### High-mass stars: fantastic beasts and where to find (and form) them

 Main Colloquium Prof. Alvaro Hacar ORATED
Institute for Astrophysics, University of Vienna

High-mass stars determine the physical and chemical evolution of the Universe. High-mass stars are typically found in dense stellar clusters and are associated to densest and more dynamic gas structures in our Galaxy. However, the precise formation mechanisms of these massive objects in clusters remain under debate, particularly in comparison to the better understood origin of low-mass objects formed in isolation. The new EMERGE ERC-StG project (http://emerge.alvarohacar.com) proposes a novel approach for the study of the initial conditions for the formation of high-mass stars with ALMA. During my talk I will present the first technical and scientific results of this project exploring where and how the origin of stars of different masses and the different modes of star formation are closely connected to the structure and properties of the gas prior to their formation.

### Numerical simulations of astrophysical plasmas through Particle-Gas Hybrid Schemes in the PLUTO Code

 Main Colloquium Prof. Andrea Mignone ORATED
Università di Torino, Italy

Recent advancements for the state-of-the-art modelling of astrophysical plasma via numerical simulations are presented. The talk focuses mainly on the development of particle-gas hybrid schemes, suitable for the numerical modeling of different astrophysical environments, such as proto-planetary disks with dust, ion kinetic processes such as diffusive shock acceleration, large-scale transport of cosmic rays (CR) electrons and related non-thermal emission signatures. The MHD-PIC hybrid equation model, for the interaction between a collisionless thermal plasma and a population of non-thermal cosmic-ray (CR) particles, is examined in detail and applications to diffusive shock acceleration are presented.

### The physical structure of massive star-forming clumps

 Promotionskolloquium Yuxin Lin ORATED

Galactic-scale surveys from millimeter to infrared wavelengths in the last decades have identified numerous massive molecular clumps (~1 pc in size, >500 M_solar in mass), which are believed to be nurseries of OB star clusters. In this thesis, we report intermediate spatial resolution (~0.1 pc) observations towards selected samples of massive clumps and reveal their physical structures, including fragmentation properties, density, temperature, dynamical state and their evolution during massive proto-cluster formation. In the first project, we analyse 350 micron dust images using APEX-SABOCA of >200 massive clumps. Combined with space-based submm/FIR archival data, we obtain 10'' dust temperature and hydrogen column density maps, providing accurate estimates of physical properties. We resolved large populations of quiescent and star-forming cores and show that a large clump mass reservoir is essential for the mass accumulation onto massive cores. In the second project, using wide-band SMA and APEX spectroscopic observations at 1 mm, we present evolutionary variations of the radial profiles of temperature, density and dynamical state of the massive clumps, corroborated by abundance ratios of molecules as evolutionary indicators. We find that most of the clump temperature profiles on spatial scales of 0.1-1 pc are strongly shaped by centrally emerging protostars. The gas densities and the gradient of gas densities increase with the clump's luminosity-to-mass ratio, and is compatible with the variations of radial profiles of the virial parameter, suggesting a gravo-turbulent picture of clump gas dynamics. The hierarchical gas structure of massive clumps are unveiled by quantifying both the radial density profiles of the bulk gas (~10^4 cm^-3) and the dense gas (>~10^6-10^8 cm^-3) structures, which indicate spatially and temporally varying dense gas conversions and star formation efficiencies. Finally, this line of work is extended to five early-stage massive clumps in W43-main, an extreme Galactic OB cluster forming massive cloud. With wide-band NOEMA and IRAM 30m observations at 3 mm, we investigate in particular the impact of cloud-cloud collisions on the enhancement of dense gas, and on the formation and fragmentation of early-stage cores. [Referees: Prof. Dr. Karl Menten, Prof. Dr. Pavel Kroupa, Prof. Dr. Ian Brock, Prof. Dr. Hubert Schorle]

### Improved techniques for pulsar data analysis

Pulsars are rapidly rotating, strongly magnetised neutron stars with spin periods ranging from few milliseconds to tens of seconds. They emit beams of electromagnetic radiation that are predominantly detected as periodic pulsations at the radio end of the spectrum. Since the first pulsar detection in 1967, there have been close to 3000 such systems discovered and they have proven to be excellent tools for a variety of scientific applications. Despite the large number of discoveries, there exist several open questions pertaining to fundamental physics as well as pulsar evolution that can be addressed by pushing the boundary of pulsars beyond the current population census. The first part of my talk will focus on radio pulsar searches conducted with the MeerKAT radio telescope. I describe a processing infrastructure built with open source tools for conducting wide field-of-view searches with hundreds of synthesised beams. I use this setup for targeted binary pulsar searches on globular cluster Terzan 5. I also introduce the MPIfR Galactic plane survey (MGPS), a commensal survey covering science cases for pulsars, fast transients, Galactic magnetic fields, continuum emission, and spectral lines. My focus will be on pulsar searches conducted on a pilot survey as part of MGPS. The second part of my talk will focus on two projects. First, I will talk about a collaboration with the technology firm SAP to use their Data Intelligence framework for comparing machine learning models for pulsar candidate classification on a large scale using 3.2 million candidates generated from the HTRU South low-latitude survey. Second, I will present an investigation of the pulse profile instability of PSR J1022+1001, a pulsar belonging to two pulsar timing arrays and thus of interest for nanohertz gravitational wave searches. In this analysis, data spanning 20 years including two different recording instruments from the Effelsberg telescope and a recording instrument from the Parkes telescopes were used. By accounting for instrumental effects like polarisation miscalibration and propagation effects like interstellar scintillation, up to 25 per cent of the instability could be explained. This indicates that the observed instability is most likely intrinsic to the pulsar. [Referees: Prof. Dr. Michael Kramer, Prof. Dr. Frank Bertoldi, Prof. Dr. Simon Stellmer, Prof. Dr. Robert Glaum]

