Arc sensitivity to cluster ellipticity, asymmetries, and substructures

Aims. We investigate how ellipticity, asymmetries and substructures separately affect the ability of galaxy clusters to produce strong lensing events, i.e. gravitational arcs, and how they influence the arc morphologies and fluxes. This is important for studies aiming, for example, at constraining cosmological parameters from statistical lensing, or at determining the inner structure of galaxy clusters through gravitational arcs. Methods. We create two-dimensional smoothed, differently elliptical and asymmetric versions of some numerical models. By subtracting these smoothed mass distributions from the corresponding numerical maps and by gradually smoothing the residuals before re-adding them to the clusters, we are able to see how the lensing properties of the clusters react to even small modification of the cluster morphology. We study in particular by how much ellipticity, asymmetries and substructures contribute to the strong lensing cross sections of clusters. We also investigate how cluster substructures affect the morphological properties of gravitational arcs, their positions and fluxes. Results. On average, we find that the contributions of ellipticity, asymmetries and substructures amount to -40%, -10% and -30% of the total strong lensing cross section, respectively. However, our analysis shows that substructures play a more important role in less elliptical and asymmetric clusters, even if located at large distances from the cluster centres (∼1 h-1 Mpc). Conversely, their effect is less important in highly asymmetric lenses. The morphology, position and flux of individual arcs are strongly affected by the presence of substructures in the clusters. Removing substructures on spatial scales ≲50 h-1 kpc, roughly corresponding to mass scales ≲5 × 1010 h-1 M⊙, alters the image multiplicity of ∼35% of the sources used in the simulations and causes position shifts larger than 5″ for ∼40% of the arcs longer than 5″. Conclusions. We conclude that any model for cluster lens cannot neglect the effects of ellipticity, asymmetries and substructures. On the other hand, the high sensitivity of gravitational arcs to deviations from regular, smooth and symmetric mass distributions suggests that strong gravitational lensing is potentially a powerful tool to measure the level of substructures and asymmetries in clusters. © ESO 2006.