Tanya Edwards (16.1.) - Peter Zeidler (18.1.) - Richard Teague (20.1.) - Jorge Abreu Vicente (25.1.) - Nina Hernitschek (26.1.) - Emanuela Giannini (8.2.) - Jakob Herpich (8.5.) - Qian Qian (10.5.) - Richard Hanson (19.5.) - Kirsten Schnuelle (27.7.)
Kirsten Schnuelle ( Germany ) 27.07.2017
Studying the Radiative Response of Circumnuclear Dust of AGNs (thesis pdf, 10 MB)
Radius measurements of dust tori around the central engine of luminous active galaxies open up the possibility of probing cosmological models out to redshifts beyond where supernovae can be used. Yet, the value of dust tori as standard candles is constrained by the substantial intrinsic scatter in the size-luminosity relation found for samples of AGNs. Indicated by single objects, a probable cause of this scatter is a non-trivial variation in the dust location with the luminosity of the central engine, due to sublimation events and subsequent long-term reformation of the hot dust surface radiating at near-infrared wavelengths. In this work, I developed a refined dust reverberation model allowing to measure torus sizes and additional observables characterizing the temperature state, stability, and distribution of the innermost, hot dust in AGNs. Optical to near-infrared photometric data were observed for 24 type 1 AGNs. Of these, I have analyzed the Seyfert 1 galaxies NGC 4151, Ark 120, NGC 5548, and NGC 3227. The derived inner torus radii fit very well into the established size-luminosity relation. While three objects are well-described by a standard model without sublimation, the data of one object could be fit satisfactorily only after allowing for dust sublimation events in the model.
Supervisor: Joerg-Uwe Pott (MPIA)
Qian Qian ( China ) 10.05.2017
Accretion and ejection in resistive GR-MHD (thesis pdf, 10 MB)
In this thesis, the accretion and ejection processes from a black hole accretion system is investigated by means of resistive general relativistic magnetohydrodynamic simulations. As a supplement to the results from prior research with non-relativistic simulations, my results confirm that the winds and outflows originated from thin accretion disks can also be observed in general relativistic simulations. In the first part, the execution of the implementation of resistivity, namely magnetic diffusivity, into the existing non-resistive general relativistic magnetohydrodynamic code HARM is illustrated. The test simulations of the new code rHARM include the comparison with analytical solution of the diffusion equation and a classic shock tube test. rHARM shows reliable performances in these tests. In the second part, rHARM is applied to investigate the evolution of magnetized tori. The results show that the existence of resistivity leads to inefficient accretion of matter from tori onto black holes by weakening the magnetorotational instability inside the tori. An indication for a critical magnetic diffusivity in this simulation setup is found beyond which no magnetorotational instability develops in the linear regime. In the third part, as the main purpose of this PhD project, rHARM is used to perform simulations of magnetically diffusive thin accretion disks that are threaded by a large-scale poloidal magnetic field around non-rotating and rotating black holes. These long-term simulations last 3000 code time units, which are about 195 rotation periods at the disk inner boundary, correspondingly. Their computational domains extend from black hole horizon to 80 Schwarzschild radii. Outflows driven from the accretion disk are clearly seen. These outflows have the typical radial velocity of 0.1 speed of light. In my analyses, I argue that these outflows are driven by the magnetic pressure gradient from the toroidal magnetic field generated by the rotation of the disk. The small ratios of the poloidal field strengths to the toroidal field strengths suggest the interpretation of the outflows as “tower jet,” rather than centrifugally driven winds (Blandford-Payne effect). Furthermore, I find direct evidence of the growths of magnetorotational instabilities inside the accretion disks, which are suppressed by the increasing levels of magnetic diffusivity. This suppression leads to inefficient accretion and ejection processes of the accretion system. Finally, the influence of rotating black holes on the accretion systems are explored. The results show an suppression effect on the black hole spin on the accretion and ejection processes in the system. The tangled field lines within the ergosphere induced by the black hole rotation produce magnetic pressure that pushes against the accreting matter from the disk. In the simulations with large spin parameters, energy extraction from the black hole (Blandford-Znajek effect) is observed, which is, nevertheless, ∼10^2 times smaller than the energy production from the disk outflow.
