The Ionization of the Diffuse Ionized Gas Halo of NGC 891

Details The Ionization of the Diffuse Ionized Gas Halo of NGC 891 The Ionization of the Diffuse Ionized Gas Halo of NGC 891

Jun 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005sptz.prop20380r&link_type=abstract

Spitzer Proposal ID #20380

Computer Science

Scientific paper

The ionization of Diffuse Ionized Gas (DIG) layers in galaxies remains a critical but unsolved problem for our understanding of galaxy energetics. Work using optical emission lines has indicated that photo-ionization by massive stars in the disk dominates the energy input, but no pure photo-ionization model can reproduce all the observed line ratios, leading to the possibility that non-ionizing heat sources and/or secondary ionization sources may be important for the energetics. However, the optical diagnostics present three main problems: sensitivity to extinction, gas temperature, and (for some crucial lines) weak emission. The MIR diagnostic ratio [Ne III]/[Ne II] provides a measurable, extinction-free diagnostic of the hardness of the ionizing spectrum with little temperature sensitivity. Thus this ratio will provide an excellent test of whether photo-ionization alone can maintain the DIG, or whether a second source of ionization is required. We therefore propose to use the SH module on the Spitzer IRS to observe two fields in the lower halo (z=1 kpc) and one field in the disk of the well studied edge-on NGC 891 to determine how this ratio changes with distance from the thin disk of ionizing sources. We will compare our results with predictions from our own 2-d and 3-d simulations of the ionization structure, where such inputs as the ionizing spectrum, spectral hardening by propagation through intervening gas, and additional heat sources can be modeled.

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