MERLIN OBSERVATIONS OF NEUTRAL HYDROGEN ABSORPTION IN THE SEYFERT NUCLEUS OF NGC-4151

We present neutral hydrogen absorption measurements of the radio sourc e associated with the nucleus of NGC4151, with an angular resolution o f similar to 0.15 arcsec. We find that only the component containing t he nucleus shows significant absorption, whereas no measurable absorpt ion is seen against other components within the 4-arcsec radio jet. He nce any neutral hydrogen disc, in which the nucleus is embedded, can b e no thicker than 50 pc. The strongest component shows a mean integrat ed absorption of 21.4+/-3 km s(-1), which corresponds to a column dens ity of (3.9+/-0.5) x 10(21) atom cm(-2) (T-spin = 100 K). There is str ucture in the absorption on scales as small as 0.1 arcsec, with column densities in excess of 6 x 10(21) atom cm(-2) over parts of the radio nucleus. The neutral hydrogen column densities are much higher than s uggested by UV absorption measurements, which implies that the optical /UV continuum nucleus is not coincident with the strongest radio compo nent. We suggest that the radio nucleus is associated with a weak VLBI component which is similar to 0.1 arcsec (similar to 7 pc) to the wes t of the deepest absorption.