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Primordial Rotating Disk Composed of ≥15 Dense Star Forming Clumps at Cosmic Dawn
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S. Fujimoto1,∗, M. Ouchi2,3,4,5, K. Kohno6,7, F. Valentino8,9, C. Giménez-Arteaga9,10, G. B. Brammer9,10,L. J. Furtak11, M. Kohandel12, M. Oguri13,14, A. Pallottini12, J. Richard15, A. Zitrin11, F. E. Bauer16,17,18,M. Boylan-Kolchin1, M. Dessauges-Zavadsky19, E. Egami20, S. L. Finkelstein1, Z. Ma20, I. Smail21,D. Watson9,10, T. A. Hutchison22, J. R. Rigby22, B. D. Welch22,23,24, Y. Ao25,26, L. D. Bradley27,G. B. Caminha28, K. I. Caputi29, D. Espada30,31, R. Endsley1, Y. Fudamoto13, J. González-López32,33,B. Hatsukade4,6,34, A. M. Koekemoer26, V. Kokorev28, N. Laporte35, M. Lee9,36, G. E. Magdis9,10,36,Y. Ono3, F. Rizzo9,10, T. Shibuya37, K. Shimasaku6,7, F. Sun15, S. Toft9,10, H. Umehata38,39,40,T. Wang41,42, and H. Yajima43Early galaxy formation, initiated by the dark matter and gas assembly, evolves through frequent mergers and feedback processes into dynamically hot, chaotic structures1. In contrast, dynamically cold, smooth rotating disks have been observed in massive evolved galaxies merely 1.4 billion years after the Big Bang2, suggesting rapid morphological and dynamical evolution in the early Universe. Probing this evolution mechanism necessitates studies of young galaxies, yet efforts have been hindered by observational limitations in both sensitivity and spatial resolution. Here we report high-resolution observations of a strongly lensed and quintuply imaged, low-luminosity, young galaxy at z = 6.072 (dubbed the Cosmic Grapes), 930 million years after the Big Bang. Magnified by gravitational lensing, the galaxy is resolved into at least 15 individual star-forming clumps with effective radii of re ≃ 10–60 parsec (pc), which dominate ≃ 70% of the galaxy’s total flux. The cool gas emission unveils a smooth, underlying rotating disk characterized by a high rotational-to-random motion ratio and a gravitationally unstable state (Toomre Q ≃ 0.2–0.3), with high surface gas densities comparable to local dusty starbursts with ≃ 103−5 solar mass (M⊙) per pc2. These gas properties suggest that the numerous star-forming clumps are formed through disk instabilities with weak feedback effects. The clumpiness of the Cosmic Grapes significantly exceeds that of galaxies at later epochs and the predictions from current simulations for early galaxies. Our findings shed new light on internal galaxy substructures and their relation to the underlying dynamics and feedback mechanisms at play during their early formation phases, potentially explaining the high abundance of bright galaxies observed in the early Universe3 and the dark matter core-cusp problem4.

21 difficulty in using the BPT diagram to distinguish the star formation and AGN in metal-poor systems with the gas-phase metallicity of Zgas 0.2Z has been argued and become clear at z > 5,94 which is likely the case in the Cosmic Grapes. In the NIRSpec IFU spectrum, we find that the line profiles of the Balmer emission lines and [O III]5008 are consistent. We thus conclude that the current data does not show any clear evidence of the presence of AGN.
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7. Properties of the individual star-forming clumps inside the Cosmic Grapes: We use SExtractor version 2.25.0101 to quantify the number of individual star-forming clumps that appear in the JWST/NIRCam maps (Extended Data Fig. 2). Here, we adopt the NIRCam/F150W map, which provides one of the highest spatial resolutions (PSF FWHM 0.05) and the best sensitivity owing to the longest exposure among our data. We run SExtractor using the default parameter values6, except for DETECT_MINAREA = 9, DETECT_THRESH = 2.0, DEBLEND_NTHRESH = 64, and DEBLEND_MINCONT = 0.0001 that are optimized to identify the small individual star-forming clumps separately. Fig. 1 shows the segmentation map obtained from the above parameter set, resulting in the identification of 15 individual star-forming clumps. Despite a different methodology adopted for clump identification, visual inspections already tell a stark difference from the number of clumps identified in lower redshift galaxies in recent JWST/NIRCa
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Given the remarkably low likelihood of capturing extraordinary phenomena through strong lensing twice, these numerous clumpy structures observed by the JWST, in conjunction with lensing effects, might be a common feature in early galaxies. In this paper, we regard N = 15 as the count of the individual star-forming clumps in the Cosmic Grapes, while this number could be increased with further high-resolution observations in the future. Using our pixel-by-pixel SED outputs and the [C II] line intensity map, we characterize the physical properties of M, SFR, and gas mass Mgas for these individual star-forming clumps. We sum the pixel-based output values for M and SFR, following the segmentation presented in Fig. 1. We note that the pixels of star-forming clump-13 did not pass the pixel mask p
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For Mgas, we first convert the [C II] luminosity L[CII] to Mgas pixel-by-pixel using the conversion factor calibrated in the literature16, and sum the values in the same manner as above. We discuss potential uncertainties in this conversion in Section 10. Note that because the ALMA beam size in the [C II] map (0.28 0.25) is larger than those in NIRCam, the Mgas estimate is only performed for star-forming clump-1, 3, 4, 6, 7, 8, and 9 whose segmentation areas are larger than the ALMA beam size. We summarize the M, SFR, and Mgas values for each star-forming clump in Extended Data Table 4. We also derive Mgas in the global galaxy scale in the same manner, resulting in 1.1+0.4 0.1 109 M, which is listed in Extended Data Table 3.
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