SCYON Abstract

Received on December 10 2007

The Cold Dark Matter Halos of Local Group Dwarf Spheroidals

Authors	Jorge Penarrubia, Alan W. McConnachie, and Julio F. Navarro
Affiliation	Department of Physics and Astronomy, University of Victoria, 3800 Finnerty Rd., Victoria, BC, V8P 5C2, Canada
Accepted by	Astrophysical Journal
Contact	jorpega@uvic.ca
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Abstract

We examine the dynamics of stellar systems embedded within cold dark matter (CDM) halos in order to assess observational constraints on the dark matter content of Local Group dwarf spheroidals (dSphs). Approximating the stellar and dark components by King and NFW models, respectively, we identify the parameters of dark halos consistent with the kinematics and spatial distribution of stars in dSphs as well as with cosmological N-body simulations. Our analysis shows, in agreement with previous work, that the total mass within the luminous radius is reasonably well constrained and approximately independent of the luminosity of the dwarf, highlighting the poor correspondence between luminosity and halo mass at the extremely faint end of the luminosity function. This result implies that the average density of dark matter is substantially higher in physically small systems such as Draco and Sculptor than in larger systems such as Fornax. Because massive CDM halos are denser than low mass ones at all radii, these results imply that Draco formed in a halo 5 times more massive than Fornax's despite being roughly 70 times fainter. Stellar velocity dispersion profiles (σ_p(R)) provide further constraints; in systems where data exist, σ_p(R) remains flat almost to the nominal "tidal" radius, implying that stars are deeply embedded within their cold dark matter halos and are quite resilient to tidal disruption. We estimate that halos would need to lose more than 90% of their original mass before tides begin affecting the kinematics of stars, but even then the peak circular velocity of the dark halo, V_max, is barely affected. We estimate that V_max is about 3 times higher than the central velocity dispersion of the stars, a result in agreement with previous estimates and that alleviates significantly the CDM "substructure crisis". We use these results to interpret the structural differences between the M31 and Milky Way (MW) dSph population and, in particular, the observation that M31 dwarfs are physically more extended by approximately a factor two than MW dwarfs of similar luminosity. Our modeling indicates that this difference in size should be reflected in their kinematics, and predicts that M31 dwarfs should have velocity dispersions up to a factor of ~2 higher than their MW counterparts. This is an eminently falsifiable prediction of CDM-motivated models of dSphs that may be verified with present observational capabilities.