Searching for Dust in Compact High-Velocity Clouds

Details Searching for Dust in Compact High-Velocity Clouds Searching for Dust in Compact High-Velocity Clouds

May 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006sptz.prop30907w&link_type=abstract

Spitzer Proposal ID #30907

Physics

Scientific paper

Although neutral hydrogen high velocity clouds (HVCs) have been known to exist for over 40 years, surprisingly little is known about their physical properties such as distance, mass and size. Whether HVCs reside in the Galactic halo or in the larger scale Local Group medium has long been a matter of debate and studies of compact HVCs are likely to provide us with a piece of the puzzle. We propose Spitzer observations of three well chosen compact HVCs with the aim of detecting dust emission from them. Detection of dust in our compact high velocity clouds will have profound implications. It would imply that a good fraction of gas in the compact HVCs is in compact, dense clumps. The current theoretical framework holds that most mass accreted onto the Milky Way is in the form of diffuse gas, resulting from the cooling of a hot halo. This picture will change significantly with positive Spitzer detecions. Moreover, the inferred mass in compact HVCs may be significantly larger than that found from HI observations alone, to be compared with about one solar mass per year accretion rate needed to explain a variety of Galactic phenomena such as the star formation rate and the distribution of stellar metallicities with age. Spitzer non-detections would be valuable as well. We expect to detect dust in these CHVCs if their properties are typical of nearby compact lower--velocity clouds. A non-detection would place these compact HVCs much farther away, more into the Local Group environment. This may be consistent with the extremely low velocity in the Local Group rest frame for one of our targets. This would support the Local Group deployment model of compact HVCs and numerial simulations. Thus, Spitzer observations of compact HVCs would play a vital role in our understanding of galaxy formation and/or the larger scale structure of the Local Group, with possible implications for the "missing satellite" problem inherent in cold dark matter simulations of structure formation.

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