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Nuclear star clusters (NSCs) are massive star clusters found ubiquitously inthe centres of galaxies, from the dwarf regime to massive ellipticals andspirals. The fraction of nucleated galaxies is as high as $>$ 90 % at$M_{\text{gal}} \sim 10^9 M_\odot$. However, how NSC formation mechanisms workin different regimes and what determines galaxy nucleation is still unclear.The dissipationless accretion of infalling globular clusters (GCs) and the insitu formation of stars directly at the galactic centre likely operate to growNSCs in most galaxies; however, their efficiency has been difficult to assessobservationally. Here, we provide, for the first time, a quantitativedetermination of the relative strength of these processes in the build-up ofindividual NSCs. Using a semi-analytical model of NSC formation based on theorbital evolution of inspiraling GCs, together with observed NSC and GC systemproperties, we derived the mass fraction of in situ born stars $f_\text{in,NSC}$ for 119 galaxies with masses from $3 \times 10^{7}$ to $3 \times 10^{11}M_\odot$, in the Local Volume, the Fornax, and Virgo galaxy clusters. Ouranalysis reveals that the NSC mass, as well as the ratio of NSC to the total GCsystem mass, are strong indicators of the dominant NSC formation channel, andnot the total galaxy stellar mass as previously suggested. More massive NSCsformed predominantly via the in situ formation of stars ($f_\text{in, NSC} \sim0.9$), while the lower-mass NSCs are expected to have formed predominantlythrough the merger of GCs ($f_\text{in, NSC} \sim 0.2$). The results of thissimple model are in agreement with recent independent estimates of the dominantNSC formation channel from recent stellar population analysis.

Disentangling the formation mechanisms of nuclear star clusters