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

Abstract

Sensitive high angular and linear resolution radio images of the 240-pc radio jet in NGC 4151, imaged at linear resolutions of 0.3 to 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 1 Royal Society University Research Fellow 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, 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 MER-LIN 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, 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 red-shifted systems with narrow linewidths also detected across E+F. In this region, average column densities are high, lying in the range 2.7 × 10 19 T S < N H < 1.7 × 10 20 T S cm −2 K −1 (T S is the spin temperature), with average radial velocities in the range 920 < V r < 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 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 favour 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 modelled by Steffen et al.

New H2O masers in Seyfert and FIR bright galaxies

“…If this is the case, the kilomaser in NGC 4151 may be produced by an interaction between the radio jet and the interstellar medium (ISM). Indeed, indications of such an interaction between the radio jet and the clumpy ISM of NGC 4151 are reported by Mundell et al (2003). NGC 4151 may then represent the second case of a nuclear kilomaser, just after M 51 (Hagiwara et al 2001b).…”

“…2.4 and Table 1), both with the maser spot and with the putative AGN of NGC 4151 (e.g. Carral et al 1990;Mundell et al 2003;Ulvestad et al 2005). …”

“…Carral et al (1990) identified six discrete knots along the jet at 15 GHz, the strongest of which (peak flux density: 12.5 mJy/beam) has a flat spectral index that is consistent with the synchrotron self absorption associated with an AGN. Higher angular resolution VLBA and phased VLA observations at 1.4 GHz resolved the strongest knot into several components that appear to be associated with changes in the orientation of a faint highly collimated jet (diameter ≤1.4 pc), as it interacts with small gas clouds (Mundell et al 2003). The identification of one of these components with the AGN is still uncertain.…”

“…From its location this peak can be identified as the sub-component E of the radio knot C4 (labels are from Mundell et al 2003, and references therein). While the debate on the exact location of the AGN in NGC 4151 is still going on, the most recent publications indicate that D, and in particular subcomponent D3, is the most promising candidate, based on the location and distribution of the Hi absorption, the structure of the radio continuum (Mundell et al 2003), and the high brightness temperature of the source (Ulvestad et al 2005). According to Mundell et al (2003), source E is instead the brightest knot of the radio counterjet that is located behind the Hi absorbing layer.…”

“…While the debate on the exact location of the AGN in NGC 4151 is still going on, the most recent publications indicate that D, and in particular subcomponent D3, is the most promising candidate, based on the location and distribution of the Hi absorption, the structure of the radio continuum (Mundell et al 2003), and the high brightness temperature of the source (Ulvestad et al 2005). According to Mundell et al (2003), source E is instead the brightest knot of the radio counterjet that is located behind the Hi absorbing layer. If this is the case, the kilomaser in NGC 4151 may be produced by an interaction between the radio jet and the interstellar medium (ISM).…”