Sangjun Cha

A High-Caliber View of the Bullet Cluster Through JWST Strong and Weak Lensing Analyses
Published in The Astrophysical Journal Letters, doi.org/10.3847/2041-8213/add2f0
NASA press release, Link to the article





The Bullet Cluster is one of the most famous merging galaxy clusters. Using high-resolution JWST imaging, we have reconstructed its mass distribution by combining strong and weak gravitational lensing data. The left figure presents a multi-wavelength view of the Bullet Cluster. Our advanced JWST-based lensing analysis can offer new insights into the dynamics of galaxy cluster mergers and the nature of dark matter.

Precision MARS Mass Reconstruction of A2744: Synergizing the Largest Strong-lensing and Densest Weak-lensing Data Sets from JWST
Published in The Astrophysical Journal, doi.org/10.3847/1538-4357/ad0cbf





Abell 2744, also known as Pandora’s Cluster, is famous for its complex merging substructures. Using deep JWST observations of strong and weak gravitational lensing, we have reconstructed a high-resolution map of Abell 2744. The right figure shows mass contours from our analysis, revealing two mass bridges. With the orientation of observed radio relics, we propose the merger axes of Abell 2744.

Publication Statistics
20 total refereed/under-review papers
6 first-author papers (6 refereed)
4 second or third author and 10 co-author papers (9 refereed, 5 under review)

First Author

• MrMARTIAN: A Multi-resolution Mass Reconstruction Algorithm Combining Free-form and Analytic Components, Cha, S., Jee, M. J., 2025, ApJ, 997, 18. doi.org/10.3847/1538-4357/ae2258
  
  
• A High-Caliber View of the Bullet Cluster Through JWST Strong and Weak Lensing Analyses, Cha, S., Cho, B. Y., Joo, H., Lee, W., HyeongHan, K., Scofield, Z. P., Finner, K., Jee, M. J., 2025, ApJL, 987, L15. doi.org/10.3847/2041-8213/add2f0
  
  
• Weak-lensing Mass Reconstruction of Galaxy Clusters with a Convolutional Neural Network. II. Application to Next-Generation Wide-Field Surveys, Cha, S., Jee, M. J., Hong, S. E., Park, S., Bak, D., Kim, T., 2025, ApJ, 981, 52. doi.org/10.3847/1538-4357/adb1b7
  
  
• Precision MARS Mass Reconstruction of A2744: Synergizing the Largest Strong-lensing and Densest Weak-lensing Data Sets from JWST, Cha, S., HyeongHan, K., Scofield, Z. P., Joo, H., Jee, M. J., 2024, ApJ, 961, 186. doi.org/10.3847/1538-4357/ad0cbf
  
  
• Model-independent Mass Reconstruction of the Hubble Frontier Field Clusters with MARS Based on Self-consistent Strong-lensing data, Cha, S., Jee, M. J., 2023, ApJ, 951, 140. doi.org/10.3847/1538-4357/acd111
  
  
• MARS: A New Maximum-entropy-regularized Strong Lensing Mass Reconstruction Method, Cha, S., Jee, M. J., 2022, ApJ, 931, 127. doi.org/10.3847/1538-4357/ac69df
  
  
  
  

Co-Author

• An Active Galaxy Cluster Merger at Cosmic Noon Revealed by JWST Weak Lensing and Multiwavelength Probes, Scofield, Z. P., Finner, K., Joo, H., Jee, M. J., Lee, W., Cha, S., Kim, J., Lin, Y.-H., Chary, R.-R., Faisst, A., and Lee, B., 2025, arXiv:2512.11022, submitted to ApJL
  
  
• Joint JWST–DECam Lensing Reveals That the Bullet Cluster Is a Minor Merger, Cho, B. Y., Jee, M. J., Joo, H., Cha, S., HyeongHan, K., 2025, arXiv:2512.03150, submitted to ApJ
  
  
• Reproducing Abell 2744 with the HyperMillennium Simulation, Wang, Q., Li, M., Gao, L., Guo, Q., Angulo, R. E., Cha, S., Cole, S., Frenk. C. S., HyeongHan, K., Li, R., Pei, W., Shan, H., Wang, J., White, S. D. M., 2025, arXiv:2510.27291, submitted to MNRAS
  
