The Cluster-EAGLE project: a comparison of dynamical mass estimators
  using simulated clusters
  Forthcoming large-scale spectroscopic surveys will soon provide data on
thousands of galaxy clusters. It is important that the systematics of the
various mass estimation techniques are well understood and calibrated. We
compare three different dynamical mass estimators using the C-EAGLE galaxy
clusters, a set of high resolution simulations with resolved galaxies a median
total mass, $M_{200c} = 10^{14.7} \, \mathrm{M_\odot}$. We quantify the bias
and scatter of the Jeans, virial, and caustic mass estimators using all
galaxies with a stellar mass $M_*> 10^9 \, \mathrm{M_\odot}$, both in the ideal
3D case and in the more realistic projected case. On average we find our mass
estimates are unbiased, though relative to the true mass within $r_{200c}$ the
scatter is large with a range of $0.09$ - $0.15$ dex. We see a slight increase
in the scatter when projecting the clusters. Selecting galaxies using the same
criteria, we find no significant difference in the mass bias or scatter when
comparing results from hydrodynamical and dark matter only simulations.
However, selecting galaxies by stellar mass reduces the bias compared to
selecting by total mass. Comparing X-ray derived hydrostatic and dynamical
masses, the former are ${\sim} 30$ per cent lower. We find a slight dependence
between substructure, measured using two different metrics, and mass bias. In
conclusion, we find that dynamical mass estimators, when averaged together, are
unbiased with a scatter of $0.11 \pm 0.02$ dex when including interloper
galaxies and with no prior knowledge of $r_{200c}$.