Supervisor: Christian Fendt (MPIA)
Jakob Herpich ( Germany ) 08.05.2017
On the Physical Origin of Radial Surface Density Profiles in Disk Galaxies (thesis pdf, 2 MB)
Observations have long established that the radial stellar surface density profiles in disk galaxies are nearly exponential (Type-I profiles). Stellar disks in numerical simulations also tend to approach an exponential profile. Deep imaging has revealed systematic deviations in the profile at large galactocentric radii. Beyond a break the profile may continue with a steeper (Type-II) or shallower (Type-III) exponential profile. In this thesis, I present numerical and analytical models that aim towards a physical understanding of how such profiles come about. I carried out numerical simulations designed to give extensive control over the physical conditions of disk galaxy formation. On this basis, I argue that the type of profile correlates with the initial spin of a galaxy’s host dark matter halo: Type-II/III disks are hosted by high-/low-spin halos. Type-I disks occur at intermediate spins. The formation mechanism for the Type-II disks is consistent with previous results in the literature. Through a very detailed analysis of the low-spin simulations I show that the formation of Type-III profiles can be linked to the formation of a strong bar in low-spin halos. Observational predictions are provided to test the presented hypotheses. The evolution of the radial disk structure can be interpreted as shuffling of the individual stars’ angular momenta. Maximizing a suitably defined entropy in stellar angular momentum space yields an analytic prediction for the radial surface density profiles, given any galactic rotation curve and the corresponding stellar mass and angular momentum of the disk. I carefully compare this result with observational data and simulated disks. It gives a fair match to observations and is in very good agreement with those simulations that provide the closest match to the model assumption of perfectly circular stellar orbits.
Supervisor: Hans-Walter Rix (MPIA)
Emanuela Giannini ( Italy ) 08.02.2017
MiNDSTEp differential photometry of the gravitationally lensed quasars WFI2033-4723, HE0047-1756 and Q2237+0305 (thesis pdf, 3 MB)
This work focusses on studying the brightness variation of gravitationally lensed multiply imaged quasars. The main goal is the optimization of the relative differential photometry procedures, which are based on the difference image analysis (DIA) method. Moreover, it aims at isolating uncorrelated flux variations among the quasar images, which can be explained as due to quasar microlensing events, and at the estimation of the time delays of the observed systems from the retrieved light curves. We present V and R photometry of the gravitationally lensed quasars WFI 2033-4723, HE 0047-1756 and Q2237+0305. The analyzed data belong to the MiNDSTEp collaboration and were taken with the 1.54m Danish telescope at ESO/La Silla from 2008 to 2012. The differential photometry is based on the already published method by Alard and Lupton as implemented in the HOTPAnTS package, and additionally uses the GALFIT package for obtaining the quasar photometry. The quasar WFI 2033-4723 shows brightness variations of ~~0.5 mag in V and R during the campaign. The two lensed components of quasar HE 0047-1756 vary by ~ 0.2 to 0.3 mag within five years. We provide for the first time an estimate of the time delay of component B with respect to A of Δt = (7.6+-1.8) days for this object. We also find evidence for a secular evolution of the magnitude difference between components A and B in both filters, which we explain as due to a long-duration microlensing event. We also find that both quasars WFI 2033-4723 and HE 0047-1756 become bluer when brighter, which is consistent with previous studies. The quasar Q2237+0305 shows impressive uncorrelated variations of the four components in both the V and R bands, with brightness variations between ~0.2 and ~1.3 mag. In particular, component D shows flux variations of ~ 1.3 mag in the V band and ~0.8 mag in the R band during the 5-year monitoring campaign, along a caustic-crossing feature of the light curve. We also find that the color of this component becomes redder by ~0.6 mag while it becomes fainter. Image C becomes brighter by ~0.7 mag between the last two monitoring seasons and this again suggests a high-magnification microlensing event.
