Introduction

Most galaxies are thought to live in physical groups with a few to perhaps one hundred gravitationally bound members. Typically, these groups of galaxies are dominated by one to a few massive spirals and/or ellipticals which are surrounded by less massive ``dwarfs''. Indeed, our own Local Group is now thought to contain 38<sup>+6</sup><sub>-2</sub> members, and is dominated by the Milky Way and M 31 (Mateo 1998) both in terms of mass and luminosity. Yet only 10% of the Local Group by number have M<sub>B</sub> < -18. The clues to the formation and evolution of the Local Group - including the Milky Way - are closely tied to understanding the dwarf galaxy population. Furthermore, understanding where the Local Group fits in the formation and evolution of the universe requires determining if the Local Group is truly an ``average'' group of galaxies. Does 90% of the population of most groups galaxies have M_B > -18? What fraction of the dwarf population is due to dwarf ellipticals and spheroidals (dE/dSphs), and what fraction is due to dwarf irregulars and dwarf spirals (dIrr/dSps)? Is there an environmental difference in this fraction depending on the morphological type of the gravitationally dominant members, or perhaps the depth of the potential well? Are most groups of galaxies truly bound systems? Are they typically in virial equilibrium? Answers to all of these questions depend on determining the complete membership of individual groups, which implies both needing to identify the members (many of which may be intrinsically quite faint) and determining their distance.

One of the nearest group of galaxies is the Centaurus A group (Cen A group). This group of galaxies not only contains NGC 5128 (Centaurus A), the nearest known radio galaxy, but according to de Vaucouleurs (1979), it has the largest spread of morphological types of any nearby group of galaxies. All of the most luminous members appear disturbed, leading Graham (1979) and van Gorkom et al. (1990) to suggest that perhaps this group has recently accreted a population of gas-rich dwarfs. Côté et al. (1997) recently completed a study of this group. They optically searched for potential members using the SRC J survey films, then confirmed membership by follow-up H and HI observations. They searched over a total area of approximately 900 square degrees, and in all, identified 27 group members. An additional member was discovered by Matthews & Gallagher (1996), bringing the total number of accepted members to 28 prior to the current study.

Given the recent, thorough, optically based study of the Cen A group, along with its intrinsic scientific interest, the Cen A group was selected as an ideal location to begin the HI Parkes All Sky Survey (HIPASS), now being conducted on the CSIRO Parkes 64 m radio telescope in Parkes, Australia. The assumed distance of the Cen A group (3.5 Mpc, based on Cepheid distances for group members NGC 5253, Sandage et al. 1994, and NGC 5128, Hui et al. 1993) is such that HIPASS should detect galaxies with HI masses of 10⁷ M, which compares very favorably with the limiting sensitivity of the HI follow-up studies of Côté et al. (1997). How does the population of group members identified via an HI search compare with the population identified through a deep optical search? Of the 28 identified Cen A group members, 21 lie within the 600 square degrees of this initial HIPASS survey, where the boundaries were selected to fit in with the scanning grid of the larger HIPASS survey. The Côté survey region was 12^h 30^m 15^h; -20

-50. This extends further north than the HIPASS region discussed here. A complete discussion of the HI properties of detected members of the Cen A group will be presented in a separate paper (Banks et al. 1999). Here we concentrate on the first results of the optical follow-up of the HIPASS observations.

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