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Reconciling galaxy cluster shapes, measured by theorists vs observers

Harvey, David and Robertson, Andrew and Tam, Sut-Ieng and Jauzac, Mathilde and Massey, Richard and Rhodes, Jason and McCarthy, Ian G (2021) 'Reconciling galaxy cluster shapes, measured by theorists vs observers.', Monthly notices of the Royal Astronomical Society, 500 (2). pp. 2627-2644.


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 (<rvir/20) regions becomes decoupled: for example with greater misalignment of the central cluster galaxy. This may indicate overly efficient implementation of feedback from active galactic nuclei. Future exploitation of cluster shapes as a function of radii will require better understanding of core baryonic processes. Exploitation of shapes on any scale will require calibration on simulations extended all the way to mock observations.

Item Type:Article
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Publisher 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.
Date accepted:06 October 2020
Date deposited:16 July 2021
Date of first online publication:15 October 2020
Date first made open access:16 July 2021

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