Influence of Projection in Cluster Cosmology Studies
  Projection tends to skew the mass-observable relation of galaxy clusters by
creating a small fraction of severely blended systems, those for which the
measured observable property of a cluster is strongly boosted relative to the
value of its primary host halo. We examine the bias in cosmological parameter
estimates caused by incorrectly assuming a Gaussian (projection-free)
mass-observable relation when the true relation is non-Gaussian due to
projection. We introduce a mixture model for projection and explore Fisher
forecasts for a survey of 5000 sq. deg. to z=1.1 and an equivalent mass
threshold of 10^13.7 h^-1 solar masses. Using a blended fraction motivated by
optical cluster finding applied to the Millennium Simulation and applying
Planck and otherwise weak priors, we find that the biases in Omega_DE and w are
significant, being factors of 2.8 and 2.4, respectively, times previous
forecast uncertainties. Incorporating eight new degrees of freedom to describe
cluster selection with projection increases the forecast uncertainty in
Omega_DE and w by similar factors. Knowledge of these additional parameters at
the 5% level limits degradation in dark energy constraints to <10% relative to
projection-free forecasts. We discuss strategies for using simulations and
complementary observations to characterize the fraction of blended clusters and
their mass selection properties.
