Environmental Dependence of Galaxy Formation in Cosmological Hydrodynamical Simulations

星系是我们可观测宇宙的基本组成部分。它们有各种各样的形状、大小和质量。值得注意的是，尽管我们已经知道银河系之外的星系的存在（以及它们不同的性质）已经有将近一个世纪了，但我们仍然没有很好的物理理解这些系统是如何随着时间的推移形成和演化的。解开星系形成之谜的一条重要线索来自这样一个事实，即我们看到不同环境下星系性质的系统性变化。例如，被称为星系团和星系团的大型引力结合的星系，在外观上通常是圆形的（或者更准确地说是椭球形的），主要由古老的红色恒星组成。相比之下，孤立的星系通常有由年轻恒星组成的突出的盘，以及大量的冷气体和尘埃。这告诉我们，当星系成为一个群体或星系团的成员时，有一个或多个物理过程可以改变星系的性质。天体物理学家提出了大量的过程，这些过程可能能够改变稠密环境中的星系。然而，由于系统的演化是高度非线性的，因此很难确定所提出的过程（如果有的话）中的哪一个是罪魁祸首。换句话说，它只能通过使用大型超级计算机直接进行数值计算来详细研究，这些计算机可以解决控制重力和流体动力学的方程。拟议中的研究将采用最先进的超级计算机模拟，为环境在星系形成和演化中所起的作用建立一个连贯的框架。先前使用这类模拟的尝试受到分辨率不足（即，计算机的功能不够强大，无法对星系进行足够详细的模拟）和对重要过程（如超新星爆炸对气体的辐射冷却和加热）的相对粗糙的近似以及对超大质量黑洞发射的流出物（现在被认为是一个关键因素）的忽视的严重阻碍。模拟将直接面对最新的观测结果。对我来说，这项拟议中的研究特别令人兴奋的是，它可能会带来重大惊喜。这项提议的研究代表了第一次使用大型流体动力学计算机模拟来研究环境在星系形成中的作用的认真尝试，这是天体物理学家工具箱中的一个新的、强大的工具。将模拟与最新一代望远镜获得的数据进行详细比较，应该会使我们对哪些转换过程对形成星系特性最重要以及它们如何详细工作的理解取得重大突破。解决这个问题将代表着朝着发展详细的星系形成物理理论迈出了重要的一步，因为今天宇宙中的大多数星系都是星系群和星系团的成员。我非常乐观地认为，在未来的几年里，我们可以真正开始了解星系形成和转变的过程，而拟议的研究将是实现这一目标的重要组成部分。

Galaxies are a fundamental building block of our observable universe. They come in a wide range of shapes, sizes, and masses. Remarkably, in spite of the fact that we have known about the existence of galaxies external to our own (and their varying properties) for almost a century, we still do not have a good physical understanding of how these systems formed and evolved over time. An important clue to the mystery of galaxy formation comes from the fact that we see systematic variations in the properties of galaxies in different environments. For example, galaxies found in large gravitationally-bound associations, called galaxy groups and clusters, are typically roundish (or more precisely ellipsoidal) in appearance and are largely composed of old, red stars. Isolated galaxies, by contrast, often have prominent discs composed of young stars, along with substantial quantities of cold gas and dust. This tells us that there are one or more physical processes that can transform the nature of galaxies when they become a member of a group or cluster. Astrophysicists have proposed a large number of processes that may be capable of transforming galaxies in dense environments. However, determining which of the proposed processes (if any) is the culprit is made difficult by the fact that the evolution of the system is highly non-linear. In other words, it can only be studied in detail through direct numerical computation using large supercomputers, which solve the equations that govern gravity and hydrodynamics. The proposed research would employ state-of-the-art supercomputer simulations to develop a coherent framework for the role that environment plays in the formation and evolution of galaxies. Previous attempts at this using such simulations were significantly hindered by lack of resolution (i.e., the computers were not powerful enough to simulate galaxies in sufficient detail) and by relatively crude approximations to important processes such as radiative cooling of the gas and heating of the gas by supernova explosions and neglect of outflows launched by supermassive black holes (which is now believed to be a crucial ingredient). The simulations will be directly confronted with the latest observations. What is particularly exciting to me about the proposed research is the potential for major surprises. The proposed research represents the first serious attempt to study the role that environment plays in galaxy formation using large hydrodynamical computer simulations, which is a new, powerful tool in the astrophysicist's tool-kit. Detailed comparisons of the simulations with data obtained from the latest generation of telescopes should lead to major breakthroughs in our understanding of which transformation processes are most important for shaping the properties of galaxies and how they work in detail. Resolving this issue would represent a major step towards developing a detailed physical theory of galaxy formation, since the majority of galaxies in the universe today are members of galaxy groups and clusters. I have a great deal of optimism that in the coming years we can really begin to understand the process of galaxy formation and transformation and that the proposed research will be an important part of achieving that goal.