2018ApJ...867...69L

Cold dark matter scenarios of hierarchical large-scale structure formation predict the existence of abundant subhalos around large galaxies. However, the number of observed dwarf galaxies is far from this theoretical prediction, suggesting that most of the subhalos could be dark or quite faint. Gravitational lensing is a powerful tool to probe the mass distribution directly irrespective of whether it is visible or dark. Time delay anomalies in strongly lensed quasar systems are complementary to flux-ratio anomalies in probing dark matter substructure in galaxies. Here we propose that lensed gravitational waves detected by the third-generation ground detectors with quite accurate time delay measurements could be a much better tool for this study than conventional techniques. Combined with good quality images of lensed host galaxies identified by the electromagnetic counterpart measurements, lensed gravitational wave signals could make the systematic errors caused by dark matter substructures detectable at levels of several percent, depending on their mass functions, internal distribution of subhalos, and lensing system configuration.