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

Abstract

We use 2MASS photometry to select blue horizontal branch (BHB) candidates covering the sky |b|>15 deg. A 12.5<J<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 deg. We insert simulated star streams into the data and conclude that the high Galactic latitude BHB candidates are consistent with having no ~5 deg 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 <1 deg. 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.

11 pages, including figures. Accepted by AJ with minor revisions

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• One Ring to Encompass them All: A giant stellar structure that surrounds the Galaxy
• Sagittarius Tidal Debris 90 kpc from the Galactic Center
• Blue Horizontal Branch Stars in the Sagittarius dwarf spheroidal galaxy
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Cited by in corpus (20)

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• Distances to Galactic high-velocity clouds. I. Cohen Stream, complex GCP, cloud g1
• Distances to Galactic high-velocity clouds. Complex C
• Broadband UBVRI Photometry of Horizontal-Branch and Metal-Poor Candidates from the HK and Hamburg/ESO Surveys. I
• The Century Survey Galactic Halo Project III: A Complete 4300 deg^2 Survey of Blue Horizontal Branch Stars in the Metal-Weak Thick Disk and Inner Halo
• Numerical Color-Magnitude Diagram Analysis of SDSS Data and Application to the New Milky Way satellites
• Kinematic Structure in the Galactic Halo at the North Galactic Pole: RR Lyrae and BHB Stars show different kinematics
• High-Velocity Clouds and the Local Intergalactic Medium
• Large-scale environment of z~5.7 CIV absorption systems I: projected distribution of galaxies
• A Two-Point Correlation Function For Galactic Halo Stars
• The stellar content of the Hamburg/ESO survey. III. Field horizontal-branch stars in the Galaxy
• The Century Survey Galactic Halo Project II: Global Properties and the Luminosity Function of Field Blue Horizontal Branch Stars
• A Catalog of Candidate Field Horizontal-Branch and A-Type Stars. III. A 2MASS-Cleaned Version
• Kinematics of the Galactic Halo from Horizontal Branch stars in the Hamburg/ESO Survey
• VVV Survey of Blue Horizontal-Branch Stars in the Bulge-Halo Transition Region of the Milky Way
• Probing the Sagittarius stream with blue horizontal branch stars
• The Identification of BHB Stars Using GALEX and other photometry
• The BHB stars in the Survey Fields of Rodgers et al. (1993): New Observations and Comparisons with other Recent Surveys