Co-evolution of nuclear star clusters, massive black holes and their host galaxies

TL;DR

This paper presents a semi-analytical model of galaxy evolution that investigates the co-evolution of nuclear star clusters, massive black holes, and their host galaxies, highlighting the roles of in-situ star formation, star cluster migration, and galaxy mergers.

Contribution

It introduces a comprehensive cosmological model that combines multiple NSC formation mechanisms and compares predictions with observational data on galaxy scaling relations.

Findings

In-situ star formation accounts for up to 80% of NSC mass.

NSC-host galaxy relations are shallower than MBH relations.

Galaxy mergers can partially erode NSCs, matching observations.

Abstract

Studying how nuclear star clusters (NSCs) form and how they are related to the growth of the central massive black holes (MBHs) and their host galaxies is fundamental for our understanding of the evolution of galaxies and the processes that have shaped their central structures. We present the results of a semi-analytical galaxy formation model that follows the evolution of dark matter halos along merger trees, as well as that of the baryonic components. This model allows us to study the evolution of NSCs in a cosmological context, by taking into account the growth of NSCs due to both dynamical friction-driven migration of stellar clusters and star formation triggered by infalling gas, while also accounting for dynamical heating from (binary) MBHs. We find that in-situ star formation contributes a significant fraction (up to ~80%) of the total mass of NSCs in our model. Both NSC growth through in-situ star formation and through star cluster migration are found to generate NSC -- host galaxy scaling correlations that are shallower than the same correlations for MBHs. We explore the role of galaxy mergers on the evolution of NSCs, and show that observational data on NSC -- host galaxy scaling relations provide evidence of partial erosion of NSCs by MBH binaries in luminous galaxies. We show that this observational feature is reproduced by our models, and we make predictions about the NSC and MBH occupation fraction in galaxies. We conclude by discussing several implications for theories of NSC formation.