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We present a simple kinematic model for the narrow-line region (NLR) of theSeyfert 1 galaxy NGC 4151, based on our previous observations of extended [OIII] emission with the Space Telescope Imaging Spectrograph (STIS). The modelis similar to a biconical radial outflow model developed for the Seyfert 2galaxy NGC 1068, except that the bicone axis is tilted much more into our lineof sight (40 degrees out of the plane of the sky instead of 5 degrees), and themaximum space velocities are lower (750 km/s instead of 1300 km/s. We findevidence for radial acceleration of the emission-line knots to a distance of160 pc, followed by deceleration that approaches the systemic velocity at adistance of 290 pc (for a distance to NGC 4151 of 13.3 Mpc). Other similaritiesto the kinematics of NGC 1068 are: 1) there are a number of high-velocityclouds that are not decelerated, suggesting that the medium responsible for thedeceleration is patchy, and 2) the bicone in NGC 4151 is at least partiallyevacuated along its axis. Together, these two Seyfert galaxies provide strongevidence for radial outflow (e.g., due to radiation and/or wind pressure) andagainst gravitational motion or expansion away from the radio jets as theprincipal kinematic component in the NLR.Comment: 31 pages, Latex, includes 11 figures in postscript, Figures  5a,5b,6a,6b in color, to appear in the Astronomical Journa

A Kinematic Model for the Narrow-Line Region in NGC 4151