IMPRS-HD alumni 2016

Alumni 2016


Shiwei Wu (1.2.)  -  Gabriel-Dominique Marleau (1.2.)  -  Kai-Martin Dittkrist (2.2.)  -   Helen Poon (3.2.)  -  Mykola Malygin (3.2.)  - Erik Bertram (3.2.)  -  Simon Bihr (20.4.)  -  Ilya Khrykin (20.4.) -  Gabriele Cologna (11.5.)  -  Maria Woellert (12.5.)  -  Cristina Garcia Vergara (25.5.)  -  Miguel Querejeta (25.5.)  -  Simone Giacche (22.6.)  -  Svea Proft (29.6.)  -  Kevin Schaal (8.7.)  -  Paolo Bianchini (19.7.)  -  Alison Mitchell (28.7.)  -  Taisiya Kopytova (23.9.)  -  Emer Brady (10.11.)  -  Reza Moetazedian (16.11.)  -  Mauricio Ortiz Alvarez (14.12.)  -  Athanasia Tsatsi (21.12.)


Athanasia Tsatsi ( Greece )                                                                                                                                        21.12.2016

Dynamical structure and evolution of merger remnants    (thesis pdf, 25 MB)

From the most massive stellar systems known to be the end products of mergers, such as early-type galaxies, to the less massive ones, such as nuclear star clusters, the recent improvements of integral-field-unit (IFU) spectr oscopic surveys have revealed that their stellar dynamics are much more complex than previously thought. These new insights are expected to reflect the complex assembly and evolutionary processes that create them and are now pointing to the need for more realistic scenarios for their formation. In the first part of this thesis, we study the connection between the present-day dynamical structure and the formation history of merger remnant early-type galaxies, by using mock observations extracted from merger simulati ons. We find that major mergers can account for the peculiar kinematic features that are often observed in early-type galaxies, such as kinematically decoupled cores, or prolate (long-axis) rotation. We find a new channel fo r the formation of kinematically decoupled cores, that can help towards explaining their observed prevalence in IFU surveys. Additionally, we present the discovery of prolate rotation in nine early-type galaxies from the CAL IFA IFU Survey, adding a significant fraction to the observed cases of such galaxies that exist so far in the literature. We show that prolate rotators can be the end-products of major mergers. In the second part, we address the role of mergers in the formation of Nuclear Star Clusters (NSCs), using simulations of consecutive infalls of globular clusters into the center of a galaxy. We find that NSCs resulting from this process show both morphological and kinematic properties that match the Milky Way NSC very well, including significant stellar rotation -- a fact that has been attributed to gas infall so far. In general, our results demonstrate that mergers can account for a variety of observed kinematic features in both early-type galaxies and NSCs and place constraints on the role of mergers in their complex assembly history.

Supervisor: Glen van de Ven (MPIA)

Mauricio Ortiz Alvarez ( Chile )                                                                                                                                 14.12.2016

Planets around giant stars: Two close-in transiting planets and one S-type planet in an eccentric binary system (thesis pdf, 19 MB)

The study of planets around giant stars is an excellent way of probing the effects of stellar evolution on planetary systems. This is because as the star evolves from its main-sequence (MS) lifetime into the red giant branch (RGB) phase, it experiences important physical changes, like for example, significant mass loss, increase of the stellar radius and luminosity variability. These effects have the potential of influencing the whole architecture of planets that might have formed previously in the system. Additionally, as giant stars are generally more massive than MS stars, the effects of stellar mass on planet formation and migration theories can be tested. In this thesis, planets around evolved stars are studied from different perspectives. First, a comprehensive radial velocity (RV) follow-up of transiting planet candidates around evolved stars – observed by the Kepler telescope – was initiated to unveil a population of close-in planets (a &< 0.5 AU) that have been undetected by previous RV surveys. This effort led to the confirmation of the planetary nature of two close-in planets, Kepler-432 b and Kepler-91 b, providing evidence that short-period planets can exist around intermediate-mass evolved stars. These results suggest that gravitational interaction, with other planets or stars, is likely to be the preferred migration channel for close-in planets in intermediate-mass giant stars. Additionally, the findings in this thesis are in agreement with the view in which the scarcity of short-period planets around clump giants is explained by an increased planet-star tidal interaction during the RGB, subsequently leading to the tidal engulfment of giant planets. In the second part of this thesis, a very interesting binary system hosting a giant planet is analyzed. The planet, HD 59686 Ab, is orbiting the primary K-giant star at ~1 au, while the stellar companion is in a very eccentric orbit (e ~0.7) with a small semi-major axis of ~13.6 au. The existence of this planet in a tight eccentric binary system severely challenges the standard view in which gas giant planets form beyond the ice line of their hosts and then experience a migration phase to reach their current positions. The planet HD 59686 Ab presented in this thesis can become a benchmark object for the study of giant planets in close-separation binaries and might allow to shed light onto different planet formation theories in tight binaries.

