Flux F measured inside a 14 circular aperture (black dots) with errors given by the 68% scatter in the six directions of observation, as a function of halo mass M_h. Additional to this “intrinsic” variation, the inhomogeneous IGM, as well as the fact that galaxies of similar masses exhibit a variety in physical properties, introduces a scatter in possible values of observed flux; the 68% PI of this scatter is represented by the red, shaded region, while the solid, red line is the double power law given in Eq. (18). The dashed, blue line shows how the flux threshold of the detectors results in a minimum, observable halo mass M_h, min, and the shaded, blue region shows how the spread in detector thresholds, combined with the spread in fluxes, results in a spread in M_h, min.