Explorations of the Milky Way's "New" Halo
Steven R Majewski
Department of Astronomy
The past decade has seen a remarkable transition in perceptions of the formation and evolution of galaxy halos, fueled by new theoretical paradigms for the growth of structure in the universe via hierarchical processes dominated by the presence of cold, dark matter (CDM). These theoretical developments have been abetted not only by the expanding panoply of observations of galaxies at high redshifts, but by milestone developments in the uncovering of evidence for past, and even ongoing, hierarchical build-up in our own Milky Way. While describing a popularly accepted overall paradigm for the growth of large structures from the accumulation of smaller ones, the current CDM models are not without problems, not least of which is the prediction of an abundance of surviving cores of past mergers orders of magnitude beyond the number of known satellites of the Milky Way, and, if such cores should indeed be present, the expectation of significant heating affects of such potentially clumpy sub-halo substructure on cold spiral disks, dwarf satellites and star clusters.
Only in the past decade or so has strong evidence for halo substructure been available, coming from complete or unbiased samples of halo stars. The advantage of complete phase space information on distant halo stars for revealing halo substructure is clear, but the task, particularly obtaining the absolute astrometry, is arduous. However, as recent models have shown, the hallmarks of strongly coherent moving groups should be recoverable within samples of halo tracer stars at sufficiently large distances from the crowded disk even if only line-of-sight velocities can be procured. Indeed, with this technique, several small distances radial velocity (RV) clumpings, suggestive of intercepted tidal streams, were found in pencil beam surveys of BHB stars. The veracity of the technique was later brilliantly demonstrated with the notable detection of an important moving group in a survey of bulge stars. The discovery of the Sagittarius (Sgr) dwarf galaxy provides the smoking gun example of halo substructure generation at 0 redshift.
Thus, the present goal of Galactic halo studies is now widely perceived to be not whether, but how much of the Galactic halo has been accreted, a question formulated in the guise of "dual halo" descriptions that envision varying balances of ELS and SZ contributions to its formation.
The goal of our proposed research is to zero in on a more complete understanding of this "new" Milky Way halo by providing substantially more observational constraints on the nature of its substructure - i.e. a study of "newly accreted" material that sets essential boundary conditions on the "newly generated" models of halo formation. We propose a fully comprehensive and integrated observational program to ascertain the true nature of the accreted halo, including the actual filling factor of tidal debris and the likely mass spectrum of accreted entities. The usefulness of tidal debris in the measurement of the Galactic mass distribution as well as in reconstructing a detailed picture of the accretion history of the Galaxy are obvious goals after a detailed mapping of tidal debris trails in phase space is in hand. The program we outline is a practicable set of experiments that are a direct extension of successful predecessor programs (described in our series of papers "Exploring Halo Structure with Giant Stars"), and that approach these questions from two directions: (1) We key on the Galactic satellite system, which consists of objects (galaxies and clusters) that are most likely to be the prototypes of previously accreted entities and among which are expected to be current contributors to the accreted halo. (2) We intend to characterize the field star population of the outer halo with a systematic search for substructure. In both cases we follow a variant of the "position+radial velocity" search paradigm described above.
More information at www.math.virginia.edu
Project Sponsored By: U.S. Nsf - Directorate Math. & Physical Sciences
Start Date: 8/15/2003
- End Date: 1/31/2007
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