tokens: We are grateful to the anonymous referee for constructive comments that significantly improved the manuscript. We are also grateful for the hospitality and financial support of the Max Planck Institute for Astrophysics that allowed IA to work directly with the MRS particle files in Garching, Germany. The Millennium Simulation databases used in this paper, and the web application providing online access to them, were constructed as part of the activities of the German Astrophysical Virtual Observatory. This work was partially supported by the National Science Foundation via grant AST-0708468.
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