Two lensed star candidates at $z\simeq4.8$ behind the galaxy cluster MACS J0647.7+7015

TL;DR

This paper reports the discovery of two highly magnified star candidates at redshift approximately 4.8 behind a galaxy cluster, using JWST data, and discusses their properties, lensing models, and implications for high-redshift star studies.

Contribution

First identification of highly magnified star candidates at z~4.8 with JWST, including detailed lensing models and analysis of their stellar properties.

Findings

Candidates are likely lensed B-type stars with temperatures of 10,000-12,000 K.

Estimated macro-magnification of at least 50-90, with caustic-crossing magnifications of 10^4-10^5.

These are among the highest-redshift star candidates observed to date.

Abstract

We report the discovery of two extremely magnified lensed star candidates behind the galaxy cluster MACS J0647.7+7015, in recent multi-band James Webb Space Telescope (JWST) NIRCam observations. The candidates are seen in a previously known, $z_{phot}\simeq4.8$ dropout giant arc that straddles the critical curve. The candidates lie near the expected critical curve position but lack clear counter images on the other side of it, suggesting these are possibly stars undergoing caustic crossings. We present revised lensing models for the cluster, including multiply imaged galaxies newly identified in the JWST data, and use them to estimate a background macro-magnification of at least $\gtrsim90$ and $\gtrsim50$ at the positions of the two candidates, respectively. With these values, we expect effective, caustic-crossing magnifications of $10^4-10^5$ for the two star candidates. The Spectral Energy Distributions (SEDs) of the two candidates match well spectra of B-type stars with best-fit surface temperatures of $\sim10,000$ K, and $\sim12,000$ K, respectively, and we show that such stars with masses $\gtrsim20$ M$_{\odot}$ and $\gtrsim50$ M$_{\odot}$, respectively, can become sufficiently magnified to be observed. We briefly discuss other alternative explanations and conclude these are likely lensed stars, but also acknowledge that the less magnified candidate may instead be or reside in a star cluster. These star candidates constitute the second highest-redshift examples to date after Earendel at $z_{phot}\simeq6.2$, establishing further the potential of studying extremely magnified stars to high redshifts with the JWST. Planned visits including NIRSpec observations will enable a more detailed view of the candidates already in the near future.