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arXiv · 2512.08054

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

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

We present 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 $z_{\rm spec}=6.2$. New JWST NIRCam imaging and high-resolution NIRSpec 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 effective radii of $R_{\rm{eff}} \sim 5$ pc, stellar masses of $M_{*} \sim 10^{6}-10^{7}\,M_{\odot}$, and high stellar mass surface densities of $\Sigma_{*} > 10^{4}\,M_{\odot}~\rm{pc}^{-2}$. While their stellar populations are very young ($\sim 5-9$ Myr), their dynamical ages exceed unity, consistent with the clusters being gravitationally bound systems. Placing the star clusters in the size vs.~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 ($\xi_{\rm{ion}}$) is slightly suppressed in this central region, potentially indicating a locally elevated Lyman continuum escape fraction facilitated by feedback-driven channels.

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Abdurro'uf, Dan Coe, Tom Resseguier, Calla Murphy, Xinfeng Xu, Angela Adamo, Namrata Roy, Alaina Henry, Vasily Kokorev, Gabriel Brammer, Seiji Fujimoto, Henry C. Ferguson, Amanda Pagul, Rogier A. Windhorst, Timothy Heckman, Jose M. Diego, Hollis B. Akins, Joseph Allingham, Ricardo O. Amorín, Danielle A. Berg, Maruša Bradač, Larry D. Bradley, Wenlei Chen, John Chisholm, Christopher J. Conselice, Pratika Dayal, Miroslava Dessauges-Zavadsky, Andreas L. Faisst, Steven L. Finkelstein, Yoshinobu Fudamoto, Lukas J. Furtak, Yuichi Harikane, Tiger Yu-Yang Hsiao, Yolanda Jimenez-Teja, Anton M. Koekemoer, Rebecca L. Larson, Ray A. Lucas, Matteo Messa, Lamiya Mowla, Minami Nakane, Gaël Noirot, Richard Pan, Massimo Pascale, Johan Richard, Massimo Ricotti, Luke Robbins, Daniel Schaerer, Fengwu Sun, Eros Vanzella, Brian Welch, Chris Willott, Adi Zitrin. 2025-12-08. 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. https://arxiv.org/abs/2512.08054

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