Physics
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop50380s&link_type=abstract
Spitzer Proposal ID #50380
Physics
Scientific paper
We propose to accurately trace molecular gas excitation and mass along the plane of the two nearby nearby edge-on spiral galaxies, NGC 4905 and 5907 through deep IRS spectroscopy of four pure rotational lines (S(3), S(2), S(1), and S(0)) of H2. Our primary goal is to investigate the physical state, mass, and heating processes in the molecular ISM in these galaxies, and compare them with our on-going study of the edge-on galaxy NGC 891. Prior modeling of the S(1) and S(0) emission from NCG 891 suggests an enormous amount of cool (T< 90K) H2 (5 to 15 times the atomic mass) for this galaxy if the ortho to para H2 ratio is 3. However, these models are hampered by the degeneracy between gas excitation and the (assumed) o/p ratio. Including the S(3) and S(2) lines in the analysis breaks the degeneracy, enabling independent determination of gas excitation for the ortho and para species, and a measure of the o/p ratio. Using three lines (S(2), S(1), and S(0)) for NGC 891, we find gas excitation near 220 K, with an o/p ratio near unity, resulting in greatly reduced molecular gas mass. We were awarded time in Cycle 4 to verify this model through deeper integrations of the S(0), S(1), and S(2) lines, and observations of the S(3) line in NGC 891. {These observations have not yet been scheduled}. Here we propose to extend our study by mapping all four lines in the disks of galaxies with different gas contents than NGC 891: the early type Sb NGC 4565 and the late type Sc NGC 5907. We are particularly intrigued with the regions outside of the CO emitting disk where the gas is likely not heated in a PDR scenario but rather by modest velocity cloud-cloud collisions in the outer galaxy. The proposed observations address the heating source as the ortho and para ratio depends on the formation temperature: a larger ratio indicates a larger formation temperature. Did H2 form in a relatively warm far-UV bathed environment near stars, or in a cold environment devoid of young stars?
Charmandaris Vassilis
Higdon Sarah
Nikola Thomas
Stacey Gordon
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