STIS Spectroscopy of the Central 14 pc of NGC 3998: Evidence for an
  Inflow
  Prior imaging of the lenticular galaxy, NGC 3998, with the Hubble Space
Telescope (HST) revealed a small, highly inclined, nuclear ionized gas disk,
the kinematics of which indicate the presence of a 270 million solar mass black
hole. Plausible kinematic models are used to constrain the size of the broad
line region (BELR) in NGC 3998 by modeling the shape of the broad H-alpha,
H-beta and H-gamma emission line profiles. The analysis indicates that the BELR
is large with an outer radius ~7 pc, regardless of whether the kinematic model
is represented by an accretion disk or a spherically symmetric inflow. The
electron temperature in the BELR is < 28,800 K consistent with photoionization
by the AGN. Indeed, the AGN is able to sustain the ionization of the BELR,
albeit with a high covering factor ranging between 20% and 100% depending on
the spectral energy distribution adopted for the AGN. The high covering factor
favors a spherical distribution for the gas as opposed to a thin disk. If the
gas density is > 7 x 10^3 cm^-3 as indicated by the broad forbidden [S II]
emission line ratio, then interpreting the broad H-alpha emission line in terms
of a steady state spherically symmetric inflow leads to a rate < 6.5 x 10^-2
Msun/yr which exceeds the inflow requirement to explain the X-ray luminosity in
terms of a radiatively inefficient inflow by a factor of < 18.
