星系中的恒星形成活动的熄灭过程

Star formation quenching in galaxies

宇宙平均恒星形成率密度在z～2达到峰值后持续下降,这意味着平均而言,星系中的恒星形成活动在逐渐减弱甚至"熄灭",这种减弱和熄灭的过程主导着过去上百亿年的星系演化历史,百亿年前占据主导地位的星爆星系在今天的宇宙中已不多见,而红星系的比例则上升为百亿年前的两倍有余.到底是哪些物理机制造成星系中恒星形成活动的熄灭?这些机制在不同的星系、环境以及宇宙时期所起的作用有何不同?这些是当前星系形成领域的关键科学问题,21世纪以来的大规模多波段图像和光谱巡天观测极大地促进了对这些问题的认识.本文试图对当前研究结果稍作总结,考虑到中央星系和卫星星系的显著区别,先对两类星系分别讨论,然后将二者综合起来,总结了导致恒星形成活动熄灭的三个主要物理过程:大质量暗物质晕中的Halo Quenching效应(下落气体激波加热、潮汐力和冲压导致的气体剥离)、棒状结构和次并合驱动的星系自演化(Morphology Quenching、活动星系核反馈)、以及中小质量星系的主并合(早型星系和经典核球的形成).需要强调的是,这种对现有观测现象的总结只能是经验性的和阶段性的,下一步既要从观测上继续深入研究(更大样本、更多波段、更高红移),以改进和完善观测结果,更重要的还要及时和充分地利用最新观测结果来检验和限制星系形成的物理模型(如半解析模型和流体动力学数值模拟).

The cosmic star formation rate density is peaked at z ～ 2 and declines continuously since then, implying that the quenching of star formation has been the dominating process in galaxy evolution in the past ～ 10 Gyr. As a result, the present day universe does not contain many star-bursting galaxies, which were the dominating population at z ～ 2. What physical processes drive the star formation quenching？ What are the different roles of proposed processes in different galaxies and environments？ Our understanding in this regard has advanced dramatically thanks to multi-band image and spectroscopic surveys conducted in the past one and a half decade. In this review I attempt to summarize the main findings, mainly but not limited to observational results. Considering the large difference between central and satellite galaxies, I will firstly discuss the two types of galaxies separately, and then try to include the different mechanisms into an empirical picture in which the star formation in galaxies gets shut down through three distinct channels： halo quenching（gravitational shock-heating and various environmental effects occurring in massive halos）, secular evolution（driven by disk instability induced by galactic bars or minor mergers, which leads to bulge growth and bulge-related morphology quenching and/or AGN feedback）, and fast quenching（major mergers and/or fast depletion of cold gas due to extremely high star formation rates）. This picture,however, can be biased or even wrong in some aspects due to my limited knowledge of the literature. A full picture of galaxy quenching requires spatially resolved measurements of both stellar and gaseous components to be obtained in large samples, covering wide ranges of galaxy mass and color, with various environmental effects being well determined. More importantly, the updated observational results should be considered in theoretical studies, timely and substantially, in order to constrain current models of galaxy formation, e.g. semi-analytic models and hydro-dymatical simulations