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Infall of substructures onto a Milky Way-like dark halo.pdf

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Infall of substructures onto a Milky Way-like dark halo

Mon. Not. R. Astron. Soc. 000, 1–9 (2007) Printed 11 March 2008 (MN LATEX style file v2.2) Infall of Substructures onto a Milky Way-like Dark Halo Yang-Shyang Li? and Amina Helmi? Kapteyn Astronomical Institute, University of Groningen, P.O. Box 800, 9700 AV Groningen, The Netherlands 11 March 2008 ABSTRACT We analyse the dynamical properties of substructures in a high-resolution dark matter simu- lation of the formation of a Milky Way-like halo in a ΛCDM cosmology. Our goal is to shed light on the dynamical peculiarities of the Milky Way satellites. Our simulations show that about 1/3 of the subhalos have been accreted in groups. We quantify this clustering by mea- suring the alignment of the angular momentum of subhalos in a group. We find that this signal is visible even for objects accreted up to z ～ 1, i.e. 8 Gyr ago, and long after the spatial coher- ence of the groups has been lost due the host tidal field. This group infall may well explain the ghostly streams proposed by Lynden-Bell Lynden-Bell to orbit the Milky Way. Our analy- ses also show that if most satellites originate in a few groups, the disk-like distribution of the Milky Way satellites would be almost inevitable. This non-random assignment of satellites to subhalos implies an environmental dependence on whether these low-mass objects are able to form stars, possibly related to the nature of reionization in the early Universe. With this picture, both the “ghostly streams” and the “disk-like configuration” are manifestations of the same phenomenon: the hierarchical growth of structure down to the smallest scales. Key words: methods: numerical - Galaxy: formation - galaxies: dwarf - galaxies: kinematics and dynamics - dark matter. 1 INTRODUCTION In cold dark-matter (CDM) cosmologies, large galaxies are the re- sult of the aggregation of smaller subunits. Some of these subunits may survive until the present day in the form of satellites, while some may be completely destroyed in the course of time,