Supervisor: Andreas Quirrenbach (LSW)

Reza Moetazedian ( Iran )                                                                                                                                                        16.11.2016

Impact of cosmological satellite galaxies on the dynamics of the Milky Way disc    (thesis pdf, 66 MB)

I present a high resolution study of the impact of realistic satellite galaxies, extracted from cosmological simulations of Milky Way-like haloes including 6 Aquarius and one Via Lactea II suites, on the dynamics of the Galactic disc. The initial conditions for the multi-component Milky Way galaxy were generated, to ensure an isolated system in a dynamical equilibrium state. After analysing the statistical properties of the subhaloes, candidates likely to impact the disc with the initial mass,tidal mass > 10^8 solar mass, were identified, inserted into our high resolution N-body simulations and evolved for 2 Gyr. I quantified the vertical heating due to such impacts by measuring the disc thickness and squared vertical velocity dispersion across the disc. According to our analysis, the strength of the heating is strongly dependent on the high mass end of the subhalo distribution from cosmological simulations. The mean increase of the vertical dispersion is approximately 20 km^2/s^2/Gyr for R > 4 kpc with a flat radial profile. Excluding Aq-F2 results with a very massive perturber with tidal mass = 6.10^10 solar mass, the mean heating is less than 12 km^^2/s^2/Gyr. These heating values correspond to 28% and 17% of the observed vertical heating rate in the solar neighbourhood, respectively. Taking into account the statistical dispersion around the mean, we miss the observed heating rate by more than 3 sigma. I observed a general flaring of the disc height in the case of all 7 simulations in the outer parts. The dynamics of the disc in the presence of satellites was also studied in the final two chapters where the selected runs were extended to 4 Gyr. The infall of satellites can be responsible for delaying/advancing the bar-formation, depending on the crossing epoch. The passage influences the bar-formation, if the passage occurs between the end of the shot noise regime T1 (0.75 Gyr) and before the exponential growth T2. The Aq-F2 run with the most massive candidate, induces localised vertical heating in the outer disc with R > 15 kpc. I also observed signatures of radial oscillatory behaviour in the outer disc plane, with features similar to that of the Monoceros ring. Disc warping is prominent after 3 Gyr of evolution, reaching median z coordinate of approximately 4 kpc. The asymmetric m = 1 mode is dominant for R > 15 kpc in the form of vertical motion, with the m = 2 mode influencing the rotation of disc particles.

Supervisor: Andreas Just (ARI)

Emer Brady ( Ireland )                                                                                                                                              10.11.2016

Radio Sources in the COSMOS Filed: Star Formation Properties of High Luminosity Radio Sources   (thesis pdf, 8 MB)

Radio wavelengths offer a dust-unobscured window to the early universe. The aim of this thesis has been to characterize the star-forming properties of the high redshift radio galaxy population. We present a preliminary catalogue of the first 610MHz map of the COSMOS feld and find that the sample properties are consistent with predictions for the low frequency radio population. Furthermore, we compare radio and IR star formation rate (SFR) tracers on a large sample of starburst galaxies and find that locally calibrated infrared (IR) and 1.4 GHz radio continuum SFR tracers produce inconsistent results for the most active starbursts. These starbursts are rare locally, but become increasingly common in the early universe. The high redshift starburst population may to be more diffuse, and hence cooler, than local counterparts. In addition, the infrared-radio correlation, observed thus far to be linear and constant for a wide range of galaxies, may exhibit redshift or luminosity evolution. We conclude that a dedicated effort to better model the IR and radio spectral energy distributions of starbursts will be essential to accurately characterize the SFR density at higher redhifts than those considered by studies to date.

Supervisor: Eva Schinnerer (MPIA)

Taisiya Kopytova ( Russia )                                                                                                                                       23.09.2016

Characterization of stellar and sub-stellar atmospheres   (thesis pdf, 1 MB)

