The build-up of galactic stellar nuclei

Nuclear star clusters are the densest stellar systems in the Universe and are found in the centres of all types of galaxies. At least 70 per cent of all galaxies - and at masses of M_gal ~ 10^9 M_sun even more than 90 per cent - are nucleated. As nuclear star clusters retain information regarding their evolutionary history imprinted in their stellar populations, they provide unique windows into the various physical processes that shape the nuclei of galaxies. The goal of this thesis is to explore the formation mechanisms of nuclear star clusters. They might form directly at the galaxy centre out of gas, explaining the presence of young stars or extended star formation histories. Additionally, the formation out of pre-enriched gas can lead to high metallicities in the formed star cluster. Alternatively, nuclear star clusters might be produced through the mergers of globular clusters. As globular clusters contain old, often metal-poor stars, this formation channel can produce old, metal-poor nuclear star clusters. There is evidence that both paths occur and also hybrid scenarios have been proposed. However, the relative contribution of either channel and their correlation with galaxy properties are not yet established because constraining the dominant formation channel with observations requires a panoramic view of all the involved stellar components. In this thesis, I employ integral-field spectroscopy of different galaxies with the MUSE instrument. I extract and analyse the spectra of the nuclear star clusters, the globular cluster population, and the underlying stellar body to obtain their kinematics and stellar population properties. Studying the elliptical galaxy FCC47, I find evidence that its massive, metal-rich, and kinematically decoupled nuclear star cluster has formed from efficient in-situ star formation. The study of two nearby nucleated dwarf galaxies gives a different result: their nuclear star clusters are significantly less enriched than the host galaxies and were likely formed out of the merger of metal-poor globular clusters. To explore how these case studies compare with the general galaxy population, I analyse the ages, metallicities, and star formation histories of nuclear star clusters in comparison to globular clusters and their hosts in 25 galaxies, mainly in the Fornax galaxy cluster. This study finds a clear transition of the dominant nuclear star cluster formation channel with both galaxy and nuclear star cluster mass. While globular cluster accretion forms the nuclear star clusters of low-mass galaxies, central star formation is responsible for the efficient mass build-up in the most massive nuclear star clusters. At intermediate masses both channels can contribute. The transition between these formation channels seems to occur at galaxy masses of M_gal ~ 10^9 M_sun and nuclear star cluster masses of M_NSC ~ 10^7 M_sun. To understand this trend from a theoretical viewpoint, I employ a semi-analytical model that describes nuclear star cluster formation from the dynamical evolution of an initial star cluster population. The mass of the so-formed nuclear star cluster depends on the available mass in star clusters and this model finds a similar trend: low-mass nuclear star clusters can be built efficiently out of globular clusters, but forming high-mass nuclear star clusters requires significant amounts of additional star formation to let them grow beyond M_NSC > 10^7 M_sun. In summary, I explore nuclear star cluster formation in individual galaxies, finding solid observational evidence for a transition in the dominant nuclear star cluster formation scenario with galaxy properties. These results are fundamental for understanding nuclear star clusters as unique probes of the evolutionary history of galaxies and they lay the basis for future