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M. E. Kaiser (The Johns Hopkins University), L. D. Bradley, II (JHU), J. B. Hutchings (Dominion Astrophysical Observatory), D. M. Crenshaw (Catholic University of America), T. R. Gull (NASA/GSFC), S. B. Kraemer (CUA), C. H. Nelson (UNLV), J. Ruiz (CUA), D. Weistrop (UNLV)
We present high spatial resolution (0.051\,\arcsec/pix) {HST} Space Telescope Imaging Spectrograph (STIS) slitless spectroscopy of the NGC4151 narrow emission-line region as a probe of the kinematic structure of the extended bicone emanating from the nucleus. Using slitless spectroscopy augmented with narrow band images, we have mapped the velocity field of the narrow line region (NLR). These measurements, at a second roll angle, confirm and extend our previous observations (Hutchings et al., 1998). In addition, we detect a set of clouds which have velocities well in excess of those predicted by pure gravitational motion. While these clouds lie preferentially near the radio axis or the radio emitting knots, none of these clouds are coincident with the radio emitting knots. Given the extremely high velocities of these clouds (400<\,\midv\mid\,<1700 km\,s-1 relative to the nuclear velocity), another physical process, such as wind driven outflow must be responsible for this kinematic motion.
Flux measurements of the individual clouds are consistent with photoionization. Both [OIII] and H\beta emission-line cloud fluxes decline roughly as the inverse square of the distance within an inner 3\arcsec (200\,pc, H\circ=75 km\,s-1Mpc-1) radius from the nucleus. Thus the ratio [OIII]/ H\beta is relatively constant within this region. For radial distances extending to 400\,pc from the nucleus, we see a slight increase, <30%, in the ratio of [OIII]/ H\beta. However, local variations are as pronounced as the the radial variations.
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Hutchings et al., 1998, ApJ, {492}, L115
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