TY - JOUR

T1 - A universal mass profile for dwarf spheroidal galaxies?

AU - Walker, Matthew G.

AU - Mateo, Mario

AU - Olszewski, Edward W.

AU - Penarrubia, Jorge

AU - Evans, N. Wyn

AU - Gilmore, Gerard

N1 - Publisher Copyright:
© 2009. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

PY - 2009/10/20

Y1 - 2009/10/20

N2 - We apply the Jeans equation to estimate masses for eight of the brightest dwarf spheroidal (dSph) galaxies. For Fornax, the dSph with the largest kinematic data set, we obtain a model-independent constraint on the maximum circular velocity, Vmax = 20+4 -3 km s-1. Although we obtain only lower limits of Vmax 10 km s-1 for the remaining dSphs, we find that in all cases the enclosed mass at the projected half-light radius is well constrained and robust to a wide range of halo models and velocity anisotropies. We derive a simple analytic formula that estimates M(rhalf ) accurately with respect to results from the full Jeans analysis. Applying this formula to the entire population of Local Group dSphs with published kinematic data, we demonstrate a correlation such that M(rhalf ) r1.4±0.4 half , or in terms of the mean density interior to the half-light radius, ρ r -1.6±0.4 half . This relation is driven by the fact that the dSph data exhibit a correlation between global velocity dispersion and half-light radius. We argue that tidal forces are unlikely to have introduced this relation, but tides may have increased the scatter and/or altered the slope. While the data are well described by mass profiles ranging over a factor of 2 in normalization (Vmax ∼ 10-20 km s-1), we consider the hypothesis that all dSphs are embedded within a "universal" dark matter halo. We show that in addition to the power law M ∞ r1.4, viable candidates include a cuspy "Navarro-Frenk-White" halo with Vmax ∼ 15 km s-1 and scale radius r0 ∼ 800 pc, as well as a cored halo with Vmax ∼ 13 km s-1 and r0 ∼ 150 pc. Finally, assuming that their measured velocity dispersions accurately reflect their masses, the smallest dSphs now allow us to resolve dSph densities at radii as small as a few tens of pc. At these small scales, we find mean densities as large as ρ 5 M pc-3 (200 GeV cm-3).

AB - We apply the Jeans equation to estimate masses for eight of the brightest dwarf spheroidal (dSph) galaxies. For Fornax, the dSph with the largest kinematic data set, we obtain a model-independent constraint on the maximum circular velocity, Vmax = 20+4 -3 km s-1. Although we obtain only lower limits of Vmax 10 km s-1 for the remaining dSphs, we find that in all cases the enclosed mass at the projected half-light radius is well constrained and robust to a wide range of halo models and velocity anisotropies. We derive a simple analytic formula that estimates M(rhalf ) accurately with respect to results from the full Jeans analysis. Applying this formula to the entire population of Local Group dSphs with published kinematic data, we demonstrate a correlation such that M(rhalf ) r1.4±0.4 half , or in terms of the mean density interior to the half-light radius, ρ r -1.6±0.4 half . This relation is driven by the fact that the dSph data exhibit a correlation between global velocity dispersion and half-light radius. We argue that tidal forces are unlikely to have introduced this relation, but tides may have increased the scatter and/or altered the slope. While the data are well described by mass profiles ranging over a factor of 2 in normalization (Vmax ∼ 10-20 km s-1), we consider the hypothesis that all dSphs are embedded within a "universal" dark matter halo. We show that in addition to the power law M ∞ r1.4, viable candidates include a cuspy "Navarro-Frenk-White" halo with Vmax ∼ 15 km s-1 and scale radius r0 ∼ 800 pc, as well as a cored halo with Vmax ∼ 13 km s-1 and r0 ∼ 150 pc. Finally, assuming that their measured velocity dispersions accurately reflect their masses, the smallest dSphs now allow us to resolve dSph densities at radii as small as a few tens of pc. At these small scales, we find mean densities as large as ρ 5 M pc-3 (200 GeV cm-3).

KW - Dwarf

KW - Galaxies

KW - Galaxies

KW - Kinematics and dynamics

KW - Local Group

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U2 - 10.1088/0004-637X/704/2/1274

DO - 10.1088/0004-637X/704/2/1274

M3 - Article

AN - SCOPUS:85021083651

SN - 0004-637X

VL - 704

SP - 1274

EP - 1287

JO - Astrophysical Journal

JF - Astrophysical Journal

IS - 2

ER -