Southern GEMS groups - I. Dynamical properties

Here, we present an investigation of the properties of 16 nearby galaxy groups and their constituent galaxies. The groups are selected from the Group Evolution Multiwavelength Study (GEMS) and all have X-ray as well as wide-field neutral hydrogen (H I) observations. Group membership is determined using a friends-of-friends algorithm on the positions and velocities from the 6-degree Field Galaxy Survey and NASA/IPAC Extragalactic Database. For each group we derive their physical properties using this membership, including: velocity dispersions (sigma(v)), virial masses (M-V), total K-band luminosities [L-K(Tot)] and early-type fractions (f(early)) and present these data for the individual groups. We find that the GEMS X-ray luminosity is proportional to the group velocity dispersions and virial masses: L-X(r(500)) proportional to sigma(3.11 +/- 0.59)(v) and L-X(r(500)) proportional to M-V(1.13 +/- 0.27), consistent with the predictions of self-similarity between group and clusters. We also find that M-V proportional to L-K(Tot)(2.0 +/- 0.9), i.e. mass grows faster than light and that the fraction of early-type galaxies in the groups is correlated with the group X-ray luminosities and velocity dispersions. We examine the brightest group galaxies (BGGs), finding that, while the luminosity of the BGG correlates with its total group luminosity, the fraction of group luminosity contained in the BGG decreases with increasing total group luminosity. This suggests that BGGs grow by mergers at early times in group evolution while the group continues to grow by accreting infalling galaxies. We form a composite galaxy group in order to examine the properties of the constituent galaxies and compare their properties with those of field galaxies. There are clear radial trends, with group galaxies becoming fainter, bluer and morphologically later types with increasing radius from the group centre, reaching field levels at radii > r(500)(> 0.7r(200)). We divide the composite group-by-group X-ray luminosity and find that galaxies in high X-ray luminosity groups [log(10) L-X(r(500)) >= 41.7 erg s(-1)] are redder with a higher giant-to-dwarf ratio and are more likely to be early-type galaxies than are those galaxies in low X-ray luminosity groups. We conclude that harassment and ram-pressure stripping processes are unlikely to cause these differences. The differences are more likely to be due to galaxy-galaxy mergers and possibly some further mechanism such as strangulation. If mergers are the dominant mechanism then the properties of galaxies in the higher X-ray luminosity groups are a result of mergers at earlier epochs in smaller mass groups that have since merged to become the structures we observe today, while lower X-ray luminosity groups are still undergoing mergers today.