How do nuclear star clusters form?

Nuclear star clusters (NSCs) are extremely dense stellar systems that reside in the centres of ~70% of galaxies, including our Milky Way. This nucleation fraction even reaches > 90% for galaxy masses ~ 10^9 M_sun. NSCs have similar sizes to globular clusters (GCs), but are even more massive and dense. NSCs often co-exist with supermassive black holes and follow distinct scaling relations with properties of the host galaxy, but it is still debated how NSCs form and grow. Generally, two main scenarios are discusse: in-situ from gas at the galactic centre or via the dissipationless accretion of GCs that spiral inwards due to dynamical friction. Most likely, a mixture of both pathways is realized in nature, but the dominant channel nor how it relates with the host galaxy are known. Constraining NSC formation in galaxies requires a complete view of both the kinematics and chemical properties of the host galaxy, the NSC, and the GC system. Such a study is challenging, but possible with modern day integral-field spectroscopy. I will present how MUSE can be used to determine the dominant NSC formation channel for individual galaxies, in conjunction with a semi-analytical model of NSC formation. These complementary approaches reveal for the first time how the NSC formation depends on properties of the host galaxy and show a transition of NSC formation via GC-inspiral to in-situ star formation with increasing NSC mass.