Study of Dark Matter Evolution of Milky Way Ancestral Galaxies in the Millennium Simulation

Jean P Walker, Rutgers University Eric Gawiser, Rutgers University Nicholas Bond, Rutgers University

Abstract The Millennium Simulation is the largest computer simulation of our universe ever made. It contains 1E10 particles of 1.18E10 Solar Masses and is able to trace the evolution of cosmic structure from eleven million years after the Big Bang to the present-day. This simulation follows the gravitational clustering of the dark matter particles that provide the majority of the mass in the universe. Dark matter assembles into ellipsoidal clumps called “halos”, and the Millennium Simulation tracks all halos of mass at least 236 billion solar masses. Although analytical techniques exist for predicting the mass distribution of the dark matter halos, Millennium allows us to track the mass distribution of “sub-halos” which are dark matter clumps large enough to host individual galaxies despite being buried within larger halos. Using the Millennium Simulation as our simulated data set, we have compiled mass distributions of dark matter sub-halos for the descendants of Lyman-Alpha Emitting galaxies (LAEs) that existing only two billion years after the Big Bang. These galaxies are thought to be the progenitors of L* galaxies such as the Milky Way (Gawiser et.al. 2007, Astrophysical Journal 671, 278). By studying the formation trees of the dark matter halos we have been able to identify the LAE descendents in the simulated present-day universe. Our results demonstrate that in extreme cases hundreds of LAEs merge into a single massive galaxy cluster today whereas other LAEs evolve in near-isolation into low-mass present-day galaxies. However, the median mass of LAE descendants supports the conclusion of Gawiser et al. that LAEs represent progenitors of typical spiral galaxies like the Milky Way.

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