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 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.
Highlights
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
Other early theoretical studies focused on gas infall and subsequent centralized star formation even in early-type galaxies include Bailey (1980) who argues that inward-flowing stellar mass loss can lead to nuclear stellar disks, and Mihos and Hernquist (1994) who point out that gas-rich mergers will produce dense stellar cores which could be identified as NSCs
Nguyen et al (2019) showed that mass-to-light ratios based on stellar population synthesis vary by less than 10% with dynamical estimates based on kinematic maps of four NSCs, suggesting that stellar population synthesis provide quite reliable masses
Summary
Providing an answer to this question is the goal of a very active research field. To date, more than 400 refereed articles have been written on NSCs, and the pace of discovery about these objects is accelerating, with $ 100 articles appearing in the past 3 years alone. The observational studies cover a wide range of topics, from measuring detailed properties of nearby NSCs (including the one in the Milky Way) to identifying large samples of nuclear star clusters in more distant galaxies of various morphological types. We point out that the black hole review of Kormendy and Ho (2013) discusses NSCs, especially in the context of how their masses scale with the properties of their host galaxies, a topic we will discuss in this review. We have organized this review into a number of topical sections: after a short summary of early NSC studies, we discuss in Sect. The physical properties of NSCs, including their morphology, masses, densities, stellar populations, and kinematics, are discussed in Sect. We note that the code and data used to make figures in this review is available at https://github.com/anilseth/nsc_review
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
