The Nuclear Regions of the Seyfert Galaxy NGC 4151 - Parsec-scale HI   Absorption and a Remarkable Radio Jet
  Sensitive high angular and linear resolution radio images of the 240-pc radio jet in NGC4151, imaged at linear resolutions of 0.3 to 2.6 pc using the VLBA and phased VLA at 21 cm, are presented and reveal for the first time a faint, highly collimated jet (diameter <1.4 pc) underlying discrete components, seen in lower resolution MERLIN and VLA images, that appear to be shock-like features associated with changes in direction as the jet interacts with small gas clouds within the central 100 pc of the galaxy. In addition, 21-cm spectral line imaging of the neutral hydrogen in the nuclear region reveals the spatial location, distribution and kinematics of the neutral gas detected previously in a lower resolution MERLIN study. Neutral hydrogen absorption is detected against component C4W (E+F) as predicted by Mundell et al, but the absorption, extending over 3 pc, is spatially and kinematically complex on sub-parsec scales, suggesting the presence of small, dense gas clouds with a wide range of velocities and column densities. The main absorption component matches that detected in the MERLIN study and is consistent with absorption through a clumpy neutral gas layer in the putative obscuring torus, with higher velocity blue- and red-shifted systems with narrow linewidths also detected across E+F. The spatial location and distribution of the absorbing gas across component E+F rules out component E as the location of the AGN (as suggested by Ulvestad et al.) and, in combination with the well-collimated continuum structures seen in component D, suggests that component D is the most likely location for the AGN. We suggest that components C and E are shocks produced in the jet as the plasma encounters, and is deviated by, dense clouds with diameters smaller than ~1.4 pc. (Abridged) 
C.G. Mundell (Astrophysics Research Institute, Liverpool John Moores   University), J.M. Wrobel (NRAO), A. Pedlar (Jodrell B...