Hubble Space TelescopeFaint Object Camera Spectroscopy of the Narrow‐Line Region of NGC 4151. I. Gas Kinematics

We present the results from a detailed kinematic analysis of bothground-based, and Hubble Space Telescope/Faint Object Camera long-slitspectroscopy at sub-arcsec spatial resolution, of the narrow-line region of NGC4151. In agreement with previous work, the extended emission gas (R > 4") isfound to be in normal rotation in the galactic plane, a behaviour that we wereable to trace even across the nuclear region, where the gas is stronglydisturbed by the interaction with the radio jet, and connects smoothly with thelarge scale rotation defined by the neutral gas emission. The HST data, at0.029" spatial resolution, allow us for the first time to truly isolate thekinematic behaviour of the individual clouds in the inner narrow-line region.We find that, underlying the perturbations introduced by the radio ejecta, thegeneral velocity field can still be well represented by planar rotation down toa radius of ~ 0.5" (30 pc), distance at which the rotation curve has itsturnover. The most striking result that emerges from our analysis is that the galaxypotential derived fitting the rotation curve changes from a "dark halo" at theENLR distances to dominated by the central mass concentration in the NLR, withan almost Keplerian fall-off in the 1"< R < 4" interval. The observed velocityof the gas at 0.5" implies a mass of M ~ 10E9 M(sol) within the inner 60 pc.The presence of a turnover in the rotation curve indicates that this centralmass concentration is extended. The first measured velocity point (outside theregion saturated by the nucleus) would imply an enclosed mass of ~ 5E7 M(sol)within R ~ 0.15" (10 pc) which represents an upper limit to any nuclear pointmass.Comment: 30 pages (aaspp4.sty), 14 figures. Fig. 1, 2 and 4 available by anonymous FTP at 143.54.2.51 (cd /pub/winge) as GIF files; or upon request to winge@if.ufrgs.br. Accepted for publication in the Astrophysical Journal (part 1