Computer Science
Scientific paper
Aug 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop..491i&link_type=abstract
Spitzer Proposal ID #491
Computer Science
Scientific paper
Are cirrus clouds a major source of molecular hydrogen emission in normal Galaxies? This question caused a considerable debate during the 2007 Spitzer Conference. After the end of the cryogenic Spitzer mission, no existing or planned observatory will be capable of answering it for the known future. To remedy this, we propose a set of Spitzer IRS (LL) pointings to observe the two lowest-lying S(0) (28.2 micron) and S(1) (17.0 micron) pure-rotational transitions of H2 towards 4 translucent 'cirrus' positions in DCld 300.2-16.9, a known source of excited H2. Two of us unexpectedly discovered H2 S(2) emission at 12.3 microns in this cloud as part of our Spitzer GO program to study the 5-15 micron PAH spectrum. Relative to the integrated PAH flux at 7.9 microns, the S(2) flux in our cloud is higher by a factor of about 6 than the S(2) flux in non-active SINGS galaxies. One hypothesis currently in favor argues that H2 emission from the disks of galaxies results from fluorescent excitation by UV photons in dense photodissociation regions with high radiation fluxes. Clearly this cannot be the case for DCld 300.2-16.9, since the UV flux incident on the cloud cannot be greater than the average interstellar value. Yet this cirrus cloud is more efficient at exciting the S(2) transition into emission than the central disks of entire galaxies! A competing scenario is that the H2 rotational lines are excited by collisions in warm pockets of gas where turbulence dissipates. A full understanding of the excitation mechanism responsible for our H2 lines is impossible without measuring the lowest transitions on the rotational ladder. Such observations would also allow us to tally the total energy expended via the rotational transitions, which we can compare with available CII and FIR measurements, both of which are the result of UV heating; as well as planned CO measurements, which trace the turbulent velocity field. We are requesting 5.3 hours to observe 4 positions using Long Low staring mode.
Bania Thomas
Boulanger Francois
Draine Bruce
Falgarone Edith
Hily-Blant Pierre
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