Parsec-scale HI absorption in the Seyfert nucleus of NGC4151

Abstract

The angular resolution of ∼ 0.15 arcsec (∼ 10 pc), achieved by MERLIN, is ideal for investigation of neutral gas in the nuclei of nearby Seyferts on scales of < 10 pc. We present, here, such observations of the radio source associated with the nucleus of NGC4151. 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. There is structure in the absorption on scales as small as 0.1 arcsec, with column densities in excess of 6 × 10²¹ atoms cm⁻² over parts of the radio nucleus. The neutral hydrogen column densities are much higher than 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. We deduce that any neutral hydrogen torus, in which the nucleus is embedded, can be no thicker than 50 pc and may have a linear (E-W) extent of < 40 pc.