# Alumni 2018

Rainer Weinberger (17.1.)  -  Roxana Chira  (22.1.) -  Adriana Pohl  (23.1.)  -  Valeriy Vasilyev  (7.2.)  -  Johannes King  (9.2.)  -  Carolin Wittmann (9.2.)  -  Clio Bertelli Motta  (20.2.)  - Taras Panamarev  (18.4.)  -  Robert Reischke (18.4.)  -  Thales Gutcke (2.5.)  -  Tim Tugendhat (16.5.)  -  Andreas Schreiber (18.5.) - Carsten Littek  (29.6.)  -  Jonas Frings (11.7.) - Chiara Mazzucchelli (12.7.)  -  Gustavo Morales (25.7.)

## Gustavo Morales ( Chile )                                                                                                                                                             25.07.2018

Stellar Tidal Streams as Cosmological Diagnostics: Comparing data and simulations at low galactic scales  (thesis pdf, 7 MB)

In hierarchical models of galaxy formation, stellar tidal streams are expected around most galaxies. Although these features may provide useful diagnostics of the LCDM model, their observational properties remain poorly constrained. Statistical analysis of the counts and properties of such features is of interest for a direct comparison against results from numerical simulations. In this work, we aim to study systematically the frequency of occurrence and other observational properties of tidal features around nearby galaxies. The approach featured here is based on a visual classification of diffuse features around a sample of nearby galaxies, using a post-processing of optical survey imaging optimized for the detection of low-surfacebrightness stellar structure. At the limiting surface brightness of this sample, 14 − 17% of the galaxies exhibit evidence of diffuse features likely to have arisen from minor merging events. For simulated images, the frequency is 16 − 19%. Our technique recovers all previously known streams in the selected sample and yields a number of new candidates. We conclude that this methodology provides a reliable foundation for the statistical analysis of diffuse circumgalactic features in wide-area imaging surveys, and for the identification of targets for follow-up studies.

Supervisors: David Martinez-Delgado, Eva Grebel (ARI)

## Chiara Mazzucchelli ( Italy )                                                                                                                                                          12.07.2018

The Physical Properties and Cosmic Environments of Quasars in the First Gyr of the Universe (thesis pdf, 10 MB)

Luminous quasars at redshift z > 6, i.e. < 1 Gyr after the Big Bang, are formidable probes of the early universe, at the edge of the Epoch of Reionization. These sources are predicted to be found in high–density peaks of the dark matter distribution at that time, surrounded by overdensities of galaxies. In this thesis, we present a search for and study of the most distant quasars, from the properties of their innermost regions, to those of their host galaxies and of their Mpc–scale environments. We search for the highest redshift quasars in the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1, PS1), discovering six new objects at z >6.5. Using optical/near–infrared spectroscopic data, we perform a homogeneous analysis of the properties of 15 quasars at z> 6.5. In short : 1) The majority of z >6.5 show large blueshifts of the broad CIV 1549 Å emission line, suggesting the presence of strong winds/outflows; 2) They already host supermassive black holes (M_BH = 0.3 - 5 x 10^9 M_sun) in their centers, which are accreting at a rate comparable to a luminosity–matched sample at z =1-3) No evolution of the Fe II/MgII abundance ratio with cosmic time is observed; 4) The sizes of their surrounding ionized bubbles weakly decrease with redshift. We present new millimeter observations of the dust continuum and of the [CII] 158 mm emission line (one of the main coolant of the intergalactic medium) in the host galaxies of four quasars, providing new accurate redshifts and [CII]/infrared luminosities. We study the Mpc–scale environment of a z =5.7 quasar, via observations with broad– and narrow–band filters. We recover no overdensities of galaxies. Among the potential explanations for these findings, are that the ionizing radiation from the quasar prevents galaxy formation, the sources in the fields are dust–obscured, or quasars do not live in the most massive dark matter halos. Finally, we report sensitive optical/near–infrared follow–up observations of gas–rich companion galaxies to four quasars at z >6, firstly detected with the Atacama Large Millimeter Array (ALMA).With the exception of one source, we detect no emission from the stellar population of these galaxies. Our limits on their stellar masses (< 10^10 M_sun) and unobscured star formation rates (

Supervisors: Fabian Walter, Hans-Walter Rix (MPIA)

