EDGE: a new model for nuclear star cluster formation in dwarf galaxies

ABSTRACT Nuclear star clusters (NSCs) are among the densest stellar systems in the Universe and are found at the centres of many spiral and elliptical galaxies, and up to ${\sim }\,$40 per cent of dwarf galaxies. However, their formation mechanisms, and possible links to globular clusters (GCs), remain debated. This paper uses cosmological simulations of dwarf galaxies at a spatial resolution of $\sim 3$ pc to present a new formation mechanism for NSCs, showing they naturally emerge in a subset dwarfs with present-day halo masses of ${\sim }\, 5\times 10^{9}\, \rm {M}_{\odot }$. The mechanism proceeds following reionization quenching that stops the supply of cold star-forming gas. Next, a major merger causes a central dense gas reservoir to form, eventually exciting rapid cooling, leading to a significant starburst. An NSC forms in this starburst that quenches star formation thereafter. The result is a nucleated dwarf that has two stellar populations with distinct age: pre- and post-reionization. Our mechanism is unique because of the low mass of the host dwarf, and because it naturally leads to NSCs that contain two stellar populations with a $\gtrsim \,$1 billion year age separation. The former means that NSCs, formed in this way, can accrete on to galaxies of almost all masses. If these accreted NSCs fall to the centre of their host galaxy, they could then seed the formation of NSCs everywhere. The latter yields a predicted colour–magnitude diagram that has two distinct main sequence turn-offs. Several GCs orbiting the Milky Way, including Omega Centauri and M54, show similar behaviour, suggesting that they may be accreted NSCs.