  Broad Line Regions (BLRs) are known to contain gravitationally-bound gas
within r~(few) 10^2-10^3 Schwarszchild radii (Rs) near supermassive black holes
(SMBHs) in Active Galactic Nuclei (AGNs). Photo-ionized by strong non-stellar
AGN continuum, this gas emits luminous ultraviolet/optical/near-infrared lines
from ionized hydrogen (and other multi-ionized atoms) that have the widest
velocity profiles observed in galaxies, uniquely indicating the deep
gravitational wells of SMBHs. Nearly all BLR studies focus on its ionized gas
phase (hereafter BLR+), with typical masses of only a few 10-100 M_Sun, despite
strong indications of neutral BLR gas reservoirs (hereafter BLR0) with
M_BLR0~10^{5-6} M_Sun. We used the photoionization code CLOUDY, with its
chemistry augmented using three-body reactions, to explore 1-D models of
dustless BLRs, focusing on the BLR0 conditions, and the abundances of its most
prevalent neutral atoms and molecules. A neutral-atom-) and molecule-rich BLR0
gas phase is found underlying the BLR+, the latter occupying only a thin outer
layer of AGN-irradiated gas column densities, while the former contains the
bulk of the BLR gas mass. Atomic carbon and oxygen as well as the CO molecule
can reach substantial abundances in the BLR0, while their lines at infrared and
submillimeter wavelengths can yield new probes of the BLR physical conditions
and dynamics, unhindered by the dust absorption from outer AGN tori that
readily absorb the BLR+ optical/far-ultraviolet (FUV) lines. We find that
neutral-atom-rich neutral-atom-rich and even molecule-rich gas can exist in the
BLR0. The corresponding spectral lines from neutral atoms and molecules promise
a new spectral window of gas dynamics in the vicinity of~SMBHs unhindered by
dust absorption. This may even offer the prospect of conducting novel tests of
general relativity in strongly curved spacetimes.
