ISO Beyond the peaks: The 2nd ISO workshop on analytical spectroscopy
- J D T Smith
- ISO Beyond the peaks: The 2nd ISO workshop on…
We report on the results of spectroscopic mapping observations carried out in the nuclear region of Centaurus A (NGC5128) over the 5.2 15 and 20-36μm spectral regions using the Infrared Spectrograph on the Spitzer Space Telescope. We have detected and mapped S(0), S(2), S(3), and S(5) pure rotational transition lines of molecular hydrogen and emissions in the fine-structure transitions of [SiII], [SIII], [FeII], [FeIII], [ArII], [SIV], [NeII], [NeV] and [OIV]. The 500 pc bipolar dust shell discovered by Quillen et al.(2006) is even more clearly seen in the 11.3μm 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 column density of the warm molecular hydrogen in the shell is N(H2) ∼ 10 cm and similar to that estimated from the continuum dust shell surface brightness. The ratio of the dust emission features at 7.7μm and 11.3μm and the ratio of the [NeII](12.8μm) and 11.3μm dust emission feature are lower in the 500 pc dust shell than in the star forming disk. 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 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](24.318μm) and [OIV](25.890μm), are detected 20–25north-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.