标题： 什么决定了星系团中椭圆星系的比例？ 显示英文标题

标题： What determines the fraction of elliptical galaxies in clusters?

摘要： 我们通过利用两种独立开发的半解析星系形成和演化模型，研究了星系团中星系形态混合与母星系团组装历史之间的相关性。 在我们的模型中，星系团成员的数量和椭圆星系成员的数量都随着星系团质量的增加而增加，使得产生的椭圆星系比例大致与星系团质量无关。 星系团椭圆星系的群体在星系恒星质量方面表现出明显的双峰分布，在质量 $\sim 10^{10}\,{\rm M}_{\odot}$ 处有一个低谷。 在我们模型的框架下，这种双峰性来源于随着恒星质量增加，星系数量显著减少，同时经历主要合并的概率相应增加的组合效应。 我们表明，测量的椭圆星系比例与母星系团组装历史之间的相关性较弱，并且在采用更长星系合并时间的模型中，这种相关性会变强。 我们认为，这是由于合并次数减少导致的 bulge 生成减少和先存椭圆星系生存概率增加的综合效应造成的，其中后者比前者更为重要。 显示英文摘要

摘要： We study the correlation between the morphological mix of cluster galaxies and the assembly history of the parent cluster by taking advantage of two independently developed semi-analytic models for galaxy formation and evolution. In our models, both the number of cluster members and that of elliptical members increase as a function of cluster mass, in such a way that the resulting elliptical fractions are approximately independent of cluster mass. The population of cluster ellipticals exhibit a marked bimodal distribution as a function of galaxy stellar mass, with a dip at masses $\sim 10^{10}\,{\rm M}_{\odot}$. In the framework of our models, this bimodality originates from the combination of a strongly decreasing number of galaxies with increasing stellar mass, and a correspondingly increasing probability of experiencing major mergers. We show that the correlation between the measured elliptical fraction and the assembly history of the parent cluster is weak, and that it becomes stronger in models that adopt longer galaxy merger times. We argue that this results from the combined effect of a decreasing bulge production due to a reduced number of mergers, and an increasing survival probability of pre-existing ellipticals, with the latter process being more important than the former.