In this work I aim characterize a number of (sub)-stellar companions. When studying isolated brown dwarfs, very low-mass stars, and directly imaged exoplanets with insignificant orbital motion, we have to rely on theoretical models to determine basic parameters such as mass, age, and consequently, effective temperature and surface gravity. While stellar and atmospheric models are rapidly evolving, we need a powerful tool to test and calibrate them. One way to test models is to compare theoretical isochrones produced by interior and atmospheric models with observed sequences in open clusters. Open clusters contain many objects of the same chemical composition and age, and spanning a range of masses. Using our own Lucky imaging observations and literature data, I constructed a single-star sequence for the Hyades open cluster. I compare the obtained sequence to a set of theoretical isochrones identifying systematic offsets and revealing probable issues in the models. However, there are many cases when it is impossible to test models before applying them to observations. One example is applying atmospheric models for constraining parameters of the coolest known Y dwarf WISE 0855-07. I demonstrate the limits of constraining effective temperature and the presence/absence of water clouds that are introduced by unknown systematic effects in models and observations. In the final chapter I introduce a novel method to take into account the above-mentioned systematics. I construct a ”systematics vector” that allows us to reveal problematic wavelength ranges when fitting atmospheric models to observed near-infrared spectra of brown dwarfs and directly imaged exoplanets. This approach plays a crucial role when retrieving abundances of brown dwarfs and exoplanets, in particularly, a C/O ratio. The latter parameter is an important key to formation scenarios of brown dwarf and exoplanets. I show the way of constraining this parameter while eliminating systematics effects, which significantly improves the reliability of a final result and our conclusions about formation history of certain exoplanets and brown dwarfs.

Supervisor: Wolfgang Brander (MPIA)

Alison Mitchell ( United Kingdom )                                                                                                                             28.07.2016

 Optical Efficiency Calibration for Inhomogeneous IACT Arrays and a Detailed Study of the Highly Extended Pulsar Wind Nebula HESS J1825-137   (thesis pdf, 12 MB)

Very High Energy (VHE) Gamma-ray astronomy using Imaging Atmospheric Cherenkov Telescopes (IACTs) is entering an era of hybrid arrays (such as H.E.S.S. II and CTA), comprising telescopes of varied specifications to enhance the accessible energy range and angular resolution. New algorithms for telescope optical efficiency calibration are developed and adapted for multi-type arrays, for both absolute light yield calibration using muons, and for relative calibration through comparison of shower images. In comparison to previous methods, the stability and flexibility of these algorithms are considerably improved. These will play an important role for the future CTA observatory. The use of multi-type IACT arrays in providing an increased understanding of astrophysical objects and environments is demonstrated through an in-depth study of the Pulsar Wind Nebula HESS J1825-137, known to be highly extended with the presence of strong energy dependent morphology. In particular, measurements of HESS J1825-137 across the entire available energy range allow the particle transport inside the nebula to be constrained, favouring advection over diffusion as the dominant mechanism. Together with X-ray data, a map of the magnetic field of the nebula can be made through spectral modelling. It will also be shown that HESS J1825-137, at 100 pc across, is one of the biggest pulsar wind nebulae to have been discovered yet.

Supervisor: Werner Hofmann (MPIK)

Paolo Bianchini ( Italy )                                                                                                                                               19.07.2016

The internal kinematics of globular clusters. An intimate view, from models to observations (thesis pdf, 75 MB !!)

Globular clusters (GCs) were long believed to be simple, non-rotating, isotropic and spherical stellar systems with all of their stars formed approximately 13 Gyr ago. However, their origin in the early epochs of galaxy formation is still debated. Growing evidence is now showing a larger degree of complexity in their structure, morphology, stellar populations and internal dynamics. The goal of this Thesis is to unveil the com- plexity of their current dynamical properties and to connect it to their formation and subsequent dynamical evolution. As a first step, I show that the study of the morphology alone is not enough in order to disentangle the formation of GCs, in particular to distinguish clusters that formed in-situ from clusters that were accreted. This motivated the detailed exploration of their internal kinematics, that provides a long lasting “fossil record” of the dynamical processes that a GC has experience during its long-term evolution. Using a combination of state-of-the-art kinematic observations and dynamical modeling, I carry out the study of the kinematic effects connected to the presence of intermediate-mass black holes, binary stars and the onset of energy equipartition. In particular, I focus on the understanding of the systematics and biases present in the integrated-light kinematic observations and Hubble Space Telescope proper motion samples, making use of mock observations constructed directly from the dynamical models. This strategy gives the direct advantage of achieving a sound interpretation of the observations and of the physical processes described by the models. Having reached a deeper understanding of the data, I set up a first step to trace the evolution of GCs, based on their current kinematics. I show that the degree of energy equipartition attained by a GC can be connected to its dynamical state and therefore used as an indicator of its formation or peculiar dynamical evolution. My work indicates that the synergy between models, observations and the study of the internal kinematics of GCs is the key to unveil their dynamical state. This will be the starting point for exploiting at full power the comprehensive amount of data that will be delivered by the Gaia mission and in the approaching era of Extremely Large Telescopes.

