Mapping the Stellar Structure of the Milky Wat Thick Disk and Halo Using SEGUE Photometry

Journal Article · Tue Apr 13 00:00:00 EDT 2010 · The Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/714/1/663· OSTI ID:3019604

de Jong, Jelte T. A. ^[1]; Yanny, Brian ^[2]; Rix, Hans-Walter ^[1]; Dolphin, Andrew E. ^[3]; Martin, Nicolas F. ^[1]; Beers, Timothy C. ^[4]

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1. Max Planck Inst. fuer Astronomie, Heidelberg (Germany)
2. Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States)
3. Raytheon Company, Tucson, AZ (United States)
4. Joint Inst. for Nuclear Astrophysics (JINA), East Lansing, MI (United States). Center for the Evolution of the Elements (JINA-CEE); Michigan State Univ., East Lansing, MI (United States)

Here, we map the stellar structure of the Galactic thick disk and halo by applying color–magnitude diagram (CMD) fitting to photometric data from the Sloan Extension for Galactic Understanding and Exploration (SEGUE) survey. The SEGUE imaging scans allow, for the first time, a comprehensive analysis of Milky Way structure at both high and low latitudes using uniform Sloan Digital Sky Survey photometry. Incorporating photometry of all relevant stars simultaneously, CMD fitting bypasses the need to choose single tracer populations. Using old stellar populations of differing metallicities as templates, we obtain a sparse three-dimensional map of the stellar mass distribution at |Z|>1 kpc. Fitting a smooth Milky Way model comprising exponential thin and thick disks and an axisymmetric power-law halo allows us to constrain the structural parameters of the thick disk and halo. The thick-disk scale height and length are well constrained at 0.75 ± 0.07 kpc and 4.1 ± 0.4 kpc, respectively. We find a stellar halo flattening within ∼25 kpc of c/a = 0.88 ± 0.03 and a power-law index of 2.75 ± 0.07 (for 7 kpc ≲ R_GC ≲ 30 kpc). The model fits yield thick-disk and stellar halo densities at the solar location of ρ_thick,☉ = 10^−2.3±0.1 M_☉ pc⁻³ and ρ_halo,☉ = 10^{−4.20±0.05} M_☉ pc⁻³, averaging over any substructures. Our analysis provides the first clear in situ evidence for a radial metallicity gradient in the Milky Way's stellar halo: within R ≲ 15 kpc the stellar halo has a mean metallicity of [Fe/H] ≃ −1.6, which shifts to [Fe/H] ≃ −2.2 at larger radii, in line with the two-component halo deduced by Carollo et al. from a local kinematic analysis. Subtraction of the best-fit smooth and symmetric model from the overall density maps reveals a wealth of substructures at all latitudes, some attributable to known streams and overdensities, and some new. A simple warp cannot account for the low latitude substructure, as overdensities occur simultaneously above and below the Galactic plane.

• Research Organization: Fermi National Accelerator Laboratory (FNAL), Batavia, IL (United States)
• Sponsoring Organization: National Science Foundation (NSF); USDOE Office of Science (SC), High Energy Physics (HEP)
• Grant/Contract Number: 89243024CSC000002; AC02-07CH11359
• Other Award/Contract Number: PHY08-22648
• OSTI ID: 3019604
• Alternate ID(s): OSTI ID: 21448901
• Report Number(s): FERMILAB-PUB--10-068-A-CD; oai:inspirehep.net:837402; arXiv:0911.3900
• Journal Information: The Astrophysical Journal, Journal Name: The Astrophysical Journal Journal Issue: 1 Vol. 714; ISSN 0004-637X; ISSN 1538-4357
• Publisher: IOP Publishing
• Country of Publication: United States
• Language: English