Stellar population variations in the milky way's stellar halo

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

    Research output: Contribution to journalArticle

    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 MilkyWay 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 ∼9000 g < 18 candidates where ∼70% of them are BHB stars. We map the BHB/MSTO ratio across ∼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 (US)
    Pages (from-to)1850-1859
    Number of pages10
    JournalAstronomical Journal
    Volume140
    Issue number6
    DOIs
    StatePublished - Dec 2010

    Fingerprint

    halos
    stars
    metal
    prediction
    dwarf galaxies
    horizontal branch stars
    blue stars
    disk galaxies
    substructures
    tropical regions
    metallicity
    sky
    color
    detection
    distribution
    test
    gradients
    predictions
    metals

    Keywords

    • Galaxies: bulges
    • Galaxies: evolution
    • Galaxies: general
    • Galaxies: spiral
    • Galaxies: stellar content

    ASJC Scopus subject areas

    • Space and Planetary Science
    • Astronomy and Astrophysics

    Cite this

    Bell, E. F., Xue, X. X., Rix, H. W., Ruhland, C., & Hogg, D. W. (2010). Stellar population variations in the milky way's stellar halo. Astronomical Journal, 140(6), 1850-1859. https://doi.org/10.1088/0004-6256/140/6/1850

    Stellar population variations in the milky way's stellar halo. / Bell, Eric F.; Xue, Xiang Xiang; Rix, Hans Walter; Ruhland, Christine; Hogg, David W.

    In: Astronomical Journal, Vol. 140, No. 6, 12.2010, p. 1850-1859.

    Research output: Contribution to journalArticle

    Bell, EF, Xue, XX, Rix, HW, Ruhland, C & Hogg, DW 2010, 'Stellar population variations in the milky way's stellar halo', Astronomical Journal, vol. 140, no. 6, pp. 1850-1859. https://doi.org/10.1088/0004-6256/140/6/1850
    Bell, Eric F. ; Xue, Xiang Xiang ; Rix, Hans Walter ; Ruhland, Christine ; Hogg, David W. / Stellar population variations in the milky way's stellar halo. In: Astronomical Journal. 2010 ; Vol. 140, No. 6. pp. 1850-1859.
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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 MilkyWay 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 ∼9000 g < 18 candidates where ∼70{\%} of them are BHB stars. We map the BHB/MSTO ratio across ∼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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