Identification of more than 40 gravitationally magnified stars in a galaxy at redshift 0.725

Abstract

Strong gravitational magnifications enable to detect faint background sources, resolve their internal structures, and even identify individual stars in distant galaxies. Highly magnified individual stars allow various appli- cations, including studies of stellar populations in dis- tant galaxies and constraining dark matter structures in the lensing plane. However, these applications have been hampered by the small number of individual stars observed, as typically one or a few stars are identified from each distant galaxy. Here, we report the discovery of more than 40 microlensed stars in a single galaxy be- hind Abell 370 at redshift of 0.725 when the Universe was half of its current age (dubbed “the Dragon arc”), using James Webb Space Telescope (JWST) observations with the time-domain technique. These events are found near the expected lensing critical curves, suggesting that these are magnified stars that appear as transients from intr- acluster stellar microlenses. Through multi-wavelength photometry, we constrain stellar types and find that many of them are consistent with red giants/supergiants magnified by factors of hundreds. This finding reveals an unprecedented high occurrence of microlensing events in the Dragon arc, and proves that JWST’s time-domain observations open up the possibility of conducting statistical studies of high-redshift stars.