Cold dark matter (CDM) models for the growth of structure in the Universe predict the growth of large structures from the accumulation of smaller ones and the persistence of long-lived, "sub-halo" substructure within large halos. However, the number of observed satellites of the Milky Way is orders of magnitude less than predicted by most CDM models, and, if these sub-halos are dark, their presence should heat dynamically cold spiral disks, dwarf satellites and star clusters. Thus, considerable problems remain in refining the CDM paradigm to explain the evolution of galaxies like the Milky Way. If the Milky Way halo formed from the merger of luminous sub-halos, we expect to see substructure in the phase space distribution of halo stars, clusters and satellites. Due to Galactic tidal effects, this substructure would be in the form of streams of stars from the disrupted parent star systems, like those recently discovered coming from the Sagittarius (Sgr) dwarf spheroidal and the star cluster Palomar 5. Tantalizing evidence that the distribution of halo stars is very lumpy has been accumulating over the past decade. If the merged CDM sub-halos were dark, the coherence of star streams like Sgr and Pal 5 place strong constraints on the sub-halo mass spectrum.
Dr. Steven Majewski and colleagues at the University of Virginia will acquire a more complete understanding of this "new" Milky Way halo by substantially expanding observational constraints on accreted substructure which set essential boundary conditions on current halo formation models. They will determine the actual filling factor of tidal debris and the likely mass spectrum of accreted entities. These measurements, in turn, permit inference of the Galactic mass distribution and the accretion history of the Galaxy. The program naturally expands upon previous work and approaches these questions from two directions: (1) The Galactic satellite system - objects that are most likely to be the prototypes of previously accreted entities, among which are current contributors to the accreted halo. First, this research team will undertake a comprehensive study of the Sgr dwarf spheroidal galaxy - the paradigm for Milky Way accretion - exploiting the clear, all-sky view of the Sgr core and tidal tails provided by the Two Micron All-Sky Survey. Second, they will continue the analysis of extended density profiles found around a number of Galactic dwarf spheroidal satellites, and ascertain whether these King profile "break populations" represent bound stellar halos or unbound tidal debris. Finally, they will expand the search for tidal disruption of Galactic satellites to the halo globular cluster system with a search for Pal 5 analogues. (2) The group will undertake a systematic search for substructure within the halo field star population using three large catalogues of K giant stars: the almost completed Grid Giant Star Survey, the companion Deep Giant Star Survey (DGSS), and a 225 square deg survey of K giants at the North Galactic Pole. In a pilot version of the DGSS, the team have undertaken observations in fields around the Magellanic Cloud periphery and uncovered substantial evidence for an outer halo that is highly networked by cold tidal streams and with no apparent contribution by a hot, pressure-supported component of halo stars. It is important to determine whether this finding is specific to regions near the Magellanic Clouds, or whether it represents the general character of the halo.
A majority of the funds in this award are for direct support of undergraduate and graduate students who will participate substantially in the research efforts. Undergraduate involvement will include individual Senior Thesis research projects and assistance with the gathering of data as part of a semi-dedicated spectroscopic program at the local observatory. Dr. Majewski will continue his active public lecture commitment, and continue to work with the University of Virginia Digital Media Lab to produce animations and other graphics that enhance the explanation of Galactic structure science to the public. ***
