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Arxiv: In-situ formation of star clusters at z > 7 via galactic disk fragmentation; shedding light on ultra-compact clusters and overmassive black holes seen by JWST Published: 11/1/2024 5:32:46 PM Updated: 11/1/2024 5:32:46 PM

Paper abstract: We investigate the nature of star formation in gas-rich galaxies at z > 7forming in a markedly overdense region, in the whereabouts of a massivevirialized halo already exceeding 10^{12} M_{\odot}. We find that not onlythe primary galaxy, but also the lower-mass companion galaxies rapidly developmassive self-gravitating compact gas disks, less than 500~pc in size, whichundergo fragmentation by gravitational instability into very massive boundclumps. Star formation proceeds fast in the clumps, which quickly turn intocompact star clusters with masses in the range 10^5-10^8 M_{\odot} andtypical half-mass radii of a few pc, reaching characteristic densities above10^5 M_{\odot} pc^{-2}. The properties of the clusters in the lowest-massgalaxy bear a striking resemblance to those recently discovered by the JamesWebb Space Telescope (JWST) in the lensed Cosmic Gems arc system at z = 10.2.We argue that, due to their extremely high stellar densities, intermediate-massblack holes (IMBHs) would form rapidly inside the clusters, which would thenswiftly sink and merge on their way to the galactic nucleus, easily growing a10^7~M_{\odot} supermassive black hole (SMBH). Due to the high fractionalmass contribution of clusters to the stellar mass of the galaxies, in the range20-40\%, the central SMBH would comprise more than 10\% of the mass ofits host galaxy, naturally explaining the overmassive SMBHs discovered by JWSTat z > 6.