Supervisor: Glenn van de Ven (MPIA)

Kevin Schaal ( Germany )                                                                                                                                                   08.07.2016

Shocks in the Illustris Universe and Discontinuous Galerkin Hydrodynamics   (thesis pdf, 33 MB)

The study of highly-nonlinear astrophysical systems and processes increasingly relies on simulation techniques. This thesis aims to develop new analysis methods for cosmological simulations and advance their accuracy by introducing novel numerical methods. We present the implementation of a shock finding algorithm for the moving-mesh code AREPO, and use it to an analyse shocks in Illustris, a cosmological state- of-the-art simulation of galaxy formation. We identify a large diversity of shock morphologies, including accretion shocks, merger shocks, as well as feedback shocks due to galactic winds and black holes; the latter produce the strongest shocks in the simulation. At late times, we measure a specific energy dissipation rate at shocks of 10^−1 erg g^−1 s^−1, fairly independent of the cosmological environment. In another project we formulate and implement a higher-order discontinuous Galerkin (DG) scheme on a structured mesh, which can be adaptively refined, for solving the equations of ideal hydrodynamics. We find that DG produces significantly less diffusion and smaller advection errors compared to a traditional finite volume scheme, and is superior in evolving rotating objects. These results demonstrate that DG offers a high potential for astrophysical applications.

Supervisor: Volker Springel (HITS)

Svea Proft ( Germany )                                                                                                                                                      29.06.2016

On the Exploration of Quasars and the Astrometric Microlensing Effect with the Gaia Satellite   (thesis pdf, 10 MB)

For two years, the Gaia satellite has been scanning the sky to obtain an unmatched positional accuracy of stars and a variety of data. In this thesis, we investigate two different kinds of questions with respect to Gaia. In our first research focus we explore Gaia data that is suitable to learn more about the structure of quasars. For this, we simulate low- and high-resolution quasar spectra as they would be generated by Gaia’s blue and red photometers and the radial velocity spectrograph (RVS). Furthermore, we investigate bright SDSS quasars with broad emission lines (BELs) redshifted into the wavelength range of Gaia’s RVS. We find out that quasar low-resolution spectra enable the study of equivalent widths, continuum variability, and the so called Baldwin effect. For about seventy bright quasars, Gaia will obtain high-resolution spectra from which BEL variability can be studied using the line profile. From our analysis we conclude that Gaia provides a large amount of data which we can use for a better understanding of variability in quasar spectra. In our second research focus, we improve the prediction of astrometric microlensing events with Gaia data. When we predict such events precisely, we could plan to observe the corresponding photometric amplification simultaneously. To improve our published search for microlensing events (Proft et al. 2011), we increase the search radius around a potential lensing star and check once more the proper motion of the lenses. Due to the revision of our program, we find fifty new potential m icrolensing events. Six have estimated positional changes that could lead to a detectable Gaia signal. Currently, the errors of our calculated quantities are too large to follow up the events in real time. We will monitor whether our microlensing predictions are correct and with future Gaia data we can make more precise predictions in order to develop observational campaigns.

Supervisor: Joachim Wambsganss (ARI)

Simone Giacche ( Italy )                                                                                                                                                      22.06.2016

Electron and positron acceleration at pulsar wind termination shocks    (thesis pdf, 4 MB)

Many astrophysical sources are very efficient particle accelerators. Among these, Pulsar Wind Nebulae (PWNe) are excellent laboratories to study the acceleration mechanism because of their proximity and brightness. In these objects, the rotational energy extracted from the central pulsar is converted in a striped-wind consisting of relativistic electrons and positrons and large-amplitude electromagnetic fields. When the wind impacts with the surrounding medium, a relativistic shock forms where the ordered flow of the pulsar becomes randomised and energetic particles emit Synchrotron and Inverse Compton radiation. We study the conversion of the pulsar wind in a strong superluminal wave. The conversion is triggered by the interaction of the wind with the shock when this is sufficiently far away from the pulsar. The shock structure is modified by the superluminal wave and the properties of the modified shock (precursor) depend on the Lorentz factor and magnetisation of the incoming wind. A relatively large fraction of electrons and positrons in the wind is energised and reflected by the turbulent electromagnetic fields in the precursor and is available for further acceleration. We suggest that the onset of the precursor is a solution to the problem of injection in the acceleration mechanism. We investigate the acceleration of the injected particles and we find that it proceeds in two distinct regimes, determined by the relative magnitude of the wavelength of the wind stripes and of the gyro-radius of energetic particles. We discuss the implications of this acceleration scenario in the context of PSR B1259-63, a pulsar powering a PWNe in a binary system.