### AHA: The Accretion History of AGN

 Main Colloquium Prof. C. Megan Urry ORATED
Yale Center for Astronomy and Astrophysics, New Haven, USA

Every massive galaxy has a supermassive black hole at its center, which has grown episodically during phases of rapid accretion. Surveys of Active Galactic Nuclei (AGN) probe this growth history and thus are important both for understanding the properties of supermassive black holes and for quantifying their impact on galaxy evolution. We show that multi-wavelength data are particularly important for mitigating strong selection biases, since most AGN are heavily obscured and thus look like inactive galaxies in optical surveys. Furthermore, a “wedding cake” combination of survey depths and areas is needed to sample the full range in luminosity and redshift. With the AHA survey at X-ray+infrared+optical wavelengths, we measured supermassive black hole growth over the last ~12 billion years, and thus the amount of energy deposited in AGN host galaxies throughout that time. Contrary to leading theoretical models, a close look at AGN host galaxies suggests that for only a minority does merger-triggered AGN “feedback” cause rapid quenching of star formation.

### Exploration of the Jupiter system with a small submillimetre wave telescope onboard the JUICE satellite

 Main Colloquium Dr. Paul Hartogh ORATED
Max-Planck-Institut für Sonnensystemforschung, Göttingen

JUICE – JUpiter ICy moons Explorer – is the first large class mission of ESA’s Cosmic Vision 2015 - 2025 programme. The JUICE satellite is planned to be launched in September 2022 and its Jupiter orbit insertion is foreseen in 2031. The primary mission in the Jupiter system will take about three years. The focus of the mission is Jupiter itself and the Galilean satellites, their internal oceans and potential habitability. Recent ground-based observations of Europa and Ganymede showed water vapour plumes, probably related to geysers on their surfaces. JUICE intends to identify the geysers, monitor their potential activity and molecular and isotopic composition in order to constrain satellite formation models and development processes (of chemical, physical and potentially biologic nature) in the interior of their oceans. Jupiter itself is seen as an archetype of a gas giant. A better understanding of its atmospheric processes will be a baseline for a better understanding of gas giants outside our solar system. JUICE will characterize the general circulation of Jupiter’s atmosphere, its meteorology, chemistry and structure between the upper cloud deck and the ionosphere and magnetosphere. The Submillimetre Wave Instrument (SWI) is one of 10 scientific instruments on the JUICE satellite. SWI covers two spectral bands between 530 and 1275 GHz. Details will be presented about the functionality and specifications of the instrument, required technology developments during the last decades and how the unique capabilities of SWI will help to answer key JUICE science questions.

### Properties of relativistic jets in X-ray binaries

 Promotionskolloquium Richa Sharma ORATED

X-ray binaries are stellar systems consisting of a compact object (either a neutron star or a black hole) accreting matter from a companion star. Some of the X-ray binaries form relativistic jets where plasma accelerates close to the speed of light. The relativistic jets have been extensively studied, and the most advanced research in this field investigates changes in the jets’ core position and short-term flaring events. The thesis approaches the former topic with a semi-analytical code and the latter topic with new observations. First, we explored and established the relationship between the core position and jet spectrum using a code of a synchrotron emitting, self-absorbed jet by varying the jet inclination angle and relativistic electron density. Then, we investigated short-term variability in the flux of the X-ray binary system LSI +61°303. We used radio and X-ray data of the source obtained with WSRT and Suzaku telescopes, respectively. Timing analysis revealed periodic oscillations of ∼ 1 h (radio) and ∼ 2.5 h (X-ray), albeit at different epochs. To explore the simultaneous radio and X-ray emission from the source, we observed it with AMI-LA and XMM–Newton telescopes. Correlation analysis established for the first time that emission at both wavelengths is highly correlated and has implications on the nature of the physical processes in the system. [Referees: Priv.-Doz. Dr. Maria Massi, Prof. Dr. Norbert Langer, Prof. Dr. Ian Brock, Prof. Dr. Zeinab Abdullah]

### The physical and chemical conditions of molecular clouds on large scales

 Promotionskolloquium Nina Brinkmann ORATED

Observations of molecular emission lines in the radio and (sub)millimetre wavelength regimes provide insights into the interiors of molecular clouds which are inaccessible by other forms of radiation. Technological advancements are now allowing observations of molecules in distant galaxies, but our ability to interpret these data are limited by our knowledge of exactly how and where molecular emission lines originate within our own galaxy. Until recent years, observational biases caused by pre-selection of either certain positions in a molecular cloud or of a few molecular lines only allowed very selective studies. This thesis describes the acquisition and analysis of the first spectroscopically unbiased data set of the northern part of the prominent Orion A molecular cloud in the 1.3 mm atmospheric window. We develop templates of distinct regions and contrast their respective emission profiles, explore the reliability of commonly used line ratios as a tool to infer physical parameters, examine correlations between different molecular species and isotopologues, and calculate line luminosities. We further report on the surprising first detection of larger-scale CF+ emission in a variety of environments and aim to explain its widespread and fairly uniform distribution. Our data set also enables us to examine the nature of the so-called radical region within Orion A and to determine whether or not it is a distinct entity characterised by unusual molecular abundances. [Referees: Prof. Dr. Karl M. Menten, Prof. Dr. Pavel Kroupa, Prof. Dr. Klaus Desch, Prof. Dr. med. Ines Gütgemann]

