Understanding and Tracing Dark Matter in the Local Universe

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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.