The Nuclear Regions of the Seyfert Galaxy NGC 4151: Parsec-Scale H I Absorption and a Remarkable Radio Jet

Sensitive high angular and linear resolution radio images of the 240 pc radio jet in NGC 4151, imaged at linear resolutions of 0.3-2.6 pc using the VLBA and phased VLA at λ21 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 shocklike features associated with changes in direction as the jet interacts with small gas clouds within the central ~100 pc of the galaxy. In addition, λ21 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 subparsec 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, close to the systemic velocity (998 km s-1) of the galaxy, and is consistent with absorption through a clumpy neutral gas layer in the putative obscuring torus, with higher velocity blue- and redshifted systems with narrow line widths also detected across E+F. In this region, average column densities are high, lying in the range 2.7 × 1019TS < NH < 1.7 × 1020TS cm-2 K-1 (TS is the spin temperature), with average radial velocities in the range 920 < Vr < 1050 km s-1. The spatial location and distribution of the absorbing gas across component E+F rules out component E as the location of the active galactic nucleus (AGN) (as suggested by Ulvestad et al.) and, in combination with the well-collimated continuum structures seen in component D, suggests that component D (possibly subcomponent D3) 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. Comparison of the radio jet structure and the distribution and kinematics of ionized gas in the narrow-line region (NLR) suggests that shock excitation by passage of the radio jet is not the dominant excitation mechanism for the NLR. We therefore favor nuclear photoionization to explain the structure of the NLR, although it is interesting to note that a small number of clouds with low-velocity and high-velocity dispersion are seen to bound the jet, particularly at positions of jet direction changes, suggesting that some NLR clouds are responsible for bending the jet. Alternatively, compression by a cocoon around the radio jet due to pressure stratification in the jet bow shock could explain the bright, compressed optical line-emitting clouds surrounding the cloud-free channel of the radio jet, as modeled by Steffen et al.