The HERACLES View of Molecular Gas in the Outer Disks of Galaxies

Details The HERACLES View of Molecular Gas in the Outer Disks of Galaxies The HERACLES View of Molecular Gas in the Outer Disks of Galaxies

Jan 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21713304s&link_type=abstract

American Astronomical Society, AAS Meeting #217, #133.04; Bulletin of the American Astronomical Society, Vol. 43, 2011

Astronomy and Astrophysics

Astronomy

Scientific paper

For 24 nearby spiral galaxies we combine observations of CO (HERACLES), HI (THINGS), IR (SINGS), and UV (GALEX) to study how the ISM forms stars in regions of low molecular gas surface density. The gas-to-stars conversion appears to be a two step process: 1) the formation of dense (molecular) gas from the (atomic) ISM and 2) the formation of stars from that dense phase. Distinguishing these processes requires obtaining high-significance measurements of molecular gas in low surface brightness, HI-dominated regions. We do this using a novel technique, leveraging HI velocity fields from THINGS and wide area coverage of HERACLES to stack CO spectra out to the optical radius. We find CO to decrease uniformly over all radii and scale remarkably linearly with tracers of star formation (IR and FUV). The H2-SFR relation is linear for individual galaxies and does not appear to depend on local gas surface density. However, we find systematic offsets between galaxies which dominate the scatter of the combined relation. Meanwhile, the H2-to-HI ratio varies by several orders of magnitude with radius and total gas surface density and, therefore, sensitively regulates the supply of star-forming molecular gas.

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