NOAO >   Observing Info >   Approved Programs >   2006A-0106

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Proposal Information for 2006A-0106


PI: Roberta M. Humphreys, University of Minnesota, roberta@umn.edu
Address: School of Physics and Astronomy, 116 Church St. SE, Minneapolis, MN 55455, USA

CoI: Juan E. Cabanela, St. Cloud State University
CoI: Jeffrey A. Larsen, U.S. Naval Academy

Title: Mapping the Asymmetric Thick Disk

Abstract: We have found a significant spatial and kinematic asymmetry in the distribution of thick disk stars in the inner part of the Galaxy (Parker et al 2003, 2004), In quadrant I, \it l ~ 20 - 60\deg and 20 to 40\deg above and below the plane, there is an excess of ~ 25% in the number of probable thick disk stars compared to the complementary fields (\it l=340 -300\deg) in quadrant IV. The region of this asymmetry covers several hundred square degrees and is therefore a major substructure in the Galaxy due to more than small scale clumpiness. There is a corresponding kinematic signature; the stars showing the excess have a much slower rate of rotation (\omega) with a lag of 80 -90 km s^-1 in the direction of Galactic rotation. Possible explanations for the spatial and kinematic asymmetry include the fossil remnant of a merger, a triaxial thick disk, and gravitational interaction with the stellar bar in the disk. We propose a program of photometric and spectroscopic observations to map the size and extent of the asymmetry along our line of sight and to determine the degree of spatial and kinematic asymmetry above and below the plane. We are requesting time for wide-field imaging to extend the completeness limits of our star counts to much fainter limiting magnitudes to determine the extent of the asymmetry feature by comparison with the star counts in quadrant I. We also request time with the Hydra MOS to obtain a more complete kinematic sample in quadrant IV for a comparison with the stellar populations in quadrant I.


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