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

Young massive clusters (M >= 10(4) M-circle dot) 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 10(7)M(circle dot) 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 clouds(12) (10(4-6)M(circle dot)), a clear relation emerges between the maximum cluster mass and the mass of the host cloud(13). 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).