HSTPROMO Internal Proper Motion Kinematics of Dwarf Spheroidal Galaxies: I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco

Eduardo Vitral; Roeland P. van der Marel; Sangmo Tony Sohn; Mattia Libralato; Andrés del Pino; Laura L. Watkins; Andrea Bellini; Matthew G. Walker; Gurtina Besla; Marcel S. Pawlowski; Gary A. Mamon

doi:10.3847/1538-4357/ad571c

paper

HSTPROMO Internal Proper Motion Kinematics of Dwarf Spheroidal Galaxies: I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco

arXiv:2407.07769 · doi:10.3847/1538-4357/ad571c

Abstract

We analyze four epochs of HST imaging over 18 years for the Draco dwarf spheroidal galaxy. We measure precise proper motions (PMs) for hundreds of stars and combine these with existing line-of-sight (LOS) velocities. This provides the first radially-resolved 3D velocity dispersion profiles for any dwarf galaxy. These constrain the intrinsic velocity anisotropy and resolve the mass-anisotropy degeneracy. We solve the Jeans equations in oblate axisymmetric geometry to infer the mass profile. We find the velocity dispersion to be radially anisotropic along the symmetry axis and tangentially anisotropic in the equatorial plane, with a globally-averaged value $β_{B} = - 0.2 0_{- 0.53}^{+ 0.28}$ , (where $1 - β_{B} \equiv ⟨ v_{tan}^{2} ⟩ / ⟨ v_{rad}^{2} ⟩$ in 3D). The logarithmic dark matter (DM) density slope over the observed radial range, $Γ_{dark}$ , is $- 0.8 3_{- 0.37}^{+ 0.32}$ , consistent with the inner cusp predicted in $Λ$ CDM cosmology. As expected given Draco's low mass and ancient star formation history, it does not appear to have been dissolved by baryonic processes. We rule out cores larger than 487, 717, 942 pc at respective 1-, 2-, 3- $σ$ confidence, thus imposing important constraints on the self-interacting DM cross-section. Spherical models yield biased estimates for both the velocity anisotropy and the inferred slope. The circular velocity at our outermost data point (900 pc) is $24.1 9_{- 2.97}^{+ 6.31} km s^{- 1} s$ . We infer a dynamical distance of $75.3 7_{- 4.00}^{+ 4.73}$ kpc, and show that Draco has a modest LOS rotation, with $⟨ v / σ ⟩ = 0.22 \pm 0.09$ . Our results provide a new stringent test of the so-called `cusp-core' problem that can be readily extended to other dwarfs.

35 pages, 18 figures, 3 Tables. Accepted for publication in ApJ. Journal version has better readability. Data is available at Zenodo: https://zenodo.org/records/11111113

HSTPROMO Internal Proper Motion Kinematics of Dwarf Spheroidal Galaxies: I. Velocity Anisotropy and Dark Matter Cusp Slope of Draco

Abstract

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