Invited Speakers

The discovery of the quasars in the 1960s led to the 'massive black hole paradigm', in which most galaxies host massive black holes of masses between millions to billions of solar masses at their nuclei, which can become active galactic nuclei and quasars when they accrete gas and stars rapidly. I will discuss the major progress that has happened in the last decades to prove the massive black hole paradigm through ever more detailed, high resolution observations, in the center of our own Galaxy, as well as in external galaxies and even in distant quasars. In the Galactic Center such high resolution observations can also be used to test General Relativity in the regime of large masses and curvatures.

In recent years, the Event Horizon Telescope (EHT) Collaboration has obtained horizon-scale images of SgrA* through millimeter interferometric observations. The Collaboration is now aiming to spatially resolve the time evolution of this radio source by reconstructing a minute-by-minute video of the black hole’s immediate surroundings at horizon scale. This is a particularly challenging task due to the sparsity of the EHT data combined with the rapid variability of SgrA*. The goal of this project is to better understand the black hole accretion dynamics and connect EHT observations with variability detected by other instruments across different wavelengths. This talk will present the VLBI imaging algorithms developed for this purpose, the extensive validation tests carried out to ensure the robustness of the results, and the possible physical interpretation of the reconstructed videos, along with all results published to date.

The time-variable emission from the accretion flow of Sgr A*, the supermassive black hole at the Galactic center, has long been examined in the radio-to-millimeter, near-infrared (NIR), and X-ray regimes of the electromagnetic spectrum. However, until now, sensitivity and angular resolution have been insufficient in the crucial mid-infrared (MIR) regime. The MIRI instrument on JWST has changed that, and we report the first MIR detection of Sgr A*. The detection was during a flare that lasted about 40 minutes, a duration similar to NIR and X-ray flares, and the source's spectral index steepened as the flare ended. The steepening suggests that synchrotron cooling is an important process for Sgr A*'s variability and implies magnetic fields strengths ~ 40–70 G in the emission zone. Observations at 1.3 mm with the Submillimeter Array revealed a counterpart flare lagging the MIR flare by ≈10 minutes. The observations can be self-consistently explained as synchrotron radiation from a single population of gradually cooling high-energy electrons accelerated through magnetic reconnection.

The flaring activity of Sgr A*, the supermassive black hole at the center of the Milky Way, provides an opportunity to study the physical processes occurring in the immediate vicinity of an event horizon. Among various interpretations, the hot-spot model - which describes localized, transient regions of enhanced emission orbiting close to the innermost stable circular orbit - has been used to account for observed flare properties, including relativistic modulation of light curves due to Doppler boosting, gravitational lensing, and light travel time effects, as well as variability in polarization signatures. In this talk, I will review the development and astrophysical motivation of the hot-spot model, tracing its evolution from early ray-tracing predictions and phenomenological treatments to more sophisticated simulations. I will show how this model has been used to interpret multi-wavelength flare observations, with an emphasis on the near-infrared and X-ray domains, as well as how it can be applied to interpret recent high-resolution astrometric and polarimetric data from the GRAVITY instrument. Finally, I will briefly discuss alternative dynamical scenarios such as magnetic reconnection or turbulence to account for the observed flaring activity of Sgr A*. The talk will conclude with a short outlook on how upcoming multi-instrument campaigns and next-generation facilities may further test these models and refine our understanding of accretion and emission mechanisms near supermassive black holes.

The Nuclear Star Cluster (NSC) of the Milky Way, surrounding the supermassive black hole Sgr A*, provides a unique opportunity to study the formation and evolution of dense stellar systems in the extreme central regions of galaxies. Despite extensive observational and theoretical efforts, key questions remain about its origin, the diversity of stellar populations it hosts, and the apparent absence of a stellar cusp. In this talk, I will present a theoretical and numerical perspective on the formation and dynamical evolution of the Galactic NSC. Using direct N-body simulations, I explore how a combination of dense and massive stellar cluster inspirals and in-situ star formation may have contributed to its assembly. I will discuss various physical processes, including stellar collisions, and how these can influence the NSC's structure and stellar content, potentially contributing to the erosion of a stellar cusp, depending on the black hole's growth and the cluster's dynamical history. I will examine how these models compare with existing observations, and how the combination of improved simulations and data from facilities such as JWST and future instruments like MOONS and the ELTs can help disentangle the formation history of the Galactic NSC—ultimately advancing our understanding of nuclear star clusters and their connection to the formation and evolution of galaxies.

