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

Orkney, Matthew D.A. ; Read, Justin I. ; Agertz, Oscar LU ; Pontzen, Andrew ; Rey, Martin P. LU ; Goater, Alex ; Taylor, Ethan ; Kim, Stacy Y. and Delorme, Maxime (2022) In Monthly Notices of the Royal Astronomical Society 515(1). p.185-200
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 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... (More)

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.

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author
; ; ; ; ; ; ; and
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Galaxies: dwarf, Galaxies: haloes, Galaxies: individual: Eridanus II, Galaxies: star clusters: general, Galaxies: structure, Methods: numerical
in
Monthly Notices of the Royal Astronomical Society
volume
515
issue
1
pages
16 pages
publisher
Oxford University Press
external identifiers
  • scopus:85134598965
ISSN
0035-8711
DOI
10.1093/mnras/stac1755
language
English
LU publication?
yes
id
fd2f1367-ee5e-406f-b684-89b4f28c1c0e
date added to LUP
2022-09-23 15:06:54
date last changed
2024-04-15 19:35:36
@article{fd2f1367-ee5e-406f-b684-89b4f28c1c0e,
  abstract     = {{<p>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.</p>}},
  author       = {{Orkney, Matthew D.A. and Read, Justin I. and Agertz, Oscar and Pontzen, Andrew and Rey, Martin P. and Goater, Alex and Taylor, Ethan and Kim, Stacy Y. and Delorme, Maxime}},
  issn         = {{0035-8711}},
  keywords     = {{Galaxies: dwarf; Galaxies: haloes; Galaxies: individual: Eridanus II; Galaxies: star clusters: general; Galaxies: structure; Methods: numerical}},
  language     = {{eng}},
  month        = {{09}},
  number       = {{1}},
  pages        = {{185--200}},
  publisher    = {{Oxford University Press}},
  series       = {{Monthly Notices of the Royal Astronomical Society}},
  title        = {{EDGE : the puzzling ellipticity of Eridanus II's star cluster and its implications for dark matter at the heart of an ultra-faint dwarf}},
  url          = {{http://dx.doi.org/10.1093/mnras/stac1755}},
  doi          = {{10.1093/mnras/stac1755}},
  volume       = {{515}},
  year         = {{2022}},
}