  Weak lensing convergence peaks are a promising tool to probe nonlinear
structure evolution at late times, providing additional cosmological
information beyond second-order statistics. Previous theoretical and
observational studies have shown that the cosmological constraints on
$\Omega_m$ and $\sigma_8$ are improved by a factor of up to ~ 2 when peak
counts and second-order statistics are combined, compared to using the latter
alone. We study the origin of lensing peaks using observational data from the
154 deg$^2$ Canada-France-Hawaii Telescope Lensing Survey. We found that while
high peaks (with height $\kappa$ >3.5 $\sigma_\kappa$, where $\sigma_\kappa$ is
the r.m.s. of the convergence $\kappa$) are typically due to one single massive
halo of ~$10^{15}M_\odot$, low peaks ($\kappa$ <~ $\sigma_\kappa$) are
associated with constellations of 2-8 smaller halos (<~$10^{13}M_\odot$). In
addition, halos responsible for forming low peaks are found to be significantly
offset from the line-of-sight towards the peak center (impact parameter >~
their virial radii), compared with ~0.25 virial radii for halos linked with
high peaks, hinting that low peaks are more immune to baryonic processes whose
impact is confined to the inner regions of the dark matter halos. Our findings
are in good agreement with results from the simulation work by Yang el al.
(2011).
