IMPRS-HD Alumni 2021

Alumni 2021

Victor Marian  (28.4.)  -  Stephan Stock  (28.4.)  -  Sabrina Gronow  (15.6.)  -  Edna Ruiz Velasco  (15.6.)  -  Felix Bosco  (1.7.)  -  Francisco Aros (6.7.)  -  Victor Ksoll  (7.7.)  -  Diana Kossakowska (8.7.) - Thomas Jackson  (19.7.)  -  Jindra Gensior  (19.7.)  -  Paul Heeren  (20.7.)  -  Martin Schlecker (21.7.)   -  Branislav Avramov  (22.7.)  -  Melanie Kaasinen  (23.7.)  -  Josefina Michea (2.11.)  -  Bahar Bidaran  (12.11.)  -  Robin Gopala Tress (17.11.)  -  Christoph Engler  (14.12.) -  Oliver Voelkel  (16.12.)

    Oliver Voelkel  (Germany)                                                                                                                               16.12.2021

On the Continuous Improvement of Global Planet Formation Models - The Consistent Formation of Planetary Embryos  ( thesis pdf )

Our knowledge on the population of exoplanets and circumstellar disks has increased drastically in the last decades. Yet many processes during the formation of planets, especially in the intermediate size range, are unobservable. This thesis focuses on linking the evolution of a circumstellar disk with a final set of planets in a globally self consistent framework. The number of embryos and their formation in current planet formation models are subject to assumptions and not to physical modeling. This inconsistency currently marks the single largest blind spot in global planet formation modeling. Within four consecutive publications, I present key improvements in global planet formation modeling. Namely the evolution of dust and pebbles, the formation of planetesimals and the formation of planetary embryos. Within this thesis I present a global planet formation model that self-consistently tracks the formation of planets from an initial disk of gas and dust during its entire lifetime. For the first time, this is achieved without far reaching assumptions on initially placed planetesimals or planetary embryos. I show that the disk consistent treatment of planetary embryo formation results in multiple distinct embryo generations during the lifetime of the circumstellar disk. A clear dichotomy between planets that form in different generations is found. The generation from which an embryo originates has far reaching implications on its composition and final planetary properties.

Supervisor:    Hubert Klahr (MPIA)

    Christoph Engler  (Germany)                                                                                                                               14.12.2021

Populations of satellite galaxies with the IllustrisTNG simulations: from galaxy clusters to the Local Group   ( thesis pdf )

In this thesis, I study populations of satellite galaxies in a $\Lambda$CDM context using the IllustrisTNG suite of cosmological magneto-hydrodynamical simulations. Utilising the entire range of IllustrisTNG allows for an unprecedented combination of statistical sample size and numerical resolution, resulting in mass ranges that cover multiple orders of magnitude for both host and satellites samples, as well as the first statistical sample of 198 high-resolution Milky Way-/Andromeda-like (MW/M31) hosts. I investigate the galaxy-halo connection for satellite and central galaxies across the mass spectrum in the stellar-to-halo mass relation as the most fundamental relationship of galaxy evolution in the cosmological standard model. I analyse the abundance of past and present-day satellite and subhalo populations around MW/M31-like hosts, find a remarkable degree of diversity, and put them into context with both observational surveys and previous simulations of similar systems. Their satellites become increasingly quenched towards smaller stellar masses as they lose their gas reservoirs more easily after infall. Thus, I not only give a detailed view on the evolution of satellite galaxies after infall and the environmental effects they experience but overcome one of the remaining challenges to the $\Lambda$CDM model: there is no missing satellites problem according to IllustrisTNG.

