Dissecting the orbital and stellar-population structure of early-type galaxies with MUSE

星系是如何形成和演化的是现代天文学中一个关键的开放性问题，在接下来的十年里，它将推动数十亿美元的国际研究基础设施投资。现代宇宙学计算机对星系形成过程的模拟表明，恒星的轨道和它们的元素丰度包含了迄今尚未开发的信息，这些信息既包括星系中恒星形成的过程，也包括它们是如何通过吸积其他伴星而成长的。这里提出的项目将利用安装在欧洲南方天文台甚大望远镜上的MUSE积分场光谱仪，充分利用在附近的天炉星系团（由申请人Sarzi领导的Fornax3D项目）中收集到的前所未有的星系质量，揭示这一“化石记录”。这样的积分场光谱数据确实可以测量在其中任何特定点观测到的平均恒星光谱，从而可以推断出同一点上恒星的平均运动，以及对平均恒星年龄和化学成分的估计。对于那些我们有理由相信恒星是在一次事件中形成的星系，这些信息对天文学家来说通常是足够的。然而，对于大多数星系来说，情况并非如此，未来的艰巨任务是从数据中确定目前存在于星系中的不同恒星族，并了解它们是如何以及何时形成的。例如，银河系和其他盘状星系是由三种主要恒星组成的。一个由老恒星组成的中心球形凸起，一个年轻恒星继续形成的薄盘，一个由老恒星组成的厚盘。对于那些不再发生恒星形成的星系，即所谓的早期型星系（etg，也被称为椭圆星系或透镜星系），情况就不太清楚了，尽管越来越多的证据表明，这些天体中的大多数确实也含有三种不同的凸起，薄和厚的成分。这也与大多数etg来自像银河系这样的盘状星系的观点相一致，尽管确切的方式仍有待了解。事实上，了解这种潜在的联系正是这个项目的目标，其目的是了解例如，恒星在etg的磁盘组件中停止形成的速度有多快，以及星系环境在其中发挥的作用，例如进入像天炉星团这样的星系群，这是典型的etg倾向于生活的地方。为了解决测量ETGs的每个磁盘组件的恒星形成历史的难题，在这里，我们将从Fornax3D合作中获得独特的建模专业知识，首先根据它们的恒星轨道和模型确定ETGs中的主要恒星家族，这些恒星可以同时匹配观测到的恒星光分布和恒星运动。然后，这些模型将允许知道在目标星系的任何一点上，每个恒星族（例如凸起恒星，厚的和薄的盘状恒星）有多少恒星以哪种方式移动，这将使它更容易（和可靠）从观察到的恒星光谱中推断出我们在同一点观察到的每个恒星族的恒星的平均年龄和化学成分。绘制ETGs褪色星盘的恒星年龄图，已经可以让我们了解导致恒星停止形成的过程，因为一些过程会随着距离星系中心的距离而留下不同的年龄趋势。

How galaxies form and evolve is a key open question in modern astronomy, driving billions of dollars of international research infrastructure investment over the next ten years. Modern cosmological computer simulations of how galaxies form show that the orbits of stars and their element-abundance hold as-yet untapped information on both on how star-formation proceeded over time in galaxies and how these also grew by accreting other companion galaxies.The project proposed here will uncover this `fossil record' by making full use of the unprecedented quality collected for galaxies in the nearby Fornax galaxy cluster (the Fornax3D project, led by applicant Sarzi) with the MUSE integral-field spectrograph installed on the Very Large Telescope of the European Southern Observatory. Such integral-field spectroscopic data allow indeed to measure the mean stellar spectrum observed at any particular point in these, which in turn allows to infer the mean motion of stars at that same point as well as an estimate of the mean stellar age and chemical content. This information is generally sufficient for Astronomers in the case of those galaxies where we have reasons to believe that stars formed over one single event. For most galaxies this is not the case, however, and the arduous task ahead is that of identifying from the data the different families of stars are currently present in galaxies, and understand how and when these came to be. For instance, the Milky Way and other disk galaxies is formed of three main stellar components. A central spherical bulge of old stars, a thin disk where young stars continue to form and relic, thicker disk of older stars. In the case of galaxies where star-formation is no longer occurring, the so-called early-type galaxies (ETGs, known also as elliptical or lenticular galaxies) the situation is less clear, although it evidence has been mounting that most of these objects do also harbour three distinct bulge, thin and thick components. This would also be in keeping with the idea that most ETGs descend from disk galaxies such as the Milky Way, although exactly how remains to be understood. Indeed, understanding this potential link is precisely the goal of this project, which aims understanding for instance how quickly stars ceased to form in the disk components ETGs and what role was played in particular by galactic environment, such as entering a group of galaxies like the Fornax cluster which is typical of where ETGs tend to live.To tackle the difficult problem of measuring the star-formation history of each of the disk components of ETGs, here we will draw from the unique modelling expertise within for Fornax3D collaboration to first identify the main families of stars in ETGs on the basis of their stellar orbits and models that can match simultaneously the observed stellar light distribution and stellar motions. These models will then allow to know how many stars move in which way at any point in the target galaxy for each stellar family (e.g. bulge stars, thick and thin disk stars), which will make it much more easier (and robust) to deduce from the observed stellar spectra the mean age and chemical content of the stars from each stellar family that we observe at that same point. Mapping the stellar age of the faded stellar disks of ETGs will already inform us on the processes that led to the cessation of star formation, as some processes will imprint different age trend with distance from the galaxy centre.