SIMBAD references

We apply a new nonparametric technique to reconstruct, with uncertainties, the projected mass distribution of the inner region of Abell 2218 using combined strong and weak lensing constraints from multiple-image systems and arclets with known redshifts. The reconstructed mass map broadly resembles previous, less detailed parametric models, but when examined in detail it shows several substructures not necessarily associated with light but strongly required by the lensing data. In particular, the highest mass peak is offset by ∼30 h^–1₅₀ kpc from the main light peak, and projected mass-to-light ratio in the directions of different cluster galaxies varies by at least a factor of 10. On comparison with mass estimates from models of the X-ray emitting gas, we find that the X-ray models underpredict the enclosed mass profile by at least a factor of 2.5; the discrepancy gets worse if we assume that mass traces light to the extent allowed by the lensing constraints.