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Reconciling galaxy cluster shapes, measured by theorists vs observers

Harvey, David; Robertson, Andrew; Tam, Sut-Ieng; Jauzac, Mathilde; Massey, Richard; Rhodes, Jason; McCarthy, Ian G

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David Harvey

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Sut-Ieng Tam
PGR Student Doctor of Philosophy

Jason Rhodes

Ian G McCarthy


If properly calibrated, the shapes of galaxy clusters can be used to investigate many physical processes: from feedback and quenching of star formation, to the nature of dark matter. Theorists frequently measure shapes using moments of inertia of simulated particles’. We instead create mock (optical, X-ray, strong- and weak-lensing) observations of the twenty-two most massive (∼1014.7 M⊙) relaxed clusters in the BAHAMAS simulations. We find that observable measures of shape are rounder. Even when moments of inertia are projected into 2D and evaluated at matched radius, they overestimate ellipticity by 56% (compared to observable strong lensing) and 430% (compared to observable weak lensing). Therefore, we propose matchable quantities and test them using observations of eight relaxed clusters from the Hubble Space Telescope and Chandra X-Ray Observatory. We also release our HST data reduction and lensing analysis software to the community. In real clusters, the ellipticity and orientation angle at all radii are strongly correlated. In simulated clusters, the ellipticity of inner (


Harvey, D., Robertson, A., Tam, S., Jauzac, M., Massey, R., Rhodes, J., & McCarthy, I. G. (2021). Reconciling galaxy cluster shapes, measured by theorists vs observers. Monthly Notices of the Royal Astronomical Society, 500(2), 2627-2644.

Journal Article Type Article
Acceptance Date Oct 6, 2020
Online Publication Date Oct 15, 2020
Publication Date 2021-01
Deposit Date Oct 16, 2020
Publicly Available Date Jul 16, 2021
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 500
Issue 2
Pages 2627-2644


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Copyright Statement
© The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.

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