Co-evolution of galaxies and their dark matter environment: constraining the standard structure formation paradigm through simulation and analysis of galaxy-galaxy lensing

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The formation and evolution of galaxies is driven primarily by the gravitational effects of the dark matter in which they are embedded. Lensing of distant galaxies by the mass associated with nearer systems offers a unique probe of the relation between galaxies and their dark matter environment. There has been much recent progress in measuring this galaxy-galaxy lensing (GGL) and more recently also galaxy-group lensing, and even better measurements will soon be provided by a new generation of wide-field imaging cameras. Quantitative interpretation of such observations is impossible without detailed numerical modeling. On small scales, GGL is sensitive to the mean mass profile of individual galaxies; on intermediate scales, it probes the galaxy content and internal structure of group and cluster halos; on large scales, it constrains the statistical relation between galaxies and dark matter. However, these aspects of the dark matter distribution affect the GGL signal on all scales and are intimately related to evolutionary processes which determine the physical properties of galaxies, hence the galaxy content of GGL surveys. We are employing a very large N-body simulation of the evolution of the dark matter distribution (the Millennium Run with ~ 1010 particles), together with galaxy formation modeling which can track the evolution of all galaxies brighter than the SMC throughout this (500h−1 Mpc)3 volume, to perform detailed simulations of GGL and galaxy-group lensing within the current concordance cosmology. By tracing light rays through this simulation we can mimic present and upcoming GGL surveys for a variety of assumptions about galaxy formation and evolution. A study of second- and higher-order auto- and cross-correlation statistics and other structure measures on our simulated galaxy + shear-field sky maps will clarify their expected signal-to-noise, their relation to the three-dimensional distribution of dark matter around galaxies, and their sensitivity to the evolutionary processes shaping the observable properties of galaxies. Such a study is required if current and future generations of GGL surveys are to test the standard paradigm, to constrain its parameters, or to show how galaxy evolution depends on dark matter environment.