) database, we study a total of
314 unique near-infrared spectra. While the FWHMs of H$\alpha$ and
near-infrared broad lines (He\textsc{i}, Pa$\beta$, Pa$\alpha$) remain unbiased
to either BLR extinction or X-ray obscuration, the H$\alpha$ broad line
luminosity is suppressed when $N_H\gtrsim10^{21}$ cm$^{-2}$, systematically
underestimating $M_{BH}$ by $0.23-0.46$ dex. Near-infrared line luminosities
should be preferred to H$\alpha$ until $N_H<10^{22}$ cm$^{-2}$, while at higher
obscuration a less biased $R_{BLR}$ proxy should be adopted. We estimate $f$
for Seyfert 1 and 2 using two obscuration-unbiased $M_{BH}$ measurements, i.e.
the stellar velocity dispersion and a BH mass prescription based on
near-infrared and X-ray, and find that the virial factors do not depend on
redshift or obscuration, but for some broad lines show a mild anti-correlation
with $M_{BH}$. Our results show the critical impact obscuration can have on BLR
characterization and the importance of the near-infrared and X-rays for a less
biased view of the BLR.
