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 ∼ 200ks sub-arcsecond resolution Chandra Advanced CCD Imaging Spectrometer (ACIS-S) observations of the nearby Seyfert 1 galaxy NGC 4151. Bright, structured soft X-ray emission is observed to extend from 30pc to 1.3 kpc in the southwest from the nucleus, much farther than seen in earlier X-ray studies. The terminus of the northeastern 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 < 200pc region, the subpixel processing of the ACIS data recovers the morphological details on scales of <30pc (<0″5) first discovered in Chandra High Resolution Camera images. The X-ray emission is more absorbed toward 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 H₂ emission and dusty spirals we find in an Hubble Space Telescope V - H color image. The agreement between the observed H₂ 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 H₂ line, although contribution from UV fluorescence or collisional excitation cannot be ruled out with current data. The discrepancy between the mass of cold molecular gas inferred from recent CO and near-infrared H₂ observations may be explained by the anomalous CO abundance in this X-ray-dominated region. The total H₂ mass derived from the X-ray observation agrees with the recent measurement by Storchi-Bergmann etal.