Key points are not available for this paper at this time.
We present the first measurement of the mass function of galaxy clusters based directly on cluster masses derived from observations of weak gravitational lensing. To investigate the degree of sample incompleteness resulting from the X-ray based selection of the target clusters, we use a sample of 50 clusters with weak lensing mass measurements to empirically determine the relation between lensing mass and X-ray luminosity and the scatter about this relation. We use a complete, volume-limited sub-sample of 35 X-ray luminous clusters of galaxies at 0. 15~ 10¹5/h Mₛun) clusters. From this, we constrain sigma₈ (Omegaₘ/0. 3) ^0. 37 = 0. 67^+0. 04-₀. ₀₅ (68% confidence limits), agreeing well with constraints from the 3-year WMAP CMB measurements and estimates of cluster abundances based on X-ray observations, but somewhat lower than constraints from ``cosmic shear'' weak lensing measurements in random fields.
Håkon Dahle (Wed,) studied this question.