The absence of stars in compact high-velocity clouds

We present the results of our search for faint Local Group dwarf galaxies in compact high-velocity clouds (HVCs). We used digitized Palomar Observatory Sky Survey (POSS) data to examine 1 deg(2) of sky around each of similar to250 northern hemisphere HVCs. The POSS images were processed to remove foreground stars and large-scale backgrounds, smoothed to enhance arcminute-sized low surface brightness features, and then compared with the original plates. Using this technique, we located 60 candidate dwarf galaxies in the similar to250 deg(2) that we surveyed. Follow-up observations of these candidates have revealed several distant clusters of galaxies and a number of possible Galactic cirrus clouds, but no Local Group dwarfs. It appears that many of the low surface brightness features in the sky survey data are plate flaws. The second-generation red POSS plates are sensitive down to surface brightness levels of 25-26 mag arcsec(-2), and our follow-up images reached 10 sigma limiting magnitudes of R = 21-23 for point sources. Given these limits, all known Local Group galaxies except four of the very diffuse, extended dwarf spheroidals located within 100 kpc of the Milky Way would have been detected had they been in our survey. Therefore, we can rule out the possibility that these HVCs are associated with normal but faint dwarf galaxies. If compact HVCs do contain stars, they must have surface brightnesses greater than or similar to 1 mag arcsec(-2) fainter than most known Local Group galaxies.