MERLIN observations of neutral hydrogen absorption in the Seyfert nucleus of NGC 4151

We present neutral hydrogen absorption measurements of the radio source associated with the nucleus of NGC 4151, with an angular resolution of ∼0.15 arcsec. We find that only the component containing the nucleus shows significant absorption, whereas no measurable absorption is seen against other components within the 4-arcsec radio jet. Hence any neutral hydrogen disc, in which the nucleus is embedded, can be no thicker than 50 pc. The strongest component shows a mean integrated absorption of $$21.4\pm3$$ km $$\textrm s^{-1}$$, which corresponds to a column density of $$(3.9\pm 0.5)\times10^{21}$$ atom cm^-2 ($$T_\textrm{spin}=100 \enspace\textrm K$$). There is structure in the absorption on scales as small as 0.1 arcsec, with column densities in excess of $$6\times10^{21}$$ atom cm^-2 over parts of the radio nucleus. The neutral hydrogen column densities are much higher than suggested by UV absorption measurements, which implies that the optical/UV continuum nucleus is not coincident with the strongest radio component. We suggest that the radio nucleus is associated with a weak VLBI component which is ∼0.1 arcsec (∼7 pc) to the west of the deepest absorption.