Probing cosmology with weak lensing peak counts

We propose counting peaks in weak lensing (WL) maps, as a function of their height, to probe models of dark energy and to constrain cosmological parameters. Because peaks can be identified in two-dimensional WL maps directly, they can provide constraints that are free from potential selection effects and biases involved in identifying and determining the masses of galaxy clusters. As a pilot study, we have run cosmological N-body simulations to produce WL convergence maps in three models with different constant values of the dark energy equation-of-state parameter, w=-0.8, -1, and -1.2, with a fixed normalization of the primordial power spectrum (corresponding to present-day normalizations of σ₈=0.742, 0.798, and 0.839, respectively). By comparing the number of WL peaks in eight convergence bins in the range of -0.1<κ<0.4, in multiple realizations of a single simulated 3×3 degree field, we show that the first (last) pair of models differ at the 95% (85%) confidence level. A survey with depth and area comparable to those expected from the Large Synoptic Survey Telescope should have a factor of ≈50 better parameter sensitivity. These results warrant further investigation, in order to assess the constraints available when marginalization over other uncertain parameters is included, and with the specifications of a realistic survey folded into the analysis. Here we find that relatively low-amplitude peaks (κ~0.03), which typically do not correspond to a single collapsed halo along the line of sight, account for most of the parameter sensitivity. We study a range of smoothing scales and source galaxy redshifts (zₛ). With a fixed source galaxy density of 15 arcmin^-2, the best results are provided by the smallest scale we can reliably simulate, 1 arcmin, and zₛ=2 provides substantially better sensitivity than zₛ≤1.5.