Imaging the IR Cirrus: Molecular Content & Small-Scale Structure

Details Imaging the IR Cirrus: Molecular Content & Small-Scale Structure Imaging the IR Cirrus: Molecular Content & Small-Scale Structure

Mar 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008sptz.prop50639d&link_type=abstract

Spitzer Proposal ID #50639

Physics

Scientific paper

The infrared cirrus is a network of diffuse interstellar clouds seen over ~50% of the sky at high Galactic latitudes. There are several compelling scientific reasons to study the cirrus with Spitzer. First, the cirrus appears to contain a substantial amount of molecular hydrogen. A correlation exists between the column density of H2 seen in absorption along extragalactic sight lines and the 100um cirrus emission adjacent to those sight lines. However, the low spatial resolution of existing all-sky IR maps and patchy nature of cirrus emission on angular scales >5' introduces a great deal of scatter into the correlation. Using cirrus flux as a proxy for H2 column density will allow us to correct large-scale HI surveys for the 'missing' molecular contribution and get accurate measurements of the total mass in this diffuse ISM phase. Second, the cirrus may delineate the disk-halo interface and be a site of enhanced cooling via H2 rotational excitation and triggered star formation. Detailed studies of cirrus morphology at small scales (<5') can tell us a great deal about the physical structure of the cirrus: thickness, particle density, and cooling rates. Additionally, small-scale cirrus structure is a significant Galactic foreground that needs to be understood and characterized for future cosmology probes such as Planck and ALMA. A small program of Spitzer observations during Cycle 4 is being carried out utilizing deep MIPS observations at 70 and 160um to map the cirrus. IRAC 8um observations trace emission from PAH molecules and small grains. In our first observation (PKS0405-12) we see cirrus emission at 8um, 70um, and 160um with clear structure down to the instrumental resolution. The cirrus intensity along the H2 absorption sight line is seen to be lower than in the low-resolution data. We propose a set of follow-on observations of eight fields. These fields have been chosen to refine the IR-H2 correlation and will additionally address the small-scale structure science driver.

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