## Jonas Frings ( Germany )                                                                                                                                                                     11.07.2018

Structure and evolution of simulated dwarf galaxies and Milky Way satellites in Cold and Warm dark matter models    (thesis pdf, 4 MB)

The satellite galaxies and dwarf galaxies in the neighborhood of the Milky Way provide us with detailed observations that can be used to test our standard model of cosmology and structure formation, the $\Lambda$CDM model. I present a sample of 27 cosmological hydrodynamical simulations with virial masses between $5\times 10^8$ and $10^{10}\,\Msun$ that are aimed to study the properties of dwarf galaxies before accretion. The simulated galaxies are able to reproduce observed scaling relations like the dispersion - size and metallicity - stellar mass relation. The stochasticity of merger induced star formation causes a large scatter in the stellar mass - halo mass relation. In galaxies of stellar masses below $10^6 \, \Msun$ stellar feedback is unable to affect the dark matter halo and hence those galaxies retain a cuspy profile. A subsample of 7 halos is used as initial conditions for simulations of satellite - host galaxy interactions in a Milky Way mass halo. The mass removal due to tidal forces creates flat stellar velocity dispersion profiles and efficiently decreases the circular velocity at $0.5\,\mathrm{kpc}$ without stripping a large amount of stars. Additionally the stripping seems to happen in a way that effectively steepens the central dark matter density slope. To investigate the effects of warm dark matter on dwarf galaxies and Milky Way satellites I repeat the study in a $3\,\mathrm{keV}$ warm dark matter scenario. I present the simulations of 21 halos in both CDM and WDM. In WDM the critical halo mass for the onset of star formation is shifted towards higher masses, while the simulations that do produce stars, reproduce the same scaling relations as their CDM counterparts. However, WDM seems to delay the bulk star formation, making galaxies in CDM look about $2\,\mathrm{Gyr}$ older. While halo concentrations are significantly lower in WDM, the central dark matter density slope is slightly steeper for the low mass end. For four halos (in WDM and CDM) I present their evolution as Milky Way satellites. In contrast to the CDM halos, WDM halos are stripped more effective due to their lower concentrations. The survival probability for WDM satellites, on the other hand, is not necessarily lower because of their steeper central slope. Again, the WDM as well as CDM satellites end up with very cuspy profiles after being stripped. I come to the conclusion that the predictions from my simulations do not challenge the $\Lambda$CDM compared to current observational data of dwarf galaxies and Milky Way satellites. An observation of a cored density profile in one of the very low mass objects, however, would force us reconsider the dark matter model. Also WDM would not pose a solution to this problem.

Supervisor: Andrea Maccio (NYU Abu Dhabi, MPIA)

## Carsten Littek ( Germany )                                                                                                                                                                       29.06.2018

Kinetic Field Theory: Momentum-Density Correlations and Fuzzy Dark Matter       (thesis pdf, 1 MB)

Building upon the recent developments of Kinetic Field Theory (KFT) for cosmic structure formation we develop a systematic way to calculate correlation functions of the momentum-density field. We show that these correlators can be calculated from the factorised generating functional after application of partial derivatives with respect to the momentum shift. For visual aid and in order to facilitate an automatic evaluation of corrections by particle interactions we introduce a diagrammatic representation of terms. We employ this formalism to calculate the 2-point momentum-density correlation tensor including initial correlations to quadratic order and completely. A comparison of the results shows that the initial correlations are responsible for the deformation of the power-spectrum on small scales rather than the particle interactions. In the spirit of the Born approximation we use an effective force term to calculate the corrections due to gravity. Our results are in good agreement with previous analytic and simulation results. Recently, Fuzzy Dark Matter models such as Ultra-Light Axions have caught a lot of interest. Their dynamics is described by the classical equations of a condensate. This introduces a quantum potential in the Euler equation and is generally repulsive. We have developed an extension to KFT treating the effects of the quantum potential on the dynamics and on the initial density fluctuation power-spectrum. We find the effects to be largest on scales in the range of 3h/Mpc > k > 0.3h/Mpc, close to the onset of non-linear structures.