### Galaxy Evolution with MeerKAT: scientific and technical synergies with VLBI

 Main Colloquium Prof. Roger Deane ORATED
University of the Witwatersrand and University of Pretoria, South Africa

MeerKAT is the world’s most sensitive radio interferometer in its class and has demonstrated its potential through several impressive and in some cases, unexpected results. This has been enabled in large part by the excellent imaging fidelity offered by the 64-antenna array. However, several attributes of the telescope and decimetre radio sky make full scientific utilisation a challenge, necessitating significant algorithmic development. I will present a suite of my group’s early MeerKAT results related to galaxy and AGN evolution, and discuss some of the synergistic scientific potential with respect to VLBI programmes. I will also discuss how technical development has been and will continue to be leveraged between MeerKAT (and MeerKAT+, SKA1-mid) with both centimetre and millimetre-wave VLBI arrays, including the African VLBI Network and the (ng)EHT.

### Fast Radio Bursts: a cosmological conundrum or why I finally left the Galaxy

 Main Colloquium Prof. Victoria Kaspi ORATED

Fast Radio Bursts (FRBs) are a newly recognized and puzzling phenomenon consisting of few-millisecond bursts of radio waves coming from cosmological distances. They are ubiquitous, with all-sky rates of roughly 1000 per day, signaling a not uncommon yet powerful explosion of presently unknown origin. Recently there has been significant observational progress on understanding FRBs and recognizing their potential as novel cosmological probes, in large part thanks to the Canadian Hydrogen Intensity Mapping Experiment Fast Radio Burst Project (CHIME/FRB). In this talk I review what is known about FRBs, and describe some of the latest CHIME/FRB results related to FRB origin and distribution in the Universe.

### Unveiling the Small Magellanic Cloud with the Australian Square Kilometre Array Pathfinder

 Main Colloquium Prof. Naomi McClure-Griffiths ORATED
The Australian National University, Canberra

The evolution of galaxies is partially regulated by their infall and outflow of gas. Many simulations of galaxy formation and evolution have highlighted the importance of feedback in reproducing the observable Universe. Huge superbubbles and outflows, formed from the stellar winds and supernovae, dominate the observed structure of neutral hydrogen within many galaxies, including the nearby Small and Large Magellanic Clouds, which we can study with a physical resolution unmatched anywhere else in the Universe. As the most numerous galaxies in the Universe, dwarf galaxies may be important candidates for populating the intergalactic medium with enriched gas. Although star formation rates in dwarf galaxies can be lower than their more massive, starburst counterparts, these low mass systems have small gravitational potential wells and thereby find it difficult to maintain their star-forming material in the presence of intense stellar feedback. In this talk I will present new atomic hydrogen (HI) data from the Australian SKA Pathfinder (ASKAP) on the Small Magellanic Cloud. Using these data we have discovered that the Small Magellanic Cloud (SMC) has massive stellar feedback driven HI outflows. The outflows are comprised of cold filamentary gas extending up ~2 kpc from the main galaxy, with temperatures of T< 500 K and widths as small as 50 pc. We estimate a significant atomic gas mass flux in the range 0.2 - 1 solar mass per year, which may contribute to feeding the Magellanic Stream. I will also discuss recent observations and future plans for the Magellanic System with Galactic ASKAP Survey.

### The Obscured Universe in the first Two Billion Years

 Special Colloquium Prof. Caitlin Casey ORATED
University of Texas, Austin, USA

Although rare in the nearby Universe, gas-rich galaxies with extraordinary star-formation rates (100-1000x the Milky Way, at >100 Msun/year) represent the typical massive galaxy in the early Universe ~10 billion years ago. These galaxies’ high star-formation rates are predominantly obscured by dust which absorbs and re-radiates >95% of the energy from massive stars in the (sub)millimeter/far-infrared, hence they are often called dusty star-forming galaxies (DSFGs). Though our physical understanding of this highly obscured, heterogeneous population has matured significantly over the past decade thanks to ALMA, the prevalence of such systems at higher redshifts (z>4) is highly uncertain. I will discuss current efforts with ALMA and future surveys to learn more about these highly obscured galaxies beyond z~4. The detection of incredibly dust-rich systems (>10^9 Msun of dust) at these early epochs is key to understanding both the early formation mechanisms of dust (from supernovae, AGB stars and via grain growth in the ISM) and the assembly history of the Universe’s most massive galaxies and their rich, over dense environments. Last, I will also discuss efforts to transform our understanding of the reionization process of the Universe using future JWST datasets aimed at studying rare galaxies and sampling the Universe’s large scale structure.

### Small Molecules, Big Impact: Investigating hydrides in the interstellar medium

 Promotionskolloquium Arshia Jacob ORATED

The nature and distribution of atomic and molecular gas in the interstellar medium (ISM) is of great interest to astrophysicists because it is this gas that provides the raw material for the formation of new stars. This thesis investigates hydride molecules in the ISM and their use as diagnostics of its different phases. Particular emphasis is given to the central CH radical, a probe of diffuse and translucent molecular clouds, including those not traced by the otherwise common CO. I will detail our searches for the rare isotopologues of CH and address questions regarding the origin of another molecule, CH2, which despite being chemically associated with the ubiquitous CH, has largely remained elusive. And lastly, I will discuss the transition from the diffuse atomic to the diffuse and translucent molecular phases of the ISM by extending our view of the chemistry of argonium, ArH+, a tracer of almost purely atomic gas. [Referees: Prof. Dr. Karl M. Menten, Prof. Dr. Pavel Kroupa, Prof. Dr. Simon Stellmer, Prof. Dr. Hubert Schorle]

