IMPRS-HD alumni 2019

Alumni 2019

Katharina Wollenberg (16.1.) - Antonio D'Isanto  (31.1.)  -  Tobias Schmidt (5.2.)  -  Vikas Joshi (6.2.)

Vikas Josji    (India)                                                                                                                                                                  06.02.2019

Reconstruction and Analysis of Highest Energy γ-Rays and its Application to Pulsar Wind Nebulae      ( thesis pdf, 35 MB )

The High Altitude Water Cherenkov (HAWC) γ-ray observatory detects cosmic- and γ-rays in the TeV energy range. HAWC was recently upgraded with a sparse detector array (the outrigger array), which increases the instrumented area by a factor of 4-5 and will improve the sensitivity at energies greater than 10 TeV. This thesis consists of a number of contributions towards the improvement of the performance of HAWC at the highest energies and the study of a prominent high energy source, 2HWC J2019+367. To decide on components of the outrigger array, simulation input is provided. A new Monte Carlo template-based reconstruction method for air shower arrays is developed. It reconstructs the core location and energy of γ-ray showers. The goodness of fit of the method is utilised to separate the cosmic- and γ-ray showers. This method significantly improves the HAWC shower reconstruction and combines the reconstruction of HAWC and the outrigger array. In-depth spectral and morphological studies of 2HWC J2019+367 are performed. 2HWC J2019+367 shows a hint of energy-dependent morphology. A new HAWC source is discovered in the vicinity associated with VER J2016+371. The preferred direction of the X-ray and TeV emission indicates their association, and their combined spectral modelling show that 2HWC J2019+367 is likely to be the TeV pulsar wind nebula of PSR J2021+3651.

Supervisor:  Jim Hinton   (MPIK)

Tobias Schmidt     (Germany)                                                                                                                                                      05.02.2019

Constraints on Quasar Emission Properties from the HeII and HI Transverse Proximity Effect      ( thesis pdf, 10 MB )

Bright quasars are powerful sources of ionizing radiation and have profound impact on the Intergalactic Medium. In particular, they create regions with enhanced ionization and therefore reduced Lyman α forest absorption in their surroundings. Observing this so-called transverse proximity effect along background sightlines provides a view of the foreground quasar from different vantage points, and hence at different lookback times compared to the line-of-sight toward Earth. One can thus constrain the emission history (lifetime, age) and emission geometry (obscuration, opening angle) of the foreground quasar based purely on geometric and light travel time arguments. Both quantities are so far poorly constrained by observations but fundamental for the understanding of Active Galactic Nuclei. To investigate the HeII transverse proximity effect, we conducted an optical spectroscopic foreground quasar survey around 22 HST/COS sightlines, leading to a sample of 20 foreground quasars. We find statistical evidence for the the HeII transverse proximity effect and infer a constraint on the quasar lifetime of > 25 Myr. From a detailed modeling, based on cosmological hydrodynamical simulations and a dedicated photoionization model including quasar obscuration and finite quasar lifetime, we derive joint constraints on age and obscuration of individual objects, indicating that one quasar is old and unobscured (tage ≈ 25 Myr, Ωobsc < 30 %) while three other are either young (tage < 10 Myr) or highly obscured (Ωobsc > 70 %). However, the models also reveal that the large scatter intrinsic to the HeII Lyα forest prohibits further progress in the field. I therefore developed a novel method that uses large numbers of HI Lyα forest spectra to map the 3D light echo of individual quasars. An end-to-end test confirms that such tomographic observations can constrain the age of hyperluminous quasars to better than 20%, requiring only 1 – 2 nights on existing 8 – 10 m facilities. The method bears potential to also constrain the quasar emission geometry and the full lightcurve over the past 100 Myr, rendering it a viable tool to investigate quasar properties.

Supervisor:  Joseph Hennawi   (MPIA/ UCSB)

Antonio D'Isanto   (Italy)                                                                                                                                                          31.01.2019

Probabilistic photometric redshift estimation in massive digital sky surveys via machine learning      ( thesis pdf, 15 MB )

The problem of photometric redshift estimation is a major subject in astronomy, since the need of estimating distances for a huge number of sources, as required by the data deluge of the recent years. The ability to estimate redshifts through spectroscopy does not scale with this avalanche of data. Photometric redshifts provide the required redshift estimates at the cost of some precision. The success of several forthcoming missions is highly dependent on the availability of photometric redshifts. The purpose of this thesis is to provide innovative methods for photometric redshift estimation. Two models are proposed. The first is fully-automatized, based on the combination of a convolutional neural network with a mixture density network, to predict probabilistic multimodal redshifts directly from images. The second model is features-based, performing a massive combination of photometric parameters to apply a forward selection in a huge feature space. The proposed models perform very efficiently compared to some of the most common models used in the literature. An important part of the work is dedicated to the correct estimation of the errors and prediction quality. The proposed models are very general and can be applied to different topics in astronomy and beyond.

Supervisor:  Kai Polsterer  (HITS)

Katharina M. J. Wollenberg   (Germany)                                                                                                                                 16.01.2019

Diversity of Population III Star Formation ( thesis pdf, 120 MB )

The aim of this thesis is to improve our understanding of the fragmentation behavior of Population III protostellar disks under the influence of rotation, turbulence, and magnetic fields. We further evaluate consequences that may be inferred for the later evolution of the star-forming halo and its surroundings with respect to protostellar ejections and in terms of the impact of radiative feedback on later chemical enrichment of neighboring halos. In the main part of this thesis, we follow the collapse of a primordial gas cloud until the formation of the first protostar and the creation of a highly gravitationally unstable protostellar disk system. We find that turbulence promotes the fragmentation of the protostellar disk and both rotation and magnetic fields can provide some stabilization against it. While the total mass growth of the collection of protostars is only mildly affected by rotation and turbulence, magnetic fields can have such a strong impact on the dynamical evolution of the disk system that accretion onto the protostars is highly disturbed and their mass growth is significantly reduced. In spite of all the differences, the disk generally breaks up into a protostellar cluster that develops a top-heavy mass function. Interactions between protostars in the cluster are highly dynamical and lead to a considerable number of protostellar ejections. We demonstrate that some of these ejected Population III protostars, even if they continue to accrete for some longer period after they have left the disk environment, continue to have masses of M < 0.8 solar mass. Hence, they have lifetimes longer than the current age of the Universe and thus describe Population III candidates that could still be observable today. In another project, we assess the role of photoevaporation of a pristine halo by a near-by Population III star prior to the supernova explosion of that star. We demonstrate that it is crucial for realistic simulations of metal enrichment to account for the photoevaporation as the radiation ablates and thins out the outer halo layers and thus makes the halo more susceptible to mixing with the metals from the supernova ejecta. In this thesis, we use both analytical estimations and numerical simulations. Further tests are conducted to investigate the performance of our numerical methods and the sensitivity of our results to the numerical resolution. We demonstrate that general trends, in particular when effects of turbulence are examined, cannot be reliably deduced from only a single numerical run. Instead a statistical analysis of an ensemble of realizations based on the same initial conditions needs to be considered.

Supervisor:  Simon Glover (ITA)

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