  By comparing sensitive Spitzer Space Telescope (SST) infrared and 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. Dust in the
region of Complex C studied here has a colder temperature
($T=10.7^{+0.9}_{-0.8}$ K - 1$\sigma$) than the local interstellar medium
($T=17.5$ K), in accordance with its great distance from the Galactic plane.
Based on the metallicity measurements and assuming diffuse Galactic
interstellar medium dust properties and dust/metals ratio, this detection could
imply gas column densities more than 5 times higher than observed in HI. We
suggest that the dust emission detected here comes from small molecular clumps,
spatially correlated with the HI but with a low surface filling factor. Our
findings imply that the HVCs' mass would be much larger than inferred from HI
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.
