Seminars and Colloquia at ESO Garching and on the campus

The radial acceleration relation allows predicting the gravitational field in most galaxies just based on the distribution of the visible matter. Its existence was first predicted by the MOND theory of modified gravity. The relation however often fails in dwarf spheroidals and the dark-matter free ultra-diffuse galaxies, which is a recently discovered rare class of galaxies whose formation mechanism is not understood well. The problematic galaxies can be described as low-surface-brightness (LSB) spheroids. In my talk, I will describe my efforts to find a MOND-like framework that would work for the LSB spheroids. The found equation allows predicting their gravitational fields just on the basis of their visible properties, like the radial acceleration relation. If the found relation is interpreted in the framework of dark matter and the standard gravity, it hints that the dark-matter free galaxies form through the standard processes acting at unusual intensities rather than by exotic mechanisms. In addition, the relation implies that dark-matter free galaxies should have very specific photometric properties, which facilitates finding more representatives of this class of rare and puzzling galaxies. I will present a few concrete ultra-diffuse galaxies predicted to be poor of dark matter.

The intergalactic medium (IGM) represents the dominant reservoir of baryons at high redshift, traces the architecture of the cosmic web dominated by dark matter, and fuels on-going galaxy evolution. The IGM has been studied using Quasi-Stellar Objects (QSO) absorption lines including the Lyman alpha forest (LAF). But because of the low surface brightness and extended, diffuse distribution, direct detection of an emission equivalent to the absorption LAF has been challenging. Using KCWI, we have detected an emission Lyman α forest (ELAF). The emission forest is highly extended, shows filamentary morphology with filaments connecting galaxies, exhibits statistics like the absorption Lyman α forest, displays spectra resembling the absorption forest, and is correlated with galaxy-traced over-densities consistent with bias like dark matter. We conclude that the ELAF may provide a new tool for tracing a significant fraction of the cosmic web of baryons and dark matter. We have also discovered a virial scaling in the Circum-Galactic Medium of nearby galaxies, demonstrating a clear transition in CGM properties moving from lower mass, star forming galaxies, to higher mass galaxies that may be beginning to quench. I will present status of the Stratospheric Cosmic Web Imager (SCWI) program, a Brinson Exploration Hub balloon experiment, focused on emission from the Circum-QSO, the Circum-Galactic Medium, and the cosmic web. SCWI offers the opportunity to image the cosmic web in the local universe for the first time and compare its properties to those at high redshift

Metal-poor galaxies provide a unique window into the physical conditions and chemical enrichment processes that govern star formation in nearly pristine environments. A subset of these systems exhibit spectra with extremely strong high-ionization emission lines that cannot be reproduced by standard stellar population models and, therefore, offer an ideal laboratory for testing the physical mechanisms that produce unusually hard ionizing radiation fields and extreme emission. These extreme emission line galaxies (EELGs) are often modeled under simplified assumptions, such as the low-density limit, and are widely used as benchmarks for interpreting elemental abundances and ionizing spectra across cosmic time. However, growing empirical evidence suggests that more extreme conditions at the heart of these sources are biasing our interpretations.

I will present new empirical methods to constrain the ionizing continua of EELGs from the JWST CLASSYIR Treasury Survey, which combines ultraviolet (UV) through mid-infrared emission lines to map the high-energy ionizing spectrum. These observations reveal radiation fields that are significantly harder and more structured than predicted by standard stellar population models, pointing to additional contributions from very massive stars, ultra-luminous X-ray sources (ULXs), and obscured AGN. At the same time, I will show that nebular conditions in these galaxies are far from uniform. Density stratification, particularly in highly ionized gas, can lead to systematic biases in temperature measurements and subsequent abundance determinations when using traditional low critical-density optical emission lines. As a result, even the long-standing “gold-standard” of metallicity measurements, the direct method, will be significantly biased in extreme environments.

Fortunately, UV diagnostics provide access to the densities and physical conditions of the high-ionization gas, enabling more robust determinations of temperatures and abundances. By combining UV and optical measurements, we can establish a physically consistent framework for interpreting local EELGs and connect them to high-redshift galaxies observed with JWST, which exhibit even more extreme ionization conditions, elevated densities, and enhanced N/O ratios. I will discuss the physical pathways that can drive rapid enrichment in relative abundances, and the implications for interpreting both local and distant galaxy populations.

Together, these results demonstrate that metal-poor EELGs expose the interconnected physics linking ionizing spectra, nebular conditions, and chemical enrichment across cosmic time, but only when interpreted with a self-consistent UV+optical framework.

Cosmic dust and Polycyclic Aromatic Hydrocarbons (PAHs) account for less than 1% of the interstellar medium by mass, yet they shape the observability of galaxies. Modeling them self-consistently in cosmological simulations of galaxy evolution is one of the main challenges in the ALMA and JWST era.

After introducing the main strategies to model the evolution of galaxies and their dust content, I will present the first cosmological galaxy formation simulation to jointly follow the evolution of the physical grain size distribution and the luminous properties of PAHs across a large volume and cosmic time. I will show how the PAH–metallicity relation emerges naturally from grain shattering in the ISM, and how PAH emission can be used to trace physical properties of galaxies such as star formation rate and molecular gas content. These results provide a physical framework to interpret recent JWST observations at cosmic noon and will help guide future missions such as PRIMA.