Abundance measurements of galaxy clusters provide powerful constraints of cosmology. The observed distribution of clusters can potentially be used to disentangle whether the accelerated cosmic expansion can be explained by a modification to Einstein's theory of gravity or whether the explanation involves a new form of 'dark' energy. Such growth of structure measurements are both complementary to and provide an important cross-check of measurements of the geometry of the universe. There are two key requirements for cosmology with galaxy clusters: a census of these systems through cosmic time and the ability to connect the measured signal with the underlying mass of the galaxy cluster. In this era of large-area millimeter and optical wavelength surveys (including the South Pole Telescope (SPT) 2500-square-degree SZ-Survey and the Dark Energy Survey (DES)) where hundreds (mm-wave) to hundreds of thousands (optical) of clusters will be detected, the most serious limitation to cluster cosmology remains understanding and calibrating observable-mass relations. Combining cluster observables across wavelengths can both test and inform our knowledge of such scaling relations. As a pilot program for future explorations of the combined SPT and DES datasets, we explore the relation between the optical-richness, lambda, and SZ-signal for a sample of 567 optically-selected clusters from the Blanco Cosmology Survey, an ~ 80 square-degree survey located within the SPT-SZ survey. In this study we detect SZ-signal at increasing significance as a function of cluster richness but find that the recovered signal falls below expectations derived from models based on X-ray samples. We explore possible biases to our recovered signal and find that contamination from cluster members -- in particular radio and dust emission from galaxies--is small and that the majority of the discrepancy at the high mass end can be explained by errors in identifying the optical centers of clusters. The toolset developed here can be combined with future cluster catalogs from the Dark Energy Survey to help improve mass-richness scaling relations and ultimately constrain cosmological models.

The XMM-Newton - Blanco Cosmology Survey project (XMM-BCS) is a coordinated X-ray, optical and mid-infrared cluster survey in a field also covered by Sunyaev-Zel dovich effect (SZE) surveys by the South Pole Telescope and the Atacama Cosmology Telescope. The aim of the project is to study the cluster population in a 14 deg2 field (center: [alpha] H"23:29:18.4, [delta] H"-54:40:33.6). The uniform multi-wavelength coverage will also allow us for the first time to comprehensively compare the selection function of the different cluster detection approaches in a single test field and perform a cross-calibration of cluster scaling relations. In this work, we present a catalog of 46 X-ray selected clusters from the initial 6 deg2 survey core. We describe the XMM-BCS source detection pipeline and derive physical properties of the clusters. We provide photometric redshift estimates derived from the BCS imaging data and spectroscopic redshift measurements for a low redshift subset of the clusters. The photometric redshift estimates are found to be unbiased and in good agreement with the spectroscopic values. Our multi-wavelength approach gives us a comprehensive look at the cluster and group population up to redshifts z H"1. The median redshift of the sample is 0.47 and the median mass M500 H"1 x 1014 M{sub {circle_dot}} (H"2 keV). From the sample, we derive the cluster log N - log S using an approximation to the survey selection function and find it in good agreement with previous studies. We compare optical mass estimates from the Southern Cosmology Survey available for part of our cluster sample with our estimates derived from the X-ray luminosity. Weak lensing masses available for a subset of the cluster sample are in agreement with our estimates. Optical masses based on cluster richness and total optical luminosity are found to be significantly higher than the X-ray values. The present results illustrate the excellent potential of medium-deep, X-ray surveys to deliver cluster samples for cosmological modelling. In combination with available multi-wavelength data in optical, near-infrared and SZE, this will allow us to probe the dependence of the selection functions on relevant cluster observables and provide thus an important input for upcoming large-area multi-wavelength cluster surveys.