Annu. Rev. Astron. Astrophys. 2000. 38: 289-335 Copyright © 2000 by Annual Reviews. All rights reserved

4. CORRELATIONS

There has been considerable interest in how the X-ray and optical properties of groups differ from those of richer clusters. Such comparisons are often limited by the poorly determined group properties. Most optical properties of groups are derived from existing redshift surveys, which typically include only the most luminous group members. Consequently, global properties such as velocity dispersion and morphological composition are subject to small number uncertainties. The properties of the hot gas also tend to be more uncertain in poorer systems than in clusters because of the lower X-ray fluxes of groups. It should also be remembered that the X-ray properties of groups and clusters are often derived over very different gas density contrasts, which further complicates the comparisons of these systems. Despite these potential problems, group and cluster comparisons have provided considerable insight into the nature of X-ray groups.

4.1. T- Relation

Because both the temperature of the intragroup medium and the velocity dispersion of the galaxies provide a measure of the gravitational potential strength, a correlation between these two quantities is expected. Although there is considerable scatter in the data, ROSAT observations are consistent with such a correlation (Figure 4; Ponman et al 1996, Mulchaey & Zabludoff 1998, Helsdon & Ponman 2000). High-temperature groups (T ~ 1 keV) appear to follow the extrapolation of the trend found for rich clusters; the ratio of specific energy in the galaxies to specific energy in the gas is approximately one (i.e. ~ 1 and T ²; Mulchaey & Zabludoff 1998, Helsdon & Ponman 2000). Ponman et al (1996), Helsdon & Ponman (2000) have claimed that the T- relation becomes much steeper for cooler groups. However, Figure 4 suggests that given the large scatter, evidence for a systematic deviation from the cluster relationship is at this point rather scarce.

Figure 4. Logarithm of the X-ray temperature versus logarithm of optical velocity dispersion for a sample of groups (circles) and clusters (triangles). The group data are taken from the literature compilation of Xue & Wu (2000), with the addition of the groups in Helsdon & Ponman (2000). The cluster data are taken from Wu et al (1999). The solid line represents the best-fit found by Wu et al (1999) for the clusters sample (using an orthogonal distance regression method). Within the large scatter, the groups are consistent with the cluster relationship.