Supervisor: John Kirk (MPIK)

Miguel Querejeta   ( Spain )                                                                                                                                               25.05.2016

Making galaxies passive: Insights from resolved studies of nearby galaxies (thesis pdf, 24 MB)

The rapid suppression of star formation, or quenching, is thought to be an important process in the evolution of the most massive galaxies, but the mechanisms involved are still hotly debated. Here, we consider two agents that control star formation and can ultimately lead to its suppression: AGN feedback and galaxy mergers.
In the first part of the thesis, we study the interplay between stellar structure, nuclear activity, and molecular gas in the context of AGN feeding and feedback. We start presenting our catalogue of stellar mass maps for more than 1500 nearby galaxies, which has been publicly released. Using the stellar mass map of the spiral galaxy M51, we show that there is sufficient molecular gas inflow to feed the AGN (~1 Msun/yr), and feedback effects which include a nuclear molecular outflow (at a rate comparable to the inflow), as well as a large-scale radio jet which pushes molecular gas laterally, inducing shocks and turbulence.
In the second part, we address the role of galaxy mergers in the buildup of a passive population of lenticular galaxies (or S0s). While lenticulars are the most common early-type galaxies in the Universe, their formation channels remain elusive. We use numerical simulations to show that even major mergers of spiral galaxies can result in lenticulars, with a bulge-disc coupling in agreement with observations. We also find that major mergers can simultaneously account for the difference in angular momentum and concentration between spirals and S0s, as recently found by the CALIFA team.
Globally, our results show that both internal processes (transport of gas and AGN feedback) and external mechanisms (mergers) have the ability to regulate and eventually suppress star formation in galaxies. Current and future instrumental advancements (ALMA, NOEMA, JWST) will permit to confirm our findings with other galaxies and further assess their relative importance.

Supervisor: Eva Schinnerer (MPIA)

Cristina Garcia Vergara   ( Chile )                                                                                                                                     25.05.2016

Quasar Environments at z ~ 4     (thesis pdf, 35 MB)

In the standard picture of structure formation, the first massive galaxies form in the highest peaks of the density field, which are the cores of massive proto-clusters. Such structures must be exceedingly rare, and thus hard to find. Luminous quasars (QSOs) at z ~ 4 are the most strongly clustered population in the Universe and this large auto-correlation demands that they reside in massive dark matter halos, associated with large overdensities of galaxies. This imply a strong QSO-galaxy cross-correlation for luminous QSOs at z ~ 4. 
In order to put the first observational constrain on the QSO-galaxy clustering properties, I present the measurement of the cross-correlation function between QSOs and both Lyman break galaxies (LBGs) and Lyman alpha emitters (LAEs) at z ~ 4.  I present VLT/FORS1 observations of 6 luminous QSOs fields. Using a novel narrow band filter technique I select LBGs associated with each quasar in a narrow redshift range of Δz ~ 0.2. I measure the QSO-LBG cross correlation function on scales of 0.1 < R < 10 h-1 Mpc (comoving), which is well fitted by a power law form with a correlation length r0 = 9.91(+3.28,-1.79) h-1 Mpc and a slope gamma = 2.05(+0.20,-0.46). This is in agreement with the theoretical expected clustering computed from the individual QSO and LBGs auto-correlation, assuming a linear bias model. I also measure the auto-correlation of LBGs in the QSO fields, which shows a stronger clustering in comparison with LBGs in random fields. The new technique used for the detection of LBGs is efficient in selecting them in a thin redshift slice, but this shows important shortcomings whereby this result should be carefully considered.
Additionally, I present VLT/FORS2 observations of 14 luminous QSO fields, designed to search for LAEs in their environments. I find that QSOs and LAEs are not correlated at z ~ 4, and the mean LAEs number density in our fields is consistent with the expected number density in random fields. This could mean either that the QSO auto-correlation length at z ~ 4 is overestimated, or that LAEs preferentially a void QSO environments on ≲10 Mpc h-1 scales.

Supervisors: Felipe Barrientos (PUC), Joe Hennawi (MPIA)

Maria Woellert ( Germany )                                                                                                                                                    12.05.2016

Stellar companions to exoplanet host stars: A Lucky Imaging survey of transiting exoplanet systems  (thesis pdf, 4 MB)

About half of all nearby stars are part of a binary or multiple star system, yet little is known about how multiplicity affects the formation and subsequent evolution of planetary systems. For instance, stellar companions may promote the inward migration of massive planets into very short orbits or cause misalignment between the planetary orbit and the stellar spin axis depending on the properties of the system. Moreover, the ignorance about the presence of depending on the properties of the system. Moreover, the ignorance about the presence of a close stellar companion to a transiting exoplanet host star may introduce biases in the derivation of planetary parameters.
In this thesis, I present the results of our Lucky Imaging search for stellar companions around more than 100 transiting exoplanet host stars. Besides characterizing the companion candidates, I calculate the probability that the companion candidates are gravitationally bound using two different statistic approaches, and I look for a correlation between binarity and hot Jupiters with aligned/misaligned orbits. I calculate the effect of unresolved stellar companions on the deduced planetary parameters, in particular the correction for the radii for two planets. Additionally, I present photometric and spectroscopic observations of the young binary HD 102077 for which I determine the orbital parameters, the spectral type of the two compo- nents, the space motion, age, and moving group membership.

