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Hubble Space Telescope Faint Object Camera Spectroscopy of the Narrow-Line Region of NGC 4151. I. Gas Kinematics

We present the results from a detailed kinematic analysis of both ground-based, and Hubble Space Telescope (HST)/Faint Object Camera long-slit spectroscopy at subarcsecond spatial resolution of the narrow-line region (NLR) of NGC 4151. In agreement with previous work, the extended emission gas (R > 4'') is found to be in normal rotation in the galactic plane, a behavior that we were able to trace even across the nuclear region, where the gas is strongly disturbed by the interaction with the radio jet and connects smoothly with the large-scale rotation defined by the neutral gas emission. The HST data, at 0.″029 spatial resolution, allow us for the first time truly to isolate the kinematic behavior of the individual clouds in the inner narrow-line region. We find that, underlying the perturbations introduced by the radio ejecta, the general velocity field can still be well represented by planar rotation down to a radius of ~0.″5 (30 pc), the distance at which the rotation curve has its turnover. The most striking result that emerges from our analysis is that the galaxy potential derived fitting the rotation curve changes from a "dark halo" at the extended narrow-line region distances to being dominated by the central mass concentration in the NLR, with an almost Keplerian falloff in the 1′′ < R < 4′′ interval. The observed velocity of the gas at 0.″5 implies a mass of M ~ 109 M☉ within the inner 60 pc. The presence of a turnover in the rotation curve indicates that this central mass concentration is extended. The first measured velocity point (outside the region saturated by the nucleus) would imply an enclosed mass of ~5 × 107 M☉ within R ~ 0.″15 (10 pc), which represents an upper limit to any nuclear point mass.