The evolution of x-ray selected clusters of galaxies.

摘要

In this thesis we study the evolution of clusters by studying x-ray selected clusters. We used two cluster catalogs from the literature; the first a subsample of the Einstein EMSS catalog at {dollar}zapprox 1/3,{dollar} the second a subsample of the HEAO-1 A-2 catalog at {dollar}zapprox 0.{dollar} We performed a multicolor optical survey of the catalogs using the UH 2.2m and the UH 0.6m N telescopes on Mauna Kea.; The x-ray data alone suggests an evolutionary effect. The luminosity functions of the two samples differ, in the sense that clusters at {dollar}z = 1/3{dollar} are more numerous and less luminous than at {dollar}z = 0{dollar}. We suggest that the {dollar}z = 1/3{dollar} clusters evolve to resemble the {dollar}z = 0{dollar} clusters.; The optical imaging data allows us to study several aspects of the evolution of clusters. We measured the properties of brightest cluster galaxies in both our samples. All, or most, have the {dollar}V-Rsb{lcub}rm c{rcub}{dollar} colors expected of an old non-evolving stellar population. However the {dollar}B-V{dollar} colors of half the {dollar}z = 1/3{dollar} brightest cluster galaxies are a half magnitude bluer than a no-evolution model would predict. We suggest the blue galaxies are due to star formation, perhaps due to cooling flows.; The morphologies of the brightest cluster galaxies may also have evolved. At {dollar}zapprox 1/3,{dollar} there are more dumbbell-like galaxies than in the {dollar}z = 0{dollar} sample. Most obvious is the triplet morphology which occurs three times in the {dollar}z = 1/3{dollar} sample and not once in the {dollar}z = 0.{dollar} These objects will coalesce on time scales of a billion years, short enough to evolve into the central galaxies seen at low redshift.; Lastly, we performed a Butcher-Oemler style analysis on our cluster data. The {dollar}z = 1/3{dollar} sample exhibits a range of blue fractions, from 0-50%. We suggest that this range of blue fractions is consistent with episodic star formation in clusters. That is, we suggest that many galaxies in the cluster undergo a star burst simultaneously, perhaps due to the merging of a subcluster clump into a main cluster.