GALAXY GROUPS

Abstract

Galaxy groups can be characterized by the radius of decoupling from cosmic\nexpansion, the radius of the caustic of second turnaround, and the velocity\ndispersion of galaxies within this latter radius. These parameters can be a\nchallenge to measure, especially for small groups with few members. In this\nstudy, results are gathered pertaining to particularly well studied groups over\nfour decades in group mass. Scaling relations anticipated from theory are\ndemonstrated and coefficients of the relationships are specified. There is an\nupdate of the relationship between light and mass for groups, confirming that\ngroups with mass of a few times 10^12 Msun are the most lit up while groups\nwith more and less mass are darker. It is demonstrated that there is an\ninteresting one-to-one correlation between the number of dwarf satellites in a\ngroup and the group mass. There is the suggestion that small variations in the\nslope of the luminosity function in groups are caused by the degree of\ndepletion of intermediate luminosity systems rather than variations in the\nnumber per unit mass of dwarfs. Finally, returning to the characteristic radii\nof groups, the ratio of first to second turnaround depends on the dark matter\nand dark energy content of the universe and a crude estimate can be made from\nthe current observations of Omega_matter ~ 0.15 in a flat topology, with a 68%\nprobability of being less than 0.44.\n