Seminars and Colloquia at ESO Garching and on the campus

Accretion plays a crucial role in our understanding of how the most extreme objects in the universe, black holes (BHs) and neutron stars (NSs), interact with and energize their surroundings through energetic jets. Neutron star low-mass X-ray binaries (NS LMXBs), systems in which a NS accretes matter from a companion star, serve as a natural laboratory for studying these processes. Even after decades of studies employing conventional spectral and timing analyses, the accretion geometry in NS LMXBs remains a subject of ongoing debate. The recent advent of sensitive polarimetry provides a powerful diagnostic tool, and by leveraging cutting-edge telescopes in the multi-wavelength regime, this study implements a novel approach to mapping the geometry of these systems. I will present our current results, which indicate that while some NS LMXBs exhibit alignment between the X-ray polarization angle (PA) and the radio PA, others display significant misalignment. I will further discuss our current efforts to understand the origin of these alignment patterns and their relationship to the jet and accretion-disk geometry, as well as to the spin and orbital axes of the binary systems, thereby offering new constraints on the formation history of NS LMXBs.

One of the central challenges to understanding planet formation is how planetary properties – their masses, orbital characteristics, bulk properties and atmospheric compositions – are connected to their formation in host protoplanetary disks (PPDs). Forming planets accrete pebbles, planetesimals, and gas over a wide range of chemical compositions as they migrate through their evolving PPDs. The basic mechanism that controls these processes is how angular momentum is removed from PPDs. Traditional models have assumed that turbulence is the main driver of such disk evolution; current ALMA observations have now largely ruled this out. Instead, powerful MHD simulations and a wide range of ALMA and JWST observations confirm that MHD disk winds likely play the dominant role. In this talk, I will briefly discuss relevant observational and theoretical advances and their consequences for a new paradigm for planet formation based on magnetized disk wind (MDW) transport of angular momentum. I will then focus on several recent advances in my group including the effects of MDWs on dust transport and evolution in PPDs, ring formation,  planetary populations that result from disk-wind driven disk evolution, and our new planetary interior structure models that we apply to compute the mass-radius relation for our formed planets and compared with observations.

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.