studies exploring nuclear star cluster formation in different environments or galaxy types.Kernsternhaufen sind die dichtesten Sternhaufen im Universum und können in den Zentren aller Galaxientypen gefunden werden. Mindestens 70 Prozent aller Galaxien - und bei Galaxien mit Massen M_gal ~ 10^9 M_sun sogar über 90 Prozent - haben einen Kernsternhaufen. Die Sterne in Kernsternhaufen tragen Informationen über die Entwicklung des Sternhaufens in sich und erlauben somit einen Einblick in die physikalischen Prozesse, die eine Rolle bei der Entstehung von Galaxienzentren spielen. Das Ziel dieser Dissertation ist es, die Entstehungsmechanismen von Kernsternhaufen zu untersuchen. Zum Beispiel können sich Kernsternhaufen aus Gas im Galaxienzentrum bilden, was junge Sterne oder komplexe Sternentstehungsgeschichten erklären kann. Zusätzlich kann die Entstehung aus angereichertem Gas zu hohen Metallizitäten führen. Alternativ können sie durch das Verschmelzen von Kugelsternhaufen entstehen. Da Kugelsternhaufen alte Sterne mit oft geringen Metallizitäten enthalten, kann so ein alter, metallarmer Kernsternhaufen entstehen. Die Datenlage zeigt, dass beide Wege vorkommen und auch Mischszenarien wurden postuliert. Allerdings ist weder die relative Gewichtung dieser verschiedenen Mechanismen, noch deren Zusammenhänge mit Galaxieneigenschaften bekannt, denn um die Entstehungsszenarien mit Beobachtungen einzuschränken, müssen alle involvierten Komponenten untersucht werden. Ich benutze integrale Feldspektroskopie mit dem MUSE Instrument um Spektren und schließlich die kinematischen und chemischen Eigenschaften von Kernsternhaufen, Kugelsternhaufen und den Galaxien zu extrahieren und analysieren. Die Untersuchung der elliptischen Galaxie FCC47 zeigt, dass deren massereicher, angereicherter und kinematisch entkoppelter Kernsternhaufen aus effizienter Sternentstehung gebildet wurde. Die Untersuchung zweier Zwerggalaxien ergibt ein anderes Resultat: ihre Kernsternhaufen sind deutlich metallärmer als die Galaxien selbst und sind wahrscheinlich durch die Verschmelzung metallarmer Kugelsternhaufen entstanden. Um die Ergebnisse dieser Fallstudien in das Gesamtbild der Galaxienpopulation einzuordnen, analysiere ich die Alter, Metallizitäten und Sternentstehungsgeschichten von Kernsternhaufen im Vergleich zu Kugelsternhaufen und den zentralen Regionen in 25 Galaxien, hauptsächlich im Fornax Galaxienhaufen. Dies zeigt einen klaren Wandel des dominanten Kernsternhaufenentstehungswegs abhängig von der Galaxien- und Kernsternhaufenmasse. Während die Verschmelzung von Kugelsternhaufen die massearmen Kernsternhaufen in Zwerggalaxien hervorbringt, entstehen massereiche Kernsternhaufen in massereichen Galaxien durch Sternentstehung und beide Wege können bei mittleren Massen beitragen. Der Übergang liegt bei Galaxienmassen von M_gal 10^9 ~ M_sun und Kernsternhaufenmassen von M_NSC ~ 10^7 M_sun. Um diesen Trend besser zu verstehen, benutze ich ein semianalytisches Modell, das die Entstehung von Kernsternhaufen durch die dynamische Entwicklung einer Kugelsternhaufenpopulation beschreibt. In diesem Modell hängt die Kernsternhaufenmasse von der verfügbaren Masse in Sternhaufen ab und es zeigt sich ein ähnlicher Trend: massearme Kernsternhaufen können aus Kugelsternhaufen entstehen, aber um massereiche Kernsternhaufen zu bilden, muss zusätzliche Sternentstehung stattfinden, da sonst Massen von M_NSC > 10^7 M_sun nicht erreicht werden können. Zusammenfassend gesagt, untersuche ich die Entstehung von Kernsternhaufen in einzelnen Galaxien und finde klare Anzeichen für einen Wandel des Entstehungswegs abhängig von Galaxieneigenschaften. Diese Resultate sind elementar um Kernsternhaufen als einzigartige Zeitzeugen der Galaxienentwicklung zu verstehen und können als Ausgangspunkt für zukünftige Studien in anderen Galaxientypen und Umgebungen genutzt werden

