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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 deg^2 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 ₃₄ 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 ₃₄ 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.
Erickson et al. (Thu,) studied this question.