2008MNRAS.384.1469Q

We report on the results of spectroscopic mapping observations carried out in the central kpc of Centaurus A (NGC 5128) over the 5.2-15 and 20-36µm spectral regions using the Infrared Spectrograph onboard the Spitzer Space Telescope. The 500-pc bipolar dust shell discovered by Quillen et al. is even more clearly seen in the 11.3-µm 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 disc 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 surface brightnesses in the dust emission features at 7.7 and 11.3 µm and the ratio of the [Neii](12.8µm) and 11.3-µm dust emission feature are lower in the 500-pc dust shell than in the star-forming disc. The clearer shell morphology at 11.3µm, warm molecular hydrogen emission in the shell, and variation in line ratios in the shell compared to those in the disc, 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](24.318µm) and [Oiv](25.890µm), are detected 20-25arcsec 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 a low-density one 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.