Seminars and Colloquia at ESO Garching and on the campus

There are many reasons that feedback may operate differently in metal-poor, extreme emission line galaxies. These differences now have great urgency, as we want to understand feedback in the metal-poor environments of the early Universe galaxies. I will discuss results we are finding with observing galactic winds in nearby, metal-poor starburst galaxies. These results include surprising discoveries regarding metal-enrichment of outflows and the presence of extreme highly ionised gas, which currently do not have clear explanations. I will then discuss the measurement of mass-loading in metal poor galaxies. I will show that substantial fractions of gas are leaving the galaxies in low velocity bubbles. These bubbles are not observable with JWST, due to the limits in spectral resolution, and thus have strong implications for interpreting high-z galaxies. These results have clear and direct application to new ESO instruments BlueMUSE and MAVIS.

JWST has enabled spectroscopy of substellar companions too faint to observe from the ground. These include planetary-mass companions (PMCs) like GJ 504 b, among the coldest known substellar companions prior to JWST launch. This object has also been the subject of extensive debate regarding its mass and planetary nature. We present the first spectroscopic data for this companion, obtaining comprehensive coverage over the 3จC5-micron range using moderate-resolution (R ~ 2,700) observations with JWST NIRSpec. Leveraging advanced post-processing techniques with a forward modeling framework, we detect the companion at high signal-to-noise (S/N>300). We also present the first successful PSF subtraction with angular differential imaging (ADI) in the NIRSpec point cloud, detecting GJ 504 b at S/N>10 and reaching contrast limits <1e-4. CCF analysis of the spectrum confirms the presence of ammonia at a 10ฆา significance, with other significant detections including CO2 (82ฆา), 13CO (36ฆา), C18O (12ฆา), and H2S (6ฆา). Joint retrieval modeling of the spectrum and previous photometry yields an effective temperature = 564 กภ 4 K, metallicity = 0.67 กภ 0.13 dex, and strong evidence for disequilibrium chemistry and salt clouds. The retrieved parameters indicate a mass ~25 MJup and an age of 2.5--4.0 Gy according to ATMO models. Lastly, we compare the abundances of GJ 504 b to its primary, obtaining a stellar abundance of sulfur (S), super-stellar carbon (C), and possibly oxygen (O). The observed metal enrichment tentatively supports planet-like formation but does not entirely exclude stellar abundances for GJ 504 b. Regardless, the strong preference for clouds by our retrievals suggest that accounting for cloud absorption is needed to accurately model the spectra of high-metallicity late-T planetary-mass companions.

UV attenuation tells a more complex story. Unlike IR, it is highly sensitive to line-of-sight column density, grain size distribution, and dust-stellar morphology, effects that can mimic a dust-poor galaxy even when substantial dust is present. This ambiguity sits at the heart of one of the most striking puzzles revealed by JWST: an unexpectedly large population of massive, UV-bright galaxies at z>10, the Blue Monsters, coexisting with dust-rich systems already in place by z~7, just ~300 Myr later.

I will present two recent efforts to disentangle dust physics from galaxy physics in this context. First, I show that ISM porosity driven by turbulence is key to reconciling UV and IR observations at z~7, while at z>10 additional mechanisms, grain growth or radiation-pressure-expelled dust, may be required (Sommovigo et al. 2026a). Second, I present a systematic study of attenuation curve shapes in IllustrisTNG, identifying new functional forms and galaxy properties that retain constraining power on attenuation curves regardless of dust mixture (Sommovigo et al. 2026b). Together, these results urge caution: much of the observed UV--IR tension may reflect incomplete dust modelling rather than genuinely new galaxy physics.

In the context of the JWST’s GARDEN (Galaxies at All Redshifts Deciphered and Explained with the NIRSpec MSA) survey, a novel slit-stepping program, we found two galaxies at z~4-5 that exhibit auroral emission lines, enabling spatially resolved measurements of electron temperature and direct oxygen abundances. Measuring spatially-resolved features in high-redshift galaxies, and comparing them with local galaxies and Galactic measurements, is key to constrain galaxy chemical evolution, especially through the variation of radial metallicity gradients. 

We build emission line, radial velocity, strong-line abundance indices, electron temperature, and direct abundance spatial maps for both galaxies, thanks to the excellent spatial resolution. From the direct abundance maps we measure their linear radial metallicity gradients. These results provide a rare measurement of a radial metallicity gradient at z>0 from direct-method abundances, and the first for a non-lensed galaxy, offering key observational support for inside-out galaxy growth with feedback-regulated chemical enrichment. Our analysis demonstrate the viability of deep JWST/NIRSpec MSA spectroscopy for spatially resolved chemical analyses 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.