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

TL;DR

This study uses high-resolution Chandra X-ray imaging to analyze the morphology of NGC 4151's circumnuclear region, revealing detailed structures, interactions with cold gas, and the influence of X-ray irradiation on molecular gas.

Contribution

It provides the first sub-30 pc scale morphological details of NGC 4151's X-ray emission and links X-ray irradiation to molecular gas excitation and distribution.

Findings

X-ray emission extends up to 1.3 kpc from the nucleus.

X-ray emission coincides with cold gas lanes and is more absorbed near the ionization cone boundaries.

X-ray irradiation likely excites the observed H_2 emission, with implications for molecular gas mass estimates.

Abstract

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.