Liang Dai

Galaxy clusters are the most powerful gravitational lenses in the known Universe. Near the lensing caustics they host some of the most dramatically magnified extragalactic sources ever detected, including extremely magnified individual super-luminous stars, and compact super star clusters. These are systems where small-scale lens substructures strongly couple to the effects from the macroscopic lens and create rich phenomenology. I will introduce the extremely magnified individual stars detected by the Hubble Space Telescope at z ~ 1-1.5, and explain the theory of intracluster microlensing. Observing and monitoring individual magnified stars will have several potential uses: (1) probing stellar-mass objects in the intracluster medium; (2) revealing a population of sub-galactic dark matter halos inside the cluster halo; (3) probing minuscule non-CDM dark matter structures through micro-caustic crossing events. In addition, highly magnified young star clusters in lensed starburst galaxies are detectable from Cosmic Noon redshifts. They have been utilized to probe galaxy-free dark matter subhalos through flux asymmetries, and the collective microlensing-induced variability of millions of cluster member stars may be detected to probe the cluster’s stellar luminosity function. The study of high magnification sources calls for observational advancements in two fronts: (i) more detected systems with space and large ground-based telescopes for building a statistically significant sample; (ii) high-cadence photometric monitoring of known extremely magnified individual stars during microlensing peak events.