Spitzer IRS mapping of the central kpc of Centaurus A

Abstract

We report on the results of infrared spectroscopic mapping observations carried out in the nuclear region of Centaurus A (NGC5128). The 500 pc bipolar dust shell discovered by Quillen et al.(2006) is even more clearly seen in the 11.3 micron dust emission feature than previous broad band imaging. The pure rotational lines of molecular hydrogen other than the S(0) line are detected above the dusty disk and associated with the oval dust shell. The molecular hydrogen transitions indicate the presence of warm gas at temperatures 250--720K. The ratio of the dust emission features at 7.7 and 11.3 micron and the ratio of the [NeII](12.8) and 11.3 dust emission feature are lower in the 500 pc dust shell than in the star forming disk. The clearer shell morphology at 11.3 micron, warm molecular hydrogen emission in the shell, and variation in line ratios in the shell compared to those in the disk, confirm spectroscopically that this shell is a separate coherent entity and is unlikely to be a chance superposition of dust filaments. The physical conditions in the shell are most similar to Galactic supernova remnants where blast waves encounter molecular clouds. The lines requiring the highest level of ionization, [NeV] and [OIV], are detected 20--25 arcsec north-east and south-west of the nucleus and at position angles near the radio jet axis. Fine structure line ratios and limits from this region suggest that the medium is low density and illuminated by a hard radiation field at low ionization parameter. These higher S molecular hydrogen pure rotational transitions are also particularly bright in the same region as the [OIV] and [NeV] emission. This suggests that the gas associated with the dust shell has been excited near the jet axis and is part of an ionization cone.

submitted to MNRAS

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• Disentangling the circumnuclear environs of Centaurus A: I. High resolution molecular gas imaging
• The outflow of gas from the Centaurus A circumnuclear disk: atomic spectral line maps from Herschel-PACS and APEX
• High-resolution mapping of the physical conditions in two nearby active galaxies based on 12CO(1-0), (2-1) and (3-2) lines
• The Light Element Abundance Distribution in NGC 5128 from Planetary Nebulae
• 3.3 μm PAH observations of the central kiloparsecs of Centaurus A
• MICONIC: JWST/MIRI-MRS reveals heavily reprocessed PAH emission in the circum-nuclear disc of Centaurus A