Limits on the OH Molecule in the Smith High-velocity Cloud

Abstract

We have used the Green Bank Telescope to search for the OH molecule at several locations in the Smith Cloud, one of the most prominent of the high-velocity clouds surrounding the Milky Way. Five positions with high H i column density were selected as targets for individual pointings, along with a square degree around a molecular cloud detected with the Planck telescope near the tip of the Smith Cloud. Gas in the Galactic disk with similar values of N HI has detectable OH emission. Although we found OH at velocities consistent with the foreground Aquila molecular cloud, nothing was found at the velocity of the Smith Cloud to an rms level of 0.7 mK (T b ) in a 1 km s−1 channel. The three positions that give the strictest limits on OH are analyzed in detail. Their combined data imply a 5σ limit on N(H2)/N HI ≤ 0.03, scaled by a factor dependent on the OH excitation temperature and background continuum T ex/(T ex − T bg). There is no evidence for far-infrared emission from dust within the Smith Cloud. These results are consistent with expectations for a low-metallicity diffuse cloud exposed to the radiation field of the Galactic halo, rather than a product of a galactic fountain.