Arc statistics in triaxial dark matter halos: Testing the collisionless cold dark matter paradigm

Statistics of lensed arcs in clusters of galaxies serve as a powerful probe of both the nonsphericity and the inner slope of dark matter halos. We develop a semianalytic method to compute the number of arcs in triaxial dark matter halos. This combines the lensing cross section from the Monte Carlo ray-tracing simulations and the probability distribution function of the axis ratios evaluated from cosmological N-body simulations. This approach enables one to incorporate both asymmetries in the projected mass density and elongations along the line of sight analytically for the first time in cosmological lensed arc statistics. As expected, triaxial dark matter halos significantly increase the number of arcs relative to spherical models; the difference amounts to more than I order of magnitude, while the value of enhancement depends on the specific properties of density profiles. Then we compare our theoretical predictions with the observed number of arcs from 38 X-ray-selected clusters. In contrast to previous claims, our triaxial dark matter halos with inner density profile rho proportional to r(-1.5) in a Lambda-dominated cold dark matter (CDM) universe reproduce observations well. Since both the central mass concentration and triaxial axis ratios (minor-to-major axis ratio similar to 0.5) required to account for the observed data are consistent with cosmological N-body simulations, our result may be interpreted to lend strong support to the collisionless CDM paradigm at the mass scale of clusters.