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

TL;DR

The paper exploits JWST imaging and spectroscopy of the strongly lensed z=6.2 galaxy Cosmic Spear to resolve parsec-scale star clusters within a high-redshift galaxy, revealing three compact, dense clusters embedded in a clumpy arc and two central clumps. Using multi-code SED fitting, spatially resolved SED mapping, and a robust lens model, it finds extremely high stellar mass densities, young ages, and dynamical binding in the clusters, and a centrally concentrated, intensely star-forming clump with low dust. The results indicate a predominantly clustered mode of star formation at this epoch (CdFE ~ 85%) and offer crucial constraints on the formation paths of ancient globular clusters and nuclear star clusters, as well as implications for ionizing photon budgets during the end of reionization. The study demonstrates the power of JWST combined with gravitational lensing to probe the assembly of compact stellar systems in the early universe and sets the stage for statistical investigations of cluster formation at high redshift.

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 $Σ_{*} > 10^{4}\,M_{\odot}~\rm{pc}^{-2}$. 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 ($ξ_{\rm{ion}}$) is slightly suppressed in this central region, potentially indicating a locally elevated Lyman continuum escape fraction facilitated by feedback-driven channels.

Figures (13)

• Figure 1: NIRCam color image (red: F277W+F356W, green: F150W+F200W, and blue: F090W+F115W) of the Cosmic Spear, at $z=6.2$, in the background of galaxy cluster MACS J0308+2645 at $z=0.356$. The strongly-lensed galaxy (shown in the two right panels) is resolved into two main components: a pair of large, bright star-forming clumps (1 and 2), and an arc containing three distinct star clusters (3, 4, and 5) which are multiply imaged into seven sources (3a, 3b, 3c, 4a, 4b, 5a, and 5b). The top-left corner inset shows a counter-image identified from the lens model. It reveals a clumpy morphology, with three distinct substructures (lower limit). The red line represents a lensing critical curve at $z=6.2$, while the white lines show contours of constant magnifications.
• Figure 2: Results of morphological modeling using GALFIT applied to eleven NIRCam filters. In the modeling, the galaxy is decomposed into ten components: the diffuse light of the host galaxy underlying the arc (i.e., tail), two bright clumps, and seven compact sources associated with the multiple images of the star clusters. In the residual images, red, white, and blue colors are for positive, zero, and negative values, respectively. The reduced chi-square ($\chi_{\nu}^{2}$) values are shown in the bottom-right corner of the residual maps.
• Figure 3: Spectrophotometric data cube constructed by combining NIRCam imaging and NIRSpec IFU data using piXedfit. This data cube is used for spatially resolved SED fitting of the clumps. To create the data cube, the datasets were spatially and spectrally aligned through PSF matching to the spatial resolution of the longest-wavelength grid of the NIRSpec G395H/F290LP data, resampling to a pixel scale of $0\farcs1$, and aligning normalization between the spectra and the photometry for consistency. The top-left panel displays the [OIII]$\lambda\,5007$ map, derived from the NIRSpec IFU data by integrating the spectra accross a narrow ($100\,\AA$) wavelength grids centered at the emission line wavelength. The remaining panels show the NIRCam images following spatial alignment and PSF matching to the NIRSpec IFU resolution. The IFU's FoV is apparent in the top-left panel and is outlined by the white box in the rest of the panels.
• Figure 4: Results of the SED fitting for the clumps, star clusters (using stacked photometry), and the counter-image, performed with bagpipes. The SED of the star clusters (3, 4, and 5) are obtained from stacking the photometry of their multiple images. In each panel, the observed photometry is shown as green circles. The best-fit model is represented by the blue squares (photometry) and the black line (spectrum).
• Figure 5: Size (i.e., effective radius) and stellar mass surface density ($\Sigma_{*}$) of the star clusters in Cosmic Spear compared with star clusters observed in galaxies at high-redshift and nearby universe: BulletArc-z11 2025Bradac, Cosmic Gems Adamo2024, Firefly Sparkler Mowla2024, Cosmic Archipelago Messa2025a, Cosmic Grapes 2025Fujimoto, Sunrise Vanzella2023, Sunburst Rivera-Thorsen2019, local young star clusters Brown2021, globular clusters in the Milky Way 2018Baumgardt, and nuclear star clusters Neumayer2020Erwin2012Georgiev2009. For the clumps in the Cosmic Grapes, we only show ones that are gravitationally bound and have an effective radius smaller than $70$ pc.