Searching for the High-Velocity Clouds in Local Group Analogs

We present the results of our search for analogs to the High-Velocity Clouds (HVCs) from our HI survey of six loose groups of galaxies analogous to the Local Group. Our survey utilized the Parkes 64m radiotelescope with its Multibeam receiver and the Australia Telscope Compact Array (ATCA) to survey the entire area of each group down to a M_HI ≤ 10⁷ M_sun. We found no HI clouds without optical counterparts, no HVC analogs, in any of the groups. Using the population of compact HVCs (CHVCs) around the Milky Way as a representative template and a detailed understanding of the completeness of our survey, we find that our nondetection of analogs implies that CHVCs must be clustered within 90 kpc of the Milky Way (and other galaxies) and have an average M_HI ∼ 10⁵ M_sun at the 95% confidence level. Uncertainties in the model may weaken these limits by no more than 50%. Our limits are some of the strongest constraints on the distribution of CHVCs around other galaxies, yet are still consistent with those of other authors. These limits imply that CHVCs are tightly clustered around individual galaxies, and contain a small reservoir of neutral hydrogen to be accreted onto the Milky Way. This research was performed while the author held a National Research Council Research Associateship at the Naval Research Laboratory. Basic research in radio astronomy at the Naval Research Laboratory is supported by the Office of Naval Research.