Mapping the Kinematics of the Narrow-Line Region in the Seyfert Galaxy NGC 4151

Using The Hubble Space Telescope's Space Telescope Imaging Spectrograph HST'sSTIS, observations of the OIII emission from the narrow-line region (NLR) ofNGC 4151 were obtained and radial velocities determined. Five orbits of HSTtime were used to obtain spectra at five parallel slit configurations, at aposition angle of 58 degrees, with spatial resolution 0.2 arcseconds across and0.1 arcseconds along each slit. A spectral resolving power of ~ 9,000 with theG430M grating gave velocity measurements accurate to ~ 34 km/s. A kinematicmodel was generated to match the radial velocities, for comparison to previouskinematic models of biconical radial outflow developed for low-dispersionspectra at two slit positions. The new high-resolution spectra permit themeasurement of accurate velocity dispersions for each radial-velocitycomponent. The full-width at half-maximum (FWHM) reaches a maximum of 1000 km/snear the nucleus, and generally decreases with increasing distance to about 100km/s in the extended narrow-line region (ENLR), starting at about 6 arcsecondsfrom the nucleus. In addition to the bright emission knots, which generally fitour model, there are faint high velocity clouds which do not fit the biconicaloutflow pattern of our kinematic model. These faint clouds occur at theturnover points of the outflowing bright clouds. We suggest possible scenariosthat could explain these rogue clouds: (1) backflow resulting from shocks and(2) outflow outside of the bicones, although the latter does not explain howthe knots are ionized and accelerated. A comparison of our observations with ahigh-resolution radio map shows that there is no evidence that the kinematicsof the NLR clouds are affected by the radio lobes that comprise the inner jet.