Introduction

The distribution and kinematics of neutral gas in galaxy groups can be studied using 21 cm emission-line observations of neutral hydrogen (HI). In particular, since HI is generally the most extended disk tracer in normal galaxies, high-resolution HI observations can be used to search for evidence of strong, mild and even weak galaxy interactions within groups. Some systems in which HI studies have revealed important interaction physics include Stephan's Quintet, for which Shostak et al. (1984) show that the bulk of the HI lies outside the optical galaxies at all velocities, Hickson compact group (HCG) 18, in which Williams & van Gorkom (1988) find HI smoothly distributed over a region twice the optical size of the group, and the Grus Quartet, in which Koribalski (1996) finds a number of tidal tails tracing possible baryonic streams between the constituent galaxies.

Most previous HI studies of small groups of galaxies have concentrated on compact groups of galaxies that satisfy the Rose (1977) or Hickson (1982) criteria. To commence an investigation into a possible relationship between these compact groups and somewhat looser groups--examples being those selected by Garcia (1993), Nolthenius (1993) and Fouqué et al. (1992)--a preliminary neutral gas imaging study of five heterogeneous groups was undertaken by Barnes (1998). One of the groups selected for imaging, LGG 138 (Garcia, 1995), turned out to be an intriguing system: two galaxies already identified as group members were found to be encircled by a hollow ring of HI. The ring differs from many previously known HI rings in that stellar material is clearly associated with parts of the ring.

© Copyright Astronomical Society of Australia 1997