Seminars and Colloquia at ESO Garching and on the campus

After decades of being mostly confined to the local Universe, the study of gas dynamics in galaxies, via a variety of emission-line gas tracers, has now become a key tool of investigation across cosmic time. The rotation of the gas allows us to trace the distribution of matter, quantify the mass and shape of the dark matter halos and study galaxy scaling relations. At the same time, gas turbulence reveals the effects of stellar feedback and disc instabilities and provides clues on the formation of the stellar thin/thick discs. I will present results on high-z rotation curves and velocity dispersions obtained through 3D reconstruction techniques of the emission-line datacubes. I will focus on ALMA observations of galaxies at z~4-5 observed in [CII] emission lines, extending to intermediate redshifts (z=1-4) using mostly CO lines. These data reveal fast rotation and relatively-low gas velocity dispersions leading to typical V/sigma values of order 10, similar to those of nearby spiral galaxies. Often, the fast rotations show the presence of mass concentrations that suggest a quick formation of stellar bulges, while the low velocity dispersions indicate that the gas turbulence is mostly fed by supernova feedback. I will discuss how the widespread presence of such “cold” discs at z~4-5 and their properties are changing our understanding of galaxy formation at early times.

The birth of stars and the formation of galaxies are cornerstones of modern astrophysics. While much is known about how galaxies globally and their stars individually form and evolve, one fundamental property that affects both remains elusive. This is problematic because this key property, the birth mass distribution of stars, referred to as the stellar initial mass function, is a key tracer of the physics of star formation that underpins almost all of the unknowns in galaxy and stellar evolution. It is perhaps the greatest source of systematic uncertainty in star and galaxy evolution. The past two decades have seen a growing variety of methods for measuring or inferring the initial mass function. This range of approaches and evolving definitions of the quantity being measured has in turn led to conflicting conclusions regarding whether or not the initial mass function is universal. Here I review this growing wealth of approaches, and highlight the importance of considering potential initial mass function variations, reinforcing the need to carefully quantify the scope and uncertainties of measurements. I present a new framework to aid the discussion of the initial mass function and promote clarity in the further development of this fundamental field.

A third of all massive stars are predicted to lose their hydrogen-rich envelope through mass transfer or common envelope ejection initiated by a binary companion star. As a result, the hot and compact helium core is exposed. These "stripped stars" are the direct progenitors of hydrogen-poor supernovae and merging binary neutron stars, but they are also so hot that they should boost the ionizing output from bursty star-forming galaxies.

Despite their importance, stripped stars remained, until recently, observationally unconfirmed since their predicted existence over half a century ago. We found the first set of stripped stars by combining ultraviolet and optical photometry with follow-up spectroscopy in the Magellanic Clouds. By fitting their spectra with a new grid of models, we could measure stellar properties and thus confirm that the predictions from binary evolution models are broadly consistent with observed stripped stars.

This discovery is a step towards understanding the role of interacting binaries in stellar populations. This is evidenced, for example, by the highly ionized gas surrounding some of these systems, shining brightly in O III and Balmer spectral lines. Directly constraining the ionizing emission and hardness of stripped stars, along with the typical gas density surrounding the stars, could lead to estimates of the escape fraction for different star types and the disentangling of stellar populations in unresolved galaxies.

This past summer, the pulsar timing array community announced strong evidence for the presence of a stochastic background of nanoHertz frequency gravitational waves. This has been the primary goal of the community for the past two decades, and it took thousands of hours of telescope time, over 500,000 pulse arrival times from ~70 millisecond pulsars, and a highly sophisticated and very computationally demanding analysis effort to accomplish. While we can't yet say for certain what is causing the gravitational waves, our best guess is a population of slowly merging super-massive black hole binaries throughout the universe. But it is possible that the signal also heralds new physics. So what does it all mean and what are we expecting next? And what other cool things can we do with all of this high-precision pulsar data?

Found to be among the most dark matter-dominated systems discovered to date, low-mass galaxies provide stringent tests of our cold dark matter model on small scales. Because they are intrinsically faint and difficult to identify and characterize, studies thus far have primarily focused on the population of dwarf galaxies orbiting the Milky Way. I will present novel observations of low-mass galaxies beyond our local galactic neighbourhood, uncovering their considerable diverseness and introducing new astrophysical puzzles. I will describe a new framework for obtaining constraints on the distribution of dark matter in low-mass galaxies by leveraging their globular cluster systems and dynamical considerations. I will also present new constraints on the statistical mapping between satellite galaxies and their host dark matter halos, utilizing a unique sample of satellite galaxies in the Local Volume from the ELVES survey. I will conclude by discussing ongoing and future surveys essential in mapping the census and properties of the general population of low-mass galaxies.

