A neutral hydrogen study of NGC 4151

Abstract

We present high-sensitivity neutral hydrogen observations of the Seyfert galaxy NGC 4151 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 × 3 arcmin2 bar. The total H I mass of the galaxy is |$3 \times 10^9 \enspace\text M_\odot$|⁠, of which |$5 \times 10^8 \enspace\text M_\odot$| is in the bar. In addition to the central bar there is a region of high column density (1.4 × 1021 cm−2) in the arm 3.5 arcmin south-west of the nucleus with a mass of |$\sim10^8\enspace \text M_\odot$|⁠. 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 1020 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 × 1020 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 ( ∼ 100○).