Journal
ASTROPHYSICAL JOURNAL
Volume 809, Issue 2, Pages -Publisher
IOP Publishing Ltd
DOI: 10.1088/0004-637X/809/2/144
Keywords
galaxies: individual (Milky Way); Galaxy: halo; Galaxy: stellar content; stars: individual (K giants)
Categories
Funding
- European Research Council under the European Union [321035]
- Alexander von Humboldt Foundation
- NSFC [11103031, 11233004, 11390371, 11003017]
- German Research Foundation (DFG) [Sonderforschungsbereich SFB 881]
- John N. Bahcall Fellowship
- W.M. Keck Foundation
- NSF [AST-1009886]
- Alfred P. Sloan Foundation
- National Science Foundation
- U.S. Department of Energy Office of Science
- University of Arizona
- Brazilian Participation Group
- Brookhaven National Laboratory
- University of Cambridge
- Carnegie Mellon University
- University of Florida
- French Participation Group
- German Participation Group
- Harvard University
- Instituto de Astrofisica de Canarias
- Michigan State/Notre Dame/JINA Participation Group
- Johns Hopkins University
- Lawrence Berkeley National Laboratory
- Max Planck Institute for Astrophysics
- Max Planck Institute for Extraterrestrial Physics
- New Mexico State University
- New York University
- Ohio State University
- Pennsylvania State University
- University of Portsmouth
- Princeton University
- Spanish Participation Group
- University of Tokyo
- University of Utah
- Vanderbilt University
- University of Virginia
- University of Washington
- Yale University
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1211989] Funding Source: National Science Foundation
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We characterize the radial density, metallicity, and flattening profile of the Milky Way's stellar halo, based on the large sample of spectroscopically confirmed giant stars from SDSS/SEGUE-2, spanning galactocentric radii 10 kpc <= r(GC) <= 80 kpc. After excising stars that were algorithmically attributed to apparent halo substructure (including the Sagittarius stream), the sample has 1757 K giants, with a typical metallicity precision of 0.2 dex and a mean distance accuracy of 16%. Compared to blue horizontal branch stars or RR Lyrae variables, giants are more readily understood tracers of the overall halo star population, with less bias in age or metallicity. The well-characterized selection function of the sample enables forward modeling of those data, based on ellipsoidal stellar density models, nu(*) (R, z), with Einasto profiles and (broken) power laws for their radial dependence, combined with a model for the metallicity gradient and the flattening profile. Among models with constant flattening, these data are reasonably well fit by an Einasto profile of n = 3.1 +/- 0.5 with an effective radius r(eff) = 15 +/- 2 kpc and a flattening of q = 0.7 +/- 0.02,. or comparably well by an equally flattened broken power. law, with radial slopes of alpha(in) = 2.1 +/- 0.3 and alpha(out) = 3.8 +/- 0.1, with a break. radius of r(break) = 18 +/- 1 kpc; this is largely consistent with earlier work. We find a modest. but significant metallicity gradient within the outer stellar halo, [Fe/H] decreasing outward. If we allow for a variable flattening q= f(r(GC)), we find the distribution of halo giants to be considerably more flattened at small radii, q(10 kpc) = 0.55 +/- 0.02, compared to q(>30 kpc) = 0.8 +/- 0.03. Remarkably, the data are then very well fit by a single power. law with index of 4.2 +/- 0.1 on the variable r(q) root R-2 + (z/q/r))(2). In this simple and better-fitting model, there is a break in flattening at similar to 20 kpc, instead of a break in the radial density function. While different parameterizations of the radial profile vary in their parameters, their implied density gradient, partial derivative ln nu(*)/partial derivative ln r, is stable along a direction intermediate between major and minor axis; this gradient is crucial in any dynamical modeling that uses halo stars as tracers.
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