Effect of the initial mass function on the dynamical SMBH mass estimate in the nucleated early-type galaxy FCC 47

Supermassive black holes (SMBHs) and nuclear star clusters (NSCs) co-exist in many galaxies. While the formation history of the black hole is essentially lost, NSCs preserve their evolutionary history imprinted onto their stellar populations and kinematics. Studying SMBHs and NSCs in tandem might help us to ultimately reveal the build-up of galaxy centres. In this study, we combine large-scale VLT/MUSE and high-resolution adaptive-optics-assisted VLT/SINFONI observations of the early-type galaxy FCC 47 with the goal being to assess the effect of a spatially (non-)variable initial mass function (IMF) on the determination of the mass of the putative SMBH in this galaxy. We achieve this by performing DYNAMITE Schwarzschild orbit-superposition modelling of the galaxy and its NSC. In order to properly take account of the stellar mass contribution to the galaxy potential, we create mass maps using a varying stellar mass-to-light ratio derived from single stellar population models with fixed and with spatially varying IMFs. Using the two mass maps, we estimate black hole masses of $(7.1^{+0.8}_{-1.1})\times 10^7\,M_{\odot}$ and $(4.4^{+1.2}_{-2.1}) \times 10^7\,M_{\odot}$ at $3\sigma$ signifance, respectively. Compared to models with constant stellar-mass-to-light ratio, the black hole masses decrease by 15% and 48%, respectively. Therefore, a varying IMF, both in its functional form and spatially across the galaxy, has a non-negligible effect on the SMBH mass estimate. Furthermore, we find that the SMBH in FCC 47 has probably not grown over-massive compared to its very over-massive NSC.Comment: 23 pages 19 Figures, accepted for publication in A&

Expanding on the Fundamental Metallicity Relation in Dwarf Galaxies with MUSE

The mass-metallicity relation (MZR) represents one of the most important scaling relations in the context of galaxy evolution, comprising a positive correlation between stellar mass and metallicity (Z). The fundamental metallicity relation (FMR) introduces a new parameter, the star formation rate (SFR), in the dependence. While several studies found that Z is anti-correlated with the SFR at fixed mass, the validity of this statement has been questioned extensively and no widely-accepted consensus has been reached yet. With this work, we investigate the FMR in nine nearby, spatially-resolved, dwarf galaxies, using gas diagnostics on integral-field spectroscopic data of the Multi Unit Spectroscopic Explorer (MUSE), pushing such investigations to lower galaxy masses and higher resolutions. We find that both the MZR and FMR exhibit different behaviours within different star forming regions of the galaxies. We find that the SFR surface density - metallicity anti-correlation is tighter in the low-mass galaxies of our sample. For all the galaxies considered, we find a SFR surface density - stellar mass surface density correlation. We propose that the main reason behind these findings is connected to the accretion mechanisms of the gas fuelling star formation -- low-mass, metal-poor galaxies accrete pristine gas from the intergalactic medium, while in more massive and metal-enriched systems the gas responsible for star formation is recycled from previous star forming episodes.Comment: 15 pages, 8 figures, accepted for publication in Astronomy & Astrophysic

The properties of dwarf spheroidal galaxies in the Cen A group

Dwarf spheroidal galaxies (dSphs) have been extensively investigated in the Local Group, but their low luminosity and surface brightness make similar work in more distant galaxy groups challenging. Modern instrumentation unlocks the possibility of scrutinizing these faint systems in other environments, expanding the parameter space of group properties. We use MUSE spectroscopy to study the properties of 14 known or suspected dSph satellites of Cen A. Twelve targets are confirmed to be group members based on their radial velocities. Two targets are background galaxies at ∼50 Mpc: KK 198 is a face-on spiral galaxy, and dw1315−45 is an ultra-diffuse galaxy with an effective radius of ∼2300 pc. The 12 confirmed dSph members of the Cen A group have old and metal-poor stellar populations and follow the stellar metallicity-luminosity relation defined by the dwarf galaxies in the Local Group. In the three brightest dwarf galaxies (KK 197, KKs 55, and KKs 58), we identify globular clusters, as well as a planetary nebula in KK 197, although its association with this galaxy and/or the extended halo of Cen A is uncertain. Using four discrete tracers, we measure the velocity dispersion and dynamical mass of KK 197. This dSph appears dark matter dominated and lies on the radial acceleration relation of star-forming galaxies within the uncertainties. It also is consistent with predictions stemming from modified Newtonian dynamics. Surprisingly, in the dwarf KK 203 we find an extended Hα ring. Careful examination of Hubble Space Telescope photometry reveals a very low level of star formation at ages between 30 and 300 Myr. The Hα emission is most likely linked to a ∼40 Myr old supernova remnant, although other possibilities for its origin cannot be entirely ruled out

Constraining nuclear star cluster formation using MUSE-AO observations of the early-type galaxy FCC 47

