High-latitude Molecular Clouds as Gamma-ray Sources for Fermi

Since publication of the large-scale Galactic CO survey of Dame, Hartmann, and Thaddeus (2001), the 1.2 meter millimeter-wave telescope at the Center for Astrophysics has continued to expand unbiased mapping to higher Galactic latitudes with a spacing of 0.25 deg (roughly every other beamwidth). So far, this survey has mapped over 200 molecular clouds at |b| > 10 deg between the Galactic longitudes of 0 and 230 deg (Dame and Thaddeus, 2004). Here we present preliminary results from a new effort, started in October 2008, to beam-width sample all of the high-latitude clouds uncovered by our unbiased mapping. As of March 2009 approximately 45,000 CO spectra have been obtained toward 150 clouds, and we expect that all isolated clouds at (δ > -15 deg) at high-latitudes (|b|>10 deg) will be mapped by June. It has been shown by Torres, Dame, and Digel (2005) that high-latitude CO clouds with an integrated luminosity above 1.7 K km/s deg² should be detected as gamma-ray sources in the first-year all-sky survey of Fermi. Since the number of such high-latitude clouds is expected to be around 100, they are expected to be the most numerous high-latitude gamma ray sources after AGN. It is therefore crucial for the diffuse gamma ray emission from these clouds to be well understood and characterized. Additionally, the new survey will be used to investigate the nature and extent of the so-called dark gas proposed by Grenier, Casandjian, and Terrier (2005). The data may also contribute to the Fermi search of dark matter in the Galactic halo, providing molecular column densities toward suspected regions of WIMP annihilation (Baltz, Taylor, and Wai 2007).