Mapping the inner halo of the Galaxy with 2MASS-selected horizontal-branch candidates

We use Two Micron All Sky Survey (2MASS) photometry to select blue horizontal-branch (BHB) candidates covering the sky, |b| > 15degrees. A 12.5< J(0) < 15.5 sample of BHB stars traces the thick disk and inner halo to d(.) < 9 kpc, with a density comparable to 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| > 50degrees. We insert simulated star streams into the data and conclude that the high Galactic latitude BHB candidates are consistent with having no similar to5degrees wide star stream with density greater than 0.33 objects deg(-2) at the 95% confidence level. The absence of observed structure suggests that 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 less than or similar to 1degrees. This structure is apparently associated with stars in the thick disk and has a physical scale of 10 - 100 pc. Interestingly, such structures are expected by cosmological simulations that predict the majority of the thick disk may arise from accretion and disruption of satellite mergers.