Mapping the Inner Halo of the Galaxy with 2MASS-Selected Horizontal-Branch Candidates

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Abstract
We use Two Micron All Sky Survey (2MASS) photometry to select blue horizontal-branch (BHB) candidates covering the sky, |b| > 15°. A 12.5 < J0 < 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| > 50°. We insert simulated star streams into the data and conclude that the high Galactic latitude BHB candidates are consistent with having no ~5° 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 θ 1°. 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.