The Cluster Mass Function from Weak Gravitational Lensing
  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<z<0.3 to constrain the abundance of very massive (M >~ 10^15/h M_sun) clusters. From this, we constrain sigma_8(Omega_m/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. 
H. Dahle