  We characterize the non-sphericity of galaxy clusters by the projected axis
ratio of spatial distribution of star, dark matter, and X-ray surface
brightness (XSB). We select 40 simulated groups and clusters of galaxies with
mass larger than 5E13 Msun from the Horizon simulation that fully incorporates
the relevant baryon physics, in particular, the AGN feedback. We find that the
baryonic physics around the central region of galaxy clusters significantly
affects the non-sphericity of dark matter distribution even beyond the central
region, approximately up to the half of the virial radius. Therefore it is very
difficult to predict the the probability density function (PDF) of the
projected axis ratio of XSB from dark-matter only N-body simulations as
attempted in previous studies. Indeed we find that the PDF derived from our
simulated clusters exhibits much better agreement with that from the observed
X-ray clusters. This indicates that our present methodology to estimate the
non-sphericity directly from the Horizon simulation is useful and promising.
Further improvements in both numerical modeling and observational data will
establish the non-sphericity of clusters as a cosmological test complementary
to more conventional statistics based on spherically averaged quantities.
