Stellar population variations in the Milky Way's stellar halo

Eric F. Bell, Xiang Xiang Xue, Hans-Walter Rix, Christine Ruhland, David W. Hogg

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    Abstract

    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.

    Original languageEnglish
    Pages (from-to)1850-1859
    Number of pages10
    JournalThe Astronomical Journal
    Volume140
    Issue number6
    DOIs
    Publication statusPublished - Dec 2010

    Keywords

    • galaxies: bulges
    • galaxies: evolution
    • galaxies: general
    • galaxies: spiral
    • galaxies: stellar content
    • DIGITAL SKY SURVEY
    • HORIZONTAL-BRANCH STARS
    • SAGITTARIUS DWARF GALAXY
    • COLD DARK-MATTER
    • A-TYPE STARS
    • GALACTIC HALO
    • BLUE STRAGGLERS
    • TIDAL STREAM
    • GLOBULAR-CLUSTERS
    • SPHEROIDAL GALAXY

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