Triaxiality in galaxy clusters: Mass versus Potential reconstructions
  Accounting for the triaxial shapes of galaxy clusters will become important
in the context of upcoming cosmological surveys. We show that, compared to the
gas density distribution, the cluster gravitational potential can be better
characterised by a simple 3D model and is more robust against fluctuations.
Perturbations in the gas density distribution can have a substantial influence
on the derived thermodynamic properties, while cluster potentials are smooth
and well-approximated by a spheroidal model. We use a statistical sample of 85
galaxy clusters from a large cosmological hydrodynamical simulation to
investigate cluster shapes as a function of radius. In particular, we examine
the shape of isodensity and isopotential shells and analyze how it is affected
by the choice of component (gas vs. potential), substructure removal (for the
gas density) and the definition of the computation domain (interior vs.
shells). We find that the orientation and axis ratios of gas isodensity
contours are degenerate with the presence of substructures and unstable against
fluctuations. We observe that, as the derived cluster shape depends on the
method used for removing the substructures, thermodynamic properties extracted
from e.g. X-ray emissivity profiles suffer from this additional, often
underestimated bias. In contrast, the shape reconstruction of the potential is
largely unaffected by these factors and converges towards simple geometric
models for both relaxed and dynamically active clusters. The observation that
cluster potentials are better represented by simple geometrical models and
reconstructed with a low level of systematics for both dynamically active and
relaxed clusters suggests that by characterising galaxy clusters by their
potential rather than by their mass, dynamically active and relaxed clusters
could be combined in cosmological studies, improving statistics and lowering
scatter.
