Spatially Resolved Physical Properties of Young Star Clusters and Star-forming Clumps in the Brightest z > 6 Galaxy, the Strongly Lensed Cosmic Spear at z = 6.2

November 2026 • 2026ApJ..1011..167A

Authors • Abdurro'uf • Coe, Dan • Resseguier, Tom • Murphy, Calla • Xu, Xinfeng • Adamo, Angela • Roy, Namrata • Henry, Alaina • Kokorev, Vasily • Brammer, Gabriel • Fujimoto, Seiji • Ferguson, Henry C. • Pagul, Amanda • Windhorst, Rogier A. • Heckman, Timothy • Diego, Jose M. • Akins, Hollis B. • Allingham, Joseph F. V. • Amorín, Ricardo O. • Berg, Danielle A. • Bradač, Maruša • Bradley, Larry D. • Chen, Wenlei • Chisholm, John • Conselice, Christopher J. • Dayal, Pratika • Dessauges-Zavadsky, Miroslava • Faisst, Andreas L. • Finkelstein, Steven L. • Fudamoto, Yoshinobu • Furtak, Lukas J. • Harikane, Yuichi • Hsiao, Tiger Yu-Yang • Jimenez-Teja, Yolanda • Koekemoer, Anton M. • Larson, Rebecca L. • Lucas, Ray A. • Messa, Matteo • Mowla, Lamiya • Nakane, Minami • Noirot, Gaël • Pan, Richard • Pascale, Massimo • Richard, Johan • Ricotti, Massimo • Robbins, Luke • Schaerer, Daniel • Sun, Fengwu • Vanzella, Eros • Welch, Brian • Willott, Chris • Zitrin, Adi

Abstract • We present a spatially resolved analysis of stellar populations in the brightest z > 6 galaxy known to date (AB mag 23), the strongly lensed MACS0308-zD1 (dubbed the ``Cosmic Spear'') at z_spec = 6.2. New JWST NIRCam imaging and high-resolution NIRSpec integral field unit (IFU) spectroscopy span the rest-frame ultraviolet to optical. The NIRCam imaging reveals bright star-forming clumps and a tail consisting of three distinct, extremely compact star clusters that are multiply imaged by gravitational lensing. The star clusters have delensed effective radii of R_eff ≲ 8 pc, stellar masses of M_* ~ 10⁶--10⁷ M_⊙, and high stellar mass surface densities of Σ_* ≳ 2 × 10⁴ M_⊙ pc^-2. While their stellar populations are very young (~6--11 Myr), their dynamical ages exceed unity, consistent with the clusters being gravitationally bound systems. Placing the star clusters in the size versus stellar mass density plane, we find they occupy a region similar to other high-redshift star clusters within galaxies observed recently with JWST, being significantly more massive and denser than local star clusters. Spatially resolved analysis of the brightest clump reveals a compact, intensely star-forming core. The ionizing photon production efficiency (ξ_ion) is slightly suppressed in this central region, potentially indicating a locally elevated Lyman continuum escape fraction facilitated by feedback-driven channels.

Links

• ELECTR https://doi.org/10.3847/1538-4357/aeab99
• PREPRINT http://arxiv.org/abs/2512.08054