In the last few decades, the astrophysics community has made a big effort to understand galaxy formation and evolution, and the interplay between internal and external processes such as the action of density waves, dynamical friction, galaxy-galaxy or galaxy-environment interactions. In spite of the huge progress in this field, one of the major unresolved issues is the role of such large-scale processes in the formation of small stellar structures in the center of galaxies.
After my PhD, I expanded my knowledge in the field of galaxy evolution as a postdoc in the Galactic Nuclei research group at MPIA (October 2019 – July 2023). The Galactic Nuclei group is an eterogeneous team analyzing nuclear regions of galaxies from different points of view and using different approaches: photometry or spectroscopy. Some of us focused on nuclei in external galaxies, some others on the Milky Way nucleus or on stripped nuclei. After that, I kept on working on galactic nuclei as one of my two main reserach lines, and I am currently leading the observational analysis of galactic nuclei in the PHANGS and BEARD surveys (see below). I am pioneering the analysis of galactic centers at small scales in high-resolution cosmological simulations with my Marie Curie project “TraNSLate: Tracing galaxy evolution with Nuclear Structures in Late-type galaxies” (see below).
Nuclear star clusters. Of particular interest in this context are compact structures located at the centers of galaxies.
With sizes of a few parsecs and masses of up to a few times 10^7 solar masses, nuclear star clusters (NSCs) are the densest stellar systems in the universe (see, e.g., review by Neumayer, Seth & Böker 2020). Most galaxies of all morphological types with stellar masses between 10^8 and 10^10 solar masses host NSCs, whose properties, such as mass and luminosity, scale with host-galaxy properties. NSCs can form via two different channels: 1) star-cluster inspiral to the center (Tremaine et al. 1975), and 2) in-situ star formation at the center of a galaxy (Brown et al. 2018). These two scenarios are not mutually exclusive, and both can contribute to NSC formation (Fahrion et al. 2021). Both channels are tightly linked to galaxy-evolution processes at larger scale, such as dynamical friction (leading to star-cluster infall), gas inflow and gas or star-cluster accretion through galaxy mergers.
OBSERVATIONAL WORK
NSCs in galaxies of different morphological types. In Pinna et al. (2021) I presented a unique adaptive-optics assisted survey of the very central regions of eleven early- (ETGs) and late-type galaxies (LTGs), providing the spatially resolved kinematics, at parsec (pc) scale, of the regions dominated by the NSCs. This work uncovered, for the first time, a trend of nuclear kinematic properties with galaxy morphology, pointing to different NSC formation scenarios: in-situ star formation as the leading mechanism in LTGs, and star-cluster mergers for ETGs. Since ETGs and LTGs have different evolutionary paths, this trend affirms a strong impact of galaxy evolution on the formation and growth of NSCs.
For the nuclear region of M33, in the top row from left to right: mean velocity V , velocity dispersion, skewness h3 and kurtosis h4. Their respective uncertainties are shown in the bottom row. The maximum and minimum values of the color bar are indicated at the bottom of each panel. The original data cube was Voronoi-binned to a signal-to-noise ratio (S/N) of 25 per bin. Different isophotes are plotted in black to guide the eye. The dashed magenta ellipse corresponds to elliptical isophote at the NSC effective radius. From Pinna et al. (2021).
NSCs in galaxies of different masses. I was also part of a systematic study of a sample of 25 unresolved NSCs hosted by ETGs (Fahrion et al. 2021 – incl. Pinna), whose integrated spectra were disentangled from the host galaxy (in a point-spread-function aperture). We fitted these spectra with full-spectral fitting and extracted age, metallicity and star-formation histories of NSCs. Their properties show trends with galaxy mass and their comparison with the host galaxy helped us reveal a diversity of formation scenarios, also depending on galaxy mass. Additionally, we have very recently analyzed a sample of nine NSCs in dwarf LTGs (Fahrion et al. – incl. Pinna – 2022), previously unexplored type. In these galaxies, we find a similar trend with galaxy mass, but lower NSC metallicities on average than ETGs of similar masses. This suggests that NSCs in LTGs may form in a different way.
