James Webb Space Telescope Feed Post

JWST Discovery of 40+ Microlensed Stars in a Magnified Galaxy, the "Dragon" behind Abell 370

Upper panel: A false-color image of the entire “Dragon” arc behind the Abell 370 cluster (Soucail et al., 1987a), using JWST filters F090W, F150W, and F200W. North is up, East to the left, and a reference angular scale of 1'' is shown by the solid horizontal bar at the bottom right corner The dashed white rectangle shows the region of interest further analyzed in Figure 2. Lower panels: F200W zoom in on a part of the Dragon in the 2022 image (left panel) and in the effective F200W in 2023 (right panel). The effective F200W image in 2023 was made using F182M and F210M images (see §3.1). Examples of the apparently bright microlensing events are indicated, where dashed half-crosses show bright sources seen only in 2022 data and solid half-crosses show sources only in 2023. Many additional microlensing events exist, but are only visible in the differential image in Figure 2. Horizontal bars in the lower right corners show 1'' scales. Abstract: Strong gravitational magnification by massive galaxy clusters enable us to detect faint background sources, resolve their detailed internal structures, and in the most extreme cases identify and study individual stars in distant galaxies. Highly magnified individual stars allow for a wide range of applications, including studies of stellar populations in distant galaxies and constraining small-scale dark matter structures. However, these applications have been hampered by the small number of events observed, as typically one or a few stars are identified from each distant galaxy. Here, we report the discovery of 46 significant microlensed stars in a single strongly-lensed high-redshift galaxy behind the Abell 370 cluster at redshift of 0.725 when the Universe was half of its current age (dubbed the ``Dragon arc'), based on two observations separated by one year with the James Webb Space Telescope ({\it JWST}). These events are mostly found near the expected lensing critical curves, suggesting that these are magnified individual stars that appear as transients from intracluster stellar microlenses. Through multi-wavelength photometry and colors, we constrain stellar types and find that many of them are consistent with red giants/supergiants magnified by factors of thousands. This finding reveals an unprecedented high occurrence of microlensing events in the Dragon arc, and proves that {\it JWST}'s time-domain observations open up the possibility of conducting statistical studies of high-redshift stars and subgalactic scale perturbations in the lensing dark matter field.