The assembly of nuclear star clusters

PhD Program

Speaker

Nils Hoyer

When

2025/07/02
14:00

Place

University of Heidelberg

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PhD Thesis defense by Nils Hoyer

Supervisors: Nadine Neumayer and Silvia Bonoli (DIPC, Ikerbasque Reserach Associate Professor)

Astronomy: Galaxy Evolution, Galaxy Centers, Star Clusters

The evolution of galaxies represents a hitherto unsolved area of modern astrophysics. Various dynamical effects play an important role in shaping galaxy centres making them extremely interesting to study in detail. Nuclear star clusters are often a key element of galaxy centres and feature both diverse and complex formation histories as well as extremely high stellar densities. Many aspects of nuclear star cluster formation are still uncertain and require new constraints to make full use of their ability as ideal laboratories for studying galaxy evolution and the growth of massive black holes that are located in their centres. An improved knowledge of nuclear star cluster assembly is especially relevant today in order to properly interpret the vast amount of upcoming data produced with all-sky surveys. I present in this dissertation various analyses of high-resolution observational data sets from space-based missions ranging between the X-ray and mid-infrared regimes and a new computer simulation of the assembly of massive star cluster populations.

The first analysis reveals tight correlations between the properties of nuclear star clusters and old globular star clusters in the Milky Way indicating a potential common formation mechanism of the two cluster types. In a second analysis I identify nuclear star clusters with variable accretion signatures from massive black holes within them and provide upper limits for lower-mass systems in case of non-detections. I demonstrate in a third project that analysing the spectral energy distribution of the nuclear star cluster in the nearby massive spiral galaxy Messier 74 constrains the assembly history of both the host galaxy and a potential massive black hole in the star cluster's centre. In addition to the projects that rely on observational data sets, I introduce a new simulation that is based on a dark matter-only computation and considers the co-formation of galaxies and massive star clusters. I show that my simulation can reproduce a number of observational quantities such as the mass function of young massive star clusters or the metallicity distribution of old globular clusters, both in nearby galaxies.

I conclude by summarising the contents of this dissertation and by presenting future efforts that build on the presented observational and numerical approaches.