A universal route for the formation of massive star clusters in giant molecular clouds

Young massive clusters (M ≥ 10⁴M_⊙) are proposed modern-day analogues of the globular clusters that were products of extreme star formation in the early Universe^1-4. The exact conditions and mechanisms under which young massive clusters form remain unknown^4,5—a fact further complicated by the extreme radiation fields produced by their numerous young stars^6-9. Here, we show that massive clusters are naturally produced in radiation-hydrodynamic simulations of isolated 10⁷M_⊙ giant molecular clouds with properties typical of the local Universe, even under the influence of radiative feedback. In all cases, these massive clusters grow to globular cluster masses within 5 million years (Myr) via a roughly equal combination of filamentary gas accretion and mergers with less massive clusters. Lowering the heavy-element abundance of the molecular cloud by a factor of ten reduces the opacity of the gas and better represents the high-redshift Universe^10,11. This results in higher gas accretion, leading to a mass increase of the largest cluster by a factor of around four. When combined with simulations of less massive molecular clouds¹² (10^4-6M_⊙), a clear relation emerges between the maximum cluster mass and the mass of the host cloud¹³. Our results indicate that young massive clusters—and potentially globular clusters—are simple power-law extensions of local cluster formation, and are insensitive to star formation thresholds. A universal picture emerges without the need for exotic formation scenarios^13-15.