Morphological analysis and SED fitting of NSCs. I have also recently co-supervised the pioneer work “PHANGS-JWST First Results: A combined HST and JWST analysis of the nuclear star cluster in NGC 628” (Hoyer, Pinna et al., 2023). This work provided the first morphological characterization and photometric analysis of a NSC with JWST and at its spatial resolution. This NSC is interestingly located in a cavity with no gas or dust (see figure). While the NSC shows a roughly constant size with wavelength (about 5 pc in radius) and a roundish shape in the optical (from HST) and near-infrared ranges (from JWST), an offset larger and flatter structure (more than double in size) is unveiled by the mid-infrared JWST data. This structure may be mainly made up of dust due to an active galactic nucleus, previously suggested by X-ray emission. This work will be followed by the fit of JWST images of four more galaxies.
NSCs in massive spiral galaxies.
Most detailed studies of NSCs, such as the ones including their stellar-population analyses, target ETGs, and NSCs in LTGs are limited to low-mass systems (Fahrion et al. – incl. Pinna – 2022, see above). Results from Kacharov et al. (2018) suggested that NSCs in massive LTGs are more metal poor than their ETG counterparts. However, this study included only four LTGs and massive spirals remain mostly unexplored. My research plans aim at filling this gap with different projects from IFS observations combined with simulations.
Blue open circles are NSCs analyzed in this work in a PSF aperture. From lower to higher galaxy masses: NGC5068, IC5332, NGC2835, NGC1385 and NGC628. The open square corresponds to the NSC in NGC628 after its spectrophotometric decomposition from the underlying galaxy.
NSCs in star-forming galaxies from PHANGS-MUSE. I am currently leading the stellar-population analysis of the NSCs in the five nucleated galaxies of the PHANGS-MUSE (Emsellem et al. 2022) sample. The PHANGS-MUSE data consist of MUSE mosaics covering the full galaxy at large scale with a high level of detail. This allows a careful comparison of NSC properties with the host galaxy. These galaxies have stellar masses between 109 and 1010.5 solar masses, range where NSCs in ETGs show high metallicities, higher than their host galaxy. In PHANGS-MUSE observations, these NSCs are not resolved, and our analysis is based on a point-spread-function (PSF) aperture. We have performed a preliminary analysis of the full sample, extracting an integrated spectrum within a PSF aperture. These results suggest that these NSCs have similar or lower metallicities than the host galaxy. Some NSCs are relatively old, which associated to the low metallicity, points to the inspiralling of one or more star clusters formed at early times, as dominant formation scenario. Some other NSCs show ongoing star formation and very young ages, suggesting in-situ formation as dominant. In these cases, we propose recent gas accretion to explain their lower metallicity than the host galaxy. Our results are in contrast with ETG counterparts at similar stellar masses, with systematically metal-rich NSCs, more metal rich than the host galaxy, for which in-situ formation was earlier proposed. Our analysis also shows that currently star-forming galaxies not necessarily form stars in their nuclei. This points to a more complex picture for massive LTGs, with no systematically preferred NSC formation channel, and reaffirms that NSCs in this type of galaxies follow their peculiar formation path, tightly linked to the evolutionary path of their host galaxy.
This preliminary analysis was done extracting spectra in a PSF aperture, including all galaxy components integrated in the line of sight. I am currently performing a spectro-photometric decomposition to separate, in the MUSE data cubes, the NSCs from other galaxy components overlapped in the line of sight. For this purpose, I am using the code C2D (Méndez Abreu et al., 2019), which performs morphological fits for quasi-monocromatic images corresponding to MUSE spectral pixels. This method allows us to extract ages and metallicities of NSCs more accurately, and compare with spatially resolved properties of the surrounding disk. In a recent work in preparation, I present the application of C2D to NSCs and show this decomposition for the galaxy M74 (NGC628), obtaining a much older and more metal poor NSC than from the PSF integrated spectrum and than the galaxy disk (Pinna et al., in prep.).
