星系团的分层效应

对星系团各类分层效应的有关问题做了概要的评述,包括成员星系在位置空间和(或)速度空间中的形态分层、光度(质量)分层和元素丰度分层的表现形式和探测途径,分层效应可能的形成机制及其对星系和星系团的结构和演化的影响。

Segregation Effects of Galaxy Clusters

Being the biggest bound systems of celestial objects,galaxy clusters are of particular importance for studies of galaxy formation,dynamical evolution of galaxies and the large scale structure of the universe.Unlike stars,galaxies show diverse shapes,i.e.different Hubble types. and the issues on segregation effects of galaxy clusters are more complicated than those of star clusters,which include morphology,mass,and abundance segregations in positional and/or ve- locity spaces,and their possible evolutionary significance for member galaxies and clusters. It is shown from observational data that the percentage of early type galaxies in a cluster increases when their radial distances from the cluster center decrease,while for late type galaxies. it is the opposite,which indicates morphology segregation in position of member galaxies in the cluster.On the other side,along the Hubble sequence E-S0-Se-S1,the velocity dispersions of member galaxies are reducing gradually,which means morphology segregation in velocity space of members in the cluster.In general,later morphological type galaxies in a cluster have larger velocity dispersion than early type ones. It has been found for solne 40 years that dwarf galaxies with lower luminosities are less concentrated than massive giant ones with higher luminosities in clusters,which shows a mass or luminosity segregation effect in radial distribution of member galaxies in clusters.Furthermore. fainter galaxies have larger velocity dispersions than brighter ones,which is so-called mass(lumi- nosily)segregation in velocity space of members in clusters,or mass velocity segregation.It has been demonstrated from some detailed studies that the mass segregation effect of galaxies in a cluster has no relationship with the morphological segregation one,although early type galaxies generally are more massive than later type ones. Because of limited data available,the research work on abundance segregation has been done mainly for some nearby clusters,especially the Virgo cluster due to its proximity.Observational evidence of abundance segregation of member galaxies is that the HI deficient Virgo spirals near the core of the cluster have higher oxygen abundances in comparison with the spirals at the pe- riphery of the cluster,which suggests the chemical evolution of galaxies in rich clusters is clearly influenced by their dense environment. The evidences for mass,morphological and abundance segregations are usually interpreted either as arising in a different formation process for galaxies of different Imninosity and/or mor- phologies,or being due to a different evolutionary history,which is called the"nature vs.nurture"problem.For a cluster galaxy,some physical processes can affect its luminosity,morphology,ve- locity and its position in the cluster,e.g.dynamical friction can slow down the velocities of massive galaxies,and these galaxies would move towards the center of the cluster,which leads to a mass segregation in positional and/or velocity spaces of galaxies.