Seminars and Colloquia at ESO Garching and on the campus

Cosmic rays (CRs) play a crucial role in the physics and chemistry of the interstellar medium (ISM). At the high densities found in molecular clouds, they represent the main ionising agent of the gas, affecting its heating and evolution. CRs ionise molecular hydrogen, quickly producing H3+, setting the gas ionisation fraction. The latter affects the timescale of ambipolar diffusion, a mechanism that allows the collapse of subcritically magnetised prestellar cores. Furthermore, CRs initiate the rich chemistry of molecular ions in molecular clouds. In this regard, it is of particular importance the formation of H2D+, the precursor of deuterated species in the gas phase, and that of He+, the first step towards ammonia formation. In this seminar, I will describe the different physical and chemical properties of star-forming regions that are affected by CRs. I will focus, in particular, on how we can observationally measure their impact through the CR ionisation rate (CRIR), discussing the different methodologies used in recent years, and I will show some of the most recent results in this regard.

The Galactic bar is a key driver of the Milky Way's internal evolution, redistributing stars and gas, influencing the structure of the central bulge. Understanding how different stellar populations interact with the bar helps unravel the Galaxy's formation history. I analyze the RR Lyrae population toward the Galactic bulge to investigate its spatial and kinematic connection to the bar. RR Lyrae stars, as old, metal-poor standard candles, provide a unique view into the early Galaxy. By refining reddening maps and extinction laws from visual to near-infrared bands, I derive accurate distances that remove bar-like artefacts caused by extinction law variations. I find that only metal-rich RR Lyrae stars align spatially with the bar, while metal-poor stars form a more spheroidal distribution. For 8456 stars with full kinematics, I compute orbits in a Milky Way-like potential, revealing that prograde motion dominates among metal-rich stars, whereas metal-poor ones exhibit an increase in the retrograde orbits. These retrograde stars have stable orbital frequencies and are highly concentrated toward the Galactic center. Comparisons with N-body+SPH simulations suggest that such a structure can emerge through secular evolution without invoking a classical bulge.

Dense molecular gas is the immediate fuel for star formation, with nearby galaxies showing a tight linear relation between dense gas mass and star formation rate. However, such studies have so far been limited to the local Universe, where star formation rates are relatively modest. At Cosmic noon (z = 2–4), galaxies form stars at rates hundreds of times higher, yet it remains uncertain whether this reflects larger gas reservoirs, higher efficiencies, or both. The PRUSSIC survey marks a major advance, greatly expanding detections of dense molecular gas tracers (HCN, HCO⁺, HNC) in galaxies at Cosmic noon and delivering the first spatially resolved view of dense gas under these extreme conditions. By connecting dense gas, total molecular gas, and star formation across cosmic time, PRUSSIC provides crucial insight into how the physical drivers of star formation evolved during the peak of the cosmic star formation rate density.

Planetary nebulae (PNe) are powerful probes of their host galaxies; however, their detection outside the Milky Way is significantly more challenging. We investigate the PN population of M33 using data from the J-PLUS survey (DR3), a 12-band photometric dataset well suited for identifying Hα emitters. While only 13 of the 143 known PNe in M33 have photometry available in the J-PLUS catalog, by performing source extraction directly on the J-PLUS images we recovered photometry for over one hundred PNe, including the 13 already cataloged, thus revealing the hidden population.
Color-color diagrams allowed us to identify possible PN candidates, most consistent with emission-line sources, and even a potential halo PN when combining radial velocities from the literature with criteria previously applied to Milky Way halo PNe.
This work represents the first attempt to explore extragalactic PNe with multi-band photometric surveys such as J-PLUS, S-PLUS, and J-PAS, highlighting their strong potential for PN research in nearby galaxies.

Our eyes are our window to the world but they can only perceive a tiny fraction of the radiation emitted by celestial objects. For centuries, astronomy relied on optical telescopes that observed only this “visible light.” Yet to truly understand the Universe, we must look beyond: we need to explore the entire electromagnetic spectrum, from radio waves to infrared, and even the high-energy ultraviolet, X-rays, and gamma rays – which require space telescopes, since our atmosphere blocks them from reaching the ground.

In this one-hour talk, I will take the audience on a journey through the often unexpected milestones that led to the development of modern astronomy, explaining along the way which cosmic sources shine in each part of the spectrum. I will conclude by discussing the “cosmic light background” – the combined glow of all celestial objects that have ever existed in cosmic history — an ancient, diffuse radiation that tells the story of the Universe’s evolution.

The Milky Way is still evolving. The accretion of gas and stars from our surroundings in the Local Group continues to shape and build the Galaxy. Multi-phase gas flows play essential roles in cycling baryons and metals through the Galactic ecosystem and fueling the Galactic gas supply. In this colloquium I will review recent work on the gas flows around the Milky Way, based on UV/optical absorption-line observations from HST and VLT, H I 21 cm observations, and hydrodynamic simulations. After introducing the use of high-velocity clouds (HVCs) as tracers of Galactic inflow and outflow, I will discuss the Galaxy’s cool nuclear outflow and the giant Fermi and eROSITA Bubbles found on either side of the Galactic Center. I will then discuss the gas content of the Magellanic System, which is interacting with the Milky Way and slowly transferring large amounts of gas to the Galaxy. This will include new results on the LMC’s gaseous halo and the distance to the Magellanic Stream. 

The discovery of high ionization emission lines in both high-redshift galaxies and nearby, metal-poor dwarf galaxies has questioned the origin of He II ionizing radiation. Current stellar population synthesis models consistently fail to reproduce the necessary ionizing fluxes, pointing to a fundamental gap in our understanding of stellar feedback and its role in cosmic reionization. Classical Wolf-Rayet (WR) stars are hot, evolved massive stars with depleted hydrogen. WR stars with prominent Nitrogen emission lines in their spectra are called WN type, further sub-divided as 'early' (WNE) when showing emission from high ionization species (e.g., He II, N V). WNE stars at low metallicity are huge contributors to the HeII ionizing flux of their host galaxy. However, the presence of WNE stars in integrated environments can be diluted, making their direct spectroscopic detection challenging. In this talk, I will directly compare stellar spectral diagnostics for the resolved WNE stars in our nearest low-metallicity dwarf galaxy, the SMC, with integrated nebular diagnostics from the Local Volume Mapper integral field survey. I will discuss the implications of our unexpected findings for population synthesis models and galaxy evolution.