The Nuclear Regions of the Seyfert Galaxy NGC 4151: Parsec‐Scale H<scp>i</scp>Absorption and a Remarkable Radio Jet

Abstract

Sensitive high angular and linear resolution radio images of the 240-pc radio\njet in NGC4151, imaged at linear resolutions of 0.3 to 2.6 pc using the VLBA\nand phased VLA at 21 cm, are presented and reveal for the first time a faint,\nhighly collimated jet (diameter &lt;1.4 pc) underlying discrete components, seen\nin lower resolution MERLIN and VLA images, that appear to be shock-like\nfeatures associated with changes in direction as the jet interacts with small\ngas clouds within the central 100 pc of the galaxy. In addition, 21-cm spectral\nline imaging of the neutral hydrogen in the nuclear region reveals the spatial\nlocation, distribution and kinematics of the neutral gas detected previously in\na lower resolution MERLIN study. Neutral hydrogen absorption is detected\nagainst component C4W (E+F) as predicted by Mundell et al, but the absorption,\nextending over 3 pc, is spatially and kinematically complex on sub-parsec\nscales, suggesting the presence of small, dense gas clouds with a wide range of\nvelocities and column densities. The main absorption component matches that\ndetected in the MERLIN study and is consistent with absorption through a clumpy\nneutral gas layer in the putative obscuring torus, with higher velocity blue-\nand red-shifted systems with narrow linewidths also detected across E+F. The\nspatial location and distribution of the absorbing gas across component E+F\nrules out component E as the location of the AGN (as suggested by Ulvestad et\nal.) and, in combination with the well-collimated continuum structures seen in\ncomponent D, suggests that component D is the most likely location for the AGN.\nWe suggest that components C and E are shocks produced in the jet as the plasma\nencounters, and is deviated by, dense clouds with diameters smaller than ~1.4\npc. (Abridged)\n