I will give an introduction to the structure and dynamics of the central 3 kpc of the Milky Way. This region hosts a complex star-forming ecosystem that is continually exchanging matter with the rest of the Galaxy through inflows and outflows. The Galactic bar efficiently transports gas from the Galactic disc towards the centre at a rate of ~1 Msun/yr, creating a ring-like accumulation of molecular gas known as the Central Molecular Zone (CMZ) at a radius R=120pc. The CMZ is the local analog of the star-forming nuclear rings commonly found at the centre of external barred galaxies, and forms by a process similar to the one that creates gaps in Saturn’s rings. Once in the ring, approximately 10% of the gas is consumed by its intense star formation activity. Star formation does not occur uniformly throughout the CMZ ring, but is more likely to occur near the sites where the bar-driven inflow is deposited. The star formation rate of the CMZ varies as a function of time, but it is currently debated whether this is due to an internal feedback cycle or to external variations in the bar-driven inflow rate. The radius of the CMZ gas ring slowly grows over Gyr timescales, and its star formation activity builds up a flattened stellar system known as the nuclear stellar disc, which currently dominates the gravitational potential of the Milky Way at 30pc<R<300pc. Most of the gas not consumed by star formation in the CMZ is ejected perpendicularly to the plane by a Galactic outflow powered either by stellar feedback and/or AGN activity, while a tiny fraction continues moving radially inward towards the circum-nuclear disc at R=few pc, and eventually into the sphere of influence of the central black hole SgrA* at R<1pc.

Recent discoveries of accreting brown dwarfs (BD) and exoplanets that appear to accrete at anomalously high rates have placed new importance on understanding the mechanisms that control their growth and formation. I will discuss my work to disentangle systematic effects from true physical variation in substellar accretion properties using the Comprehensive Archive of Substellar and Planetary Accretion Rates (CASPAR). CASPAR consists of >1000 measured Ṁs from ~800 T-Tauri stars, BDs, and planetary mass companions (PMC), making it the largest compiled sample of Ṁs for accreting objects to-date. I will show that systematically rederiving physical and accretion properties for all objects in the database has a negligible effect on the scatter in the M-Ṁ relation while showing that the remaining broad scatter is attributable to physical effects such as age, mass, and variability.

There has been a tremendous leap forward in our understanding of the formation of planetary systems thanks to substantial advances in our observational capabilities. Observatories like ALMA are routinely revealing the presence of complex structures in the gas and dust which are forming planets, much of which has been associated with young, embedded protoplanets. Such observations are unpinning sigificant developments in the theory of how, and from what, planets form. In this talk I will provide an overview of our current understanding of the physical, chemical and dynamical structure of protoplanetary disks with a particular focus on how we are beginning to detect the presence of young planets, only recently formed. I will present new results from the exoALMA program, an ALMA Large Program that is undertaking an extensive planet-hunting campaign in the sub-mm, and the related projects on facilities like JWST, VLT and Magellan. To conclude, I will discuss future facilitiies, and detail how, in the coming decade, we will begin to push into the terrestrail planet forming regions of these disks and understand the formation of Earth-like planets. 

Low-mass star-forming regions are blooming in emission from abundant complex organic molecules (carbon-containing molecules of at least 6 atoms). Unbiased spectral surveys and the advent of state-of-the-art interferometers like ALMA have tremendously expanded our understanding of the chemical composition of protostellar regions. The earliest stage of star formation, the prestellar core, is the birthplace of complex organic molecules under interstellar physical conditions. Upon gravitational collapse, a young protostar with a protoplanetary disk is formed. The concurrent heating and UV irradiation boost the production of complex organics. It is thought that the largest reservoir of complex organics is in interstellar ices, which can now be directly probed by the JWST. Meanwhile, thermal desorption in the warm inner regions around protostars allows us to readily observe such species in the gas with ALMA. In the outer parts of a protoplanetary disk, solid complex organics become integrated into forming comets and planets.

Our Solar System was once too an infant low-mass protostar embedded in its natal cloud. The most pristine relics of this time that survive to this day are comets. Recently, cometary science experienced a significant boost as a result of the large wealth of data coming from the ESA Rosetta mission that escorted comet 67P/Churyumov-Gerasimenko for two years. In my talk, I will highlight recent observational investigations of complex organics from cores to protostars, including studies of methanol isotopologs in the prestellar core L1544 and the comprehensive chemical inventory of the low-mass star-forming region IRAS 16293-2422. I will present the chemical trail that connects the earliest phases of star formation with comets in our Solar System. I will address the story told by the comet’s volatile inventory and isotopic ratios about the connections with protostellar and prestellar phases, thereby bring forward the idea that comets of our Solar System reflect to a degree the complex organic composition of the innate core that birthed our Sun.

Unlike the Hertzsprung–Russell diagram for stars, there remains no formal classification for exoplanets composed of varying proportions of fluids, rock+metals and ice. Still, as with stars, planetary mass and composition – expressed in geochemical and cosmochemical terms – mold bulk physical characteristics and evolutionary paths. Here, I show how combining geodynamics with astrophysical observations provides insights into rocky exoplanet characteristics such as silicate mantle viscosity and intrinsic heat production vs. age. I test the general predictability of such geochemical models with an example from recent atmospheric retrieval data collected from an ultra-hot Jupiter in the WASP-76 system. I conclude with a geochemical evaluation of spectral data of moderately volatile vs. moderately refractory lithophile elements reported from some polluted white dwarfs and what this means for the ultimate fates of rocky planets around Sun-like stars

How much do different physical processes in the interstellar medium -- in particular dynamics, magnetic field and density structure -- influence the formation of massive stars? I will show observational results covering scales of dynamical cloud-cloud collisions to collapsing star-forming regions. Employing studies from mm wavelengths (SMA, NOEMA, ALMA, 30m) to the mid-infrared (JWST), the characterisation of magnetic field, density structure and accretion processes will be discussed.