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.