Context. Nuclear star clusters (NSCs) are found in at least 70% of all galaxies, but their formation path is still unclear. In the most common scenarios, NSCs form in-situ from the galaxy's central gas reservoir, through the merging of globular clusters (GCs), or through a combination of both. Aims: As the scenarios pose different expectations for angular momentum and stellar population properties of the NSC in comparison to the host galaxy and the GC system, it is necessary to characterise the stellar light, NSC, and GCs simultaneously. The large NSC (r_eff = 66 pc) and rich GC system of the early-type Fornax cluster galaxy FCC 47 (NGC 1336) render this galaxy an ideal laboratory to constrain NSC formation. Methods: Using Multi Unit Spectroscopic Explorer science verification data assisted by adaptive optics, we obtained maps for the stellar kinematics and stellar-population properties of FCC 47. We extracted the spectra of the central NSC and determined line-of-sight velocities of 24 GCs and metallicities of five. Results: The galaxy shows the following kinematically decoupled components (KDCs): a disk and a NSC. Our orbit-based dynamical Schwarzschild model revealed that the NSC is a distinct kinematic feature and it constitutes the peak of metallicity and old ages in FCC 47. The main body consists of two counter-rotating populations and is dominated by a more metal-poor population. The GC system is bimodal with a dominant metal-poor population and the total GC system mass is \u30317% of the NSC mass (\u3037 7 10^8 M_sun). Conclusions: The rotation, high metallicity, and high mass of the NSC cannot be explained by GC-inspiral alone. It most likely requires additional, quickly quenched, in-situ formation. The presence of two KDCs likely are evidence of a major merger that has significantly altered the structure of FCC 47, indicating the important role of galaxy mergers in forming the complex kinematics in the galaxy-NSC system

Metal-poor nuclear star clusters in two dwarf galaxies near Centaurus A suggesting formation from the in-spiraling of globular clusters

Studies of nucleated dwarf galaxies can constrain the scenarios for the formation and evolution of nuclear star clusters (NSC) in low-mass galaxies and give us insights on the origin of ultra compact dwarf galaxies (UCDs). We report the discovery of a NSC in the dwarf galaxy KKs58 and investigate its properties together with those of another NSC in KK197. Both NSCs are hosted by dwarf elliptical galaxies of the Centaurus group. Combining ESO VLT MUSE data with photometry from VLT FORS2, CTIO Blanco DECam, and HST ACS, as well as high-resolution spectroscopy from VLT UVES, we analyse the photometric, kinematic and stellar population properties of the NSCs and their host galaxies. We confirm membership of the NSCs based on their radial velocities and location close to the galaxy centres. We also confirm the membership of two globular clusters (GCs) and detect oblate rotation in the main body of KK197. Based on high signal-to-noise spectra taken with MUSE of the NSCs of both KKs58 and KK197 we measure low metallicities, [Fe/H] = −1.75 ± 0.06 dex and [Fe/H] = −1.84 ± 0.05 dex, and stellar masses of 7.3 × 105 M and 1.0 × 106 M , respectively. Both NSCs are more metal-poor than their hosts that have metallicities of −1.35±0.23 dex (KKs58) and −0.84±0.12 dex (KK197). This can be interpreted as NSC formation via the in-spiral of GCs. The masses, sizes and metallicities of the two NSCs place them among other NSCs, but also among the known UCDs of the Centaurus group. This indicates that NSCs might constitute the progenitors of a part of the low-mass UCDs, although their properties are almost indistinguishable from typical GCs.OM is grateful to the Swiss National Science Foundation for financial support. GvdV acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement No 724857 (Consolidator Grant ArcheoDyn). HJ acknowledges support from the Australian Research Council through the Discovery Project DP150100862

Looking into the faintEst WIth MUSE (LEWIS): on the nature of ultra-diffuse galaxies in the Hydra-I cluster.I. Project description and preliminary results

