A search for dust in high-velocity clouds

IRAS observations are used to look for infrared emission from dust in high-velocity clouds (HVCs). None of these clouds is detected. The upper limit that can be set on the IR emission per hydrogen atom from this type of H I cloud is significantly smaller than the measured emission from low-velocity H I clouds observed elsewhere. This result implies either that the dust in the HVCs studied is cooler than that in low-velocity hydrogen clouds or that the dust abundance is at least three times lower. The corresponding explanations are that either the HVCs are far above the galactic plane (a lower limit of 10 kpc is found if the dust abundance is normal) or that the dust inside the clouds has been depleted.