Supervisor: Wolfgang Brandner (MPIA)

Gabriele Cologna ( Italy )                                                                                                                                                        11.05.2016

TeV observations of hard spectrum active galactic nuclei with HESS-I and HESS-II (thesis pdf, 20 MB)

In my PhD thesis, I present the results of the analysis of three point-like gamma-ray emitters observed at very high energies with the High Energy Stereoscopic System (H.E.S.S.): the two well known blazars 1ES 0229+200 and Mrk 501, and HESS J1745-290. The nature of the latter source is unclear, although it is likely linked to Sgr A∗, the black hole at the center of the Galaxy. I make use of multiwavelength observations in order to assess the physical properties of the three sources and of the ambient medium, both via spectral and variability studies and by comparison with models and predictions found in the literature. I also present systematic comparisons of the gamma-ray energies as reconstructed for different sub-arrays of the H.E.S.S. system.
I search for correlated variability in simultaneous x-ray and very high energy observations of Sgr A∗ in order to understand whether or not the gamma-ray emission of HESS J1745-290 originates from the black hole, making it an active galactic nucleus. The correlation of the flux variability in different energy bands for 1ES 0229+200 supports the interpretation of a synchrotron self Compton emission model for this source. The refined spectrum of the inverse Compton peak helps constraining the strength of the intergalactic magnetic field. The hard intrinsic spectrum of Mrk 501 shows a dependency on the flux state. The spectrum can be used to constrain the extragalactic background light intensity in the mid-infrared for the redshift of the source. Multiwavelength observations suggest the presence of at least two emission zones or mechanisms in order to explain the broadband emission.

Supervisor: Stefan Wagner (LSW)

Ilya Khrykin   ( Russia )                                                                                                                                                             20.04.2016

Understanding the Sources of He II Reionization   (thesis pdf, 60 MB)

The lifetime of quasars is a fundamental parameter for understanding the physical processes governing the growth of supermassive black holes, their co-evolution with galaxies, and the reionization history of the intergalactic helium. However, despite cosmological importance, current estimates of quasar lifetime are uncertain by at least two orders of magnitude, preventing significant progress in our understanding of the above questions. This thesis presents theoretical progress toward constraining quasar lifetimes and properties of helium reionization. A combination of cosmological hydrodynamical simulations and 1D radiative transfer algorithm is used to investigate the structure and evolution of the He II Lya proximity zones around quasars at redshift z = 3-4 and draw the following conclusions.
In the first part of this thesis I show that the time evolution of the proximity zones can be described by a simple analytical model for the approach of the fraction of singly ionized helium (He II fraction) to ionization equilibrium, and use this picture to illustrate how the transmission profile depends on the quasar lifetime, quasar luminosity, and the the average He II fraction (or equivalently the metagalactic He II ionizing background). Due to density fluctuations I advocate stacking existing He II quasar spectra at z ∼ 3, and show that the shape of this average proximity zone pro- file is sensitive to lifetimes as long as ~ 30 Myr. I find that at higher redshift z ~ 4 where the He II fraction is poorly constrained, degeneracies will make it challenging to determine these parameters independently.
In the second part of this thesis I describe the method to constrain the quasar lifetime and the He II fraction (redshifts of the He II reionization) at z ~ 4 using the thermal state of the intergalactic medium around quasars. I show how the temperature of intergalactic gas increases due to quasar activity, producing thermal proximity effect in the Lyα absorption spectra. I investigate how the amplitude and the extent of thermal proximity effect depend on the amount of singly ionized helium in the IGM prior to quasar activity and on quasar lifetime by measuring the power spectrum of the H I Lya absorption spectra. I propose to use Markov Chain Monte Carlo algorithm to estimate the accuracy of the power spectrum measurement, and illustrate how with the mock sample of 50 high-resolution quasar spectra the He II fraction and quasar lifetime can be estimated with ~ 5 - 10% and 2 - 5% precision, respectively.

Supervisor: Joe Hennawi (MPIA)

Simon Bihr   ( Germany )                                                                                                                                                             20.04.2016

Molecular Cloud Formation out of the Atomic Phase    (thesis pdf, 60 MB)