### Cosmological simulations as laboratories to test theories on galaxy formation and particle physics

 Main Colloquium Dr. Sylvia Plöckinger ORATED
Lorentz Institute for theoretical physics, Leiden University, The Netherlands

Simulations are an invaluable tool in astrophysics to test theories relating to processes that take millions and billions of years. Within the last decades, cosmological simulations have drastically improved on their power to compare simulation with observations: from simulations only tracing the gravitational effects of cosmic structure formation ("dark matter only / N-body simulations") to fully hydro-dynamic simulations that form galaxy populations resembling observed galaxies. An upcoming new generation of cosmological simulations further advances the opportunities for a more direct comparison to observed galaxies. This is not only achieved by an increased mass and spatial resolution, but also by an innovative physical and chemical model of the gas phases of the interstellar medium (ionized, neutral atomic, molecular). This allows to self-consistently model the multi-phase interstellar medium (ISM) and produce more realistic mock observations. I will present first results of these new simulations and show how a multi-phase ISM can impact some of the small-scale challenges of the current standard cosmological model. This improved model for baryon physics also allows us to better study the impact on the formation of dwarf galaxies on the assumed dark matter particle. This will be illustrated by performing statistical test on the galaxy populations within cold and warm (sterile neutrino) dark matter simulations.

### Resolved dust continuum and CII measurements of high-redshift protocluster members

 Master Colloquium Aparna Venkateshwaran ORATED

SPT2349-56 is a protocluster discovered via the 2500 deg2 South Pole Telescope (SPT) survey (Williamson et al., 2011). In this thesis, we study the kinematics and morphologies of the galaxies that are found in the core of SPT2349-56 using high-resolution (~ 1.7 kpc scale at z = 4.303) 850-micrometer dust continuum and [CII] data. Kinematical modelling of the galaxies using the 3DFIT task in 3DBAROLO gives a mean V/sigma = 5.0 +- 2.3. This implies that the galaxies agree with simulations that suggest galaxies at this redshift should be dynamically hotter, or dispersion dominated. To study the surface brightness profiles of the SPT sample, we use the ELLPROF task in 3DBAROLO and a PYTHON code to first construct the observed 850-micrometer dust and [CII] gas profiles. We then fit beam-convolved Sérsic functions to the observed profiles and derive the intrinsic brightness profiles. Lastly, we compare the intrinsic profiles to the intrinsic 160-micrometer dust profiles of three local (z ~ 0.1) starburst-galaxies – Centaurus A, NGC253 and NGC4945. [Referees: Prof. Dr. Karl Menten, Prof. Dr. Pavel Kroupa]

### TBD

 Master Colloquium Aparna Venkateshwaran CANCELED

TBD

### Empirical Formulae and Coincidences in the Gravity’s Dark Sector

 Main Colloquium Dr. Hongsheng Zhao ORATED
University of St Andrews, UK

A rare coincidence of scales in standard particle physics is needed to explain why \$\Lambda\$ or the negative pressure of cosmological dark energy (DE) coincides with the positive pressure \$P_0\$ of random motion of dark matter (DM) in bright galaxies. As a specific proof of concept we construct a smooth extension of GR with a simple kinetic Lagrangian of a four-velocity vector field \$V^\mu\$. We check whether the pressure from the V-field can bend space-time sufficiently to replace the roles of DE, Cold DM and heavy neutrinos in explaining anomalous accelerations at all scales.

### A dust-unbiased view of high-redshift star formation

 Main Colloquium Prof. Jacqueline Hodge ORATED
Leiden Observatory, the Netherlands

A substantial fraction of the Universe's star formation is heavily enshrouded by dust, and this basic fact has long been a hindrance to the development of a complete picture of galaxy evolution. Now, thanks to the advent of the Atacama Large Millimeter Array (ALMA) and the upgrades to the Karl G. Jansky Very Large Array (VLA), our view of dusty star formation at high-redshift is undergoing a radical transformation. I will present recent ALMA and JVLA observations of the gas, dust, and dust-unbiased star formation in high-redshift galaxies down to ~kpc and even sub-kpc scales, providing key insight into the morphology, dynamics, dark matter halo masses, and ISM physics in these systems. These observations have important implications for our understanding of the formation of the dustiest galaxies in the universe, which will soon be revealed in unprecedented detail by JWST.

### Polarimetric view of magnetically arrested accretion flows onto a black hole

 Special Colloquium Prof. Monika Moscibrodzka ORATED

The Event Horizon Telescope Collaboration, who produced the first ever image of a black hole, has just revealed a new view of the massive object at the centre of the M87 galaxy: how it looks in polarised light. This is the first time astronomers have been able to resolve polarisation this close to the edge of a black hole. A detailed theoretical understanding of near black hole processes is now crucial to interpret these new observations. I will review our current efforts to model polarimetric properties of light produced in synchrotron processes in plasma falling towards the event horizon and on theoretical insigths from the new observations of M87 core. The particular focus will be given to modeling and understanding the circular polarization of near horizon emission and its spatial and temporal handedness stability.

### Feeding of supermassive black holes in galactic nuclei by supernova expanding shells

 Main Colloquium Prof. Jan Palous ORATED
Astronomical Institute of the Academy of Sciences of the Czech Republic, Prague

We explore the possibility of feeding the Super Massive Black Holes in galactic centers by expanding Supernova remnants. The shell expansion is simulated with hydrodynamical codes FLASH and RING using different approximations describing the thin shell expansion in the ISM. The simulations are performed in the homogeneous and also in the inhomogeneous turbulent medium. It shows that the supernovae occurring at specific places near the galaxy rotational axis can feed the central accretion disk surrounding the SMBH. The mass deposited into the central parsec by individual supernovae varies between 10 and 1000 solar masses depending on the ambient density and the distribution of supernovae in space. Supernovae occurring in the aftermath of a starburst event near a galactic center can supply within 30 Myr two to three order of magnitude more mass into the central parsec. The fate of that mass splits between the growth of the SMBH and outflow from the nuclear disk.