Supervisor:    Thorsten Lisker (ARI), Annalisa Pillepich  (MPIA)

    Bahar Bidaran  (Iran)                                                                                                                               12.11.2021

Kinematics and stellar populations of early-type dwarf galaxies in the Virgo cluster: how relevant is pre-processing in galaxy groups?     ( thesis pdf, 10 MB )

This thesis is dedicated to answering one particular question: how relevant is pre-processing in shaping present-day properties of early-type dwarf galaxies (dEs) in massive clusters? With the intention of doing so, we present the analysis of the kinematics and stellar population properties of nine early-type dwarf galaxies (dEs) of the Virgo cluster. According to the results of Lisker et al. (2018), these nine dEs were accreted onto Virgo as gravitationally bound members of a massive galaxy group (M_{star} ∼ 10^13 M_{sun}) along the observer’s line of sight about 2-3 Gyr ago. We confirm a similar accretion time by investigating their distribution on the projected phase-space diagram. We performed full-spectrum fitting on the spatially binned MUSE/VLT data of these dEs to investigate their kinematics. We quantified their stellar rotation using specific angular momentum (λR) and derived diverse kinematic properties in our sample, despite their similar accretion time onto Virgo and stellar mass range. While half of our sample members show higher degrees of rotation, similar to low-mass star-forming field galaxies, the rest show kinematic profiles comparable with Virgo dEs with relatively larger average infall time. Furthermore, we employed Lick absorption features in each dEs’ MUSE data and performed a four-step fitting procedure (based on a χ2 minimization approach) to derive their stellar population properties. We detected relatively younger and metalpoorer stellar populations in these dEs that are noticeably α-enhanced than equally massive dEs in the Virgo and Coma clusters. Similar to other studies on cluster dEs, we detected flat age gradients, negative metallicity gradients, and positive [α/Fe] gradients for our sample dEs. By performing the full-spectrum fitting technique on their averaged MUSE spectra, we detected the presence of a significant peak in the star formation activity of some of our sample members, occurring at an epoch consistent with or even more recent than their accretion time onto Virgo. Through this thesis, we interpret our results by concurrently considering the role of pre-processing and early effects of the Virgo’s environment in shaping the observed properties of this unique Virgo dEs sample.

Supervisor:    Eva Grebel (ARI)

    Robin Gopala Tress  (Germany)                                                                                                                               17.11.2021

ISM dynamics in simulated galaxies: bridging the scales  ( thesis pdf, 88 MB )

The interstellar medium (ISM) and in particular giant molecular clouds (GMCs) are complex and dynamic entities, shaped by internal and external agents like stellar feedback and the galactic environment in which they reside. The aim of this thesis is to model the ISM to understand the connection of the smallest GMC scales to the large galactic scales and study the role of the environment in regulating their dynamics. We perform high resolution hydrodynamic simulations of the ISM in peculiar and rather extreme galactic configurations where we can stress test the ISM response to these environments. Our ISM model and resolution is fine-tuned to capture all important GMC physics while still retaining the large dynamic range in spatial scales necessary to follow them in the galactic environment. In the first part of this thesis I focus on the gas dynamics of an M51-like galaxy encounter. I describe how the interaction affects the global ISM and star formation properties and I proceed with an analysis of the cloud population. In the second part the focus falls on the central barred region of a Milky Way model. I describe the complex gas flows in this extreme environment and analyse the properties of the molecular ISM and the resulting star formation. These simulations are valuable tools to interpret observational data of the region.

Supervisor:    Ralf Klessen (ITA)

    Josefina Michea  (Chile)                                                                                                                               02.11.2021

Brought to Light: Disk and Clump Substructures in Dwarf Early-Type Galaxies  ( thesis pdf, 50 MB )