This thesis is dedicated to study observationally the formation of molecular cloud formation out of the atomic phase. In the framework of 'The HI, OH, Recombination Line survey of the Milky Way' (THOR), we used the Very Large Array (VLA) to map the 21 cm HI line, 4 OH transitions, up to 19 H alpha recombination lines and the continuum from 1 to 2 GHz of a significant fraction of the Milky Way. The goal of this thesis is to characterize and study the atomic and molecular content of molecular clouds. Observations of the giant molecular cloud complex associated with the W43 star formation complex revealed large quantities of optically thick HI emission. We used strong continuum sources to measure the optical depth and used this information to correct the HI column density and hence the HI mass. Without these corrections, the mass estimate is at least a factor of 2.4 too low. In addition, we observed HI column densities up to N(HI) ~ 150 M_sun/pc^2, which is in contradiction to current cloud formation models. Furthermore, we present a catalog of ~4400 continuum sources, extracted from the first half of the THOR survey. Due to the broad bandwidth from 1 to 2 GHz, we are able to determine a reliable spectral index for ~1800 bright sources, which allows us to distinguish between thermal and non-thermal radiation. Using this information, we can confirm four super nova remnant candidates. Beside the direct scientific studies of these evolved sources, this catalog is the basis for prospective absorption studies of the HI and OH lines. By extracting HI self absorption (HISA) features of the molecular filament GMF38.1-32.4, we examined directly the cold and dense atomic hydrogen and compared it to the molecular counterpart. We studied the kinematics and column density probability distribution functions (PDFs) and found a log-normal shape for them, which indicates turbulent motion. Furthermore, we might observe several evolutionary stages within the filament, which will be helpful to validate theoretical models and simulations.

Supervisor: Henrik Beuther (MPIA)

Erik Bertram   ( Germany )                                                                                                                                                          03.02.2016

The Role of Turbulence in the Process of Star Formation   (thesis pdf, 44 MB)

The aim of this thesis is to study the role of interstellar turbulence in the process of star formation. We demonstrate that supersonic turbulent motions significantly affect various properties of the interstellar medium (ISM). Therefore, we run numerical simulations of molecular clouds in different environments. In particular, we study typical clouds located in the Milky Way disk as well as clouds which can be found in more extreme regions in our Galaxy, e.g. in the Central Molecular Zone (CMZ) near the Galactic Center. In addition, we perform radiative transfer calculations of numerous diagnostic fine structure lines and compare our results with observational measurements. Furthermore, we analyze the influence of the turbulence on different observables, e.g. on the structure functions, the ∆-variance, the power spectra as well as the star formation efficiencies. We also study the impact of turbulent motions on the chemistry and the different phases of the ISM. Our studies about Milky Way disk clouds show that the statistical properties of the turbulence are significantly influenced by the individual gas tracers. Moreover, our investigations about CMZ-like clouds show that high levels of turbulence can significantly suppress, but never inhibit star formation, owing to local compression of gas by turbulent shocks. Finally, we show that various atomic tracers accurately reflect most of the physical properties of both the H2 and the total gas of the cloud and that they provide a very useful alternative to common molecular lines when we study the ISM in the CMZ.

Supervisors:  Ralf Klessen, Simon Glover  (ITA)

Mykola Malygin   ( Russia )                                                                                                                                                         03.02.2016

Gas Opacity in Planet and Star Formation   (thesis pdf, 44 MB)

Opacity plays a key role in determining the structure of the heterogeneous CSM and its observable spectrum. Planet and star formation changes dust density on various spatial and time scales through gravity and turbulence, radiation field and chemistry. The following thesis presents calculations of gas opacity in the suitable parameter space, a methodology of utilising it in hybrid-type radiative transfer schemes, as well as first results on radiative transfer and thermal relaxation calculations.
The largest set of dust-free opacity in the CSM regime is presented. Improved frequency sampling yielded substantially larger (up to a factor of $10^4$ in comparison to some previous works) Planck means. The two-temperature Planck means important for determining the temperatures in the optically thin irradiated gases were tabulated. The equilibrium temperature in such media is degenerate and depends on the thermal history of the gas.
The suggested recipe to estimate the thermal relaxation time accounts for collisional coupling, which limits the relaxation in low-density regions. A fundamental lower limit to the relaxation time is set by the LTE emissivity giving $\Omega t\sim10^{-4}$ at the T Tau phase. The locations of onset of linear and the locations of operation of subcritical hydrodynamic instabilities, reckoned as turbulence drivers in magnetically inactive zones, are constrained by the relaxation criterion.

Supervisors: Hubert Klahr (MPIA)

Helen Poon   ( China )                                                                                                                                                                  03.02.2016

H.E.S.S. Observations of Sgr A* (thesis pdf, 12 MB)

The Galactic Centre has been studied with the High Energy Stereoscopic System (H.E.S.S.) for over 10 years, showing a bright, complex gamma-ray morphology. Other than a strong point-like very-high-energy gamma-ray source coincident with the supermassive black hole Sgr A*, previous analyses also revealed a diffuse ridge of gamma-ray emission, indicative of a powerful cosmic-ray accelerator in this region. It has been widely studied in the multiwavelength, exhibiting flares from radio to X-ray, but in the VHE regime it has always been stable. A gas cloud called G2 was found travelling straight to the Galactic Centre in 2011. Accretion rate was expected to increase which might possibly lead to flares in different wavebands. The addition of a fifth telescope with 600 m2 mirror area to the centre of the H.E.S.S. array has increased the energy range accessible, allowing observations to take place down to 100 GeV and potentially below. The sensitivity is also increased which means it has better ability to detect VHE flares from the Galactic Centre. In this work, long-term study of Sgr A* with H.E.S.S. I and data involving H.E.S.S. II around pericentre passage time of G2 is presented. Varibility tests are also applied to check for transient events. The overall spectrum and spectra for individual years have been stable over the years. The results of variability tests also revealed stability in the flux.

