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

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 and offset are naturally explained if EriII has a central dark matter core, but such a core may be challenging to explain in a ΛCDM cosmology. In this paper, we revisit the survival and evolution of EriII's SC, focusing for the first time on its puzzlingly large ellipticity (0.31+0.05-0.06). We perform a suite of 960 direct N-body simulations of SCs, orbiting within a range of spherical background potentials fit to ultra-faint dwarf (UFD) galaxy simulations. We find only two scenarios that come close to explaining EriII's SC. In the first scenario, EriII has a low-density dark matter core (of size ∼70 pc and density ≲ 2× 108, M⊙ , kpc-3). In this model, the high ellipticity of EriII's SC is set at birth, with the lack of tidal forces in the core allowing its ellipticity to remain frozen for long times. In the second scenario, EriII's SC orbits in a partial core, with its high ellipticity owing to its imminent tidal destruction. However, this latter model struggles to reproduce the large size of EriII's SC, and it predicts substantial tidal tails around EriII's SC that should have already been seen in the data. This leads us to favour the cored model. We discuss potential caveats to these findings, and the implications of the cored model for galaxy formation and the nature of dark matter.

NATURVETENSKAP Fysik Astronomi, astrofysik och kosmologi hsv//swe
NATURAL SCIENCES Physical Sciences Astronomy, Astrophysics and Cosmology hsv//eng
Galaxies: dwarf
Galaxies: haloes
Galaxies: individual: Eridanus II
Galaxies: star clusters: general
Galaxies: structure
Methods: numerical

Read, Justin I.University of Surrey (author)
Agertz, OscarLund University,Lunds universitet,Astronomi - Genomgår omorganisation,Institutionen för astronomi och teoretisk fysik - Genomgår omorganisation,Naturvetenskapliga fakulteten,Lund Observatory - Undergoing reorganization,Department of Astronomy and Theoretical Physics - Undergoing reorganization,Faculty of Science(Swepub:lu)os6277ag (author)
Pontzen, AndrewUniversity College London (author)
Rey, Martin P.Lund University,Lunds universitet,Astronomi - Genomgår omorganisation,Institutionen för astronomi och teoretisk fysik - Genomgår omorganisation,Naturvetenskapliga fakulteten,Lund Observatory - Undergoing reorganization,Department of Astronomy and Theoretical Physics - Undergoing reorganization,Faculty of Science,University of Oxford(Swepub:lu)ma0444re (author)
Goater, AlexUniversity of Surrey (author)
Taylor, EthanUniversity of Surrey (author)
Kim, Stacy Y.University of Surrey (author)
Delorme, MaximeUniversity of Paris-Saclay (author)
University of BarcelonaUniversity of Surrey (creator_code:org_t)

In:Monthly Notices of the Royal Astronomical Society: Oxford University Press (OUP)515:1, s. 185-2000035-87111365-2966

By the author/editor: Orkney, Matthew ...; Read, Justin I.; Agertz, Oscar; Pontzen, Andrew; Rey, Martin P.; Goater, Alex; show more...; Taylor, Ethan; Kim, Stacy Y.; Delorme, Maxime; show less...

About the subject

NATURAL SCIENCES: NATURAL SCIENCES; and Physical Science ...; and Astronomy Astrop ...

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