My research focuses on nuclear star clusters (NSCs). These objects are the densest stellar systems known and are located in galaxy centers. NSCs appear most frequently in galaxies of approximately a billion solar masses, which is about a tenth of the stellar mass of the Milky Way. Studying nuclear star clusters is interesting for several reasons:

Nuclear region of NGC 628
Color-composite image of the central 950pc × 950pc of NGC 628, a nearby grand-design spiral galaxy using imaging data obtained from NIRCam onboard the James Webb Space Telescope. The nuclear star cluster is visible as a bright central cluster embedded in a more extended spherical component.

Increased interest in NSC resulted in analyses of statistically significant data sets over the last decade, pushing the number of known clusters beyond 1000. Nevertheless, many questions about the details of their formation, evolution, and relation to host galaxies and black holes remain unanswered. The goal of my research is to investigate further these subjects on the basis of high-resolution observational data and semi-anayitcal modelling techniques.

The remainder of this webpage describes some of my most recent projects. While the respective publications yield additional information, please feel free to contact me directly if you have questions.

Newly detected nuclear star clusters in low-mass galaxies

The review paper by Neumayer, Seth & Böker (2020) established that globular cluster (GC) migration dominates over in-situ star formation in dwarf galaxies. However, it is still unclear whether NSCs in these galaxies are (a) a merger product of GCs, or (b) an individual GC located at the galaxy center. Using archival HST data, I analysed 21 newly discovered NSCs within the Local Volume (up to approximately eleven million parsecs) and combined them with literature values to answer this question.

I find a great similarity between the least massive NSCs and GCs of the Milky Way when considering their mass and structural parameters like the effective radius or ellipticity. This result corroborates the link between both types of clusters and indicates that in-situ star formation is not important for these parameters. Indications for the relevance of GC mergers as the formation of NSCs are present in the surface density versus stellar mass parameter space but the overlap between both cluster types is too great to be certain. In addition, I find an environmental dependence of the scaling relation between the NSC and host galaxy stellar mass in that, at the low-mass end, clusters in a dense environment are more massive than in a loose environment. It was argued that this observation is most likely due to differences in the environments of the birthplaces of the star clusters at high redshift.

This research was re-submitted to Monthly Notices of the Royal Astronomical Society and is available on arxiv.

Structural properties of newly detected star clusters in the Local Volume   
Ellipticity (left) and effective radius (right) of the newly discovered NSCs (green), NSCs from the literature (blue and gray), and the Milky Way GC population (red). The clear overlap between the newly discovered NSCs and the GC population suggest a tight connection.
Nuclear star cluster versus host galaxy stellar mass comparison   
NSC versus host galaxy stellar mass indicating the environmental dependence consider field galaxies (blue) and galaxies in the Virgo galaxy cluster (orange).

The nuclear star cluster of NGC 628

The James Webb Space Telescope (JWST) observes the universe in the near- and mid-infrared wavelength regime. The large effective diameter of its primary mirror offers high-resolution data and makes it the ideal observatory to extent investigations of NSCs performed with data from the Hubble Space Telescope (HST) towards larger wavelengths. In this research project, I combined archival HST with newly obtained JWST imaging data to study the NSC of NGC 628, a nearby grand-design spiral galaxy located at a distance of approximately ten million parsecs. For the first time, this analysis yields structural and photometric parameters of the cluster from ultraviolet to mid-infrared wavelengths.

It was found that the NSC has a mass of about ten million solar masses with a half-light radius of five parsecs and is composed of an old stellar population. In the mid-infrared regime, the structural parameters change and the measured photometry cannot be explained by the main stellar population. I discussed five different scenarios for the origin of the emission but found that none of them can sufficiently explain the observations. The nature of the mid-infrared structure remains unknown.

This research is accepted by the Astrophysical Journal Letters and is available on arxiv.

Structural properties of the NSC of NGC 628   
Effective radius (left) and ellipticity (right) of the NSC of NGC 628 in different filterbands. The data up to the near-infrared trace the NSC while the source of the mid-infrared emission is unknown.
Spectral flux density   
Spectral flux density of the NSC. The fit up to the near-infrared data (solid line) worsens if the mid-infrared data are included (dashed line).

The occupation fraction of galaxies

How frequent are nuclear star clusters? Only recently, large-scale surveys addressed this question for galaxies in the Coma, Fornax, and Virgo galaxy clusters. However, no survey was available for the field environment. The goal of this project was to obtain the occupation fraction for field galaxies in the Local Volume (distance up to approximately 11 million parsecs), which is assumed to well represent the field.

Using archival HST data for more than 600 galaxies covering the whole stellar mass and Hubble type range, it was found that the occupation fraction is lower than for dense cluster environments for galaxy masses between a few hundred-thousand and a billion solar masses. Below and above these galaxy masses no significant differences were found. Furthermore, elliptical dwarf galaxies appear to be nucleated more frequently than their late-type counterparts. There is evidence in the literature that there exists a similar difference in the average number of GCs, corroborating the tight relation between both cluster types. The results show the biased formation scenarios of GCs and NSCs in different environments, most likely related to variations in gas pressures in their host galaxies during cluster formation.

This research was published by Monthly Notices of the Royal Astronomical Society and is available as an open access document.

Occupation fraction as a function of stellar mass   
NSC occupation fraction versus host galaxy stellar mass for different environments. Between a few hundred-thousand and about one billion solar masses, the fraction depends on environment in that it is highest for the densest environment (Coma galaxy cluster) and smallest for a field environment (Local Volume).
Occupation fraction as a function of stellar mass and type   
NSC occupation fraction as a function of galaxy stellar mass, split by the host galaxies' Hubble type. The environmental dependence for dwarf elliptical galaxies still persists but a secondary dependence on Hubble type emerges.