The radial profile and flattening of the Milky Way's stellar halo to 80 kpc from the SEGUE K-giant Survey release_ieinemilxbclxilme5pzsvp5i4

by Xiang-Xiang Xue, Hans-Walter Rix, Zhibo Ma, Heather Morrison, Jo Bovy, Branimir Sesar, William Janesh

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We characterise the radial density, metallicity and flattening profile of the Milky Way's stellar halo, based on the large sample of 1757 spectroscopically confirmed giant stars from SDSS/SEGUE-2 after excising stars that were algorithmically attributed to apparent halo substructure (including the Sagittarius stream). Compared to BHB stars or RR Lyrae, 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 modelling of those data, based on ellipsoidal stellar density models, ν_* (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 α_in=2.1± 0.3 and α_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)∼ 0.57, compared to q(>30 kpc)∼ 0.8. Remarkably, the data are then very well fit by a single power-law of index ∼ 4.2±0.1 of the variable r_q≡√(R^2+(z/q(r))^2). In this simple and better fitting model, there is a break in flattening at ∼ 20 kpc, instead of a break in the radial density function.
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Type  article
Stage   accepted
Date   2015-08-20
Version   v2
Language   en ?
arXiv  1506.06144v2
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