Two Lensed Star Candidates at z ≃ 4.8 behind the Galaxy Cluster MACS J0647.7+7015

Abstract

We report the discovery of two extremely magnified lensed star candidates behind the galaxy cluster MACS J0647.7+015 using recent multiband James Webb Space Telescope (JWST) NIRCam observations. The star candidates are seen in a previously known, z phot ≃ 4.8 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 background macro-magnifications of at least ≳90 and ≳50 at the positions of the two candidates, respectively. With these values, we expect effective, caustic-crossing magnifications of ∼[103–105] for the two star candidates. The spectral energy distributions of the two candidates match well the spectra of B-type stars with best-fit surface temperatures of ∼10,000 K, and ∼12,000 K, respectively, and we show that such stars with masses ≳20 M ⊙ and ≳50 M ⊙, respectively, can become sufficiently magnified to be observable. We briefly discuss other alternative explanations and conclude that these objects are likely lensed stars, but also acknowledge that the less-magnified candidate may alternatively reside in a star cluster. These star candidates constitute the second highest-redshift examples to date after Earendel at z phot ≃ 6.2, establishing further the potential of studying extremely magnified stars at high redshifts with JWST. Planned future observations, including with NIRSpec, will enable a more detailed view of these candidates in the near future.

Spatially Resolved Stellar Populations of 0.3 < z < 6.0 Galaxies in WHL 0137–08 and MACS 0647+70 Clusters as Revealed by JWST: How Do Galaxies Grow and Quench over Cosmic Time?

“…The processed imaging data along with the photometric catalog are publicly available. 24 These data products have also been used in some recent studies (Bradley et al 2022b;Welch et al 2022b;Hsiao et al 2022;Meena et al 2023;Vanzella et al 2022).…”

“…These lens models have been used in previous studies in CLASH (e.g., Coe et al 2013;Zitrin et al 2015;Chan et al 2017). Now with the addition of JWST NIRCam imaging data, which add on many new strongly lensed multiple-image candidates (thanks to its high spatial resolution and depth), a This new method has also been implemented in several clusters We used this lens model constructed by Meena et al (2023) for galaxies in the MACS 0647+70 field. We correct the M * and SFR obtained from SED fitting for the lensing magnification by dividing them with μ.…”

The GLASS-JWST Early Release Science Program. III. Strong-lensing Model of Abell 2744 and Its Infalling Regions

“…While independent models based on high-quality spectrophotometric data tend to be robust in the low-magnification regime (μ  3; see, e.g., Meneghetti et al 2017;Priewe et al 2017), such a requirement becomes progressively more important near the critical lines, at μ  10. In the most extreme cases, for sources found within a fraction of arcseconds of a critical line (e.g., Chen et al 2022;Diego et al 2022;Welch et al 2022;Meena et al 2023), including caustic crossing events (Kelly et al 2018), magnification values can be on the order of hundreds or more, implying that the interpretation on the nature of the source relies heavily on the lens model accuracy.…”

Exotic image formation in strong gravitational lensing by clusters of galaxies – II. Uncertainties

“…Hence, once a sufficiently large sample of HU image formations is identified in the sky, observations with JWST can lead to the identification of a large number of small-scale clumps in these HU image formations, increasing our chances of detecting substructures lensing. Observation of clusters lenses with JWST is also beneficial in making more accurate lens models by identifying new lensed systems (e.g., Pascale et al 2022;Diego et al 2023;Meena et al 2023) and allowing us to better differentiate the effect of the overall cluster lens and possible substructure (near HU image) itself.…”