### Simulating star formation with the radiation transport code TreeRay

 SFB Colloquium Dr. Richard Wünsch ORATED
Astronomical institute of the Czech Academy of Sciences, Prague

I will present a novel fast radiation transport algorithm TreeRay, suitable to work together with hydrodynamic codes. It is based on reverse ray tracing combined with tree-based accelerated integration. TreeRay is implemented in the adaptive mesh refinement code FLASH, however, the method itself is independent of the host code and can be implemented in any grid based or particle based hydrodynamics code. A key advantage of TreeRay is that its computational cost is independent of the number of sources, making it suitable for simulations with many point sources (e.g. massive star clusters) as well as simulations where diffuse emission is important. A very efficient communication and tree-walk strategy enables TreeRay to achieve almost ideal parallel scalings. TreeRay can easily be extended with modules to treat radiative transfer at different wavelengths and to implement related physical processes. I will focus on the module calculating ionising radiation using the On-the-Spot approximation and describe various tests demonstrating that complicated simulations of star clusters with feedback from multiple massive stars become feasible with TreeRay. Finally, I will show several applications where TreeRay has been used, including project SILCC (SImulating Life Cycle of Clouds) - complex simulations of the interstellar medium in a part of the galactic disk, designed to capture the full matter cycle between gas and stars.

### The build-up of galactic nuclei: how do black holes get there?

 Main Colloquium Dr. Nadine Neumayer ORATED
Max-Planck-Institut für Astronomie, Heidelberg

The centers of massive galaxies are special in many ways, not least because apparently all of them host supermassive black holes. Since the discovery of a number of relations linking the mass of this central black hole to the large scale properties of the surrounding galaxy bulge it has been suspected that the growth of the central black hole is intimately connected to the evolution of its host galaxy. However, at lower masses, and especially for bulgeless galaxies, the situation is much less clear. Interestingly, these galaxies often host massive star clusters at their centers, and unlike black holes, these nuclear star clusters provide a visible record of the accretion of stars and gas into the nucleus. I will present our ongoing observing programme of the nearest nuclear star clusters, including the ones in our Milky Way and the Sagittarius dwarf galaxy. These observations provide important information on the formation mechanism of nuclear star clusters, allow us to measure potential black hole masses and give clues on how black holes get to the centres of galaxies.

### Direct imaging of transient relativistic outflows from neutron star mergers with VLBI

 Main Colloquium Prof. Adam Deller ORATED
Swinburne University of Technology, Hawthorn, Australia

The detection in gravitational waves of two merging neutron stars by LIGO/Virgo in 2017 fired the starting gun on a new era of multi-messenger GW/EM astronomy. Observations of GW170817 provided a wealth of information, first identifying its host galaxy and distance via the short-lived optical/NIR kilonova that followed the merger, and later with an extended campaign monitoring the relativistic outflow launched in the final throes of the merger. Radio and X-ray light curve modelling provided considerable insight as to the nature of this outflow, but even half a year after the merger, it was still unclear whether the highly relativistic jet launched by the merger had been contained by the mildly relativistic cocoon of material it slammed into, or if it had in fact punched free. The smoking gun evidence for a successful jet, of the kind observed in short GRBs, came only when milliarcsecond resolution radio imaging precisely measured the apparent superluminal motion of the source position. In this talk, I will revisit the results of VLBI observations of GW170817 and show how these were compared to hydrodynamical and analytical models to constrain the jet parameters and system viewing angle. I will show how this information was used to provide an improved standard siren measurement of the Hubble constant compared to the GW data alone, and review how improving VLBI instrumentation (higher sensitivities, wider fields-of-view) will enable similar measurements to be made for more distant merger afterglows in future LIGO/Virgo runs.

### Constraining the Cosmic Baryon Cycle with ALMA

 Main Colloquium Dr. Fabian Walter ORATED
MPIA, Heidelberg

I will report on some of the results emerging from the ALMA large program ASPECS that obtained deep (sub-)millimeter imaging of the Hubble Ultra-Deep Field (H-UDF). The observations provide a census of dust and molecular gas in the H-UDF, down to masses that are typical of main-sequence galaxies at redshifts 1-3. The resulting data enable a great range of studies, from the characterisation of individual galaxies, capitalizing on the unique multi-wavelength database available for the H-UDF, to CO excitation studies that constrain the gas properties. Stacking analyses in dust continuum and CO line emission, using available H-UDF galaxy catalogues and precise redshifts from VLT/MUSE, helped to constrain the emission of galaxy samples that are too faint to be detected individually. The nature of the observations (full ALMA spectral scans) provides a census of dust and molecular gas in the cosmic volume defined by the H-UDF. The resulting cosmic molecular gas density as a function of redshift shows an order of magnitude decrease from z~2 to z=0. This is markably different from the atomic gas phase that shows a rather flat redshift dependence. With other measurements from the literature, these results are used to put additional observational constraints on the gas (net) accretion flows, needed to explain the build-up of stellar mass in galaxies, and are compared to cosmological galaxy formation simulations.

