D. J. Pisano (ATNF); B. Gibson (Swinburne University of Technology); D. Barnes (University of Melbourne); L. Staveley-Smith (ATNF); K. Freeman (Research School of Astronomy and Astrophysics, Australian National University)

Like people, galaxies have a tendency to gather together. Galaxies have their small towns - groups with only a few galaxies loosely spaced, and their big cities - giant clusters with hundreds or thousands of galaxies tightly bunched while spanning a large area. Roughly two thirds of all galaxies reside in a group of some sort, while the remaining third live in isolation as hermits. These groupings of galaxies are an important building block for structure in the Universe. Not only do the majority of galaxies reside in them, but most of the baryonic (normal) matter in the Universe is likely to be contained in groups as warm and hot gas. While many astronomers have studied the dense clusters and compact groups of galaxies, very few have studied the more diffuse components of structure known as loose groups.

Loose groups of galaxies are collections of a few (typically two or three) large galaxies and tens of smaller galaxies. The large galaxies in these groups are well spaced, with separations of order 100,000 parsecs, and they are spread out over a volume of roughly one cubic Megaparsec. They tend to be dominated by spiral galaxies, but some contain one or two large elliptical galaxies. Our Galaxy, the Milky Way, is part of a loose group known as the Local Group of galaxies. Although we are inside this group, there is much we still do not know about the Local Group and other similar loose groups. In particular, we do not know whether groups of galaxies, and the galaxies in them, are still forming.

Over the past few years, a 30-year old suggestion, that the high-velocity clouds (HVCs), clouds of neutral hydrogen (HI) spread around the sky at velocities inconsistent with the rotation of the Milky Way, are primordial material associated with the formation of the Local Group and its galaxies, has been revived. In 1999, Leo Blitz and collaborators as well as Robert Braun and Butler Burton proposed that at least some of the HVCs may reside at distances of up to a million parsecs from the Milky Way and contain of order 10 million solar masses of HI. For this hypothesis, the HVCs contain primordial material still falling into the Local Group and building up the galaxies in it. In this case, other groups similar to the Local Group should contain analogous HI clouds also lacking stars.

In an attempt to test these predictions we have set out to map HI in a sample of loose groups of galaxies with similar content to the Local Group using the Parkes telescope with its multibeam receiver. Using the narrowband mode, we have mapped the entire regions around four loose groups of galaxies. Two of the groups, LGG 93 and LGG 180, are at a distance of approximately 11 million parsecs, contain only spiral and irregular galaxies, and span 25 square degrees on the sky. The multibeam in its narrowband mode allows for rapid mapping of a large area of the sky down to low mass limits, below one million solar masses of HI at these distances, permitting the detection of possible analogues of HVCs in these two other groups. In addition to searching for HI clouds associated with galaxy formation, we are also making a census of the gas-rich galaxies in these groups and are examining the dynamical structure of the groups. The relative number of small galaxies and the presence of dynamically distinct components in the group (sub-groups) can be used to constrain the various models of galaxy formation and dark matter. Together these datasets will shed a great deal of light on how galaxies and groups form and evolve.

Observations of LGG 93 revealed the six previously known galaxies in the group, plus four new HI detections with HI masses of 10-100 million solar masses. Of the new detections, one is clearly identifiable as a dwarf galaxy, ESO 200-45, while for the other three the poor angular resolution of Parkes makes it impossible to identify a single optical counterpart. An example is shown in Figure 1.

For LGG 180, we detected all 10 group galaxies, plus three new HI detections, all of which have uniquely identifiable optical counterparts (ESO 434-G17, ESO 373-G6, and ESO 434-G8). These objects have slightly higher HI masses (100-1000 million solar masses). The HI detection of ESO 434-G17 is shown in Figure 2. There is still much work to be done to constrain the models of HVCs and galaxy formation.

The detections in both groups should be mapped at higher resolution with the Australia Telescope Compact Array to assure that they are single objects, and accurately identify them with optical counterparts. Optical imaging to place firm limits on the stellar content associated with the hydrogen clouds is also essential. Finally, observations of more loose groups using the Parkes telescope will assure that these groups are not anomalous in their properties.

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