Seminars and Colloquia at ESO Garching and on the campus

Observations of dense protostellar cores indicate high levels of magnetization, strong enough in MHD simulations to suppress the formation of rotationally-supported protostellar disks. However, large disks continue to be detected around a growing number of young protostars, suggesting that additional physical mechanisms are needed to enable disk formation in magnetized environments. Non-ideal MHD effects provide one possible solution, but their observational signatures are difficult to detect and quantify directly. Since
cosmic-ray ionization influences the ionization fraction, and therefore the efficiency of non-ideal MHD effects, measurements of the cosmic-ray ionization rate and ionization fraction can provide indirect constraints on the role of non-ideal MHD during disk formation. Here, we present JCMT observations of DCO+ and H13CO+ toward 16 dense protostellar cores with well-constrained disk radii from the eDisk ALMA Large Program. Using these observations, we estimate the cosmic-ray ionization rate and ionization fraction in order to statistically assess the importance of non-ideal MHD in the formation of protostellar disks.

Euclid is expected to discover approximately 170,000 strong gravitational lenses over its nominal survey, increasing the number of known systems by roughly two orders of magnitude. This unprecedented sample will enable not only statistical studies of the overall lens population, but also detailed investigations subclasses, such as double-source-plane lenses and edge-on spiral galaxies, which provide powerful constraints on the distribution of dark matter. For edge-on lenses, we model the baryonic component (informed by the light) separately from the dark matter halo, allowing the two mass distributions to be constrained independently. First results indicate a dark matter density profile that is shallower than the canonical NFW profile (with an uncertainty of ~10-25% per object from photometric data only), with a halo that is close to round in projection. Models that omit a dark matter halo require unphysically high bulge mass-to-light ratios, reinforcing the standard dark matter interpretation over alternatives such as MOND.

Massive star clusters contain a substantial population of stars with unusual abundances of light elements, with chemical patterns indicative of hot hydrogen burning. The source of this material, the large number of stars with this signature, and its presence only in massive clusters all remain unsolved puzzles of this 'multiple population' problem. In this discussion, I will argue that a complete solution must involve a combined understanding of the stellar populations found in massive star clusters and how those stellar populations are modified, and can themselves modify, the complex star cluster assembly process.

Globular clusters (GCs) in massive galaxies have proven to be powerful tracers of merger histories, star formation

episodes, and the conditions of the early Universe. These insights mainly arise from scaling relations between host

galaxy properties and GC system parameters. However, massive galaxies, contrary to dwarfs, have assembled most

of their stellar mass via ex situ formation, implying that a substantial fraction of their GC systems originated outside

the main galaxy. To perform the study in a clean regime where ex-situ processes are minimized, we focus on dwarf

galaxies (DGs, M⋆ < 109.5M⊙), where stellar mass forms predominantly in situ, thereby providing a perspective

directly tied to the galaxy’s intrinsic formation history. Therefore, we expect that DGs host GCs systems that reflect

the initial conditions of galaxy formation. In this context, we select a sample of irregular DGs, located outside the

Local Group but within 20 Mpc. These objects are in low-density environments (isolated or in small groups) and

have stellar masses ranging from 107.5 − 109.0 M⊙. We also adopt a multi-wavelength approach, combining optical

(griz, DELVE), mid-infrared (3.6 and 4.5 µm, S4G), and radio (21 cm, HIPASS) data. We select GC candidates using

griz photometry and SSP-based color cuts. Preliminary results indicate an average of 6 GCs per galaxy and a GC

luminosity function that deviates from the gaussian shape typically observed in massive galaxies. We find a linear

correlation between GC system mass and dark matter halo mass, as predicted for massive galaxies, an indicative of

a co-evolution between the two. We report hints of bimodality in the GC populations, which is unexpected for such

systems. This feature may indicate past merger events, earlier episodes of enhanced star formation, or a combination

of both. To confirm this finding we analyse multi-object spectroscopic observations from Gemini/GMOS and present

preliminary results. Finally, we perform SED fitting on the GCs to constrain their ages and metallicities. The results

are consistent with the bimodality. Our results suggest that GC populations in dwarf galaxies may be more complex

than previously thought. These complexities may offer valuable insights into mass assembly of low mass systems,

and the importance of mergers in low-density environments

Gravitational waves (GWs) have a very weak absorption cross section. This makes them difficult to observe on the one hand, but it  is also for this reason they carry pristine information from their generation mechanism. Their observation may allow  glimpses  back to the highest redshifts and energies in the Universe. 

After a brief introduction to the significance of the observation of a stochastic gravitational wave background, I shall  concentrate on two possibilities: Generating gravitational waves from violent processes in the post-inflationary Universe and generating them during inflation. I shall show that in both cases different universal GW spectra are generated. I shall also discuss what we may learn from their observation.