### The complex chemistry of young stars

 SFB Colloquium Prof. Jes Jørgensen ORATED
Niels Bohr Institute, University of Copenhagen

The environments in which young stars form show a rich and varied chemistry. Understanding how, when and where complex organic and potentially prebiotic molecules are formed in these regions is a fundamental goal of astrochemistry. With the Atacama Large Millimeter/submillimeter Array (ALMA) we are moving into a new territory for these studies with ALMA's high sensitivity for faint lines, high spectral resolution which limits line confusion, and high angular resolution making it possible to study the structure of young protostars on solar-system scales. In this talk I will present some of our recent ALMA results concerning the chemistry taking place in star forming regions. I will discuss the implications of these results on the formation of complex organic molecules around young protostars and the link between the birth environments of these protostars, the conditions in their protoplanetary disks and eventually the chemistry in emerging solar systems.

### Black Hole Physics at the Horizon Scale

 Main Colloquium Prof. Feryal Özel ORATED
University of Arizona

Recent observational advances with the Event Horizon Telescope, GRAVITY, and LIGO/VIRGO have opened up new avenues for studying black hole physics at horizon scales. In this talk, I will discuss what we have learned about the spacetimes of astrophysical black holes and how strong-field gravity is imprinted on their images. I will also present how the observations help us model and understand the heating and acceleration of plasmas on horizon scales.

### Cosmic flows crank up the tension in cosmology

 Main Colloquium Prof. Mike Hudson ORATED

Peculiar velocities - deviations from Hubble expansion - are the only practical probe of the growth of matter density fluctuations on very large scales in the nearby Universe. I will discuss recent measurements of cosmological parameters based on our approach of predicting peculiar velocities from the density field. This yields interesting results for the rms density fluctuations on 8 Mpc/h scales and their growth rate that may require new physics beyond the standard Lambda Cold Dark Matter model.

 Promotionskolloquium Marilyn Cruces ORATED

I present the study of two radio transient phenomena: pulsars and Fast Radio Bursts (FRBs). Pulsars are superb natural laboratories for fundamental physics such as placing limits on the equation of state of ultra-dense matter and testing theories of gravity in the strong-field regime. FRBs are bright flashes of millisecond duration detected exclusively at radio frequencies. Their astrophysical origin is yet unknown. I present the ongoing pulsar survey using the world largest single-dish radio telescope, the Five-hundred-meter Aperture Spherical radio Telescope (FAST). In the follow-up observing campaign at Effelsberg, I monitored 10 pulsars discovered in the early commissioning phase of FAST. A highlight is PSR J2338+4818, the widest pulsar--white dwarf binary system with the most massive companion known to-date. In the following section, I model the long-term evolution of millisecond pulsars to understand their lower magnetic field strengths when compared to normal pulsars. I show that the observed magnetic field strengths of pulsars in binary systems can be well reproduced by a magnetic field decay driven by ambipolar diffusion. Lastly, I present an extensive multi-wavelength campaign on FRB121102, the first discovered repeating FRB. With 36 bursts detected with Effelsberg over four years, I find a periodic window of activity of 161+/-5 days. [Referees: Prof. Dr. Michael Kramer, Prof. Dr. Norbert Langer, Prof. Dr. Simon Stellmer, and Prof. Dr. Volkmar Gieselmann]

### A radio pinwheel associated with WR147

 Main Colloquium Prof. Luis Rodríguez ORATED
Instituto de Radioastronomia y Astrofisica, UNAM, Mexico

Wolf-Rayet (WR) stars are evolved massive stars, characterized by high luminosities and fast and dense stellar winds. We have detected a radio continuum pinwheel associated with WR 147, a nitrogen-rich WR star. These structures have been detected in the infrared around a handful of late-type carbon-rich WR stars with massive companions, where dust has formed in the zone where the two winds collide and produced a plume of dense gas and dust that is carried out with the WR wind. As the binary system rotates, an Archimedean spiral is produced. The resulting pinwheel contains information on wind speeds, wind-momentum ratio, and orbital parameters. However, WR 147 is a WN star and the formation of dust is unlikely, so a different emission mechanism must be at work. Our analysis of the data suggests that in this case the emission is dominantly of a nonthermal nature (synchrotron). It is possible that the pinwheels associated with WN stars will be detectable only as nonthermal emitters at radio wavelengths. From the characteristics of the pinwheel we estimate a period of 1.7 yr for the binary system (the WN star and a companion yet undetected directly) that is responsible for the pinwheel.

### The Restless Universe

 Main Colloquium Prof. Shrinivas Kulkarni ORATED
California Institute of Technology, Pasadena, USA

The Universe began only with hydrogen and helium. It is cosmic explosions which build up the periodic table. Astronomers have now identified several classes of cosmic explosions of which supernovae constitute the largest group. The Palomar Transient Factory was an innovative 2-telescope experiment, and its successor, the Zwicky Transient Factory (ZTF), is a high tech project with gigantic CCD cameras, sophisticated algorithms (employing machine and deep learning) and robust pipelines, and squarely aimed to systematically find "blips and booms in the middle of the night". The speaker will talk about the great returns and surprises from this project: super-luminous supernovae, new classes of transients, new light on progenitors of supernovae, detection of gamma-ray bursts by purely optical techniques, troves of compact binary stars of interest to the LISA mission, asteroids in the interior of Venus and more. ZTF is now considered to be the stepping stone for LSST.

