The impact of baryons on massive galaxy clusters: halo structure and
  cluster mass estimates
  We use the BAHAMAS and MACSIS hydrodynamic simulations to quantify the impact
of baryons on the mass distribution and dynamics of massive galaxy clusters, as
well as the bias in X-ray and weak lensing mass estimates. These simulations
use the sub-grid physics models calibrated in the BAHAMAS project, which
include feedback from both supernovae and active galactic nuclei. They form a
cluster population covering almost two orders of magnitude in mass, with more
than 3,500 clusters with masses greater than $10^{14}\,\mathrm{M}_\odot$ at
$z=0$. We start by characterising the clusters in terms of their spin, shape
and density profile, before considering the bias in both weak lensing and
hydrostatic mass estimates. Whilst including baryonic effects leads to more
spherical, centrally concentrated clusters, the median weak lensing mass bias
is unaffected by the presence of baryons. In both the dark matter only and
hydrodynamic simulations, the weak lensing measurements underestimate cluster
masses by ${\approx}10\%$ for clusters with
$M_{200}{\leq}10^{15}\mathrm{M}_\odot$ and this bias tends to zero at higher
masses. We also consider the hydrostatic bias when using both the true density
and temperature profiles, and those derived from X-ray spectroscopy. When using
spectroscopic temperatures and densities, the hydrostatic bias decreases as a
function of mass, leading to a bias of ${\approx}40\%$ for clusters with
$M_{500}{\geq}10^{15}\,\mathrm{M}_\odot$. This is due to the presence of cooler
gas in the cluster outskirts. Using mass weighted temperatures and the true
density profile reduces this bias to $5{-}15\%$.
