Identification of $>$40 gravitationally magnified stars in a galaxy at redshift of 0.725 (2404.08045v2)

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 applications, including studies of stellar populations in distant 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 behind 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 intracluster 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 {\it JWST}'s time-domain observations open up the possibility of conducting statistical studies of high-redshift stars.

• The paper reports 46 microlensing events in a strongly lensed galaxy, marking an unprecedented detection of individual stars at high redshift.
• The study employs JWST NIRCam imaging over multiple epochs to identify transient microlensing events aligned with critical curves in Abell 370.
• The discovery advances our understanding of stellar evolution and dark matter distribution in distant galaxies, paving the way for future observations.

JWST Discovery of 40+ Microlensed Stars in a Magnified Galaxy

The paper reports on the discovery of an unprecedented number of microlensed stars in a strongly lensed high-redshift galaxy, dubbed the "Dragon," located behind the galaxy cluster Abell 370. Using the James Webb Space Telescope (JWST), the researchers identified 46 microlensed stars across two observational epochs, marking a significant advancement in our ability to detect and analyze individual stellar phenomena at cosmic distances.

Observational Overview

The observations leveraged the gravitational lensing effect produced by the massive Abell 370 cluster to significantly magnify the background Dragon galaxy at redshift 0.725. The high magnification allows for the detection of faint sources, including individual stars, which are typically challenging to observe at such distances.

Methodology

The research utilized images captured by JWST's NIRCam across multiple filters. By carefully analyzing the differences between images taken one year apart, the authors were able to identify transient events indicative of microlensing. Microlensing is expected to occur in regions where gravitational lensing creates critical curves, and this paper found that many events were indeed aligned with such features in the cluster Abell 370.

Results

The paper details the discovery of 46 significant microlensing events within the Dragon arc, a record number relative to previous observations. Notably, microlensing events typically feature only one or few stars per distant galaxy, making this high occurrence rate particularly noteworthy. The paper's comprehensive approach involved cross-referencing multiple lens models to confirm the positioning of these events relative to expected critical curves.

The detected stars predominantly exhibited red colors, leading the authors to conclude they are likely red giants or supergiants magnified by factors of thousands. This alignment with existing stellar population models provides new insights into the types of stars that populate distant galaxies and enhances our understanding of stellar evolution at high redshift.

Analysis of Notable Events

Among the detected events, notable cases like "event α" were located significantly off the main arc and were explored in greater detail. While initially considered a microlensed star, spectroscopic analysis suggested alternate possibilities, such as a Type Ia supernova within the foreground cluster or a "hostless" supernova at a higher redshift, implying a different origin for this particular transient.

Implications and Future Directions

The discovery of such a large number of microlensing events in a single galaxy opens up new potential for statistical studies of high-redshift stars and provides implications for the understanding of dark matter distribution at the subgalactic scale. This work highlights the critical role of JWST's advanced capabilities in the field of observational cosmology and astrophysics.

Future JWST observations hold the promise of even more refined data, allowing researchers to build on these findings and further constrain the characteristics of dark matter and the distribution of stellar populations in the universe. This paper marks a step forward in preparing for a new era of astrophysical investigations made possible by these advanced observational platforms.