Seminars and Colloquia at ESO Garching and on the campus

The intermediate-mass black hole (IMBH) regime is still poorly constrained, with few detections between 150 and 10^5 Msun. This poses a challenge to our understanding of supermassive black hole formation in the early universe.
An IMBH in ω Centauri, the Milky Way’s most massive globular cluster, has been suspected for almost two decades, but all previous detections have been questioned due to their assumptions and the possible mass contribution of a central cluster of stellar mass black holes.
I will present a new astrometric catalog for the inner region of ω Centauri, containing 1.4 million proper motion measurements based on 20 years of Hubble Space Telescope observations.
Our catalog is supplemented with precise HST photometry in 7 filters, allowing the separation of its complex subpopulations. The catalog is publicly available, providing the largest kinematic dataset for any star cluster.
Our new catalog revealed 7 fast-moving stars in the innermost 3 arcseconds (0.08 pc) of ω Centauri. The inferred velocities of these stars are significantly higher than the expected central escape velocity of the star cluster, so their presence can only be explained by being bound to an IMBH. From the velocities, we can infer a firm lower limit of the black hole mass of ∼8,200 Msun. In addition, we compare the full distribution of stellar velocities to N-Body models that suggest the presence of an IMBH with M≲50,000 Msun. These results confirm ω Centauri hosts an IMBH which makes this the nearest known massive black hole and, after the Milky Way center, only the second where we can track the orbits of multiple individual bound companions.

The element compositions of resolved stars formed at distinct epochs record the chemical evolution trajectory of a galaxy. As this evolution is influenced by the galaxy-wide stellar initial mass function (gwIMF), the abundance profiles of stars offer a means to estimate the gwIMF. In this presentation, I will demonstrate the application of this methodology using the dwarf galaxy Sculptor as a case study. Specifically, I will elucidate how the gwIMF of long-lived low-mass stars intricately shapes the observed stellar metallicity distribution of a galaxy, thereby allowing for the estimation of low-mass gwIMF via galaxy chemical evolution modeling. Our analysis suggests that dwarf galaxies, characterized by low stellar metallicities and a low star formation rates, exhibits a shallower gwIMF slope for low-mass stars and a steeper gwIMF slope for massive stars (bottom- and top-light IMF). Furthermore, we have compared our findings with those derived from independent IMF estimation techniques including stellar population synthesis and star counting, illustrating a coherent and systematic IMF variation.

Lambda Cold Dark Matter (LCDM) is the most successful theory for the formation of structure in the Universe. Although its predictions have been verified on large scales, they are still contested on the scale of dwarf galaxies, whose dynamical properties are often cited as evidence for the need to revise some of LCDM’s basic tenets. In this context, I will discuss the recent discovery of the faintest galaxies known to date, and how their properties may be used to place constraints on the clustering of dark matter on the smallest galactic and sub-galactic scales, as well as on the viability of some of the proposed alternatives to LCDM.

I will present recent and ongoing explorations regarding cold clouds in the circumgalactic media of (simulated) z=0 Milky Way-like galaxies. We find that these CGMs are typically filled with >~100s-1000s of such cold gas structures, possibly analogs of high-velocity clouds (HVCs) observed in the Milky Way sky. These objects primarily originate as a result of cold gas outflows from the central galaxy and/or precipitation of the warm-hot phase of the CGM. Clouds arising as a result of stripping of cold gas from satellites are rare in our sample (<5%). Lastly, we find that properties of clouds are diverse, and may furthermore depend closely on their source of origin.

Stellar multiplicity is ubiquitous at early stages of star formation. This unavoidably modifies the initial conditions of protoplanetary discs and hence planet formation. The high level of stellar multiplicity translates into a high number of binaries and triple stellar systems in active star forming regions. In this lunch talk, we will briefly cover disc dynamics within multiple stellar systems, mainly binaries and triples. In addition, based on numerical simulations, we will discuss how to interpret complex disc morphologies such as spirals, warps, and streamers as signposts of ongoing gravitational interaction between young stars. We will illustrate these claims with representative examples of observed multiple stellar systems with discs (e.g. ALMA, VLT, GRAVITY, Gaia). To conclude, we will briefly discuss the current open question regarding planet formation in multiple stellar systems given the emergence of planetary architectures in such systems.