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W. R. Brown, M. J. Geller, S. J. Kenyon (SAO), T. C. Beers (MSU), M. J. Kurtz, J. B. Roll (SAO)
We use 2MASS photometry to select blue horizontal branch (BHB) candidates covering the sky |b| > 15\circ. A 12.5 < J0 < 15.5 sample of BHB stars traces the thick disk and inner halo to d\sun ~q 9 kpc, with a density 3-5 times that of M giant stars. We base our sample selection strategy on the Century Survey Galactic Halo Project, a survey that provides a complete, spectroscopically-identified sample of blue stars to a similar depth as the 2MASS catalog. We show that a -0.20 < (J-H)0 < 0.10, -0.10 < (H-K)0 < 0.10 color-selected sample of stars is 65% complete for BHB stars, and is composed of 47% BHB stars. We apply this photometric selection to the full 2MASS catalog, and see no spatial overdensities of BHB candidates at high Galactic latitude |b| > 50\circ. We insert simulated star streams into the data and conclude that the high Galactic latitude BHB candidates are consistent with having no ~5\circ wide star stream with density greater than 0.33 objects deg-2 at the 95% confidence level. The absence of structure suggests there have been no major accretion events in the inner halo in the last few Gyr. However, at low Galactic latitudes a two-point angular correlation analysis reveals structure on angular scales \theta \lesssim 1\circ. This structure is apparently associated with stars in the thick disk, and has a physical scale of 10-100 pc. One possible explanation for this structure is provided by cosmological simulations that predict the majority of the thick disk may arise from accretion and disruption of satellite mergers.
The author(s) of this abstract have provided an email address for comments about the abstract: wbrown@cfa.harvard.edu
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Bulletin of the American Astronomical Society, 35#5
© 2003. The American Astronomical Soceity.