Supervisor: Matthias Bartelmann (ITA)

## Andreas Schreiber ( Germany )                                                                                                                                                        18.05.2018

Diffusion Limited Planetesimal Formation    (thesis pdf, 60 MB)

Planets are surprisingly abundant in our own solar system, but also in extrasolar systems. It is striking to find no explanation for them, as dust in protoplanetary disks was found to not outgrow metres in size. The growth barrier of dust to km-sized planetesimals thus states a missing link onto their formation mechanisms. It is evident for planetesimals to have been present in the early solar system, as their remnants prowl the solar system today in the form of asteroids, Kuiper belt objects, and comets. Of them, many were found to be pristine, giving a hint on what once populated the early solar nebula. Studying the sizes of these pristine objects revealed for all of them a characteristic diameter of 100 km. It is stunning to find this feature independent of distance from the Sun in most pristine object families, hence this feature has to be an imprint of their formation mechanism. This thesis derives a formation criterion for planetesimals out of particle cloud collapse within protoplanetary disks. The found mechanism is capable of reproducing the characteristic sizes of these pristine objects, as it is to first order independent of radial distance from the star. By comparing collapse timescale with turbulent particle diffusion timescale, a minimum size criterion for a dust cloud to collapse is found and investigated. Naturally, dust cloud collapse happens at high dust-to-gas ratios, thus the streaming instability is a good candidate for this turbulent process. Hence, the streaming instability is studied in 2-d and 3-d simulations at dust-to-gas ratios well above unity and on typical collapse length scales. This study found a new instability, namely the azimuthal streaming instability. It operates in the radial-azimuthal plane and has characteristics similar to the streaming instability, thus its name. Subsequent collapse simulations in 2-d and 3-d proved the diffusion limited planetesimal formation to produce planetesimals right at the expected 100 km diameter. It is the conclusion of this thesis to have shown a fundamental concept to be applied in future studies on planetesimals. It has the prospect to make verifiable predictions which can proof this mechanism to have shaped the solar system as we see it today.

Supervisor: Hubert Klahr (MPIA)

## Tim Tugendhat ( Germany )                                                                                                                                                                   15.05.2018

On the Impact of Intrinsic Alignments of Galaxies on Measurements of Weak Gravitational Lensing      (thesis pdf, 8 MB)

Future weak lensing surveys will be too precise for their own good: due to their impressive statistical precision, their systematics must be treated with extreme care. In particular, intrinsic alignments (IA) of galaxies, which cause galaxy ellipticities to be correlated, are indistinguishable from gravitational lensing. I present two established models for IAs and will link them to the physical properties of two galaxy morphologies, namely spiral and elliptical galaxies. Their amplitudes relative to the lensing spectrum is determined and a realistic mix of galaxies and thus utilised to predict the impact of IAs on a Euclid-like tomographic survey and its inferences on a cosmological parameter set. It will be shown that IAs will give rise to significant systematic errors (biases) in future surveys. Furthermore, I present a method for suppressing the IA signal by using colour information and separating the survey into subsamples of blue and red galaxies. This method can also be used to suppress the lensing signal instead, enabling the treatment of just IAs. Finally, I will show a possible way of constrain deviations from General Relativity (Gravitational Slip) by using the model for elliptical galaxies and leveraging its signal against the one from a known weak lensing signal.

Supervisor: Bjoern Malte Schaefer (ARI)

## Thales Gutcke ( Germany / USA )                                                                                                                                                        02.05.2018

The quenching of star formation in galaxies  (thesis pdf, 20 MB)

This thesis is concerned with investigating what makes star formation inefficient in galaxies. Cosmological, hydrodynamical simulations of galaxy formation show that the energy produced in stars, supernova explosions and active galactic nuclei must couple back into a galaxy’s interstellar medium to prevent excess star formation. However, the physical processes at work in this feedback loop are not well understood. This thesis unravels the details of the baryon cycle to constrain the strength of feedback. The first part explores a phenomenological model of star formation quenching in massive galaxies, showing that gas starvation is a viable pathway to realistic elliptical galaxies. In the second part, a state-of-the-art implementation of stellar feedback is put to the test by comparing the chemical composition of the circum-galactic medium with the latest observations. The simulations exhibit a deficiency in highly ionized oxygen, indicating that models of thermally coupled feedback may be insufficient. The last part delves into the star formation prescription itself, since this directly affects the resulting stellar feedback cycle. An empirical model of a metallicity-dependent stellar initial mass function reveals the significant uncertainty resulting from the common assumption of its universality. Thus, this analysis links star formation processes with stellar feedback and shows how they affect the baryon cycle of entire galaxy ecosystems.

