Infant Galaxy Clusters at Low Redshifts

AbstractThe population and population composition of galaxies in galaxy clusters atpresent reflect the mass of the clusters and the mass growth of the galaxies in thepast. We investigate them for six clusters. We show that galaxies in massive clustersstopped growing at redshifts of ∼4. Moreover, we find that some small galaxy clusters(groups) have too many massive galaxies for their apparent masses. One possibilityis that these groups are much more massive and in a phase just before virialization.If this is the case, they should be called ‘infant galaxy clusters’ that will be maturedclusters in the dynamical time-scale (∼10 9 yrs).Keywords:galaxies: clusters—galaxies: halos—large-scale structure of universes1. IntroductionClusters of galaxies are the most massive objects in the universe. It is generally be-lieved that dark matter constitutes a large fraction of the mass in the universe, and galaxiesand clusters form in dark matter-dominated halos (dark halos). In the standard picture ofhierarchical structure formation of the universe, small objects are the first to form and thesethen amalgamate into progressively larger system (Blumenthal et al. 1984). This means thata dark halo containing a galaxy cluster has formed via merging and accretion of smaller darkhalos containing galaxies or galaxy groups (small galaxy clusters). From now on, we do notdiscriminate a dark halo containing a cluster from the cluster unless otherwise mentioned; thesame rule is applied for a group and a galaxy.The mass distribution function (MDF) of galaxies absorbed into but not disrupted in acluster is defined by the number of the galaxies in a given mass range and shows the populationcomposition of the galaxies with different masses. The MDF tells us the average time whenthe galaxies stopped growing as follows. The mass of galaxies increases with time as matteraccretes to them through gravity. As a result, the number of massive galaxies increases and the1