Seminars and Colloquia at ESO Garching and on the campus

The origin of cosmic-rays remains one of the biggest mysteries in astrophysics. Cosmic rays can carry away large fractions of the total energy of their sources, contributing to the energy density of the interstellar medium as much as starlight does. Despite their importance, there are big gaps in our understanding of where and how cosmic rays are accelerated, especially as their energy increases.  Extreme particle acceleration requires rather exotic astrophysical environments, such as the powerful outflows associated with accretion onto black holes ("jets"). In fact, the jets produced by supermassive black holes in the center of distant galaxies are often invoked to explain the most energetic cosmic rays detected. But what about our own Galaxy?  In my talk I will introduce an emerging class of particle accelerators: jets produced by stellar-mass black holes ("X-ray binaries" ) in our own Galactic backyard. Although their relevance was long theorized, it is only recently that the detection of the gamma-ray emission produced by the accelerated particles has began to reveal their potential as particle accelerators. I will describe the current observational evidence that some X-ray binary jets can accelerate particles to energies up to and above 1 PeV and briefly discuss what to expect from the class as a whole and from observations with future and current facilities

Massive stars (> 8 Msun) are likely found in binaries or higher order multiples throughout their lives; however, due to observational challenges, the relative importance of the formation mechanisms giving rise to this multiplicity are not well constrained. The youngest multiple systems, whose system parameters best constrain theoretical models, are the most deeply embedded, and in the high-mass regime only a few examples of young multiples -that have not yet developed ultracompact/hypercompact HII regions or bright IR emission- have been identified. Using high angular resolution (~350 AU) ALMA observations taken as part of the The Complex Chemistry in hot Cores with ALMA (CoCCoA) survey I will present a case study of multiplicity in the massive star forming environment NGC 6334-43. We identify nine protostellar and prestellar sources that reside within a single filamentary structure, whose derived system parameters are consistent with a bound system in a simple stability analysis. 

Streamers, high-aspect-ratio velocity-coherent structure of gas (and sometimes dust) that are falling towards a source, may play a crucial role in a variety of star formation processes. In contrast to numerous identifications towards low-mass sources, there is a comparatively sparse sample of streamers in high-mass star-forming regions. We aim to expand this sample through a search for streamers in H13CO+ emission across nine high-mass star forming regions observed as part of the CoCCoA survey. I will present ongoing work to this end, where we have obtained reliable Vlsr measurements for ~55 sources by fitting synthetic spectra to their CH3CN emission and analysed the complex H13CO+ emission via Gaussian decomposition and subsequent clustering into position-position-velocity coherent structures. I will also show early results of fitting candidate streamers using an MCMC implementation of TIPSY. 

The UV 2175 Å bump is one of the most enigmatic features of galaxy extinction and attenuation curves. Although its physical origin remains uncertain, laboratory and theoretical studies suggest that the carriers of the bump are likely polycyclic aromatic hydrocarbons (PAHs). However, observational evidence directly linking PAHs to the UV attenuation bump is still scarce, especially beyond the local Universe. In this talk, I will present the first statistically significant study of the connection between PAH emission and the UV 2175 Å bump in individual galaxies at cosmic noon (1.5 < z < 2.7). Our sample consists of ~50 galaxies with spectroscopic measurements of the UV bump from VLT/MUSE. PAH emission is traced using JWST/MIRI photometry, while gas-phase metallicities are derived from Keck/MOSFIRE and JWST/NIRSpec spectroscopy. Additional galaxy properties are obtained through SED fitting to multi-wavelength photometry spanning HST, JWST, Herschel, and ALMA data. We find a clear correlation between PAH emission and UV bump strength, providing strong observational support for the idea that PAHs are the primary carriers of the bump. Both PAH emission and bump strength also correlate with stellar mass and gas-phase metallicity, such that lower-mass, lower-metallicity galaxies tend to exhibit weaker PAH emission and weaker UV bumps. These results extend well-known relations observed in the local Universe to galaxies at cosmic noon. At high PAH abundances, however, we observe substantial scatter in the bump strength. We show that this scatter is linked to variations in dust optical depth, suggesting that dust geometry/clumpiness also plays an important role in shaping the observed UV bump.

Finally, we show that the UV bump strength is best described by a combination of PAH abundance, gas-phase metallicity, dust optical depth, and specific star formation rate, pointing toward a complex interplay between dust (and, therefore, PAH) formation and destruction mechanisms in galaxies across cosmic time.

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.