Nuclear star clusters

Abstract

We review the current knowledge about nuclear star clusters (NSCs), the spectacularly dense and massive assemblies of stars found at the centers of most galaxies. Recent observational and theoretical works suggest that many NSC properties, including their masses, densities, and stellar populations, vary with the properties of their host galaxies. Understanding the formation, growth, and ultimate fate of NSCs, therefore, is crucial for a complete picture of galaxy evolution. Throughout the review, we attempt to combine and distill the available evidence into a coherent picture of NSC evolution. Combined, this evidence points to a clear transition mass in galaxies of $$\sim 10^9\,M_\odot$$ ∼ 10 9 M ⊙ where the characteristics of nuclear star clusters change. We argue that at lower masses, NSCs are formed primarily from globular clusters that inspiral into the center of the galaxy, while at higher masses, star formation within the nucleus forms the bulk of the NSC. We also discuss the co-existence of NSCs and central black holes, and how their growth may be linked. The extreme densities of NSCs and their interaction with massive black holes lead to a wide range of unique phenomena including tidal disruption and gravitational-wave events. Finally, we review the evidence that many NSCs end up in the halos of massive galaxies stripped of the stars that surrounded them, thus providing valuable tracers of the galaxies’ accretion histories.

Simulations of the Milky Way’s Central Molecular Zone – II. Star formation

“…Such star formation might also be related to the formation of the nuclear stellar cluster (NSC; see e.g. Genzel, Eisenhauer & Gillessen 2010;Schödel et al 2014;Gallego-Cano et al 2020) by providing in situ newly born stars and, since such stars are rotating, it might contribute to its observed rotation (Feldmeier et al 2014;Chatzopoulos et al 2015;Feldmeier-Krause et al 2015;Tsatsi et al 2017;Neumayer, Seth & Boeker 2020). Fig.…”

The accreted nuclear clusters of the Milky Way

“…The Sequoia, G-E/S, Sagittarius Dwarf and the H99 streams have estimated stellar masses (at the time of accretion) of ∼ 1-3×10 8 M ⊙ (e.g., Kruijssen et al 2020). Neumayer et al (2020) have compiled a list of NSCs in the local Universe, along with the properties of their host galaxies. They found that at these host galaxy stellar masses, 75 − 80 per cent of galaxies host NSCs (though this may be a lower limit given the observational challenges in identifying NSCs).…”

“…Nuclear star clusters (NSCs) are some of the densest and most massive clusters known, and occupy the central parts (i.e., nuclear region) of many galaxies. For galaxies with stellar masses above 10 8 M ⊙ , the majority have readily identifiable nuclear clusters (for a recent review, see Neumayer et al 2020) and for galaxies with masses as low as 10 6 M ⊙ the occupation fraction of NSCs remains above 10 per cent. NSCs are unique amongst star clusters as they are the only type (including open clusters, globular clusters, and young massive clusters) that hosts clear evidence for multiple generations of star formation within them or extended star-formation histories (SFHs; see the recent review by Bastian & Lardo 2018).…”

GALACTICNUCLEUS: A high angular-resolution JHKs imaging survey of the Galactic centre

“…It hosts a supermassive black hole, Sgr A*, in its dynamical centre that is embedded in a nuclear star cluster (e.g. Schödel et al 2014;Neumayer et al 2020).…”