Warm H2 at the Edge of NGC 891: Determining the Excitation and Mass of the Most Common Molecule

Details Warm H2 at the Edge of NGC 891: Determining the Excitation and Mass of the Most Common Molecule Warm H2 at the Edge of NGC 891: Determining the Excitation and Mass of the Most Common Molecule

May 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007sptz.prop40877s&link_type=abstract

Spitzer Proposal ID #40877

Physics

Scientific paper

We propose to obtain deep IRS spectra of the outer disk of the nearby edge on spiral galaxy NGC 891. Our primary goal is to investigate the physical state, mass, and heating processes in the molecular ISM through investigations of four pure rotational lines of H2. Pioneering work in the S(0) and S(1) lines with ISO indicate large quantities of warm (T ~ 120 K) molecular clouds along the disk of the galaxy ? fully half of the cold H2 component traced in CO. Furthermore, the ISO results suggest vast quantities of cool (T < 90 K) molecular gas at radii beyond 8 kpc ? 5 to 15 times the mass of the atomic gas, and perhaps enough to solve the missing mass problem within the disk of the galaxy! These are exciting results that need to be verified. The key element, however, is that the large masses depend on an assumed ortho to para H2 ratio, and uncertain gas excitation due to limited lines. For example, GTO time spectra in the S(1) and S(0) lines are consistent with the ISO results, but adding in the S(2) line leads to an interpretation invoking a small ortho to para ratio (~ 0.65), and larger gas temperatures, resulting in 1/10 the mass of warm molecular gas! Unfortunately, our spectra are not deep enough to detect the S(2) line in the outer galaxy, so we are unable to properly model the gas excitation there. This proposal is to go at least 5 times deeper in the most interesting regions of the outer galaxy to strongly detect the S(2) line. We also plan to scour the outer regions (R > 12 kpc) for cool molecular gas by deep integrations in the S(1) and S(0) lines. Finally, we plan a sparse map of most of the disk to detect the S(3) line using the Short-Lo spectrometer. The S(0), S(2), and S(1) and S(3) transitions constrain the excitation of para and ortho H2 respectively, and together yield the o/p ratio. The four lines will therefore solidly establish the mass of warm and cool molecular gas in NGC 891. Are there massive quantities of cool gas in the outer disks of galaxies?

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