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SIM Lite and Dark Matter
The concordance Lambda-Cold Dark Matter model of the Universe has been very successful in describing large scale structure but is still challenged by observations on galactic scales. Fortunately, CDM models and their various proposed alternatives make a rich variety of testable predictions that make the Local Group and our own Milky Way galaxy laboratories for exploring dark matter (DM).
One of the key points of debate about DM has been whether it is cold or warm. Dwarf spheroidal galaxies (e.g. Leo and Draco) provide a great laboratory to test predictions for each possibility. In the case of cold dark matter (CDM), which may be composed of axions or neutralinos, there will be a cusp in the DM density at the center of a dwarf spheroidal. In the case of warm dark matter (WDM), like sterile neutrinos, there will be a flattened core at the center. Precision proper motion measurements of stars in dwarf spheroidal galaxies made by SIM Lite, combined with line of sight radial velocity measurements, will unambiguously determine whether these small galaxies are dominated by warm or cold DM, as illustrated in the figure.
 Tidal stream stars that are stripped from the Milky Way’s satellites have been identified as far away as 100 kpc and so they trace the total mass distribution at all radii of our galaxy. They can not only tell us about the DM distribution of the Milky Way but also if there are any DM clumps in the halo of our galaxy. Such clumps would be a new feature of DM and would be detected via the dispersion seen in the tidal streams. Warm and cold DM predictions will also be able to be tested if such lumps are detected. CDM predicts that there must be a large numbers of small sub-halos (an impressive 10^13) while WDM predicts a much more modest 100 sub-halos.
Hyper velocity stars are a recent discovery and their large distance and extreme galactocentric velocity ( >>600 km/s) indicate that they originated near the black hole at the center of our Galaxy. Their 3-D motions provide a means of measuring the shape of the total mass distribution of the Milky Way to large distances, shedding light on both the luminous and dark matter in our galaxy in a way that can only be accomplished by the accuracy afforded by SIMLite.
For more information on SIM investigations of Dark Matter click here.
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