### A Commensal Radio Astronomy FAST Survey (CRAFTS)

 Main Colloquium Prof. Di Li ORATED
National Astronomical Observatories, Chinese Academy of Sciences, Beijing, China

We will release the first internationally open call-for-proposal for the The Five-hundred-meter Aperture Spherical radio Telescope (FAST) in the coming April. Previously, more than 100 PI-led science programs and five major surveys have been approved by the science committee. These observation have resulted in more than 40 publications, including two on the Nature magazine. I report a few highlights here, particularly from the CRAFTS survey, which is an unprecedented large-scale commensal survey. Enabled by a novel calibration technique, CRAFTS simultaneously records pulsar, Galactic HI, extra-galactic HI, and transient data streams. CRAFTS has discovered more than 150 new pulsars, including at least one DNS system. We have imaged about 5% of the full sky, including the Lockman hole, the Orion region, etc. CRAFTS has also resulted in 5 new FRBs, including one high DM repeater that has been localized.

### Space-VLBI studies of the parsec-scale jet in the quasar 3C 345

 Promotionskolloquium Felix Pötzl ORATED
MPIfR

Supermassive black holes in the centres of radio-loud active galactic nuclei (AGN) can produce collimated relativistic outflows (jets). Magnetic fields are thought to play a key role in the formation and collimation of these jets, but the details are much debated. In my PhD thesis work I investigate mainly two methods of estimating the magnetic field strength and morphology on scales of a few 1000 gravitational radii of the central black hole. I will present the concept for high-accuracy core shift measurements at mm-wavelengths with source-frequency phase referencing observations, as well as an analysis of high-resolution space-VLBI data in full polarisation of the bright quasar 3C 345 obtained within the RadioAstron mission. The latter gives us insight on the magnetic field, where I present an analysis of the polarisation structure, the spectral index, Faraday rotation, as well as the brightness temperature of the source. This provides us with the picture of a jet with predominantly toroidal magnetic field, where plane-perpendicular shocks quench the magnetic field perpendicular to the jet direction, which also causes the jet to be partly out of equipartition. [Referees: Prof. Dr. Anton Zensus, Prof. Dr. Andreas Eckart, Prof. Dr. Susanne Crewell, and Dr. Steffen Rost]

### Targeted fast radio burst searches with the Effelsberg 100-m radio telescope

Fast radio bursts (FRBs) are bright, energetic, millisecond duration radio transients of extragalactic origin. While over 100 FRBs have been detected, their origin is still unknown. Most FRB sources have only been seen to produce a single burst, but repeated bursts have been detected from 22. During this colloquium, I will discuss the main projects of my PhD work. The first project is on observations of superluminous supernovae (SLSNe) and long gamma-ray bursts (LGRBs), where we observed 10 SLSNe/LGRBs for 50 hours in search for repeating FRB sources using the C+ (5-9 GHz) receiver on Effelsberg. SLSNe and LGRBs were targeted due to their host galaxy sharing similarities to the host galaxy of the first discovered repeating FRB, FRB121102. The second project is on observations of FRB121102, where we investigate its long-term rotation measure (RM) evolution. We obtain 16 new RMs from November 2017 to August 2019, expanding the RM sample of FRB121102 up to 2.5 years. During this time, the RM of FRB121102 has showed a significant decrease in RM. We compare the RM evolution of FRB121102 to models that estimate the RM evolution from within a supernova remnant, and the RM evolution of the Galactic center magnetar, J1745-2900. [Referees: Prof. Dr. Michael Kramer, Prof. Dr. Norbert Langer, Prof. Dr. Simon Stellmer, and Prof. Dr. Robert Glaum]

### Tracing the origins of interstellar objects

 Main Colloquium Dr. Coryn Bailer-Jones ORATED
Max-Planck-Institut für Astronomie, Heidelberg

The first two interstellar objects (ISOs), 1I/'Oumuamua and 2I/Borisov, were discovered in 2017 and 2019 as they passed within a couple of astronomical units of the Earth and Sun. Using ground-based and HST data to reconstruct their orbits, together with Gaia data to trace back the positions of stars in the solar neighbourhood, we searched for stars that encountered these ISOs in the past (Bailer-Jones et al. 2018, 2020). A close, slow encounter could indicate that the ISO originated from that stellar system. We identified some such encounters, in particular one at 0.07 pc for 2I/Borisov. But how likely is this to be the parent star? What factors do we need to take into account to determine this? What are the prospects for more confident detections in the future? And what are ISOs anyway?

### From Schwarzschild's singularity to collapsing stars: The history and interpretation of Penrose's singularity theorem

 Main Colloquium Prof. Dennis Lehmkuhl ORATED
Institute of Philosophy, University of Bonn

The Nobel Prize of 2020 was awarded to Roger Penrose for his singularity theorem of 1965, which the Nobel foundation interpreted as ''the discovery that black hole formation is a robust prediction of the general theory of relativity.'' However, the 1965 paper does not mention the term "black hole" but speaks of gravitational collapse and spacetime singularities, starting with remarks on Schwarzschild's 1916 solution to the Einstein field equations. In this talk, I will put Penrose's singularity theorem in its historical context, starting with Einstein's and Schwarzschild's interpretation of the Schwarzschild metric in the late 1910s and 1920s, and discuss how the metric was linked to the question of gravitational collapse by Oppenheimer and Snyder in the late 1930s, and reconsidered by Wheeler and others in the 1950s and 1960s. I will describe which conceptual and technical advances Penrose had to invent and combine in order to come up with his singularity theorem to go beyond considerations of specific spacetimes like that of Schwarzschild, and show why the theorem was such a game-changer. Finally, I will discuss different possible interpretations of the theorem.

