EDGE: the puzzling ellipticity of Eridanus II’s star cluster and its implications for dark matter at the heart of an ultra-faint dwarf

• DOI: 10.1093/mnras/stac1755
• Publication Date: 2022-06-29T21:06:01Z
• Authors: Matthew D A Orkney; Justin I Read; Oscar Agertz; Andrew Pontzen; Martin P Rey; Alex Goater; Ethan Taylor; Stacy Y Kim; Maxime Delorme
• Keywords: galaxies: individual: Eridanus II; Methods: numerical; Galaxies: dwarf; FOS: Physical sciences; galaxies: dwarf; Galaxies: individual: Eridanus II; Astrophysics - Astrophysics of Galaxies; methods: numerical; galaxies: haloes; Galaxies: structure; Astrophysics of Galaxies (astro-ph.GA); galaxies: star clusters: general; galaxies: structure; Galaxies: haloes; [PHYS.ASTR] Physics [physics]/Astrophysics [astro-ph]; [PHYS.ASTR]Physics [physics]/Astrophysics [astro-ph]; Galaxies: star clusters: general

ABSTRACT The Eridanus II (EriII) ‘ultra-faint’ dwarf has a large (15 pc) and low-mass (4.3 × 103 M⊙) star cluster (SC) offset from its centre by 23 ± 3 pc in projection. Its size are naturally explained if EriII central dark matter core, but such core may be challenging to explain ΛCDM cosmology. In this paper, we revisit the survival evolution of EriII’s SC, focusing for first time on puzzlingly ellipticity ($0.31^{+0.05}_{-0.06}$). We perform suite 960 direct N-body simulations SCs, orbiting within range spherical background potentials fit ultra-faint (UFD) galaxy simulations. find only two scenarios that come close explaining SC. scenario, low-density (of ${\sim}70\, \text{pc}$ density $\lesssim 2\times 10^8\, \text{M}_{\odot }\, \text{kpc}^{-3}$). model, high SC is set at birth, with lack tidal forces allowing remain frozen long times. second orbits partial owing imminent destruction. However, latter model struggles reproduce it predicts substantial tails around should have already been seen data. This leads us favour cored model. discuss potential caveats these findings, implications formation nature matter.