  
• Cosmology with supernova Encore in the strong lensing cluster MACS J0138-2155: Lens model comparison and H0 measurement, Suyu, S. H., Acebron, A., Grillo, C., Bergamini, P., Caminha, G. B., Cha, S., Diego, J. M., Ertl, S., Foo, N., Frye, B. L., Fudamoto, Y., Granata, G., Halkola, A., Jee, M. J., Kamieneski, P. S., Koekemoer, A. M., Meena, A. K., Newman, A. B., Nishida, S., Oguri, M., Rosati, P., Schuldt, S., Zitrin, A., Cañameras, R., Hayes, E. E., Larison, C., Mamuzic, E., Millon, M., Pierel, J. D. R., Tortorelli, L., and Wang, H., 2025, arXiv:2509.12319, submitted to A&A
  
  
• Cosmology with supernova Encore in the strong lensing cluster MACS J0138-2155: Time delays & Hubble constant measurement, Pierel, J. D. R., Hayes, E. E., Millon, M., Larison, C., Mamuzic, E., Acebron, A., Agrawal, A., Bergamini, P., Cha, S., Dhawan, S., Diego, J. M., Frye, B. L., Gilman, D., Granata, G., Grillo, C., Jee, M. J., Kamieneski, P. S., Koekemoer, A. M., Meena, A. K., Newman, A. B., Oguri, M., Padilla-Gonzalez, E., Poidevin, F., Rosati, P., Schuldt, S., Strolger, L. G., Suyu, S. H., Thorp, S., and Zitrin, A., 2025, arXiv:2509.12301, accepted in ApJ
  
  
• JWST Discovery of Strong Lensing from a Galaxy Cluster at Cosmic Noon: Giant Arcs and a Highly Concentrated Core of XLSSC 122, Finner, K., Cha, S., Scofield, Z. P., Jee, M. J., Lin, Y.-. heng ., Joo, H., Park, H., Morishita, T., Faisst, A., Lee, B., Wang, W., Chary, R.-R., 2025, ApJL, 994, L35 doi.org/10.3847/2041-8213/ae1d80
  
  
• Is Earendel a Star?: Investigating the Sunrise Arc Using JWST Strong and Weak Gravitational Lensing Analyses, Scofield, Z. P., Jee, M. J., Cha, S., Park, H., 2025, ApJ, 993, 226. doi.org/10.3847/1538-4357/ae07da
  
  
• A dynamical mass measure of an inactive black hole in the distant universe, Newman, A. B., Gu, M., Belli, S., Ellis, R. S., Gangula, S., Greene, J. E., Walsh, J. L., Suyu, S. H., Ertl, S., Caminha, G., Granata, G., Grillo, C., Schuldt, S., Barone, T. M., Bird, S., Glazebrook, K., Jafariyazani, M., Kriek, M., Matthews, A., Morishita, T., Nanayakkara, T., Pierel, J. D. R., Acebron, A., Bergamini, P., Cha, S., Diego, J. M., Foo, N., Frye, B., Fudamoto, Y., Jee, M. J., Kamieneski, P. S., Koekemoer, A. M., Meena, A. K., Nishida, S., Oguri, M., Rosati, P., Zitrin, A., 2025, arXiv:2503.17478, submitted
  
  
• Cosmology with Supernova Encore in the strong lensing cluster MACS J0138-2155: photometry, cluster members, and lens mass model, Ertl, S., Suyu, S. H., Schuldt, S., Granata, G., Grillo, C., Caminha, G. B., Acebron, A., Bergamini, P., Cañameras, R., Cha, S., Diego, J. M., Foo, N., Frye, B. L., Fudamoto, Y., Halkola, A., Jee, M. J., Kamieneski, P. S., Koekemoer, A. M., Meena, A. K., Nishida, S., Oguri, M., Pierel, J. D. R., Rosati, P., Tortorelli, L., Wang, H., Zitrin, A., 2025, A&A, 702, A157 doi.org/10.1051/0004-6361/202554477
  
  
• SN H0pe: The First Measurement of H₀ from a Multiply Imaged Type Ia Supernova, Discovered by JWST, Pascale, M., Frye, B. L., Pierel, J. D. R., Chen, W., Kelly, P. L., Cohen, S. H., Windhorst, R. A., Riess, A. G., Kamieneski, P. S., Diego, J. M., Meena, A. K., Cha, S., Oguri, M., Zitrin, A., Jee, M. J., Foo, N., Leimbach, R., Koekemoer, A. M., Conselice, C. J., Dai, L., Goobar, A., Siebert, M. R., Strolger, L., Willner, S. P., 2025, ApJ, 979, 13. doi.org/10.3847/1538-4357/ad9928
  