## Robert Reischke ( Germany )                                                                                                                                                                18.04.2018

Cosmology as a probe of gravity with future surveys      (thesis pdf, 7 MB)

The subject of this thesis are different aspects of cosmology as a probe of the underlying gravitational theory with future surveys. In the first part of this work we discuss the parameter dependence of covariance matrices of the power spectrum estimator of the large-scale structure. Its variation across parameter space is calculated analytically by constructing a suitable basis and is then compared with numerical simulations. The method presented is applicable to any matrix-valued function which is everywhere positive-definite. The second part investigates the influence of tidal gravitational fields on the formation of dark matter halos at peaks in the density field of the large-scale structure. We extend the spherical collapse model to incorporate the influence of shear and rotation by treating them as inhomogeneities in the non-linear evolution equation. We investigate the statistics of the tidal field and how it is inherited to the statistics of the critical over-density δc. It is shown that the collapse in a tidal field will always proceed faster than the collapse in a homogeneous background. The last part investigates the combination of observations of weak gravitational lensing, galaxy clustering and the cosmic microwave background and the cross-correlations between the probes to investigate scalar-tensor theories of gravity. We carry out a Fisher analysis as well as a Monte-Carlo-Markov-chain to estimate the expected statistical errors. The analysis shows that gravitational theories can be constrained very well with future surveys.

Supervisor: Bjoern-Malte Schaefer (ARI)

## Taras Panamarev ( Kazakhstan )                                                                                                                                                          18.04.2018

Dynamical interaction of supermassive black holes with the surrounding stellar system (thesis pdf, 6 MB)

We use high resolution direct N-body simulation methods to study the stellar dynamics in the Galactic centre (GC) and in active galactic nuclei (AGN). We follow the evolution of the GC from the assumed in-situ formation of the nuclear star cluster (NSC) up to 5 Gyr using one million particles taking into account single and binary stellar evolution. We investigate 3D stellar density distributions, obtain rate of tidal disruption events and estimate the number of extreme mass-ratio inspirals. We examine the survivability of binary stars in the NSC and discuss the contribution of binaries with compact objects in presence of pulsars and Supernovae Ia rates in the GC. We use more simplified model to study the stellar dynamics in AGN. The analysis of two simulations with 128000 particles (with and without the accretion disk (AD)) shows that the interaction of the NSC with the AD leads to formation of a stellar disc in the central part of the NSC.We derive the mass and size of the formed stellar disc and discuss possible existence of such discs in some nearby galaxies.

Supervisor: Andreas Just (ARI)

## Clio Bertelli Motta ( Italy )                                                                                                                                                        20.02.2018

The footprints of stellar evolution on the chemical composition of the Galactic old open cluster M67  (thesis pdf, 40 MB)

In this work we investigate how stellar evolutionary processes change the surface chemical composition of stars. As a test-bench, we use the old open cluster M67, for whose stars high-resolution spectroscopic data are available in many different evolutionary stages, from the main sequence to the red clump. In particular, we use data retrieved from the archives of large spectroscopic surveys such as APOGEE and Gaia-ESO. First we investigate the effects of the so-called first dredge-up on the surface [C/N] abundance of M67 stars. We then analyse variations in the surface abundances of several elements from the main-sequence to the red-giant phase of M67 stars and discuss how these can be explained by atomic diffusion effects. We also present the results of these investigations in the broader context of Galactic archaeology studies. Furthermore, we investigate the chemical composition of three blue straggler stars and two evolved blue straggler stars in M67 in order to find hints for their formation scenario and discuss the results from the point of view of stellar evolution. Finally, we present an experiment based on TGAS data for the study of the dynamical evolution of OB associations.

