University of Michigan
The distribution of the most massive galaxy clusters is particularly sensitive to cosmological parameters, making them ideal probes of cosmology. I will briefly discuss the success of the Millennium Gas Simulations in matching the X-ray properties of core-excised galaxy clusters. I will then present the merger histories of the total population of massive halos in the MGS. We find that halo properties are correlated over long timescales, and that assembly bias, in the form of halo formation time, imprints itself on the X-ray properties at redshift zero. This may introduce a bias towards clusters with earlier formation times in flux-limited X-ray or SZ surveys.