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 5000deg² to z=1.1 and an equivalent mass threshold of 10^13.7h^-1M_⊙. 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 Ω_DE and w are significant, being factors of 2.8 and 2.4, respectively, times previous forecast uncertainties. Incorporating 8 new degrees of freedom to describe cluster selection with projection increases the forecast uncertainty in Ω_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.