Understanding and Tracing Dark Matter in the Local Universe

暗物质将我们在宇宙大尺度上看到的可见结构结合在一起，提供了密集的“晕”，在这些晕中形成了单个星系、星系群和星系团。然而，暗物质的基本性质仍然未知，建立它与粒子物理标准模型的其他部分的关系仍然是宇宙学的重中之重。在大尺度上，暗物质的聚集被很好地理解，观测结果和理论预测相吻合。在小尺度上，我们的观测探测器要少得多，暗物质的行为也更有争议。新的和即将到来的调查有望极大地改善这种情况，对星系群集和引力透镜进行更灵敏的测量。理论学家面临的挑战是跟上形势，做出足够准确的预测，以充分利用这些数据。这是我提出的研究计划的目标，它包括三个主要组成部分。

Dark matter binds together the visible structure we see on large scales in the Universe, providing the dense 'halos' within which individual galaxies and groups and clusters of galaxies form. The fundamental nature of dark matter remains unknown, however, and establishing its relation to the rest of the Standard Model of particle physics remains a top priority in cosmology. On large scales, the clustering of dark matter is well understood, and observations and theoretical predictions match. On small scales, we have far fewer observational probes, and the behaviour of dark matter is more controversial. New and forthcoming surveys promise to improve this situation dramatically, with much more sensitive measurements of galaxy clustering and gravitational lensing. The challenge for theorists is to keep up, making predictions accurate enough to allow a full use of this data. This is the goal of my proposed research program, which includes three main components. The first component is to understand how dark matter halos evolve as they grow through repeated mergers. I have started a program of simulations of major halo mergers, to understand how the structural properties of halos relate to their formation history. The next steps are to apply our findings to cosmological simulations and translate the results into observational tests. We also intend to study minor mergers, which determine how individual galaxies evolve in groups and clusters. The second component is to trace dark matter substructure and characterize the efficiency of galaxy formation on small scales, by defining local samples of faint satellite galaxies and comparing them to simulations. The Milky Way has fewer dwarf satellites than predicted by the simplest theoretical models, the famous 'missing satellite problem'. In galaxy clusters, however, the picture is quite different, and dwarfs seem abundant. To resolve where and why galaxy formation tapers off, we need larger samples of satellite populations, across a broad range of environments, together with predictions from theory. My group has developed techniques to measure satellite abundance, and plans to apply these to several forthcoming surveys. The final component of the program is to study the density of dark matter in and around individual galaxies. Theory predicts that galaxies in the inner regions of clusters should form earlier and have higher dark matter densities. We have recently tested this prediction in the Virgo cluster, and have detected a significant gradient in density with radial position. This technique provides a new, direct estimate of the assembly epoch of individual galaxies. We propose to apply it to other nearby galaxy clusters, but also to high-redshift samples with density constraints from dynamics or lensing. Together, these components should provide a richer theoretical understanding of halos and subhalos, new tools to study galaxy formation, and tighter constraints on the properties of dark matter.