The Highest Resolution Chandra View of Photoionization and Jet-Cloud
  Interaction in the Nuclear Region of NGC 4151
  We report high resolution imaging of the nucleus of the Seyfert 1 galaxy NGC
4151 obtained with a 50 ks Chandra HRC observation. The HRC image resolves the
emission on spatial scales of 0.5", ~30 pc, showing an extended X-ray
morphology overall consistent with the narrow line region (NLR) seen in optical
line emission. Removal of the bright point-like nuclear source and image
deconvolution techniques both reveal X-ray enhancements that closely match the
substructures seen in the Hubble Space Telescope [OIII] image and prominent
knots in the radio jet. We find that most of the NLR clouds in NGC 4151 have
[OIII] to soft X-ray ratio ~10, despite the distance of the clouds from the
nucleus. This ratio is consistent with the values observed in NLRs of some
Seyfert 2 galaxies, which indicates a uniform ionization parameter even at
large radii and a density decreasing as $r^{-2}$ as expected for a nuclear wind
scenario. The [OIII]/X-ray ratios at the location of radio knots show an excess
of X-ray emission, suggesting shock heating in addition to photoionization. We
examine various mechanisms for the X-ray emission and find that, in contrast to
jet-related X-ray emission in more powerful AGN, the observed jet parameters in
NGC 4151 are inconsistent with synchrotron emission, synchrotron self-Compton,
inverse Compton of CMB photons or galaxy optical light. Instead, our results
favor thermal emission from the interaction between radio outflow and NLR gas
clouds as the origin for the X-ray emission associated with the jet. This
supports previous claims that frequent jet-ISM interaction may explain why jets
in Seyfert galaxies appear small, slow, and thermally dominated, distinct from
those kpc scale jets in the radio galaxies.
