JWST's PEARLS: a new lens model for ACT-CL J0102$-$4915, "EL Gordo'', and the first red supergiant star at cosmological distances discovered by JWST (2210.06514v1)

Abstract: The first JWST data on the massive colliding cluster El Gordo confirm 23 known families of multiply lensed images and identify 8 new members of these families. Based on these families, which have been confirmed spectroscopically by MUSE, we derived an initial lens model. This model guided the identification of 37 additional families of multiply lensed galaxies, among which 28 are entirely new systems, and 9 were previously known. The initial lens model determined geometric redshifts for the 37 new systems. The geometric redshifts agree reasonably well with spectroscopic or photometric redshifts when those are available. The geometric redshifts enable two additional models that include all 60 families of multiply lensed galaxies spanning a redshift range $2<z\<6$. The derived dark-matter distribution confirms the double-peak configuration of mass found by earlier work with the southern and northern clumps having similar masses. We confirm that El Gordo is the most massive known cluster at $z\>0.8$ and has an estimated virial mass close the maximum mass allowed by standard cosmological models. The JWST images also reveal the presence of small-mass perturbers that produce small lensing distortions. The smallest of these is consistent with being a dwarf galaxy at $z=0.87$ and has an estimated mass of $3.8\times10^9$~\Msol, making it the smallest substructure found at $z>0.5$. The JWST images also show several candidate caustic-crossing events. One of them is detected at high significance at the expected position of the critical curve and is likely a red supergiant star at $z=2.1878$. This would be the first red supergiant found at cosmological distances. The cluster lensing should magnify background objects at $z>6$, making more of them visible than in blank fields of similar size, but there appears to be a deficiency of such objects.

• The paper refines the lens model for El Gordo by confirming 23 and discovering 37 new lensed systems through JWST’s deep observations.
• The paper uncovers dual dark matter clumps and identifies a dwarf galaxy (~3.8×10^9 M☉), providing detailed insights into the cluster’s mass substructure.
• The paper reports the first detection of a red supergiant star at cosmological distances (z=2.19), magnified by over 4000 times, advancing our understanding of stellar evolution.

Overview of "JWST's PEARLS: A New Lens Model for ACT-CL J0102−4915, 'El Gordo,' and the First Red Supergiant Star at Cosmological Distances Discovered by JWST"

The paper by Diego et al. explores the substantial new findings from the James Webb Space Telescope (JWST) observations of the massive galaxy cluster ACT-CL J0102−4915, also known as "El Gordo." This paper is a major step forward in understanding the cluster's complex structure and composition through the refined lens modeling achieved with JWST's high-resolution and deep observations.

Key Findings

1. Lens Model and Multiply Lensed Systems: The JWST data confirm 23 multiply lensed systems previously identified and reveal 37 new lensed galaxy systems, significantly increasing the number of constraints for lens modeling. The innovative use of these constraints led to the development of a powerful lens model capable of dissecting the mass distribution with unprecedented detail.
2. Mass Distribution in El Gordo: The lens model confirms El Gordo's dual mass peaks corresponding to its bimodal galaxy distribution, consistent with expectations from X-ray observations. The cluster includes at least two dense clumps of dark matter and has a virial mass estimate close to the maximum allowed by the standard cosmological models. This finding is crucial in ongoing discussions regarding the compatibility of observed massive clusters with the current cosmological framework.
3. Small-Scale Structures and Perturbers: JWST's sensitivity allowed the detection of small-mass perturbers, providing detailed substructure information. Notably, a dwarf galaxy with a mass of approximately $3.8 \times 10^9 M_\odot$ was identified, representing one of the smallest known substructures at a redshift greater than 0.5.
4. Detection of Individual Stars: A remarkable discovery is the first identification of a red supergiant star at a cosmological distance, named "Quyllur," at a redshift of 2.1878. This star, potentially magnified by factors over 4000 due to lensing, provides insights into stellar evolution and magnification effects mediated by galaxy clusters.
5. Possible Deficiency of High-Redshift Galaxies: The enhanced detection capability of JWST under the magnifying influence of El Gordo should naturally lead to the discovery of more high-redshift objects. However, the analysis suggests a scarcity of such detections, raising questions about the population distribution, detection limits, or cosmic evolution features.

Implications and Future Directions

The research offers rich implications both practically and theoretically. From a practical standpoint, the accurate mass models provided by JWST deepen our understanding of cluster mass distributions and enhance our ability to convert lensing information into precise cosmic measurements. Theoretically, the observed and modeled characteristics of El Gordo push the boundaries of the ΛCDM cosmological model, especially concerning the formation and evolution of massive structures at high redshifts.

Future developments in cosmic observations will benefit greatly from the methods and findings in this paper. For instance, further analysis combining strong and weak lensing data could yield a more robust mass estimate over larger radii. Additionally, with continued JWST observations, the paper could extend to include other similar clusters, helping to paint a more comprehensive picture of galaxy cluster evolution and dark matter distribution across the universe.

In summary, the research by Diego et al. harnesses JWST's advanced observational capabilities to provide profound insights into galaxy cluster physics and cosmology. It highlights the transformative potential of next-generation telescopes in enhancing our understanding of the universe, from individual stars at great distances to the largest gravitationally bound structures known.