Vorträge, Seminare, Ereignisse

Abstract
Understanding how the heavy elements are produced in the early Universe remains one of the open questions in astrophysics. We seek to trace the first chemical enrichment from the First Stars (Pop III stars), which are long gone, and which we due to observational limitations cannot observe directly. Hence, we are forced to explore the properties and physics of this first population of stars through indirect measurements of the second generation stars. Bona fide second generation stars preserve to a great extent the gases and thus the traces of the Pop III stars in their surfaces. As we observe the old metal-poor stars in greater numbers we see that these are typically C-rich and often contain varying amounts of heavy elements. The amount of heavy element content is a direct tell about the Carbon Enhanced Metal-Poor (CEMP) stars nature and origin. It therefore becomes of utmost importance to understand the source of the heavy element production. Until the recent years, the heavy element nucleosynthesis could only be explored indirectly, however, thanks to a-LIGO's gravitational wave detections and the following ground-based observations of the electromagnetic wave signal, we can for the first time directly study the formation of radioactive heavy element nucleosynthesis taking place in the rapid neutron-capture process hosted by a kilonova. I will describe the manifold nature of CEMP stars, their origin, and link this to the r-process.

Abstract
We are currently witnessing a golden age in Galactic halo science. Largely thanks to the Gaia mission, we now have phase-space plus chemical information for significant numbers of halo stars, globular clusters and satellite dwarf galaxies in the Milky Way. These halo populations can be used to uncover the assembly history of the Galaxy, probe the nature of dark matter, and scrutinise cosmological models. In this talk I will describe recent efforts to address these fundamental questions using a combination of new observational data and state-of-the-art cosmological simulations. In particular, I will discuss the total mass and mass profile of the Milky Way, the last major accretion event that shaped the Galaxy's history, and the (surviving and destroyed) dwarf satellite luminosity function.

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AGN feedback is a key ingredient in galaxy formation models and is now widely considered to be one of the main drivers in regulating the growth and assembly of massive galaxies. In my talk I will describe several efforts in our group to understand the power, reach and impact of AGN feedback processes and how they may impact the build-up of galaxies. Using the SDSS-IV MaNGA survey at low-z, we have found that AGN signatures and winds can be easily hidden in the integrated spectrum of a galaxy. We show that outflow and feedback signatures in low-luminosity, low-z AGN may so far have been underestimated. At higher redshift, we have found that outflows can indeed suppress star formation in their hosts, consistent with the AGN having a `negative' impact. However, both star formation and quasar activity peaks at z ~ 2-3 where AGN are expected to impact the build-up of stellar mass the most. Our team recently discovered a unique population of luminous high-z (2 < z < 4) extremely red quasars (ERQs) with extreme outflow properties, including blueshifted [OIII] lines at speeds up to 6000 km/s and unusual Lya profiles. ERQs are the ideal population to obtain a census of the overall mass and energy budget of both outflow and infall/feeding from the CGM, an essential requirement to probe the detailed and full feedback loop. Finally, I will also introduce the JWST ERS Program "Q3D" (PI: Wylezalek) which will make use of the IFU capabilities of NIRSpec and MIRI and through which we will study the impact of three carefully selected luminous quasars on their hosts. Our program will provide a scientific dataset of broad interest serving as a pathfinder for JWST science investigations in IFU mode.

Abstract
With its high sensitivity, excellent angular resolution, and wide spectral coverage, ALMA is revolutionizing our view of galaxies in the nearby universe. ALMA is particularly important for studying the dense molecular gas that is the fuel for star formation. Radio continuum emission from ALMA is also an important measure of the star formation rate, particularly in galaxies with high visual extinction such as starburst galaxies and luminous infrared galaxies. Finally, the ALMA archive contains an ever-growing collection of data that can be mined and combined to produce large samples of targets that can match or exceed the amount of observing invested in a single ALMA large program. I will describe our work on the link between dense gas and star formation for a sample of 9 nearby galaxies from the ALMA archive, which includes measuring the resolved Kennicutt-Schmidt star formation law at extreme star formation rate surface densities and identifying a new molecular line that appears to be an excellent tracer of the densest star forming gas.

Abstract
Understanding the first steps in the formation of stars and protoplanetary disks is a great unsolved problem of modern astrophysics. Observationally, the key to constraining theoretical models lies in high-resolution studies of the youngest protostars. \;
I will present the state of the art regarding observations and modeling of the youngest protostellar disks, observed less than 0.1 Myrs after the onset of protostellar formation. \;
In a first part, I will show how our millimeter dust continuum interferometric data suggests that most (>\;75%) protostellar disks observed less than 0.1 Myrs after the onset of protostellar formation are only found at very small radii <\;60 au, which favors magnetized models for protostellar disk formation. I will also present our ALMA observations of a very young solar-type protostar suggesting a disk is currently forming in counter-rotation with respect to the protostellar core rotation, and discuss potential scenarii to understand this oddity. I will show our SMA and ALMA observations of the magnetic field topology in a sample of young protostars and compare all observed protostellar properties to the typical outcome of models for protostellar formation. I will argue that our observations of small disks, counter-rotating disks and organized magnetic fields in the youngest star-forming cores question the established paradigm of disk formation as a simple consequence of angular momentum conservation during the main accretion phase: they instead highlight the need to investigate magnetized models in order to unveil the mechanisms responsible for protostellar disk properties.
In a second part, I will show both our observations and models of the dust continuum emission call for significant grain growth (grains larger than 10 microns and up to submillimeter sizes) at radii 100-1000 au in the protostellar envelopes observed less than tenth of a million years after the onset of collapse. I will describe new avenues to explore dust pristine properties and describe better in the future, for example, the initial conditions for the formation of planetesimals.

Abstract
Saturn’s icy moon Enceladus harbours a global ocean, which lies under an ice crust of just a few kilometres thickness and above a rocky core. Through warm cracks in the crust a cryo-volcanic plume ejects ice grains and vapour into space providing access to materials originating from the ocean. The ocean is 30–55 km deep and provides an environment of mild salinity and alkaline pH. Hydrothermal activity is suspected to be occurring at the bottom of the ocean and also deep inside the water-percolated porous core. The energy is delivered by tidal dissipation. Two mass spectrometers aboard the Cassini spacecraft, the Cosmic Dust Analyzer (CDA) and the Ion and Neutral Gas Spectrometer (INMS) frequently carried out compositional in situ measurements of plume material emerging from the subsurface of Enceladus. Our latest results now show that, in addition to volatile organic compounds, some emitted ice grains contain concentrated macromolecular organic material with molecular masses clearly above 200u. Moreover, the mass spectra of the two instruments provide key constraints on the macromolecular structure. We suggest that the detected organic compounds and other materials found in the plume originate from Enceladus' hydrothermally active rocky core. Thermal ocean convection together with bubbles of volatile gases, transports these materials from the moon’s hydrothermal core up to the ocean surface. There, a spray of salty water together with droplets of solid organic nucleation cores - generated by bubble bursting and subsequently coated with ice from vapor freezing - are ejected into space.

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KoCo Signature Speaker