I will briefly review the observational evidence of young stars in the central parsec of the Milky Way whose origin is still not fully understood. I will discuss the hypothesis of in-situ star formation in a dense gaseous disc. While being positively accepted in general, the hypothesis has to prove its viability by passing the observational tests. To do so, it is necessary to trace the initial state of the young stellar disc to the evolved one. I will present results of N-body integrations considering evolution of an initially thin and coherent stellar disc due to both internal two-body relaxation and secular evolution as well as the influence of other components of the Galactic center - the old stellar cusp, gas, and putative cluster of compact objects. I will put the model into context of observations of not only the young stars in the Galactic Center, but also other related objects, like the hyper-velocity stars that are being observed far in the Galactic halo.

The Galactic Center remains a puzzling region. In this talk, we consider three observables: (1) the surprisingly high ionization rate in the Central Molecular Zone; (2) the GC gamma-ray emission, including the Fermi Bubbles; and (3) M31 as a Milky Way analogue with diffuse GeV emission extending well beyond a few kpc. Taken together, these datasets reveal a tension between ionization and γ-ray constraints and highlight the possible relevance of non-standard propagation scenarios. We discuss the implications of this combined view and outline future directions.

The Central Molecular Zone (CMZ) of the Milky Way is among the most scrutinized regions of the sky, providing a unique nearby laboratory for understanding the interplay between gas dynamics, star formation, and feedback under extreme conditions. Despite decades of observations, key questions remain about the CMZ’s 3D structure, how gas is converted into stars and planets, and the processes that regulate mass and energy flows. In this talk, I will review our evolving understanding of the CMZ, highlighting new results from recent surveys which offer unprecedented resolution and sensitivity across many orders of magnitude in spatial scales. I will discuss how these programs are reshaping our view of star formation, feedback, and chemical complexity in the CMZ, outline current challenges in interpreting these datasets, and look ahead to how complementary numerical and astrochemical modeling efforts are poised to unlock further insight into our Galactic nucleus.

Isotopic ratios of elements (e.g. D/H, 12C/13C and 14N/15N ratios) measured within interstellar molecular clouds depend on the chemical evolution of the galaxy due to stellar nucleosynthesis and on their physical conditions, and thus they can provide unique constraints to the history of star formation in galaxies. In this talk, I will review the isotopic ratios measured towards the Galactic center, and I will present the most sensitive observations ever obtained of H-, C-, N-, and S-isotopic ratios towards the molecular cloud G.0.693-0.027. The results show 12C/13C of 40 and 14N/15N of 600, two times higher than those expected from the extrapolation of Galactic disk isotopic ratios trend. Astrochemical models have been used to exclude the presence of chemical isotopic fractionation effects because of the high kinetic temperature of the molecular cloud (about 100 K). In addition, the D/H ratio can be used to pinpoint the locations of the future generation of stellar clusters. This method, when applied for the Galactic center molecular clouds, indicates that the degree of deuteration of different molecules, such as N2H+ and HCO+, and their line profiles can be used to reveal the different gas components in the line of sight to the CMZ. Finally, I will also discuss about the nucleosynthesis history and the possible contamination by low-metallicity stars from the Galactic disk through the bars, leading to the measured isotopic ratios in the Galactic center interstellar medium.

The nuclear stellar disc (NSD) is a flattened, dense and rotating stellar structure with total mass of 10^9 Msun that dominates the gravitational field of the Milky Way at Galactocentric radius R<300 pc. Embedded at its centre is the Nuclear Star Cluster (NSC), a spheroidal, moderately flattened and mildly rotating structure with a total mass of 2-3x10^7 Msun and a half-light radius R~5pc. Both the nuclear disc and nuclear cluster are believed to form predominantly from in-situ star formation - the nuclear disc from gas that is transported inward by the Galactic bar that stalls at R~120pc in the ring-like gas accumulation known as the Central Molecular Zone (CMZ), the nuclear cluster from a small fraction of gas that manages to leave the central molecular zone and make its way down to the innermost few parsecs. I will review the structure, dynamics, and evolution of the nuclear disc and cluster, highlighting their connection. I will also discuss the role of the nuclear disc as a tracer of the secular evolution of the Galactic bar. I will end by outlining open questions in the field and future prospects.