Supervisor: Joachim Wambsganss (ARI)
Nina Hernitschek ( Germany ) 26.01.2017
Astrophysical Modeling of Time-Domain Surveys (thesis pdf, 30 MB)
The goal of this work is to develop and apply algorithmic approaches for astrophysical modeling of time- domain surveys. Such approaches are necessary to exploit ongoing and future all-sky time-domain surveys. I focus on quantifying and characterizing source variability based on sparsely and irregularly sampled, non-simultaneous multi-band light curves, with an application to the Pan-STARRS1 (PS1) 3 pi survey: variability amplitudes and timescales are estimated via light curve structure functions. Using PS1 3 pi data on the SDSS "Stripe 82" area whose classification is available, a supervised machine-learning classifier is trained to identify QSOs and RR Lyrae based on their variability and mean colors. This leads to quite complete and pure variability-selected samples of QSO and RR Lyrae (away from the Galactic disk), that are unmatched in their combination of area, depth and fidelity. The sample entails: 4.8 x 10^4 likely RR Lyrae in the Galactic halo, and 3.7 x 10^6 likely QSO. The resulting map of RR Lyrae candidates across 3/4 of the sky reveals targets to 130 kpc, with distances precise to 3%. In particular, the sample leads to an unprecedented map of distance and width of Sagittarius stream, as traced by RR Lyrae. Furthermore, the role of PS1 3 pi as pilot survey for the upcoming LSST survey is discussed.
Supervisor: Hans-Walter Rix (MPIA)
Jorge Abreu Vicente ( Spain ) 25.01.2017
Molecular Cloud Structure at Galactic Scales (thesis pdf, 60 MB)
Molecular clouds are the sites were stars are born and they play a crucial role in galactic evolution. Despite their main role on star formation and galaxy evolution, physics of molecular clouds are still poorly understood. Particularly, the processes controlling the formation, structure, and evolution of molecular clouds are still a matter of debate and so are the processes that regulate their star–forming activity. Previous to the beginning of this thesis, observational studies of molecular cloud structure and accurate measurements of star–forming activity in molecular clouds existed only for the Solar neighborhood, proving a very limited range of Galactic environments. Extending these studies to larger distances is crucial. This thesis is dedicated to provide the observational assets needed to obtain a Galactic picture of the processes involved in the molecular cloud structure and star– formation. We present the first systematic study of molecular cloud structure and evolution including molecular clouds in nearby spiral arms. We present a census of filamentary–shaped molecular clouds that are thought to be connected to the spiral Galactic structure. Finally, we also develop a new technique that improves the quality of the existing observational data to obtain more accurate observational assets, crucial in the study of molecular cloud structure.
Supervisor: Thomas Henning (MPIA)
Richard Teague ( UK ) 20.01.2017
Tracing the Earliest Stages of Planet Formation through Modelling and Sub-mm Observations (thesis pdf, 20 MB)
This thesis explores the utility of molecular line emission as a tool to unravel the physical structures and processes involved in planet formation.
Observations of molecular ions, HCO+ and DCO+, in the disk of DM Tauri allow for a study of the ionization structure. These constraints are an essential ingredient in modelling the physical and chemical evolution of a disk which directly impact the efficiency of planet formation.
We also present the first spatially resolved direct measurement of turbulence in a protoplanetary disk using CO, CN and CS molecular line emission. Such a measurement is vital in identifying the physical mechanisms driving turbulence. In addition, we perform a thorough analysis of all uncertainties involved when determining turbulent velocities in disks and demonstrate that all measurements of turbulence will be ultimately limited by the precision to which the gas temperature can be derived.
