Quasar Mesolensing—Direct Probe to Substructures around Galaxies

Recently, ``CDM crisis'' is under discussion. The main point of this crisisis that number of substructures presented by cosmological N-body simulationsbased on CDM scenario for structure formation is much larger than observedsubstructures. Therefore, it is crucial for this crisis to discriminate whetherexpected number of CDM substructures really exist but non-luminous or do notexist. In this paper, we present a new idea to detect such invisible substructuresby utilizing a gravitational lensing. Here, we consider quasars that aregravitationally lensed by a foreground galaxy. A substructure around thelensing galaxy may superposed on one of the lensed images of such quasars. Inthis situation, additional image splitting should occur in the image behind thesubstructure, and further multiple images are created. This is ``quasarmesolensing''. We estimate separation and time delay between further multiple images due toquasar mesolensing. The expected value is $1 \sim 30$ milli-arcsecond for theseparation and future fine resolution imaging enable us to find invisiblesubstructures, and is $1 \sim 10^3$ second for the time delay and high-speedmonitoring of such quasar will be able to find ``echo''-like variation due toquasar mesolensing in intrinsic variability of the quasar. Furthermore, weevaluate that the optical depth for the quasar mesolensing is $\sim 0.1$.Consequently, if we monitor a few multiple quasars, we can find ``echo''-likevariation in one of the images after intrinsic flux variations of quasars.Comment: 31 pages (including 11 figures) Accepted to PAS