  Cosmological simulations of galaxy clusters typically find that the weight of
a cluster at a given radius is not balanced entirely by the thermal gas
pressure of the hot ICM, with theoretical studies emphasizing the role of
random turbulent motions to provide the necessary additional pressure support.
Using a set of high-resolution, hydrodynamical simulations of galaxy clusters
that include radiative cooling and star formation, we find instead that in the
most relaxed clusters rotational support exceeds that from random turbulent
motions for radii 0.1 - 0.5 r_500, and that the observed clusters are much
rounder than the simulated, relaxed clusters within ~ 0.4 r_500. Moreover,
while the observed clusters display an average ellipticity profile that does
not vary significantly with radius, the ellipticity of the relaxed CDM clusters
declines markedly with increasing radius, suggesting that the ICM of the
observed clusters rotates less rapidly than that of the relaxed CDM clusters
out to ~ 0.6 r_500. We also find the ellipticity profile of a simulated cluster
without radiative cooling is in much better agreement with the observations,
implying that over-cooling has a substantial impact on the gas dynamics and
morphology out to larger radii than previously recognized. It also suggests
that the 10%-20% systematic errors from non-thermal gas pressure support
reported for simulated cluster masses, obtained from fitting simulated X-ray
data over large radial ranges within r_500, may need to be revised downward.
These results demonstrate the utility of X-ray ellipticity profiles as a probe
of ICM rotation and over-cooling which should be used to constrain future
cosmological cluster simulations.