Finally, the CS emission profile from TW Hydrae displays a dip-like feature coincident with features observed in scattered light observations of the disk, suggesting a common origin. extensive modelling demonstrates that this may be the first detection of a surface density perturbation through molecular line emission, potentially the manifestation of an embedded protoplanet or a strong magneto-rotational instability.
Supervisor: Thomas Henning (MPIA)
Peter Zeidler ( Germany ) 18.01.2017
Revealing the secrets of Westerlund 2 - A young massive star cluster observed with the Hubble Space Telescope (thesis pdf, 42 MB)
We present a detailed study of the Galactic young massive star cluster Westerlund 2 (Wd2) using an optical and near-infrared high resolution multi-band survey observed with the Hubble Space Telescope. Images obtained in Hα and Paβ filters allowed us to derive a high-resolution pixel-to-pixel E(B − V)_g gas extinction map. This map helped us to individually deredden the stellar photometry, to identify the cluster population, and to determine the properties of Wd2, such as distance (d = 4.16 kpc), total-to-selective extinction (RV = 3.95), and age (1.04 ± 0.72Myr). We identified 240 bona fide pre-main-sequence Hα excess emitters indicating active disk accretion. A careful analysis of the radial dependence of the Hα excess emission shows a 60% lower mass accretion rate in the cluster center, indicating a more rapid disk dispersal in close proximity to the massive OB-stars. We performed state-of-the-art artificial star tests to study the completeness-corrected spatial distribution of the stars. This revealed that Wd2 consists of two subclusters. Additionally, we determined the present-day mass function (PDMF) with a slope of G = −1.53 ± 0.05, which translates to a total stellar cluster mass of (3.6±0.3)·10^4 M_sun. The spatial analysis of the PDMF and the young age of Wd2 indicates that the cluster is, most likely primordial, mass segregated. A spatially uniformly distributed low-mass (< 0.15 M_sun) population, extending into the gas and dust cloud, as well as a confined region of reddened stars, most likely caused by a foreground CO cloud, suggests that cloud-cloud collision might be the origin of the formation of Wd2.
Supervisor: Eva Grebel (ARI)
Tanya Edwards ( UK ) 16.01.2017
Separation of gamm-Ray, Electron and Proton induced Air Showers applied to Diffuse Emission Studies with H.E.S.S. (thesis pdf, 10 MB)
A fundamental issue in ground-based gamma-ray astronomy is the identification of γ-ray events among the overwhelming background of air showers induced by charged cosmic rays. Reconstruction techniques exist to distinguish most of the background of hadrons but an irreducible background of electrons and gamma-like protons still remain. I present here a new technique making use of high-altitude Cherenkov light emitted by the charged primary particle and air shower development properties. This method provides a way to distinguish between electrons and gamma rays on a statistical basis. In addition to this, the remaining proton background can also be identified. The technique was developed, tested and applied to studies using the High Energy Stereoscopic System (H.E.S.S.) located in Namibia. The analysis method is especially important in the detection of diffuse signals and eliminates the necessity of a background region in the field of view. The technique was applied to three scientific studies. The latitude profile of the Galactic diffuse gamma-ray emission was analysed. A width of σ = 0.25±0.05 deg (0.20±0.06 deg) for energies of 380 to 900 GeV (1 to 6 TeV) was determined. The cosmic electron spectrum was measured between 0.38 and 14 TeV and a broken power law was fit to the data. The spectrum steepens from an index of 3.08±0.06 to 3.72±0.12 at a break in energy of 1.11±0.04 TeV. In addition, upper limits on the maximum y-ray contamination from the Isotropic γ-Ray Background was placed at 4 x 10^3 (5 x 10^3) MeV cm^-2 s^-1 sr^-1 for energies of 1 to 6 TeV (380 to 900 GeV).
Supervisor: Werner Hofmann (MPIK)