Dwarf early-type galaxies (ETGs) display a rich diversity in their photometric, structural, and dynamical properties. In this work, we address their structural complexity by studying with deep imaging two data sets of dwarf ETGs that are members of the Virgo and Fornax galaxy clusters. These dwarf ETGs are characterized by having disk-like or clump-like substructure features that lie mostly hidden within the bright diffuse light of the galaxies. We present a newly developed method that aims to robustly identify and extract the substructure features embedded in these dwarf ETGs. The method consists in an iterative procedure that gradually separates a galaxy image into two components: the bright, dominant, diffuse component, and the much fainter, underlying substructure component. By applying it to the two dwarf ETG data sets, we quantify their substructure features, and find that they contribute only between 2% to 10% of the total galaxy light within two effective radii. We test the reliability of the method, and prove that it is accurate in recovering the substructures we introduce in mock galaxy images, even at low substructure-to-total light fractions of a few percent. To showcase potential applications of the method, we subject the extracted substructure components of the Virgo and Fornax data sets to a Fourier analysis and to a color analysis, respectively. Our results indicate that the Virgo cluster has a larger fraction of dwarf ETGs with disk substructures compared to the Fornax cluster, and that embedded spiral arm features are common in disky dwarf ETGs in Virgo but completely absent in Fornax. We propose that these differences are a reflection of the particular characteristics of each distinct cluster environment. We also find that disk substructures tend to be fainter and redder than clump substructures, possibly indicating that disky and clumpy dwarf ETGs may have had different origins and/or evolutionary histories. However, both sub-classes are consistent with being transition-type dwarf galaxies, where environmentally-driven processes are transforming them from late-types to early-types. Through this work, we conclude that the cluster environment must play a major role in shaping and transforming the dwarf galaxy populations in galaxy clusters.

Supervisor:    Eva Grebel (ARI)

    Melanie Kaasinen  (Australia)                                                                                                                             23.07.2021

The Molecular Interstellar Medium in Distant Galaxies  ( thesis pdf, 40 MB )

With the increased capabilities of sub-/millimeter facilities over the last decade it has become possible to observe the medium out of which most of the stars in today's Universe formed, the molecular interstellar medium in distant galaxies. In this thesis, I have tested what can be learnt from such observations, particularly of dust-continuum and CO emission. I compared the molecular gas masses inferred from observations of the ground transition of CO to those inferred from dust-continuum emission, finding that dust-continuum emission can be used to reliably infer the molecular gas content of massive star-forming galaxies at the peak epoch of star formation. However, by comparing the dust and CO emission at resolutions of a few kiloparsec, I found that dust-continuum emission is a poor tracer of molecular gas in the outskirts of typical star-forming galaxies at the same epoch. I also found that at least some distant star-forming galaxies host large, centrally-concentrated molecular gas reservoirs. Finally, I simulated molecular clouds with the aim of testing the impact of the Cosmic Microwave Background on observations of the emission from CO and atomic carbon in distant galaxies, highlighting the dominance of this background in the distant Universe.

Supervisor:    Fabian Walter (MPIA) , Simon Glover (ITA)

    Branislav Avramov   (Serbia)                                                                                                                              22.07.2021

Dynamical evolution of massive black holes in galactic nuclei  ( thesis pdf, 13 MB )

In this work, I investigate the dynamical evolution of supermassive black hole (SMBH) binaries and triples using high-resolution N -body simulations. Starting from cosmological initial condi- tions, I characterize the effects of stellar dynamics on the formation and evolution of SMBH binaries. I perform numerical tests and benchmarks of the novel N -body code φ-GRAPE-hybrid and discuss how numerical parameters affect the evolution of SMBH binaries. I then simulate a hard SMBH binary system embedded in a triaxial and gas-poor galaxy merger remnant. I investigate the properties of loss cone stars that interact strongly with the SMBH binary and find that the summed energy changes of these stars match very well with the overall binary energy change, demonstrating that stellar interactions are the primary drivers of SMBH binary hardening in triaxial, gas-poor systems. I also investigate the orbital and phase-space properties of these stars and distinguish three different populations based on their apocenter. Then, I investigate the population of SMBH triple systems in the IllustrisTNG100-1 cosmological simulation. Using the tree code bonsai2 , I perform zoom simulations of six of these triple SMBH candidates. I then present the orbital evolution of the black holes following the mergers of their host galaxies and examine prospects for their coalescence.

