An HiSurvey of Six Local Group Analogs. I. Survey Description and the Search for High‐Velocity Clouds

We have conducted an HI 21 cm emission-line survey using the Parkes 20cmmultibeam instrument and the Australia Telescope Compact Array (ATCA) of sixloose groups of galaxies chosen to be analogs to the Local Group. The goal ofthis survey is to make a census of the HI-rich galaxies and high-velocityclouds (HVCs) within these groups and compare these populations with those inthe Local Group. The Parkes observations covered the entire volume of eachgroup with a rms M(HI) sensitivity of 4-10x10^5 M(sun) per 3.3 km/s channel.All potential sources detected in the Parkes data were confirmed with ATCAobservations at ~2' resolution and the same M(sun) sensitivity. All theconfirmed sources have associated stellar counterparts; no starless HIclouds--HVC analogs--were found in the six groups. In this paper, we present adescription of the survey parameters, its sensitivity and completeness. Usingthe population of compact HVCs (CHVCs) around the Milky Way as a templatecoupled with the detailed knowledge of our survey parameters, we infer that ournon-detection of CHVC analogs implies that, if similar populations exist in thesix groups studied, the CHVCs must be clustered within 90 kpc of groupgalaxies, with average M(HI) < 4x10^5 M(sun) at the 95% confidence level. Thecorollary is that the same must apply to Milky Way CHVCs. This is consistentwith our previous results from a smaller sample of groups, and in accordancewith recent observational and theoretical constraints from other authors. Theseresults confirm that there is very little neutral matter around galaxies, andthat any substantial reservoir of baryons must be in other phases.Comment: 10 pages, ApJ accepte