Abundance trends as a function of metallicity for different stellar populations in the Galactic Center provide new insights that complement other studies. Since various elements trace distinct nucleosynthetic channels with unique evolutionary timescales, these trends can vary depending on factors such as the star formation rate and gas inflow, thereby revealing differences between stellar populations. I will present a project in which we have succeeded in measuring detailed abundances of nearly two dozen elements for stars in the Galactic Center structures. We find predominantly metal-rich stars, with abundance trends consistent with those of the inner bulge, suggesting similar evolutionary histories. However, we identify a significant and unexplained enhancement in Na abundances. Further studies are required to determine possible evolutionary links to the complex stellar system Liller 1 and metal-rich globular clusters. We find no evidence of typical globular cluster abundance signatures in our Nuclear Stellar Disk stars with sub-solar metallicities.

The Nuclear Stellar Disk (NSD) is the most prolific star-forming region in the Galaxy when averaged by volume, with an estimated star formation rate of 0.2–0.8 M☉/yr. Recent studies suggest that approximately 10^6 M☉ of stars have formed in the past 〜30,Myr. However, apart from the Nuclear Star Cluster, only two stellar clusters are currently known in the NSD, the Arches and the Quintuplet, which together account for less than 10% of the expected mass of young stars formed during this period. This discrepancy is referred to as the missing cluster problem. The extreme stellar crowding in this region makes it nearly impossible to detect even dense and massive clusters through local overdensity analyses. Nevertheless, they can be identified using proper motions, since members of a cluster/association share similar motions with significantly lower velocity dispersion than the surrounding field population. In this talk, we present a high-angular-resolution, low–proper-motion-uncertainty catalog covering a substantial fraction of the NSD, along with a technique to detect stellar clusters in this extreme environment.

December 9, 2021 marked the launch of IXPE (Imaging X-ray Polarimetry Explorer), the first modern X-ray polarimeter in over 40 years. By adding polarimetry to spectral, imaging, and timing techniques, IXPE opened a new window onto high-energy astrophysics. Among its targets was Sgr A*, the supermassive black hole at the center of the Milky Way, to investigate the origin of X-ray emission from nearby molecular clouds. In this talk, I will show that IXPE detected polarized X-ray emission with a degree of 31 ± 11% and a polarization angle of 48° ± 11°. These measurements are consistent with a reflection scenario, where past flares from Sgr A* illuminated the clouds. The polarization angle points back to Sgr A*, while the high polarization degree suggests that, about 200 years ago, Sgr A* briefly reached a luminosity comparable to that of a Seyfert galaxy.

The X-Ray Imaging and Spectroscopy Mission (XRISM) realizes the first-ever non-dispersive high-resolution X-ray spectroscopy, which enables us to precisely determine the ion temperatures, ionization states, and velocity structures of diffuse sources typified by supernova remnants. XRISM observed the supernova remnant Sgr A East in February 2024. The Fe-K spectrum of Sgr A East showed narrow (~109 eV in 1 sigma) He-like lines as well as low-ionized (Be-like, Li-like, etc.) lines and enhanced forbidden-to-resonance ratio (~1.4). Our findings suggest that the plasma is in a highly "over-ionized" state, indicating that the remnant experienced a rapid ionization or thermalization, neither of which is expected in the standard evolution. We conclude that one of the possible origins of this over-ionization is a bursting activity of Sgr A* a few thousand years ago (XRISM Collaboration, 2024, PASJ, 74, 1). I will review what we have learnt with XRISM about Sgr A East and the surrounding environment.

For decades, chemical studies in external galaxies have been limited by sensitivity, resolution, bandwidth or a combination of them. A decade of ALMA operations, made the detailed study of the molecular abundances in nearby galaxies a routine job. We can finally bridge the ISM chemistry from Galactic to extragalactic environments, through one to one comparison, albeit probing significantly different scales. The ALMA large program ALCHEMI imaged the chemistry of the starburst galaxy NGC 253 at an unprecedented combination of sensitivity and resolution, achieving a detailed spatially resolved chemical inventory. We probed molecular differences among regions within its CMZ, and to compare with Galactic environments ranging from hot cores, quiescent molecular clouds and comets. This comparison can be put in the context of available data towards other extragalactic environments. The wideband sensitivity upgrade of ALMA will expand similar comparisons towards a wider range of Galactic and extragalactic objects.