Supervisor:    Andreas Just  (ARI)

    Martin Schlecker  (Germany)                                                                                                                              22.07.2021

The Architectures of Planetary Systems: Population Synthesis Meets Observations  ( thesis pdf, 25 MB )

Advancing our understanding of planet formation is a prime motivation for the search for exoplanets. While it is now widely recognized that multiple planets per system are common, their mutual relationships are still largely unexplored. This thesis investigates such relationships by confronting simulated planet populations with observed planetary systems. To draw conclusions about the formation environments of planets that we observe today, the causal connections between their properties and those of their natal protoplanetary disks must first be established. In a data-driven approach, I identify the most predictive initial conditions of a planet formation model and show that N-body interactions affect primarily low-mass planets. These insights are then used to study the relations between super-Earths on short orbits and outer giant planets. I find a connection between the com- position of simulated planets and the architectures of their systems. This gives rise to the testable hypothesis that high-density inner super-Earths point to a giant companion in the same system. The analysis also suggests that dynamically active giant planets frequently destroy systems of inner super-Earths. This is compatible with the discovery of one of the most eccentric warm Jupiters known that I present in this thesis. I demonstrate in a tidal evolution analysis that this planet is not the progenitor of a hot Jupiter during its high-eccentricity migration. To explore variations of these trends as a function of stellar host mass, I confront the CARMENES M dwarf survey with a synthetic population of planets around low-mass stars. A striking discrepancy is the observed existence of giant planets around very low- mass stars, which can not be reproduced by our model. Future planet formation theories must explain also this peculiar finding.

Supervisor:    Thomas Henning   (MPIA)

    Paul Heeren   (Germany)                                                                                                                                   20.07.2021

Testing Planet Candidates around Giant Stars: Computation and Analysis of High Precision Radial Velocities    ( thesis pdf, 20 MB )

The radial velocity (RV), or Doppler, technique is one of the most successful methods in the search for exoplanets; with more than two decades of RV measurements acquired for some stars, and thanks to a precision around 1 m s − 1 and better reached by modern spectrographs, it allows to explore an ever greater variety of planetary systems. In this PhD dissertation, I present my contributions to the RV survey of G- and K-giant stars, which is conducted by the Exoplanet Group at the Landessternwarte (LSW) Heidelberg. The aim is to track planet candidates in the sample, and thus strengthen our understanding of planet occurrence rates around these types of stars and discern between planetary signatures and false positives caused by intrinsic stellar variations. My work can be split into two parts: First, I was involved in the Waltz telescope project, which will act as a successor to the CAT telescope at Lick observatory and allow to continue the RV survey of giant stars with an LSW-owned telescope. I describe my work on opto-mechanical components which enabled to reach first light on-sky, and I present results from early observations. My main task in the project was the build-up of the Waltz DRS (data reduction software), which will be used to reduce acquired spectra and extract RVs. I detail the structure of the software and implementation of mathematical methods, and discuss first test results on early Waltz and archived Lick spectra. The second part of my work concerns the analysis of the highly eccentric stellar binary Epsilon Cygni, which is part of the K-giant sample. RV data obtained at Lick, with the SONG telescope on Tenerife, and from the literature show short-period variations in addition to the long signal caused by the stellar companion, which might hint at a planetary companion on a S-type orbit around the primary. I present Keplerian and dynamical models and constrain the orbit of the stellar companion; however, in combination with a stability analysis of the system, the models deem the planet hypothesis to be highly unlikely. I examine possible alternative explanations for the short-period RV variations and find tidally induced stellar oscillations as a plausible cause.

Supervisor:    Andreas Quirrenbach   (LSW)

    Thomas Jackson   (United Kingdom)                                                                                                                                19.07.2021

The Assembly and Size Evolutionary Processes of Low Redshift, Massive, Central Galaxies  ( thesis pdf, 10 MB )

