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

Matthew D. A. Orkney; Justin I. Read; Oscar Agertz; Andrew Pontzen; Martin P. Rey; Alex Goater; Ethan Taylor; Stacy Y. Kim; Maxime Delorme

doi:10.1093/mnras/stac1755

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

Matthew D. A. Orkney, Justin I. Read, Oscar Agertz, Andrew Pontzen, Martin P. Rey, Alex Goater, Ethan Taylor, Stacy Y. Kim, Maxime Delorme

Department of Physics

Research output: Contribution to journal › Article › peer-review

7 Citations (SciVal)

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 low-density dark matter core (of size ${\sim}70\, \text{pc}$ and density $\lesssim 2\times 10^8\, \text{M}_{\odot }\, \text{kpc}^{-3}$). 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....

Original language	English
Journal	Monthly Notices of the Royal Astronomical Society
Early online date	29 Jun 2022
DOIs	https://doi.org/10.1093/mnras/stac1755
Publication status	Published - 30 Sept 2022

Data Availability Statement

Data available upon reasonable request.

Acknowledgements

We acknowledge Prof. Josh Simon for his helpful guidance, Prof. Mark Gieles for his useful advice on globular cluster evolution, Sebastiaan Zoutendijk for sharing his constraints on the density profile of Eridanus II, and the anonymous reviewer for their valuable comments.

In addition to software already mentioned, this work has made use of the public software python (Van Rossum & Drake 2009), Numpy (Van der Walt, Colbert & Varoquaux 2011), scipy (Virtanen et al. 2020) and matplotlib (Hunter 2007).

Funding

MO acknowledges the UKRI Science and Technology Facilities Council (STFC) for support (grant ST/R505134/1). This work was performed using the DIRAC Data Intensive service at Leicester, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). The equipment was funded by BEIS capital funding via STFC capital grants ST/K000373/1 and ST/R002363/1 and STFC DIRAC Operations grant ST/R001014/1. DIRAC is part of the National e-Infrastructure. OA acknowledges support from the Knut and Alice Wallenberg Foundation and the Swedish Research Council (grant 2019-04659). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 818085 GMGalaxies. AP was supported by the Royal Society. MR is supported by the Beecroft Fellowship funded by Adrian Beecroft and the Knut and Alice Wallenberg Foundation. ET acknowledges the UKRI Science and Technology Facilities Council (STFC) for support (grant ST/V50712X/1). MD acknowledges support by ERC-Syg 810218 WHOLE SUN.

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10.1093/mnras/stac1755

Engineering Dwarf Galaxy at the Edge of galaxy formation
Rey, M. (CoPI), Read, J. I. (PI), Agertz, O. (CoPI) & Pontzen, A. (CoPI)
1/04/19 → 1/04/27
Project: Research-related funding

Cite this

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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",

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 low-density dark matter core (of size ${\sim}70\, \text{pc}$ and density $\lesssim 2\times 10^8\, \text{M}_{\odot }\, \text{kpc}^{-3}$). 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....",

author = "Orkney, {Matthew D. A.} and Read, {Justin I.} and Oscar Agertz and Andrew Pontzen and Rey, {Martin P.} and Alex Goater and Ethan Taylor and Kim, {Stacy Y.} and Maxime Delorme",

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doi = "10.1093/mnras/stac1755",

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T1 - EDGE: the puzzling ellipticity of Eridanus II's star cluster and its implications for dark matter at the heart of an ultra-faint dwarf

AU - Orkney, Matthew D. A.

AU - Read, Justin I.

AU - Agertz, Oscar

AU - Pontzen, Andrew

AU - Rey, Martin P.

AU - Goater, Alex

AU - Taylor, Ethan

AU - Kim, Stacy Y.

AU - Delorme, Maxime

PY - 2022/9/30

Y1 - 2022/9/30

N2 - 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 ${\sim}70\, \text{pc}$ and density $\lesssim 2\times 10^8\, \text{M}_{\odot }\, \text{kpc}^{-3}$). 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....

AB - 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 ${\sim}70\, \text{pc}$ and density $\lesssim 2\times 10^8\, \text{M}_{\odot }\, \text{kpc}^{-3}$). 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....

U2 - 10.1093/mnras/stac1755

DO - 10.1093/mnras/stac1755

M3 - Article

SN - 0035-8711

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

ER -

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

Abstract

Data Availability Statement

Acknowledgements

Funding

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Engineering Dwarf Galaxy at the Edge of galaxy formation

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