The Nuclear Regions of the Seyfert Galaxy NGC 4151: Parsec‐Scale HiAbsorption and a Remarkable Radio Jet

Sensitive high angular and linear resolution radio images of the 240-pc radiojet in NGC4151, imaged at linear resolutions of 0.3 to 2.6 pc using the VLBAand 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, seenin lower resolution MERLIN and VLA images, that appear to be shock-likefeatures associated with changes in direction as the jet interacts with smallgas clouds within the central 100 pc of the galaxy. In addition, 21-cm spectralline imaging of the neutral hydrogen in the nuclear region reveals the spatiallocation, distribution and kinematics of the neutral gas detected previously ina lower resolution MERLIN study. Neutral hydrogen absorption is detectedagainst component C4W (E+F) as predicted by Mundell et al, but the absorption,extending over 3 pc, is spatially and kinematically complex on sub-parsecscales, suggesting the presence of small, dense gas clouds with a wide range ofvelocities and column densities. The main absorption component matches thatdetected in the MERLIN study and is consistent with absorption through a clumpyneutral gas layer in the putative obscuring torus, with higher velocity blue-and red-shifted systems with narrow linewidths also detected across E+F. Thespatial location and distribution of the absorbing gas across component E+Frules out component E as the location of the AGN (as suggested by Ulvestad etal.) and, in combination with the well-collimated continuum structures seen incomponent 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 plasmaencounters, and is deviated by, dense clouds with diameters smaller than ~1.4pc. (Abridged)Comment: 30 pages including 7 figures and 2 tables; accepted for publication in Ap