Environmental Dependence of Galaxy Formation in Cosmological Hydrodynamical Simulations

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.