E+A星系的研究进展

Research Progress on E+A(Post-Starburst) Galaxies

E+A星系的光谱具有很强的巴耳末吸收线,缺乏与恒星形成相关的发射线,将典型的椭圆星系(E)和A型恒星的光谱进行线性组合就能够拟合出这类星系的光谱。它们的颜色、形态、星族年龄等参数介于典型的早型和晚型星系之间。E+A星系近期经历了星暴活动,在星系演化进程中,它们处于晚型到早型的过渡阶段,可能在演变过程中扮演着重要角色。介绍了E+A星系的基本观测特征,并讨论了它们的形成机制、演化去向、所处的环境以及它们与AGN的关系。

The most typical characteristics of E＋A galaxies are that their spectra show strong Balmer absorption lines but no significant emission lines associated with ongoing star formation. The spectra can be roughly fitted by a combination of an old elliptical galaxy （E） and a young A-type star spectrum, so named E＋A galaxies. The existence of strong Balmer absorption lines indicates that E＋A galaxies have undergone starburst activities, and the shortage of emission lines implies that they have quenched their star formation recently. Therefore, E＋A galaxies are also interpreted as post-starburst galaxies. On the evolutionary path of some galaxies from gas-rich, star-forming, disk-dominated blue ones into gas-poor, quiescent, bulge-dominated red ones, E＋A galaxies, which are in the intermediate phase, play an important role in our understanding of galaxy formation and evolution. E＋A galaxies originate mainly from merging/interaction in low density re- gions , especially wet merging between gas-rich galaxies, but a few cluster-related physical mechanisms also play some role, such as ram pressure stripping star-forming gas by high- temperature and high-density intracluster media. Many physical parameters of E＋A galaxies are between typical early-type and late type galaxies, or close to early-type, which suggest they will eventually evolve into elliptical galaxies. Although poor groups and field environ- ment are preferred by E＋A galaxies to reside in, a fraction of them located in rich groups and clusters whose members are almost early-type galaxies. Feedback from AGNs may be responsible for quenching star formation on the path of forming E＋A galaxies. Spectral features of LINER/Seyferts and observational evidences for feedback from AGNs are in- deed found in many E＋A galaxies recently. However, this mechanism does not work in less massive galaxies. This paper presents the basic observational characteristics of E＋A galaxies, and reviews their formation mechanism, evolutionary destination, environment and the connections with AGNs. Some problems are reviewed, .ie. the method for picking out homogeneous E＋A galaxies, the connections between E＋A and other transition galaxies, the detailed histories of star formation at the very starburst and post-starburst stages.