A Spitzer-MIPS search for dust in compact high-velocity HI clouds
  We employ three-band Spitzer-MIPS observations to search for cold dust
emission in three neutral hydrogen compact high-velocity clouds (CHVCs) in the
vicinity of the Milky Way. Far-infrared emission correlated with HI column
density was previously reported in HVC Complex C, indicating that this object
contains dust heated by the Galactic radiation field at its distance of ~10kpc.
Assuming published Spitzer, IRAS, and Planck IR-HI correlations for Complex C,
our Spitzer observations are of sufficient depth to directly detect 160um dust
emission in the CHVCs if it is present at the same level as in Complex C, but
no emission is detected in any of the targets. For one of the targets (CHVC289)
which has well-localized HI clumps, we therefore conclude that it is
fundamentally different from Complex C, with either a lower dust-to-gas ratio
or a greater distance from the Galactic disk (and consequently cooler dust
temperature). Firm conclusions cannot be drawn for the other two
Spitzer-observed CHVCs since their small-scale HI structures are not
sufficiently well known; nonetheless, no extended dust emission is apparent
despite their relatively high HI column densities. The lack of dust emission in
CHVC289 suggests that at least some compact high-velocity clouds objects may
exhibit very low dust-to-gas ratios and/or greater Galactocentric distances
than large HVC complexes.