I present a study into the stellar mass assembly and size growth of low redshift, massive, central galaxies using a three-stage process. Stage one investigates the stellar mass assembly times of ~ 90,000 galaxies from the SDSS. Stellar mass is the main driver of assembly, with secondary dependencies on environment. A comparison to simulations shows that despite reproducing isolated trends, simulations struggle to reproduce secondary trends. Stage two investigates the nature of low surface brightness (LSB) features around a sub-sample (236) of these galaxies using deep, multi-band imaging from Subaru-HSC. High levels of interaction features are found from minor mergers, imprinted on stellar mass profiles. The LSB material properties such as stellar mass and colours are similar to SDSS satellites. This provides the first direct observational evidence of minor mergers driving the size evolution of galaxies. Stage three uses stellar population fitting of IFU data for a further sub-sample (15) of these galaxies. Subtle differences in age and metallicity profiles in galaxies of different interaction classes are found, however no difference in fractions of ex-situ stars are found. Differences in the stellar populations of galaxies are found to be preferentially linked to the assembly histories, not interaction morphologies.

Supervisor:    Anna Pasquali   (ARI)

    Gindra Gensior   (Germany)                                                                                                                                19.07.2021

The influence of the galactic gravitational potential on star formation and the interstellar medium  ( thesis pdf, 10 MB )

Empirically, the star formation rate of a galaxy depends only on its molecular gas mass. However, recent observations indicate that star formation in stellar spheroids systematically falls below these scaling relations. It has been suggested that the global galactic environment can affect the gas dynamics such that star formation is heavily suppressed. This is investigated using a suite of hydrodynamic simulations of isolated galaxies, which includes a new sub-grid model that captures the influence of galactic dynamics on star formation. The parameter space covered by the simulations ranges from disc galaxies to spheroids, with initial gas fractions varying from 1-20 per cent. This enables a detailed exploration of how differences in the gravitational potential affect the gas properties and star formation rate, as well as how it interlinks with the gas fraction. The shear generated by the the deep gravitational potential of spheroids is found to drive turbulence in the gas, leading to a dynamical suppression of fragmentation and star formation. This dynamical suppression of star formation is enhanced at higher stellar surface densities and lower gas fractions. This implies that the physics of star formation can limit and regulate the baryon cycle at low redshifts and high galaxy masses.

Supervisor:    Diederik Kruijssen   (ARI)

    Diana Elizabeth Kossakowska   (USA)                                                                                                                         08.07.2021

Modeling and Determining Origins of Signals found in Radial Velocity Data  ( thesis pdf, 70 MB )

Thanks to the current TESS space mission, the number of exoplanets with both precise radius and mass measurements is continually growing, providing more promising candidates for the next era in the exoplanet field of atmosphere characterization. A key ingredient is to understand all the signals that appear in the data, to ensure that we obtain the correct planetary parameters. I was involved in developing the juliet package, which allows for easy model comparison between models of combined transit and radial velocity datasets. The methodology is based on modern statistical concepts, such as Bayesian statistics, Nested Sampling, and Gaussian Processes, which are presently gaining popularity among the community. Within the CARMENES consortium, I was heavily engaged in demonstrating how to construct the appropriate models and also how to interpret the results. Hence, for this thesis, I present a guideline on how to build an intuition using this code, covering the small nuances and common misconceptions.

I employed juliet on a variety of planets/systems: TOI-150 b, one of the most eccentric transiting hot Jupiters known to date whose obliquity can be measured; TOI-163 b, another hot Jupiter with an inflated radius ideal for future atmospheric characterization in the JWST continuous viewing zone; TOI-1201 b, currently one of the most optimal transiting mini-Neptunes for atmospheric characterization to aid in determining the origins of these planets; and AD Leo, a case study of a highly-active star that continues to puzzle how stellar activity influences radial velocity measurements as it often masquerades as a planet. These projects showcase the wide application of the code as well as the importance of implementing the correct methodology to contribute well-constrained planetary parameters. To end the thesis, I discuss the need for a better understanding of how stellar activity affects radial velocities to better mitigate the effects, and as well I urge the community to take a standardized approach for modeling in order to avoid misleading interpretations.

