Journal
ASTRONOMICAL JOURNAL
Volume 140, Issue 6, Pages 1850-1859Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-6256/140/6/1850
Keywords
galaxies: bulges; galaxies: evolution; galaxies: general; galaxies: spiral; galaxies: stellar content
Categories
Funding
- National Science Foundation [AST 1008342]
- Deutsche Forschungsgemeinschaft
- Alfred P. Sloan Foundation
- National Aeronautics and Space Administration
- National Science Foundation
- U.S. Department of Energy
- Japanese Monbukagakusho
- Max Planck Society
- University of Chicago
- Fermilab
- Institute for Advanced Study
- Japan Participation Group
- Johns Hopkins University
- Los Alamos National Laboratory
- Max-Planck-Institute for Astronomy (MPIA)
- Max-Planck-Institute for Astrophysics (MPA)
- New Mexico State University
- University of Pittsburgh
- Princeton University
- United States Naval Observatory
- University of Washington
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1008342] Funding Source: National Science Foundation
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If the stellar halos of disk galaxies are built up from the disruption of dwarf galaxies, models predict highly structured variations in the stellar populations within these halos. We test this prediction by studying the ratio of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor populations) to main-sequence turn-off stars (MSTO stars; a feature of all populations) in the stellar halo of the Milky Way using data from the Sloan Digital Sky Survey. We develop and apply an improved technique to select BHB stars using ugr color information alone, yielding a sample of similar to 9000 g < 18 candidates where similar to 70% of them are BHB stars. We map the BHB/MSTO ratio across similar to 1/4 of the sky at the distance resolution permitted by the absolute magnitude distribution of MSTO stars. We find large variations of the BHB/MSTO star ratio in the stellar halo. Previously identified, stream-like halo structures have distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo features, e. g., the low-latitude structure, appear to be almost completely devoid of BHB stars, whereas other structures appear to be rich in BHB stars. The Sagittarius tidal stream shows an apparent variation in the BHB/MSTO ratio along its extent, which we interpret in terms of population gradients within the progenitor dwarf galaxy. Our detection of coherent stellar population variations between different stellar halo substructures provides yet more support to cosmologically motivated models for stellar halo growth.
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