Do dark matter halos explain lensing peaks?

TLDR

Low‑significance peaks (S/N 2–3) carry key cosmological information in N‑body simulations, whereas Camelus relies mainly on high‑significance peaks (S/N > 3). The study evaluates Camelus, a halo‑based model for weak‑lensing peak counts, against N‑body simulations across 162 cosmologies. Camelus underestimates low‑S/N peaks by ~50 % and overestimates negative‑S/N peaks compared to N‑body, but shape noise reduces discrepancies to <20 %; it also yields larger, cosmology‑sensitive covariances, leading to ~30 % wider Ωm–σ8 confidence regions, underscoring the need for a cosmology‑dependent covariance to improve its predictive power.

Abstract

We have investigated a recently proposed halo-based model, Camelus, for predicting weak-lensing peak counts, and compared its results over a collection of 162 cosmologies with those from N-body simulations. While counts from both models agree for peaks with $\mathcal{S/N}>1$ (where $\mathcal{S/N}$ is the ratio of the peak height to the r.m.s. shape noise), we find $\approx 50\%$ fewer counts for peaks near $\mathcal{S/N}=0$ and significantly higher counts in the negative $\mathcal{S/N}$ tail. Adding shape noise reduces the differences to within $20\%$ for all cosmologies. We also found larger covariances that are more sensitive to cosmological parameters. As a result, credibility regions in the ${\Omega_m, \sigma_8\}$ are $\approx 30\%$ larger. Even though the credible contours are commensurate, each model draws its predictive power from different types of peaks. Low peaks, especially those with $2<\mathcal{S/N}<3$, convey important cosmological information in N-body data, as shown in \cite{DietrichHartlap, Kratochvil2010}, but \textsc{Camelus} constrains cosmology almost exclusively from high significance peaks $(\mathcal{S/N}>3)$. Our results confirm the importance of using a cosmology-dependent covariance with at least a 14\% improvement in parameter constraints. We identified the covariance estimation as the main driver behind differences in inference, and suggest possible ways to make Camelus even more useful as a highly accurate peak count emulator.