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.

First Author

• 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

• 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, arXiv:2504.08879, submitted to ApJ
  
  
• 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., Suyu, S. H., Ertl, S., Caminha, G. B., Granata, G., Grillo, C., Schuldt, S., Barone, T., 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. L., 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, Etrl, 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, arXiv:2503.09718, submitted to A&A
  
  
• 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., Willer, 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 Galaxy Cluster Mergers by Combining Strong and Weak Gravitational Lensing in the JWST Era / MPA / Germany
• 2024 Probing Galaxy Clusters by Combining Strong and Weak Gravitational Lensing in the JWST Era / NOIRLab, University of Arizona / 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)
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