Supervisors: Anna Pasquali, Eva Grebel (ARI)

## Johannes King ( Germany )                                                                                                                                                           09.02.2018

Hochenergetische Gammastrahlung aus dem Galaktischen Zentrum     (thesis pdf, 10 MB)

Multi-wavelength observations of the centre of our Galaxy have revealed a number of energetic processes that are related to the central supermassive black hole. Gamma-ray astronomy contributes to these observations with measurements from satellite and ground based instruments such as Fermi-LAT and HESS. These measurements can make important contributions towards answering some of the key open questions. An analysis of the source HESS J1745-290, potentially linked to acceleration of cosmic rays to PeV-energies, is presented in this thesis. The analysis is performed using the HESS II array, which includes the largest existing Cherenkov telescope, in order to measure photon energies below 100 GeV. Furthermore, the Galactic centre is a promising target for indirect dark matter searches. Such a search is performed in this thesis based on HESS and Fermi-LAT data . Due to the limited angular resolution of current instruments in gamma-ray astronomy, spectral analysis is often the only tool for the investigation of gamma-ray sources. Therefore this thesis includes studies to test which spectral analysis methods can lead to biased parameter estimators.

Supervisor:   Werner Hofmann (MPIK)

## Carolin Wittmann ( Germany )                                                                                                                                                      09.02.2018

Ultra-compact and ultra-diffuse stellar systems in nearby galaxy clusters: signs of environmental influence?      (thesis pdf, 40 MB)

In this thesis we investigate ultra-compact and ultra-diffuse stellar systems in the cores of the nearby Perseus and Fornax galaxy clusters for signs of environmental influences. We search for possible disturbances of their stellar structures by examining their light distributions in deep optical wide field imaging data. In the Fornax cluster we analyse a sample of 355 spectroscopically confirmed compact stellar systems. Our data reveal that many objects show distorted outer structures, although we do not find long tidal streams around any of them. We investigate their spatial and phase-space distributions, and interpret our results in the framework of proposed formation scenarios. In the Perseus cluster we identify a population of 89 diffuse low surface brightness galaxy candidates for which we perform photometry. The majority of the diffuse candidates appear unperturbed based on their stellar structures. We find, however, that galaxies with large sizes seem to be absent in the dense cluster core region. We discuss possible implications for the dark matter content of these systems and compare our sample to faint low surface brightness galaxies in the Coma cluster. Our data reveal a few low-mass galaxies with tidal tails or disturbed morphology, and several diffuse streams and tidal debris. Nevertheless, the number of recent galaxy disruption events seems to be very low in both the Perseus and Fornax galaxy cluster cores, indicating that most of the low-mass galaxy population was probably shaped at earlier epochs.

Supervisor: Thorsten Lisker (ARI)

## Valeriy Vasilyev ( Russia )                                                                                                                                                            07.02.2018

Dynamical model atmospheres for the abundance analysis of pulsating stars (thesis pdf, 15 MB)

The chemical composition of Cepheid variables can provide information on the chemo-dynamical evolution of the Galaxy and beyond. The standard method for determining atmospheric parameters and abundances of Cepheids is based on one-dimensional plane-parallel hydrostatic model atmospheres, where convection is treated by Mixing Length Theory. The aim of the thesis is to investigate the impact of the atmospheric dynamics on observable spectroscopic properties. Two approaches are followed: firstly, I construct one-dimensional pulsating atmosphere models implementing a non-local, time-dependent theory of convection, and secondly check the validity of the quasi-static approach against a t wo-dimensional dynamical Cepheid model.
The spectroscopic analysis of the classical Cepheid KQ Scorpii with my one-dimensional model showed that pulsations do not produce strong enough velocity gradients in the line-formation region to explain the estimated microturbulent velocities. The spectroscopic investigation of the two-dimensional Cepheid model allowed to explain the residual line-of-sight velocity of Galactic Cepheids, long known as the “K-term”, by lineshifts of convective origin. Moreover, hydrostatic 1D model atmospheres can provide unbiased estimates of stellar parameters and abundances of Cepheids for particular phases of their pulsations. Summarizing, the main result is a change of paradigm in the context of spectroscopic investigations of Cepheids toward a greater importance of convection than thought previously.

