The case for a cold dark matter cusp in Draco

J. I. Read; M. G. Walker; P. Steger

doi:10.1093/mnras/sty2286

paper

The case for a cold dark matter cusp in Draco

arXiv:1805.06934 · doi:10.1093/mnras/sty2286

Abstract

We use a new mass modelling method, GravSphere, to measure the central dark matter density profile of the Draco dwarf spheroidal galaxy. Draco's star formation shut down long ago, making it a prime candidate for hosting a 'pristine' dark matter cusp, unaffected by stellar feedback during galaxy formation. We first test GravSphere on a suite of tidally stripped mock 'Draco'-like dwarfs. We show that we are able to correctly infer the dark matter density profile of both cusped and cored mocks within our 95% confidence intervals. While we obtain only a weak inference on the logarithmic slope of these density profiles, we are able to obtain a robust inference of the amplitude of the inner dark matter density at 150pc, $ρ_{DM} (150 pc)$ . We show that, combined with constraints on the density profile at larger radii, this is sufficient to distinguish a $Λ$ Cold Dark Matter ( $Λ$ CDM) cusp $-$ that has $ρ_{DM} (150 pc) > 1.8 \times 1 0^{8} M_{⊙} kpc^{- 3}$ $-$ from alternative dark matter models that have lower inner densities. We then apply GravSphere to the real Draco data. We find that Draco has an inner dark matter density of $ρ_{DM} (150 pc) = 2. 4_{- 0.6}^{+ 0.5} \times 1 0^{8} M_{⊙} kpc^{- 3}$ , consistent with a $Λ$ CDM cusp. Using a velocity independent SIDM model, calibrated on $Λ$ SIDM cosmological simulations, we show that Draco's high central density gives an upper bound on the SIDM cross section of $σ / m < 0.57 cm^{2} g^{- 1}$ at 99% confidence. We conclude that the inner density of nearby dwarf galaxies like Draco provides a new and competitive probe of dark matter models.

arXiv comment19 pages, 11 Figures. Final version accepted for publication in MNRAS

The case for a cold dark matter cusp in Draco

Abstract

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