Probing the epoch of formation of slowly rotating galaxies

当今宇宙中大约五分之一的恒星质量被锁定在大型星系中，这些星系有两个非常特殊的特性：它们不形成恒星，它们的恒星在星系体内没有有序的运动。这些系统被认为是星系演化的最终产物，与其他时代的星系非常不同。提出的项目是关于了解星系是如何达到这种最终状态的：它们是如何停止形成恒星并变得静止的，它们是如何将形状从圆盘变为球体的，以及它们的恒星是如何失去从恒星形成气体中继承的角动量的？该项目以红移空间（z<1）的特定范围为目标，涵盖了过去80亿年的星系演化。数值模拟预测，这将是所有星系中相当一部分从“快速旋转星系”（具有高角动量）转变为“慢旋转星系”（具有低角动量）的时代，到目前为止，我们还没有清晰的图像。目前的证据表明，遥远宇宙中星系中的恒星（高红移）大部分形成于旋转的气体盘中，因此继承了高角动量和相对有序的旋转。一些已知的高红移大质量星系已经停止形成恒星的例子表明，在这样的系统中，恒星有有序的运动和高角动量。这意味着遥远宇宙的恒星形成淬火过程特征不会显著改变星系的内部动力结构。在高红移和低红移的星系之间的差异表明，在其间的几十亿年中，星系的内部结构发生了强烈的演变。然而，我们几乎没有证据表明这种转变是如何发生的，促进它的主要过程是什么，以及它与恒星形成的停止有什么关系。该项目将绘制星系间的恒星运动学（有序和随机运动），测量恒星角动量，并确定星系的内在形状，作为时间的函数（红移）。此外，该项目将确定恒星形成的片段，并将其与星系内部动力结构变化的证据相匹配。将进行这项工作的星系样本是前所未有的大，包括数百个用VLT上的积分场单元（IFU） MUSE观测到的物体。这确保了该项目将实现上述目标，并有助于与通过过去几年的基准IFU研究（例如ATLAS3D， Califa, Sami, MaNGA）获得的当地系统的特性进行比较。最后，该项目将为恒星角动量红移演化的模型和数值模拟提供第一个观测约束。

About one fifth of all stellar mass in the present-day Universe is locked in large galaxies that have two very special properties: they do not form stars and their stars do not show an ordered motion within the body of the galaxy. These systems are considered the end products of galaxy evolution, and are very different from galaxies at all other epochs. The proposed project is about understanding how galaxies attain this end state: how they stop forming stars and become quiescent, how they change their shapes from disks to spheroids, and how their stars lose the angular momentum inherited from the star-forming gas?The project targets a specific range in the redshift space (z<1), covering the last 8 billion years of galaxy evolution. Numerical simulations predict this to be the epoch when a substantial fraction of all galaxies changes from “fast rotators” (having high angular momentum) to “slow rotators” (having low angular momentum), a transformation of which we have no clear picture so far. The current evidence suggests that stars in galaxies in the distant universe (high redshift) largely form within rotating disks of gas, and therefore inherit the high angular momentum and relatively ordered rotation. A few known examples of massive galaxies at high redshifts, that had already stopped forming stars, indicate that stars in such systems have ordered motions and high angular momenta. This implies that star formation quenching processes characteristic for the distant universe do not significantly change the internal dynamical structure of galaxies. The differences between galaxies at high and low redshifts indicate there is a strong evolution of the internal structure of galaxies during the intervening several billion years. Yet, we have very little evidence of how this transformation happens, what are the main processes that facilitate it, and how it is related to the cessation of star formation. The project is going to map stellar kinematics (ordered and random motions) across galaxies, measure the stellar angular momentum and determine the intrinsic shape of galaxies, as a function of time (redshift). Furthermore, the project will determine the episodes of star formation and match it with the evidence for changes in internal dynamical structure of galaxies. The sample of galaxies for which this will be done is unprecedentedly large, comprising several hundreds of objects observed with the integral-field unit (IFU) MUSE on the VLT. This ensures the project will achieve its goals stated above, as well as facilitate a comparison with properties of local systems obtained through benchmark IFU studies of past years (e.g. ATLAS3D, Califa, Sami, MaNGA). Finally, the project will provide the first observational constraints for models and numerical simulations on the redshift evolution of the stellar angular momentum.