Supervisor:    Martin Kürster   (MPIA)

    Victor Ksoll   (Germany)                                                                                                                                           07.07.2021

Characterising Pre-Main-Sequence Stars in the Large Magellanic Cloud with Machine and Deep Learning Techniques  ( thesis pdf, 70 MB )

The Large Magellanic Cloud (LMC) exhibits an extraordinary star-forming activity, providing excellent targets for star formation research. Photometric observations with the Hubble Space Telescope (HST) allow for deep, high-resolution studies of young stellar clusters and still-forming pre-main-sequence (PMS) stars in the LMC. In this thesis we study two LMC star-forming complexes, the Tarantula Nebula and N44. Using HST photometry of the Tarantula Nebula from the "Hubble Tarantula Treasury Project" (HTTP), we devise a machine-learning (ML) classification procedure to identify PMS stars from photometry and recover the PMS population captured by the HTTP survey. We introduce new HST observations of N44, the "Measuring Young Stars in Space and Time" (MYSST) survey, identify N44’s PMS content with our ML classification procedure, and conduct a clustering analysis of the identified PMS stars. Additionally, we develop a conditional invertible neural network approach to predict stellar physical parameters from photometric observations, based on the PARSEC stellar evolution models. We perform a test on HST observations of the Milky Way clusters Westerlund 2 and NGC 6397, and successfully confirm previous findings on e.g. the age of Westerlund 2. For NGC 6397, however, we identify discrepancies between the PARSEC stellar evolution models and HST observations that prevent accurate predictions.

Supervisor:    Ralf Klessen   (ITA)

    Francisco Aros Pinochet   (Chile)                                                                                                                             06.07.2021

Towards a robust detection of intermediate-mass black holes in globular clusters  ( thesis pdf, 20 MB )

Intermediate-mass black holes (IMBHs) with masses between $10^2$ to $10^5$ solar masses could be the key to explaining the formation of supermassive black holes in the centre of galaxies at high-redshift. The possible formation scenarios of IMBHs point towards dense stellar systems as favoured birthplaces. If an IMBH resides at the centre of such dense stellar systems, the surrounding stars under the IMBH’s dynamical influence will show characteristic kinematic signatures. During the last two decades, the community has made significant efforts to find IMBHs through these kinematic signatures, particularly in the centre of the Milky Way’s globular clusters (GCs). To date, however, no robust detection has been made.

In this thesis, we explore the internal dynamics of GCs to understand the current limitations for detecting IMBHs and propose alternative lines of evidence of their presence. With this purpose, we use numerical simulations of GCs with and without a central IMBH and apply commonly used dynamical models to estimate the mass of the possible central IMBH. We focus on the limitations of the dynamical modelling itself, particularly those due to common assumptions such as a constant velocity anisotropy and mass-to-light ratio. From an observational viewpoint, we explore the role of binaries in the observed kinematics and the connection between the observed binary fraction and the presence of an IMBH. Finally, we study the implications of the dynamical modelling limitations and the effects of binary systems on the scaling relations, which connect the mass of the central IMBHs with their host stellar system properties.

Supervisor:    Glenn van de Ven   (MPIA / U. of Vienna)

    Felix Bosco   (Germany)                                                                                                                             01.07.2021

Probing the Growth of Black Holes at the Limit of Large Telescopes  ( thesis pdf, 6 MB )

Direct kinematical measurements of black hole (BH) masses require to resolve the sphere of influence at sub-milliarcsecond scales. Here, two novel observational approaches are studied that exhaust the resolution limit of modern large telescopes in order to allow for tighter constraints on masses and growth mechanisms of BHs. The first approach applies holographic speckle imaging techniques to near-infrared images after partial atmospheric turbulence correction. Based on simulations and observational data, we show that the combination of techniques recovers the diffraction limit of 8 m-class telescopes for stars too faint for classical speckle imaging. This approach will allow for tracing BHs directly via stellar kinematics. The second technique uses the spectroastrometric signal of the quasar broad emission line region in order to constrain its geometric and kinematic structure. We extract this signal from adaptive-optics-assisted near-infrared spectroscopy with an 8 m telescope and carefully study the uncertainties. The comparison of the data to our model allows us to report on the first tentative detection of the spectroastrometric signal of a luminous quasar and thereby for constraining its BH mass, the first direct measurement beyond a redshift of z = 2 and out of the detection range of reverberation mapping or optical interferometry.