Supervisors: Hans-Guenther Luwdig, Norbert Christlieb (LSW)

## Adriana Pohl ( Germany )                                                                                                                                                             23.01.2018

Structure of Planet-forming Disks: Multi-wavelength Polarization Diagnostics   (thesis pdf, 10 MB)

The study of dynamic processes that drive the evolution of planet-forming disks is fundamental to understand the origin and diversity of planetary systems. This requires observations at high spatial resolution and sensitivity, which nowadays typically reveal intriguing disk substructures including gaps, rings, spirals, and shadows. This thesis investigates the capability of polarization observations at multiple wavelengths to trace the earliest stages of planet formation. In-depth radiative transfer calculations are carried out in order to link numerical simulations of dust and gas evolution in disks with their observational indicators. This approach demonstrates that measuring polarization is a powerful tool to identify the shaping effects that possible embedded planets have on the density distribution of different dust grain sizes. On the observational part, this work presents several case studies of individual planet-forming disks that were observed with polarimetric imaging by the VLT/SPHERE instrument and subsequently modeled to quantify their structure. A particular focus is the characterization of spiral and ring/gap structures in the context of dust growth, planet-disk interactions, and dust dynamics near ice lines. Furthermore, a modeling study of marginally gravitationally unstable disks is presented to study the influence of the disk self-gravity on the shape and contrast of planet-induced spiral arms in scattered light images. Additionally, it is demonstrated that polarized emission of disks at millimeter wavelengths can be caused by self-scattered thermal dust emission. It is shown that the latter is a viable method to constrain grain properties and identify dust concentrations of different origin. New ALMA observations are presented that offer the first look at a dust trap in polarized scattered light in the sub-millimeter range.

Supervisor: Thomas Henning (MPIA)

## Roxana Chira ( Germany )                                                                                                                                                            22.01.2018

On Filaments within Molecular Clouds and their Connection to Star Formation     (thesis pdf, 15 MB)

In recent years, there have been many studies on the omnipresence and structures of filaments in star-forming regions, as well as the role of their fragmentation in the process of star formation. However, only a few comprehensive studies have analysed the evolution of filaments and their distribution with the Galactic disk where the filaments form self-consistently as part of large-scale molecular cloud evolution. In this thesis, I study the effect of inclination on dust observations of filaments to evaluate whether the variations would enable the identification of further filaments in existing dust surveys. I address the early evolution of pc-scale filaments that form within individual clouds and focus on the questions how and when the filaments fragment, and how the fragmentation relates to typically used observables of the filaments. I perform dust radiative transfer calculations on models of cylinders and reconstructions of observed star-forming regions. For evaluating the equilibrium state of filaments and the nature of their fragmentation I examine three simulated molecular clouds formed in kpc-scale numerical simulations modelling a self-gravitating, magnetised, stratified, supernova-driven interstellar medium. I find that the observables of filaments in dust emission are on average on small scales influenced by inclination; yet the variations strongly depend on the structure of the object. The first fragments appear when the line masses of the simulated filaments lie well below the critical line mass of Ostriker’s isolated hydrostatic equilibrium solution. This indicate that, although the turbulence of the entire clouds is mostly driven by gravitational contraction, fragmentation does not occur do
to gravitational instability, but is supported by colliding flow motions. I conclude that there is no single quantity in my analysis that can uniquely trace the inclination and 3D structure of a filament based on dust observations alone. A simple model of an isolated, isothermal cylinder may not provide a good approach for fragmentation analysis, independently of the dominant driving source of the parental cloud.

Supervisor: Thomas Henning (MPIA)

## Rainer Weinberger ( Germany )                                                                                                                                                     17.01.2018

Supermassive black holes and their feedback effects in galaxy formation   (thesis pdf, 9 MB)

Supermassive black holes play a key role in modern galaxy formation research. They are conjectured to be present in almost all massive galaxies, and through the release of enormous amounts of energy triggered by gas accretion, they are able to substantially change the properties of the host galaxy. To which extent and how the interaction mechanisms work is an open question. In this thesis, I review the current state of galaxy formation research with a focus on cosmological simulations of structure formation as well as the basic theories of supermassive black holes as far as they are important for galaxy formation. Subsequently, I discuss a new model for black hole growth and feedback in cosmological simulations, along with its application in large cosmological volume simulations. I show how supermassive black holes affect the formation and evolution of their host galaxy as well as their own growth. Furthermore, I present a model for supermassive black hole jets in a galaxy cluster environment. Applying this model, I study the coupling between the jet and the surrounding intra-cluster gas.

Supervisor: Volker Springel (HITS)

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