Looking into the faintEst WIth MUSE (LEWIS) is an ESO large observing programme aimed at obtaining the first homogeneous integral-field spectroscopic survey of 30 extremely low-surface brightness (LSB) galaxies in the Hydra I cluster of galaxies, with MUSE at ESO-VLT. The majority of LSB galaxies in the sample (22 in total) are ultra-diffuse galaxies (UDGs). The distribution of systemic velocities Vsys ranges between 2317 km/s and 5198 km/s and is centred on the mean velocity of Hydra I (Vsys = 3683 $\pm$ 46 km/s). Considering the mean velocity and the velocity dispersion of the cluster, 17 out of 20 targets are confirmed cluster members. To assess the quality of the data and demonstrate the feasibility of the science goals, we report the preliminary results obtained for one of the sample galaxies, UDG11. For this target, we derived the stellar kinematics, including the 2-dimensional maps of line-of-sight velocity and velocity dispersion, constrained age and metallicity, and studied the globular cluster (GC) population hosted by the UDG. Results are compared with the available measurements for UDGs and dwarf galaxies in literature. By fitting the stacked spectrum inside one effective radius, we find that UDG11 has a velocity dispersion $\sigma = 20 \pm 8$ km/s, it is old ($10\pm1$ Gyr), metal-poor ([M/H]=-1.17$\pm$0.11 dex) and has a total dynamical mass-to-light ratio M$/L_V\sim 14$, comparable to those observed for classical dwarf galaxies. The spatially resolved stellar kinematics maps suggest that UDG11 does not show a significant velocity gradient along either major or minor photometric axes. We find two GCs kinematically associated with UDG11. The estimated total number of GCs in UDG11, corrected for the spectroscopic completeness limit, is $N_{GC}= 5.9^{+2.2}_ {-1.8}$, which corresponds to a GC specific frequency of $S_N = 8.4^{+3.2}_{-2.7}$.Comment: Accepted for publication in Astronomy and Astrophysic

Evaluating the feasibility of interpretable machine learning for globular cluster detection

Extragalactic globular clusters (GCs) are important tracers of galaxy formation and evolution because their properties, luminosity functions, and radial distributions hold valuable information about the assembly history of their host galaxies. Obtaining GC catalogues from photometric data involves several steps which will likely become too time-consuming to perform on the large data volumes that are expected from upcoming wide-field imaging projects such as Euclid. In this work, we explore the feasibility of various machine learning methods to aid the search for GCs in extensive databases. We use archival Hubble Space Telescope data in the F475W and F850LP bands of 141 early-type galaxies in the Fornax and Virgo galaxy clusters. Using existing GC catalogues to label the data, we obtained an extensive data set of 84929 sources containing 18556 GCs and we trained several machine learning methods both on image and tabular data containing physically relevant features extracted from the images. We find that our evaluated machine learning models are capable of producing catalogues of a similar quality as the existing ones which were constructed from mixture modelling and structural fitting. The best performing methods, ensemble-based models such as random forests, and convolutional neural networks recover ∼90−94% of GCs while producing an acceptable amount of false detections (∼6−8%), with some falsely detected sources being identifiable as GCs which have not been labelled as such in the used catalogues. In the magnitude range 22 < m4_g ≤ 24.5 mag, 98−99% of GCs are recovered. We even find such high performance levels when training on Virgo and evaluating on Fornax data (and vice versa), illustrating that the models are transferable to environments with different conditions, such as different distances than in the used training data. Apart from performance metrics, we demonstrate how interpretable methods can be utilised to better understand model predictions, recovering that magnitudes, colours, and sizes are important properties for identifying GCs. Moreover, comparing colour distributions from our detected sources to the reference distributions from input catalogues finds great agreement and the mean colour is recovered even for systems with fewer than 20 GCs. These are encouraging results, indicating that similar methods trained on an informative sub-sample can be applied for creating GC catalogues for a large number of galaxies, with tools being available for increasing the transparency and reliability of said methods

Extending the extinction law in 30 Doradus to the infrared with JWST

We measured the extinction law in the 30 Dor star formation region in the Large Magellanic Cloud using Early Release Observations (EROs) taken with Near-Infrared Camera (NIRCam) on board the JWST, thereby extending previous studies carried out with the Hubble Space Telescope to the infrared. We used red clump stars to derive the direction of the reddening vector in twelve bands and we present the extinction law in this massive star forming region from 0.3 to 4.7 μm. At wavelengths longer than 1 μm, we find a ratio of total and selective extinction twice as high as in the diffuse Milky Way interstellar medium and a change in the relative slope from the optical to the infrared domain. Additionally, we derive an infrared extinction map and find that extinction closely follows the structure of the highly embedded regions of 30 Dor