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High-velocity clouds (HVCs) are clouds of HI seen around the Milky Way with velocities inconsistent with Galactic rotation, have unknown distances and masses and controversial origins. One possibility is that HVCs are associated with the small dark matter halos seen in models of galaxy formation and distributed at distances of 150 kpc - 1 Mpc. We report on our attempts to detect the analogs to such putative extragalactic clouds in three groups of galaxies similar to our own Local Group using the ATNF Parkes telescope and Compact Array. Eleven dwarf galaxies were found, but no HI clouds lacking stars were detected. Using the population of compact HVCs around the Milky Way as a template, we find that our non-detection of analogs implies that they must be clustered within 160 kpc of the Milky Way (and other galaxies) with an average HI mass <4x10^5 M(sun) at the 95% confidence level. This is in accordance with recent limits derived by other authors. If our groups are true analogs to the Local Group, then this makes the original Blitz et al. and Braun & Burton picture of HVCs residing out to 1 Mpc from the Milky Way extremely unlikely. The total HI mass in HVCs, < 10^8 M(sun), implies that there is not a large reservoir of neutral hydrogen waiting to be accreted onto the Milky Way. Any substantial reservoir of baryonic matter must be mostly ionized or condensed enough as to be undetectable.
Motivated by the apparent order-of-magnitude discrepancy between the observed number of Local Group satellite galaxies, and that predicted by Lambda-CDM hierarchical clustering cosmologies, we explore an alternate suggestion - perhaps the missing sat
A set of HI sources extracted from the north Galactic polar region by the ongoing ALFALFA survey has properties that are consistent with the interpretation that they are associated with isolated minihalos in the outskirts of the Local Group (LG). Unl
We have conducted an HI 21 cm emission-line survey using the Parkes 20cm multibeam instrument and the Australia Telescope Compact Array (ATCA) of six loose groups of galaxies chosen to be analogs to the Local Group. The goal of this survey is to make
To date, all of the reported high velocity stars (HVSs), which are believed to be ejected from the Galactic center, are blue and therefore almost certainly young. Old-population HVSs could be much more numerous than the young ones that have been disc
Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the Local Group model and would have HI volume densities of order 0.0003/cm^3. Clouds with these volume densities and the observed neutral hydrogen column densities will be larg