Seyfert Galaxies.

NGC 4151 is a weakly-barred spiral galaxy (type PSXT2). It is also one of the best-studied examples of a nearby active galactic nucleus (AGN), and one of the original nuclei noted by Seyfert (1943). The Seyfert type is possibly intermediate between 1 and 2. Pedlar et al. (1992) investigated the

large-scale HI structure of the galaxy which covers a velocity range of about 920-1080 kms^-1 in emission and absorption; the galaxy is viewed nearly face-on ( $i \sim 20^{\circ}$). They find that the most striking feature of the Hsc i emission structure is a `fat' bar, or oval, which occupies the central 5 kpc. The distortions of the velocity field are most likely caused by the non-circular gas flows in the bar. No evidence for tidal interactions with the companions at a distance of $\ga$20 has been found. Pedlar et al. suggest that the massive, gas-rich bar of NGC 4151 could have caused the inflow of gas to the centre (similar to the model I discuss for the nearby starburst galaxies in Section 4.1), and that the neutral hydrogen gas present within the few hundred parsec of the nucleus (as shown by the HI absorption measurements) constitutes a fuelling reservoir for the AGN.

Using the VLA B-array HI absorption toward the nuclear region of NGC 4151 was detected by Dickey (1986), with a centre velocity of 997 kms^-1 and width of 52$\pm$15 kms^-1. To resolve the nuclear region Mundell et al. (1995) obtained Merlin HI absorption measurements with an angular resolution of 015. They found absorption (FWHM $\sim$90 kms^-1, centred at 993$\pm$6 kms^-1) against the nucleus only. Several other, slightly weaker continuum sources which are part of a jet pointing toward us show no absorption. The authors therefore conclude that the neutral hydrogen disk in which the nucleus is embedded can be no thicker than 50 pc.

The HI survey of Seyfert galaxies by Heckman, Balick & Sullivan (1978) shows how the low angular resolution of even the biggest single-dish telescopes makes it nearly impossible to detect HI absorption against the nuclear continuum sources of galaxies. Out of 58 Seyfert and Seyfert-like galaxies, 25 were detected in HI emission and only three in HI absorption.