Cosmological information in weak lensing peaks

Recent studies have shown that the number counts of convergence peaks N(κ) in weak lensing (WL) maps, expected from large forthcoming surveys, can be a useful probe of cosmology. We follow up on this finding, and use a suite of WL convergence maps, obtained from ray-tracing N-body simulations, to study (i) the physical origin of WL peaks with different heights, and (ii) whether the peaks contain information beyond the convergence power spectrum P_. In agreement with earlier work, we find that high peaks (with amplitudes 3.5σ, where σ is the r.m.s. of the convergence κ) are typically dominated by a single massive halo. In contrast, medium-height peaks (≈0.5--1.5σ) cannot be attributed to a single collapsed dark matter halo, and are instead created by the projection of multiple (typically, 4--8) halos along the line of sight, and by random galaxy shape noise. Nevertheless, these peaks dominate the sensitivity to the cosmological parameters w, σ₈, and Ωₘ. We find that the peak-height distribution and its dependence on cosmology differ significantly from predictions in a Gaussian random field. We directly compute the marginalized errors on w, σ₈, and Ωₘ from the N(κ)+P_ combination, including redshift tomography with source galaxies at zₛ=1 and zₛ=2. We find that the N(κ)+P_ combination has approximately twice the cosmological sensitivity compared to P_ alone. These results demonstrate that N(κ) contains non-Gaussian information complementary to the power spectrum.