Supervisor:    Joerg-Uwe Pott    (MPIA)

    Edna Ruiz Velasco   (Mexico)                                                                                                                             15.06.2021

Search and first detection of very-high-energy photons in gamma-ray bursts: an analysis with HAWC and H.E.S.S.  ( thesis pdf, 10 MB )

This thesis is devoted to the study of extremely energetic short-timescale astrophysical events, Gamma-ray bursts (GRBs). GRBs exhibit broad-band bright non-thermal emission, which was analysed using two major experiments: the High Altitude Water Cherenkov observatory (HAWC) and the High Energy Stereoscopic System (H.E.S.S.). The two experiments are in many respects complementary for the observation of very high energy (VHE) gamma-ray emission from GRBs, and in this work the respective advantages were exploited to maximise the sensitivity to VHE signals. After the analysis of several tens of GRBs observed using H.E.S.S. until 2017, where no significant emission was detected, improvements in the observation strategy of H.E.S.S. allowed the detection of GRB~180729B and GRB~190829A. These detections are presented in context with multi-wavelength data, proposing plausible emission mechanisms, thus concluding a decade-long search for these elusive phenomena at VHE.

In the second part, novel methods to improve the accuracy of the HAWC detector simulation are presented, including better modelling of the detector efficiencies and electronics. A model that accounts for the detector response and the GRB flux evolution has been developed to estimate the optimal integration time for VHE searches with HAWC. Thanks to these improvements, it is possible to exploit the wide field of view and high duty cycle of HAWC for the search of VHE emission in several tens of GRBs. Preliminary evidence for emission is found in one of the GRBs studied, and upper limits are obtained for all the GRBs analysed and placed in context of the X-ray properties of these events. Finally, the limits and detections presented in this work are placed within the framework of the current understanding of GRBs and prospects for future and present VHE gamma-ray detectors are presented.

Supervisor:    Jim Hinton  (MPIK)

    Sabrina Gronow   (Germany)                                                                                                                             15.06.2021

Contribution of Type Ia supernovae to the chemical enrichment of the Milky Way: explosions of sub-Chandrasekhar mass white dwarfs  ( thesis pdf, 10 MB )

Type Ia supernovae (SNe Ia) are important for galactic chemical evolution (GCE) because they produce heavy elements. Sub-Chandrasekhar mass carbon-oxygen white dwarfs with helium shells are favored progenitors for SNe Ia. This thesis investigates the double detonation explosion scenario. A focus lies on an accurate calculation of the detonation propagation in the white dwarf shell and the assumption of core-shell mixing. Parameter studies were conducted to analyse whether variations found in observables of SNe Ia can be reproduced and to provide (metallicity-dependent) nucleosynthetic yields for subsequent radiative transfer calculations and GCE models. Three-dimensional simulations were carried out using the Arepo code. A previously neglected carbon detonation ignition mechanism was found showing that the helium detonation wave convergence is sufficient to ignite carbon in a core-shell transition region. The study shows that various luminosities coinciding with SNe Ia can be reproduced. Metallicity-dependent yields illustrate that a high stellar metallicity shifts the production to stable isotopes while supporting the manganese production. GCE models suggest that the inclusion of this explosion type allows to account for about 80% of the solar manganese abundance. The correlation of [Mn/Fe] with metallicity in the solar neighborhood is supported by the inclusion of metallicity-dependent SNe Ia yields.

