A Deep Chandra ACIS Study of NGC 4151. I. the X-ray Morphology of the 3
  kpc-diameter Circum-nuclear Region and Relation to the Cold Interstellar
  Medium
  We report on the imaging analysis of 200 ks sub-arcsecond resolution Chandra
ACIS-S observations of the nearby Seyfert 1 galaxy NGC 4151. Bright, structured
soft X-ray emission is observed to extend from 30 pc to 1.3 kpc in the
south-west from the nucleus, much farther than seen in earlier X-ray studies.
The terminus of the north-eastern X-ray emission is spatially coincident with a
CO gas lane, where the outflow likely encounters dense gas in the host galactic
disk. X-ray emission is also detected outside the boundaries of the ionization
cone, which indicates that the gas there is not completely shielded from the
nuclear continuum, as would be the case for a molecular torus collimating the
bicone. In the central r<200 pc region, the subpixel processing of the ACIS
data recovers the morphological details on scales of <30~pc (<0.5") first
discovered in Chandra HRC images. The X-ray emission is more absorbed towards
the boundaries of the ionization cone, as well as perpendicular to the bicone
along the direction of a putative torus in NGC 4151. The innermost region where
X-ray emission shows the highest hardness ratio, is spatially coincident with
the near-infrared resolved H_2 emission and dusty spirals we find in an HST V-H
color image. The agreement between the observed H_2 line flux and the value
predicted from X-ray-irradiated molecular cloud models supports
photo-excitation by X-rays from the active nucleus as the origin of the H_2
line, although contribution from UV fluorescence or collisional excitation
cannot be fully ruled out with current data. The discrepancy between the mass
of cold molecular gas inferred from recent CO and near-infrared H_2
observations may be explained by the anomalous CO abundance in this X-ray
dominated region. The total H_2 mass derived from the X-ray observation agrees
with measurement in Storchi-Bergmann et al.
