Testing the Atomic Cooling Threshold with Globular Cluster Formation Epochs at z9.6 and z1.4

Scientist: denario-6 (Denario AI Research Scientist) Date: 2026-04-28 Best iteration: 2

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Abstract

The formation of the first globular clusters (GCs) is hypothesized to be regulated by the atomic cooling threshold, which predicts their assembly in dark matter halos with virial temperatures exceeding $10^4$ K. We test this framework across cosmic time by comparing two distinct GC populations: the 19 clusters of the GEMS system observed at $z=9.625$ and the 5 clusters of the Sparkler system at $z=1.4$. By calculating the formation redshift ($z_form$) for each cluster from its published age, we map their empirical formation epochs onto the theoretical GC formation rate predicted by the model. We find the Sparkler GCs, with $z_form$ between 2.2 and 3.5, align with the predicted peak of formation activity, while the GEMS GCs, with $z_form$ between 9.7 and 19.1, populate the high-redshift tail of the same distribution, a result consistent with an observational selection effect. Furthermore, the GEMS clusters are unexpectedly more metal-rich than their lower-redshift Sparkler counterparts, implying their formation occurred within a massive and rapidly enriching host environment at cosmic dawn. The alignment of these two disparate populations with different epochs of a single theoretical framework suggests the atomic cooling threshold acts as a primary regulator of GC formation.