The First Detection of Dust Emission in a High-Velocity Cloud

Abstract

By comparing sensitive Spitzer Space Telescope infrared observations with Green Bank Telescope 21 cm observations, we are able to report the first detection of dust emission in Complex C, the largest high-velocity cloud in the sky. The dust in the region of Complex C studied here has a colder temperature (K; ϩ0.9 T p 10.7 Ϫ0.8 1 j) than the local interstellar medium (K), in accordance with its great distance from the Galactic T p 17.5 plane. Based on the metallicity measurements and assuming diffuse Galactic interstellar medium dust properties and a dust-to-metals ratio, this detection could imply gas column densities more than 5 times higher than observed in H i. We suggest that the dust emission detected here comes from small molecular clumps, spatially correlated with the H i but with a low surface filling factor. Our findings imply that the mass of high-velocity clouds would be much larger than inferred from H i observations and that most of the gas falling on the Milky Way would be in cold and dense clumps rather than in a diffuse phase.