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±3| km |s⁻¹|, which corresponds to a column density of |(3.9± 0.5)×10²¹| atom cm-2 (|Tₛpin=100 K|). There is structure in the absorption on scales as small as 0.1 arcsec, with column densities in excess of |6×10²¹| 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.
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Mundell et al. (1995) studied this question.