Meso-structure in three strong-lensing systems

We map substructure in three strong-lensing systems having particularly good image data: the galaxy lens MG J0414+ 053 and the clusters SDSS J1004+ 411 and ACO1689. Our method is to first reconstruct the lens as a pixelated mass map and then subtract off the symmetric part ( in the galaxy case) or a projected Navarro- Frenk- White profile ( for the cluster lenses). In all three systems we find extended irregular structures, or meso- structures, having of order 10% of the total mass. In J0414+ 053, the meso- structure suggests a tidal tail connecting the main lens with a nearby galaxy; however, this interpretation is tentative. In the clusters, the identification of meso- structure is more secure, especially in ACO 1689, where two independent sets of lensed images imply very similar meso- structure. In all three cases, the meso- structures are correlated with galaxies but much more extended and massive than the stellar components of single galaxies. Such extended structures cannot plausibly persist in such high-density regions without being mixed; the crossing times are too short. The meso-structures therefore appear to be merging or otherwise dynamically evolving systems.