This shows that this method gives cleaner spectra for NSCs disentangling them from the surrounding galaxy. Our results show a NSC formed a long time ago, which remained frozen and di not grow with time in the cavity shown by JWST (Hoyer, Pinna et al., 2023, see above). This project is part of my MSCA fellowship (see below).
NSCs in massive spirals with MEGARA@GTC. While PHANGS-MUSE gives us a panoramic view of the galaxies hosting NSCs, we zoom right into the centers with MEGARA, a fiber integral-field spectrograph mounted at GTC in la Palma. I am leading two different projects focusing on stellar populations of NSCs in massive spirals with MEGARA, in which I apply a very similar approach as for the PHANGS-MUSE data (see above):
- NSCs in bulgeless galaxies from the BEARD survey. I am a recently appointed member of the BEARD international collaboration (June 2023, P.I.: J. Méndez Abreu), aiming at understanding the formation and evolution of bulgeless galaxies with a variety of different (photometric and spectroscopic) datasets. I am leading the science about galactic nuclei, and performing the PSF fitting, spectro-photometric decomposition and stellar-population analysis of nuclear star clusters with MEGARA data.
- “Nuclear-star-cluster formation in late-type galaxies: spatially resolving their stellar populations with MEGARA”, including three very nearby galaxies observed with the ENO-GTC program 75-GTC68/22B (P.I. García-Lorenzo B., subm. 04/04/2022). This project is part of a PhD thesis that I will co-supervise in collaboration with Dr. Begoña García Lorenzo. I am performing the same analysis as for in the BEARD sample.
GALACTIC CENTERS FROM COSMOLOGICAL SIMULATIONS
Maria Skłodowska-Curie (MSCA) Fellowship: TraNSLate
I am leading the highly innovative project «TraNSLate: Tracing galaxy evolution with Nuclear Structures in Late-type galaxies«, funded with a MSCA fellowship (from Sep 2024). This project will combine high-resolution cosmological simulations with state-of-the-art integral-field spectroscopy (IFS) observations to conclude on the connection between galaxy evolution and the mass assembly of central regions. The role of different large-scale processes, such as gas or star accretion, in the buildup of these regions will be quantified in 50 existing simulated galaxies from the NIHAO project (Wang et al., 2015; Buck et al., 2020). IFS observations of the central regions of eight spiral galaxies will be analyzed and interpreted with the help of results from the simulations. However, these observations have much higher resolution than current cosmological simulations. TraNSLate will also deliver one pilot simulation at much higher resolution (about a factor of 10 better than the best current NIHAO simulation, with a mass resolution of few hundreds solar masses, coupled with a softening length of tens of pc), and a detailed plan for a complete set of 20 simulations at unprecedented resolution. The pilot simulation is a less computationally expensive case to be realistically run as part of a two-year MSCA project, while the full set of 20 simulations will require a longer-term position to be entirely run. We have already run the dark-matter-only simulation of the pilot galaxy, and we are currently running the first tests. This is done within the Research Group Numerical Astrophysics: Galaxy Formation and Evolution (P.I. Claudio dalla Vecchia), in close collaboration with A. Negri (ULL/IAC), C. Brook (ULL/IAC) and A. Macciò (NYU Abu Dhabi).
Plans for the future.
After my Marie Curie fellowship, I plan to build my own team to perform, analyze and publicly release the full set of new TraNSLateHR simulations. The main specific objective of the project will be to finally witness the formation of observed nuclear stellar structures (e.g., clusters, disks, rings) in simulations, and quantify the relative contribution of different large-scale processes – such as gas inflow followed by in-situ star formation and stellar migration – in the buildup of these structures. On the other hand, it will provide a new set of zoom-in cosmological simulations at unprecedented resolution, which will be released and used for a very large variety of science goals. TraNSLateHR will have a huge impact not only in the field of galaxy evolution in general, but also in the theoretical community, setting a new resolution standard for cosmological simulations.