Computer Science
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007sptz.prop40284r&link_type=abstract
Spitzer Proposal ID #40284
Computer Science
Scientific paper
The majority of ionized gas in galaxies is in a diffuse phase - the so-called Diffuse Ionized Gas (DIG) - and its maintenance represents a significant power requirement for galaxies. In the past fifteen years, it has been found that DIG layers in spirals are suprisingly thick, and the way in which galaxies maintain these thick ionized layers is not well understood. Diagnostic optical emission line ratios suggest that the dominant ionizing source is photo-ionization by the thin disk of massive stars, but there is evidence that additional sources of ionization in these DIG 'halos' is required. The degree to which such sources are necessary, the nature of the sources, and environment-dependent variations all rely on unambiguous ionization diagnostics. Unfortunately, the optical ratios suffer from complications due to gas temperature, abundance and extinction effects. Spitzer offers access to the [Ne III]/[Ne II] ratio - a diagnostic free of all these complications. A successful pilot IRS study of NGC 891 by us showed the feasibility of detecting these lines in faint gaseous halos and already revealed severe difficulties for pure photo-ionization models. However, this study featured only two positions in the halo of a single spiral. Here we propose to extend this study to observe positions in the halos of three nearby edge-on spiral galaxies, where the optical ratios indicate a variety of ionization requirements, allowing more general conclusions to be drawn about the maintenance of DIG layers, as well as the interpretation of the optical ratios. We will compare results with our own state-of-the-art photo-ionization and shock-ionization models. Our pilot study also featured the first spectroscopic detections of PAHs in an external galaxy halo, and the proposed observations will extend our knowledge of the PAH population in halo environments.
Benjamin Robert
Rand Richard
Wood Kenneth
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