Reconciling galaxy cluster shapes, measured by theorists vs observers
  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 ($\sim10^{14.7}\,M_\odot$) 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
{\emph Hubble Space Telescope} and {\emph 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
($<r_{\mathrm{vir}}/20$) 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.