  
• Weak-lensing detection of intracluster filaments in the Coma cluster, HyeongHan, K., Jee, M. J., Cha, S., Cho, H., 2024, NatAs, 8, 377. doi.org/10.1038/s41550-023-02164-w
  
  
• Weak-lensing Analysis of the Complex Cluster Merger A746 with Subaru/Hyper Suprime-Cam, HyeongHan, K., Cho, H., Jee, M. J., Wittman, D., Cha, S., Lee, W., Finner, K., Rajpurohit, K., Brüggen, M., Forman, W., Jones, C., van Weeren, R., Botteon, A., Lovisari, L., Stroe, A., Domínguez-Fernández, P., O’Sullivan, E., Vrtilek, J., 2024, ApJ, 962, 100. doi.org/10.3847/1538-4357/ad1bcc
  
  
• Weak-lensing Mass Bias in Merging Galaxy Clusters, Lee, W., Cha, S., Jee, M. J., Nagai, D., King, L., ZuHone, J., Chadayammuri, U., Felix, S., Finner, K., 2023, ApJ, 945, 71. doi.org/10.3847/1538-4357/acb76b
  
  
• Weak-lensing Mass Reconstruction of Galaxy Clusters with a Convolutional Neural Network, Hong, S. E., Park, S., Jee, M. J., Bak, D., Cha, S., 2021, ApJ, 923, 266. doi.org/10.3847/1538-4357/ac3090
  

• 2025 Probing Dark Matter and Cluster Merger with JWST Strong and Weak Lensing: Case Studies of the Bullet Cluster and Abell 2744 / 33rd Texas Symposium on Relativistic Astrophysics / USA / Contributed (accepted)
• 2025 Weak-lensing Detection and Reconstruction of Filamentary Structures in the Rubin Era: A Case Study of Abell 2744 / Korean Rubin Science Workshop 2025 / South Korea / Contributed
• 2025 From Cluster Cores to Filaments: Gravitational Lensing Studies of Galaxy Clusters / CfA Galaxy Clusters Group Meetings / USA
• 2025 Lensing Analysis of Merging Clusters in the JWST Era: The Bullet Cluster and Abell 2744 / 2025 Merging Cluster Workshop / South Korea / Contributed
• 2025 JWST Lensing Analysis of Merging Galaxy Clusters: The Bullet Cluster and Abell 2744 / CL2025: Entering a Golden Age of Galaxy Cluster Studies / Taiwan / Contributed
• 2025 Probing Galaxy Cluster Mergers by Combining Strong and Weak Gravitational Lensing in the JWST Era / MPA Cosmology Seminar / Germany
• 2024 Probing Galaxy Clusters by Combining Strong and Weak Gravitational Lensing in the JWST Era / NOIRLab Friday Scientific Lunch Talks / USA
• 2024 Probing Galaxy Clusters by Combining Strong and Weak Lensing in the JWST Era: Mass Reconstruction of Abell 2744 / The 11th KIAS Workshop on Cosmology and Structure Formation / South Korea / Contributed
• 2024 Do Globular Cluster Trace Dark Matter? / 2024 KAS Fall Meeting / South Korea / Contributed
• 2024 Constraining Cosmological Parameters through Strong Lensing / 2024 KAS Spring Meeting / South Korea / Contributed
• 2023 Precision MARS Mass Reconstruction of Abell 2744: Combining Large Strong and Weak Lensing Datasets from JWST / 2023 KAS Fall Meeting / South Korea / Contributed
• 2022 Wide-field Weak-lensing Mass Reconstruction with Improved Convolutional Neural Network / 2022 KAS Fall Meeting / South Korea / Contributed
• 2022 MARS Probe of Hubble Frontier Fields Clusters / IAUGA 2022 / South Korea / Contributed
  

Integrated Undergraduate-and-Graduate Program Scholarship (Three-year full-tuition)
Yonsei University
2021 – 2023

Brain Korea 21 Plus Fellowship
Yonsei University
2021 – 2023

Yonsei Merit Academic Paper Award
Yonsei University
2023

Doctoral Student Research Fellowship (∼ $ 40,000)
Title: A research on Cosmological Parameter Prediction in Modern Cosmology Combining Strong Gravitational Lensing and Deep-Learning technique
National Research Foundation of Korea
2024 - 2026

Excellent Academic Paper Award
Yonsei University
2024

Korea National Police Agency Auxiliary Police
Served as part of the mandatory military service in South Korea
2016 – 2017