2026/07/27 by Raul Jimenez, Elena Tomasetti, Carmela Lardo +1
#astro-ph.GA #astro-ph.CO
Origins of globular clusters (GCs) are linked to the assembly of their host galaxies. We analyze star-cluster populations in two strongly lensed systems that bracket Cosmic Dawn and Cosmic Noon: the Cosmic Gems arc (GEMS) at z=9.625, among the first galaxies, and the Sparkler at z=1.378. New STARRED deconvolution photometry of GEMS provides SEDs for ten unique, doubly imaged cluster candidates, while a homogeneous Bayesian analysis places both populations on a common cosmological timeline. The GEMS clusters formed at z\rm form≈ 10--11 (median ≃10.2), consistent with halo assembly at or above the atomic-cooling scale. Their photometry requires low metallicities: individual clusters are consistent with [Z/\rm H] \lesssim -1.2, and the data exclude [Z/\rm H]≥-0.5, though they cannot distinguish reliably below [Z/\rm H] ≃ -1.5. This conclusion is unchanged when using stellar-population models including binary evolution---important for ultraviolet emission at this age---yielding similarly low metallicities, [Z/\rm H]=-2.2 to -2.7. The formal estimate, [Z/\rm H] = -2.3±0.3, is consistent with the Milky Way GC metallicity floor, though its value remains prior-dependent. The Sparkler clusters formed ∼2.5 Gyr later, at z\rm form≈ 2--3.5 in a Cosmic Noon dwarf galaxy, and are more enriched ([Z/\rm H] ≈ -0.5). Comparison with Milky Way GCs places GEMS in an exceptionally early, metal-poor regime and the Sparkler among later, more enriched populations, though neither association uniquely determines an in-situ or ex-situ origin. Closed-box and gas-regulator calculations show both systems are compatible with limited pre-enrichment followed by rapid enrichment and accretion-regulated growth. Together, they probe distinct cluster-forming environments from Cosmic Dawn to Cosmic Noon.