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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.