### Gaia Early Data Release 3

 Main Colloquium Prof. Stefan Jordan ORATED
Zentrum für Astronomie der Universität Heidelberg

On December 3, 2020, the first (early) part of Gaia's third star catalogue will be published. Based on 34 months of Gaia observations it contains a significant increase in the number of entries compared to Gaia DR2: position measurements of more than 1.8 billion stars as well as proper motions, parallaxes, and broad-band photometry for about 1.4 billion stars. Additionally, both the precision and accuracy of the astrometric data has significantly improved, especially the determination of proper motions. Together with the release of the star catalogue, four papers are published, impressively demonstrating the scientific potential of the Gaia EDR3 catalogue.

### The sharpest view of blazar jets through space and mm VLBI observations

 Main Colloquium Dr. Jose Gómez ORATED
Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain

Mass accretion onto the supermassive black holes that power AGN leads to the formation of highly collimated relativistic jets that radiate across the entire electromagnetic spectrum, from radio to γ-rays. Over the past decades, AGN jets have been repeatedly studied through centimeter VLBI observations, but the resolution provided is just a step behind the necessary to resolve the compact regions in the vicinity of the central black hole where jets are formed, collimated, and accelerated. This limitation has recently been overcome by two technological improvements. On one side, the space VLBI mission RadioAstron has allowed us to increase the virtual size of our VLBI telescopes to as large as the distance to the Moon, achieving angular resolutions as small as 20 microarcseconds. On the other side, the participation of ALMA in millimeter VLBI arrays, such as the EHT and GMVA, has not only allowed the EHT to capture the first image of a black hole, but also holds the potential to actually address for the first time the fundamental questions of how gravity works in the strong-field regime near the event horizon, how accretion leads to the formation of relativistic jets, and what are the sites and mechanisms for the production of high energy emission in blazars. In this talk we summarize the results from our RadioAstron Key Science Program, which in combination with quasi-simultaneous GMVA and EHT observations provide us the sharpest view of blazars jets to probe the magnetic field and innermost jet regions in a sample of the brightest blazars to test jet formation models.

### Astrochemistry and Compositions of Planetary Systems

 Main Colloquium Prof. Karin Öberg ORATED
Harvard University, Cambridge, USA

The past decades have revealed that planets are incredibly common, and incredibly diverse. The origins of planets and their compositions are intimately linked to the chemical environments within which planets assemble, i.e. to the chemistry of planet-forming disks. The arrival of ALMA has provided observational access to disk chemistry, revealing disk snowlines, abundant organic molecules, and curious chemical gradients and sub-structures across the planet and comet forming zones. The most recent development is the execution of the ALMA Large Program MAPS (Molecules with ALMA on Planet-forming Scales), which has enabled us to zoom in on disk chemical structures at scales of 10-30 au. In parallel to these observational developments, astrochemistry models and laboratory experiments are providing new clues on what chemistry is likely to occur in different disk environments. I will present highlights from the MAPS program, as well as some recent laboratory astrochemistry discoveries. I will discuss how these new observational and laboratory data are affecting our understanding of the chemistry of planet formation, the chemical habitability of mature planetary systems, and the history of our own Solar System.

### The Chemistry of Planet Formation

 SFB Colloquium Dr. Catherine Walsh ORATED
University of Leeds, UK

Protoplanetary disks around young stars are the factories of planetary systems. These structures contain all the material - dust, gas, and ice - that will build planets and other bodies such as comets. Hence, understanding the physics and chemistry of disks provides much needed insight into the conditions under which planets form, and determining their molecular content reveals the raw ingredients of planetary atmospheres. In this colloquium I will present early results from the first ALMA Large Program dedicated to the observation of molecular line emission from protoplanetary disks around nearby young stars at high angular resolution (0.1'' - 0.3''), titled Molecules with ALMA on Planet-Forming Scales or MAPS. I will present images that reveal intriguing sub-structure in emergent line emission from key organic molecules such as CO, C2H, HCN, and CH3CN.  I will discuss the link between known dust substructure and the observed line emission, and will present results from quantitative analyses of source properties such as radial mass distribution, chemical structure, ionisation structure, and elemental composition of the gas.  I will also discuss how observations in the gas-phase of large organic molecules provide insight into the composition of the icy-comet building reservoir around other stars. Finally I will discuss how early results from MAPS have provided the most detailed studies to date of the chemistry of planet formation.

### Cosmological simulations as laboratories to test theories on galaxy formation and particle physics

 Main Colloquium Dr. Sylvia Plöckinger CANCELED
Lorentz Institute for theoretical physics, Leiden University, The Netherlands

Simulations are an invaluable tool in astrophysics to test theories relating to processes that take millions and billions of years. Within the last decades, cosmological simulations have drastically improved on their power to compare simulation with observations: from simulations only tracing the gravitational effects of cosmic structure formation ("dark matter only / N-body simulations") to fully hydro-dynamic simulations that form galaxy populations resembling observed galaxies. An upcoming new generation of cosmological simulations further advances the opportunities for a more direct comparison to observed galaxies. This is not only achieved by an increased mass and spatial resolution, but also by an innovative physical and chemical model of the gas phases of the interstellar medium (ionized, neutral atomic, molecular). This allows to self-consistently model the multi-phase interstellar medium (ISM) and produce more realistic mock observations. I will present first results of these new simulations and show how a multi-phase ISM can impact some of the small-scale challenges of the current standard cosmological model. This improved model for baryon physics also allows us to better study the impact on the formation of dwarf galaxies on the assumed dark matter particle. This will be illustrated by performing statistical test on the galaxy populations within cold and warm (sterile neutrino) dark matter simulations.

### The SEDIGISM survey: Morphology of molecular clouds

 Master Colloquium Kartik Rajan Neralwar ORATED