Supervisor:    Friedrich Roepke  (HITS)

    Stephan Stock   (Germany)                                                                                                                             28.04.2021

Analysis of High-Precision Spectroscopic Data in Search of Exoplanet Signals Close to the Stellar Noise Limit   ( thesis pdf, 70 MB )

Due to their low mass and low temperature, M-dwarf stars are promising targets for the detection of planets in the host star’s temperate zone with the radial velocity (RV) method. However, M dwarfs typically exhibit stellar activity, producing noise in the RV data which can mask the signals from existing planets or, as well as aliasing caused by data sampling, can lead to false candidates or to wrongly determined planetary parameters. I present RV analyses for four M dwarfs with the aim of detecting and characterizing potential planetary signals. To overcome the aliasing problems, I developed the public tool AliasFinder, which is an improvement of an already known method for distinguishing aliases from actual signals. For the disentanglement of stellar activity and planetary signals, and for their modeling, I used current state-of-the-art analysis techniques, e.g., Bayesian statistics and Gaussian process (GP) regression. The properties and the performance of GP models have been investigated and tested on simulated RV data of active M dwarfs. These GP models, along with additional data taken by CARMENES, have resulted in improved planetary parameters for four previously known planets (the multiplanetary system YZ Ceti and the single-planet system of Lalande 21185) and two new planet discoveries (the temperate super-Earth GJ 251 b and the warm super-Earth HD 238090 b). The analyses, results,and partially unique approaches in this thesis demonstrate that it is possible to detect and model planetary RV signals at the limit of the stellar astrophysical noise by applying rigorous statistical models, along with prior information from auxiliary data and further simulations.

Supervisor:    Sabine Reffert  (MPIA)

    Victor Marian   (Austria)                                                                                                                             28.04.2021

The Intricate Connection Between Major Mergers and AGN with the Highest Eddington Ratios   ( thesis pdf, 25 MB )

Over the past decade, research has revealed contradictory results on whether mergers of galaxies of similar mass, so-called major mergers, are the dominant or at least an essential mechanism for the formation of active galactic nuclei (AGN) and the growth of supermassive black holes (SMBH) in galaxies. In this work, it is investigated whether such a connection exists for the ‘most plausible’ objects – broad line AGN with the highest Eddington ratios. The first sample is based on HST/WFC3 observations and focuses on these objects that possess an Eddington Ratio >0.7 at z=2, the peak epoch of black hole activity. To validate the results and to identify possible discrepancies between observations and simulations, a comparison study is performed on similar objects modeled by the large-scale cosmological simulation ILLUSTRIS-TNG. An additional sample consists of local z=0.2 AGN with an Eddington ratio >0.3, observed with the VLT/FORS. All AGN samples are compared to inactive galaxies of similar stellar mass and redshift. For all three individual studies, the applied methodology is identical: sources are visually identified and classified into merging and unperturbed galaxies. An excess for the respective AGN host galaxies in the subsequently derived major merger fractions would then indicate that AGN are predominantly triggered by such galaxy mergers. The analysis of the observed galaxies at z=2 yields merger fractions of 0.24 ± 0.09 and 0.19 ± 0.04 for the AGN hosts and inactive galaxies, respectively. In addition to the visual assessment, the ILLUSTRIS-TNG data set is analyzed by evaluating the galaxies’ merger histories and accounting for typical observational uncertainties. The resulting best estimates give 0.27 ± 0.07 and 0.22 ± 0.01, indicating an excellent agreement between the two studies at this redshift. In contrast, with 0.41 ± 0.12 and 0.08 ± 0.06, the fractions at z=0.2 show a clear excess in the incidence of major mergers for the AGN host galaxies. The results are analyzed extensively, including an investigation of possible influences due to selection effects, a time lag between the visibility of galaxy mergers and the AGN, as well as the methodology used. While there is no significant evidence that such major mergers are the dominant process for the existence of the studied AGN at z=2, such gravitational encounters appear to be an important mechanism for comparable AGN at z=0.2. This may be due to the fact that the gas required for SMBH to achieve such high accretion rates is less abundant in galaxies at low redshifts, leading to major mergers playing a more dominant role in the local Universe. Nevertheless, regardless of redshift, for a minimum of 50% of this particular AGN population, the origin remains unclear.

Supervisor:    Knud Jahnke  (MPIA)

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