Supervisors: Werner Hofmann (MPIK)

Kai-Martin Dittkrist   ( Germany )                                                                                                                                            02.02.2016

Aspects of planet formation - a model of migration and SADFACE: a 1-dimensional vertically integrated disk model for planet population synthesis calculations (thesis pdf, 15 MB)

We discuss two aspects of planet formation in this thesis. At first we present a model of type I migration which includes new results of outward migration of low mass planets. The second part contains SADFACE, a 1-dimensional vertically integrated disk model for planet population synthesis calculations. Results of the first part is that with the inclusion of thus modes of outward migration no artificial fudge factor to reduce the speed of type I migration is needed. We find two to three times as many ”cold” planets when we include outward migration than without. Results of the second part is that in the scope of the model we can find initial conditions that fit observed disks quite well. We also present likelihood of initial conditions which can be further used for planet population synthesis calculations. We also find that rotation and temperature of a cloud are correlated and can not be independently chosen.

Supervisors: Hubert Klahr (MPIA)

Gabriel-Dominique Marleau   ( Canada) 01.02.2016

The post-formation entropy of gas giants: Radiative properties of the accretion shock and constraints from observations   (thesis pdf, 5 MB)

We study the radiative properties and in particular the radiation loss efficiency of the planetary accretion shock by performing radiation-hydrodynamics simulations of accretion onto a planet at snapshots during its formation. We use the state-of-the-art code PLUTO in spherical symmetry and both one- and two-temperature flux limited diffusion. We take tabulated gas and dust opacities and use a constant equation of state to isolate radiation transport effects.
We find that, for a significant subset of the formation parameter space, the schock is isothermal and essentially the entire kinetic energy is converted to radiation. The fraction brought into the planet is negligible compared to the internal luminosity, which appears to favour the so-called ‘cold-start’ assumption.
We also study what constraints direct detections can provide on the post-formation entropy. First, we attempt to produce accurate cooling tracks by using self-consistently the sophisticated BT-Settl-2010 atmospheric models. However, we find that this is not possible because of due to dust phase transitions at Teff ≈ 1500 K. Secondly, we set lower bounds on the post-formation entropy of some well-studied gas giants. We find that they most likely did not form as classical coldest starts, but that κ Andromeda b may be undergoing a ‘deuterium flash’.

Supervisors: Hubert Klahr (MPIA)

Shiwei Wu   ( China )                                                                                                                                                                   01.02.2016

A Spectroscopic Survey of Young Massive Star-Forming Regions (thesis pdf, 44 MB)

Massive stars form on different scales, ranging from large, dispersed OB associations to compact, dense starburst clusters. The question whether one single star formation mechanism is responsible for this wide range of properties is not answered yet. The complex structure of regions of massive star formation, and the involved short timescales provide a challenge for our understanding of their birth and early evolution. In this thesis, I study the formation process of the massive stellar content in two of the most massive and luminous star forming regions in our Galaxy: W49 and W51. I analyse near-infrared (NIR) observations obtained with state-of-the-art ground based telescopes. NIR spectroscopic observations provide reliable classification on the nature of massive stars still heavily embedded in Giant Molecular Clouds (GMCs). With the derived physical properties of the massive stars, we aim to investigate and determine the star formation history of the star forming regions. A very massive star (VMS) (M > 100 M⊙) is discovered in the central cluster of W49. It is classified as an O2-3.5If* star based on its K-band spectrum. By comparing with Geneva stellar evolutionary models, the initial mass of this star is estimated as between 100 M⊙ and 180 M⊙. With the complete spectroscopic observations of W49, thirteen O type stars a s well as two Young Stellar Objects (YSOs) are identified. The age of the cluster is estimated as ~1.5 Myr, with star formation still ongoing in different parts of the region. The stellar content of W51 is also studied in this thesis. Evidence has been found that the star formation in W51 has started ~5 Myr ago and still active until now. The distinct environments and properties of sub-clusters are discussed. Despite the fact that the W51 and W49 GMCs have similar mass, we find very different massive stellar population and star formation history. W51 does not contain any VMS, while W49 has 3-4 stars more massive than 100 M⊙. This might be related to differences in the star formation process between these two regions.

Supervisors: Arjan Bik, Thomas Henning (MPIA)

 
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