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Original languageEnglish
Article number20
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 20 Nov 2011


We introduce a method for measuring the slopes of mass profiles within dwarf spheroidal (dSph) galaxies directly from stellar spectroscopic data and without adopting a dark matter halo model. Our method combines two recent results: (1) spherically symmetric, equilibrium Jeans models imply that the product of half-light radius and (squared) stellar velocity dispersion provides an estimate of the mass enclosed within the half-light radius of a dSph stellar component, and (2) some dSphs have chemodynamically distinct stellar subcomponents that independently trace the same gravitational potential. We devise a statistical method that uses measurements of stellar positions, velocities, and spectral indices to distinguish two dSph stellar subcomponents and to estimate their individual half-light radii and velocity dispersions. For a dSph with two detected stellar subcomponents, we obtain estimates of masses enclosed at two discrete points in the same mass profile, immediately defining a slope. Applied to published spectroscopic data, our method distinguishes stellar subcomponents in the Fornax and Sculptor dSphs, for which we measure slopes Γ ≡ Δlog M/Δlog r = 2.61+0.43 –0.37 and Γ = 2.95+0.51 –0.39, respectively. These values are consistent with "cores" of constant density within the central few hundred parsecs of each galaxy and rule out "cuspy" Navarro-Frenk-White (NFW) profiles (dlog M/dlog r ≤ 2 at all radii) with a significance gsim 96% and gsim 99%, respectively. Tests with synthetic data indicate that our method tends systematically to overestimate the mass of the inner stellar subcomponent to a greater degree than that of the outer stellar subcomponent, and therefore to underestimate the slope Γ (implying that the stated NFW exclusion levels are conservative).

    Research areas

  • dark matter , galaxies: dwarf, galaxies: fundamental parameters, galaxies: kinematics and dynamics

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