Cosmological information in weak lensing peaks

Recent studies have shown that the number counts of convergence peaks N(kappa) 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-l. In agreement with earlier work, we find that high peaks (with amplitudes greater than or similar to 3.5 sigma, where sigma is the r.m.s. of the convergence kappa) are typically dominated by a single massive halo. In contrast, medium-height peaks (approximate to 0.5-1.5 sigma) 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, sigma(8), and Omega(m). 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, sigma(8), and Omega(m) from the N(kappa) + P-l combination, including redshift tomography with source galaxies at z(s) = 1 and z(s) = 2. We find that the N(kappa) + P-l combination has approximately twice the cosmological sensitivity compared to P-l alone. These results demonstrate that N(kappa) contains non-Gaussian information complementary to the power spectrum.