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Title:
Cores and cusps in the dwarf spheroidals
Authors:
Evans, N. W.; An, J.; Walker, M. G.
Affiliation:
AA(Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA), AB(Dark Cosmology Centre, Niels Bohr Institutet, Københavns Universitet, Juliane Maries Vej 30, DK-2100 Copenhagen Ø, Denmark; Niels Bohr International Academy, Niels Bohr Institutet, Københavns Universitet, Blegdamsvej 17, DK-2100 Copenhagen Ø, Denmark), AC(Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA)
Publication:
Monthly Notices of the Royal Astronomical Society: Letters, Volume 393, Issue 1, pp. L50-L54. (MNRAS Homepage)
Publication Date:
02/2009
Origin:
MNRAS
MNRAS Keywords:
galaxies: kinematics and dynamics, galaxies: structure
DOI:
10.1111/j.1745-3933.2008.00596.x
Bibliographic Code:
2009MNRAS.393L..50E

Abstract

We consider the problem of determining the structure of the dark halo of nearby dwarf spheroidal galaxies (dSphs) from the spherical Jeans equations. Whether the dark haloes are cusped or cored at the centre is an important strategic problem in modern astronomy. The observational data comprise the line-of-sight velocity dispersion of a luminous tracer population. We show that when such data are analysed to find the dark matter density with the spherical Poisson and Jeans equations, then the generic solution is a dark halo density that is cusped like an isothermal (ρD ~ r-2). Although milder cusps (like the Navarro-Frenk-White ρD ~ r-1) and even cores are possible, they are not generic. Such solutions exist only if the anisotropy parameter β and the logarithmic slope of the stellar density γl satisfy the constraint γl = 2β at the centre or if the radial velocity dispersion falls to zero at the centre. So, for example, a dSph with an exponential light profile can exist in Navarro-Frenk-White halo and have a flat velocity dispersion, but anisotropy in general drives the dark halo solution to an isothermal cusp. The identified cusp or core is therefore a consequence of the assumptions (particularly of spherical symmetry and isotropy), and not the data.
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