Journal
ASTROPHYSICAL JOURNAL
Volume 738, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/738/1/79
Keywords
galaxies: individual (MilkyWay); Galaxy: halo; Galaxy: structure; stars: horizontal-branch; stars: kinematics and dynamics
Categories
Funding
- Alfred P. Sloan Foundation
- National Science Foundation
- U.S. Department of Energy
- University of Arizona
- Brazilian Participation Grou
- Brookhaven National Laboratory
- University of Cambridge
- University of Florida
- French Participation Group
- German Participation Group
- Instituto de Astrofisica de Canarias
- Michigan State/Notre Dame/JINA Participation Group
- Johns Hopkins University
- Lawrence Berkeley National Laboratory
- Max Planck Institute for Astrophysics
- New Mexico State University
- NewYork University
- Ohio State University
- University of Portsmouth
- Princeton University
- University of Tokyo
- University of Utah
- Vanderbilt University
- University of Virginia
- University of Washington
- Yale University
- Max-Planck-Institute for Astronomy
- National Natural Science Foundation of China (NSFC) [10821061, 10876040, 10973021]
- Young Researcher Grant of National Astronomical Observatories
- Chinese Academy of Sciences
- National Basic Research Program of China [2007CB815103]
- NSF [AST 1008342, AST-0098435]
- U.S. National Science Foundation [PHY 02-16783, PHY 08-22648]
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [1008342, 1009886] Funding Source: National Science Foundation
- Science and Technology Facilities Council [ST/H00243X/1] Funding Source: researchfish
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We present and analyze the positions, distances, and radial velocities for over 4000 blue horizontal-branch (BHB) stars in the Milky Way's halo, drawn from SDSS DR8. We search for position-velocity substructure in these data, a signature of the hierarchical assembly of the stellar halo. Using a cumulative close pair distribution as a statistic in the four-dimensional space of sky position, distance, and velocity, we quantify the presence of position-velocity substructure at high statistical significance among the BHB stars: pairs of BHB stars that are close in position on the sky tend to have more similar distances and radial velocities compared to a random sampling of these overall distributions. We make analogous mock observations of 11 numerical halo formation simulations, in which the stellar halo is entirely composed of disrupted satellite debris, and find a level of substructure comparable to that seen in the actually observed BHB star sample. This result quantitatively confirms the hierarchical build-up of the stellar halo through a signature in phase (position-velocity) space. In detail, the structure present in the BHB stars is somewhat less prominent than that seen in most simulated halos, quite possibly because BHB stars represent an older sub-population. BHB stars located beyond 20 kpc from the Galactic center exhibit stronger substructure than at r(gc) < 20 kpc.
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