case western reserve university



Cosmology and the Milky Way

James Bullock (Harvard/CfA)

If the favored Cold Dark Matter (CDM) model of structure formation is correct, then the first objects to collapse in the Universe are low-mass systems, which fall together to form progressively larger structures over time. Our Galaxy system, for example, should have accreted and subsequently tidally destroyed ~100 low-mass galaxies in the past ~12 Gyr. The roughly 10 satellite galaxies we see around the Milky Way today correspond to the residual, surviving population of these early-collapse objects. Put in this context, near-field observations of the Milky Way and its environment offer a powerful probe of cosmology and galaxy formation on small scales and at early times. I discuss a program to simulate the formation of the Milky Way system within the context the LCDM "concordance" cosmological model. One interesting result is that the debris from accreted, disrupted, low-mass galaxies should produce an extended stellar cloud of material about the main galaxy that has similar characteristics to the observed "stellar halo" component of our Milky Way. A stellar halo formed in this manner should be dominated by spatially coherent substructure at large radii, and this should be detectable by several ongoing and upcoming surveys. Searches of this kind offer a direct test of whether cosmology is indeed hierarchical on small scales, where the current paradigm is facing its most serious challenges. More generally, Galactic observations help test ideas about star formation in early-forming, low-mass galaxies and are sensitive to the shape of the initial inflationary power spectrum and to the nature of dark matter.