A Large Dark Matter Core in the Fornax Dwarf Spheroidal Galaxy?

We use measurements of the stellar velocity dispersion profile of the Fornax dwarf spheroidal galaxy to derive constraints on its dark matter distribution. Although the data are unable to distinguish between models with small cores and those with cusps, we show that a large ≳1 kpc dark matter core in Fornax is highly implausible. Irrespective of the origin of the core, reasonable dynamical limits on the mass of the Fornax halo constrain its core radius to be no larger than ~700 pc. We derive an upper limit of r_core ≲ 300 pc by demanding that the central phase-space density of Fornax not exceed that directly inferred from the rotation curves of low-mass spiral galaxies. Furthermore, if the halo is composed of warm dark matter, then phase-space constraints force the core to be quite small in order to avoid conservative limits from the Lyα forest power spectrum, r_core ≲ 85 pc. We discuss our results in the context of the idea that the extended globular cluster distribution in Fornax can be explained by the presence of a large ~1.5 kpc core. A self-consistent core of this size would be drastically inconsistent with the expectations of standard warm or cold dark matter models and would also require an unreasonably massive dark matter halo, with V_max ≃ 200 km s^-1.