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

arXiv:2512.08054 · astro-ph.GA · Submitted 2025-12-08 · Read on arXiv

astro-ph.GA

Submitted: 2025-12-08

Updated: 2026-09-18

Comments: 29 pages, 13 figures, accepted for publication in ApJ

Code: https://github.com/spacetelescope/drizzlepac

Project page: https://jwst-venus.github.io

License: http://creativecommons.org/licenses/by/4.0/

The gist: 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 spec=6.2.

Terminology

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 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 delensed effective radii of R 8 pc, stellar masses of M* about 10 6-10 7,M, and high stellar mass surface densities of Σ* 2 times 10 4,M-2. While their stellar populations are very young (about 6-11 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 (ξ) 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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