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J. D. Simon, L. Blitz (University of California at Berkeley)
We present results from 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 square degree of sky around each of the ~300 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 to the original plates. Using this technique, we located 73 candidate dwarf galaxies in the 300 degrees2 of sky that we surveyed. Followup observations of these candidates have revealed several distant clusters of galaxies and a number of 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 POSS plates are sensitive down to surface brightness levels of 25-26 magnitudes per square arcsecond, and our followup images reached 10\sigma limiting magnitudes of R=22-23 for point sources. Given these limits, all known Local Group objects, except three of the very diffuse, extended dwarf spheroidals (and Sagittarius) located within 100 kpc of the Milky Way, would have been discovered had they been in our survey. Therefore, we can rule out the possibility that any of these HVCs contain normal but faint dwarf galaxies. If compact HVCs contain stars, they must have surface brightnesses at least 1 magnitude per square arcsecond fainter than any other known Local Group galaxies.
This research was partially supported by the National Science Foundation.