A NEUTRAL HYDROGEN STUDY OF NGC-4151

We present high-sensitivity neutral hydrogen observations of the Seyfert galaxy NGC4151 with a linear resolution of 800 pc. Despite the intense activity in the nucleus we see little evidence of large-scale peculiar motions, although there are significant departures from coplanar circular motion in a central 2 x 3 arcmin2 bar. The total H I mass of the galaxy is 3 x 10(9) M., of which 5 x 10(8) M. is in the bar. In addition to the central bar there is a region of high column density (1.4 x 10(21) cm-2) in the arm 3.5 arcmin south-west of the nucleus with a mass of approximately 10(8) M.. We have used velocity measurements in the outer part of the galaxy to determine an average heliocentric recession velocity of 997 +/- 3 km s-1. There are local minima in the H I distribution to the north-west and south-east of the active nucleus, where the column densities are less than 10(20) cm-2. These features are not due to contributions from H I in absorption, but a strong absorption feature is detected against the radio continuum nucleus which implies a neutral hydrogen column density of 6 x 10(20) cm-2. The absorption appears to be in part of a bridge of neutral hydrogen which crosses the active nucleus and could be associated with its fuelling mechanism. There is close agreement between the velocities of the optical 'extended narrow-line region' and the neutral hydrogen in the bar, implying that the ENLR is simply a component of neutral gas close to the plane of the galaxy, photoionized by anisotropic UV continuum emission from the nucleus. This has implications for schemes attempting to unify Seyfert 1 and 2 nuclei in terms of orientation. As the galactic disc is almost face-on, it is not clear why NGC 4151 is seen as a Seyfert 1 unless the collimated UV has a very wide opening angle (approximately 100-degrees).