IMPRS-HD alumni 2019

Alumni 2019

Katharina Wollenberg (16.1.) - Antonio D'Isanto  (31.1.)  -  Tobias Schmidt (5.2.)  -  Vikas Joshi (6.2.)  -  Anna-Christina Eilers (3.5.)  -  Neven Tomicic (15.5.)

Neven Tomicic    (Croatia)                                                                                                                                                      15.05.2019

Probing the cold phase of the interstellar medium and star formation in nearby galaxies   ( thesis pdf, 30 MB )

Properly measuring the spatial distribution of the star formation rate (SFR) in galaxies helps us to understand the driving forces for the star formation in galaxies, effects on their interstellar media (ISM), and their evolution. However, this is hindered by the uncertainties in estimating SFRs and calibrating the SFR prescriptions. These uncertainties are caused by not properly measuring the attenuation of light, probing large spatial scales, or averaging over large sample of galaxies. Additionally, the physical factors that set the efficiency whith which galaxies convert gas into stars (star formation efficiency; SFE), and their role in galactic evolution, are not yet fully understood. Variations in the SFE are difficult to disentangle from uncertainties of estimated SFRs.
The main goal of this thesis is to use optical integral field unit (IFU) observations of nearby galaxies, in order to probe the cold phase of their ISM at sub-kpc scales. We aim to measure the attenuation, thus to properly estimate the SFRs and calibrate the SFR prescriptions. We also estimate variations of SFE across the disk of an interacting galaxy. Using IFU observations of the outskirts of the Andromeda galaxy (M31) at sub-kpc scales (from 10 pc to kpc), we derive the Balmer line attenuation. By comparing attenuation with the dust mass surface density, we derive the 3-dimensional spatial distribution of dust and ionized gas in M31. Our results indicate that the vertical dust/gas distribution from the central areas of nearby galaxies differs from the outskirts of M31. From this evidence, we hypothesize that the vertical dust/gas distribution in galactic disks varies as a function of the galactocentric distance. Following that, we use extinction corrected Balmer line emission as a reference SFR tracer in a combination with ultraviolet and near-infrared images, to calibrate hybrid SFR prescriptions. We find that the hybrid SFR prescriptions do not change with spatial scale or with the subtraction of a diffuse component. However, our SFR prescriptions observed in M31 differ from the prescriptions in the literature. This indicates that the SFR prescriptions are not universal and that they may vary with the inclination and the galactocentric radius, due to varying dust/gas distributions.
Our IFU observations of the interacting galaxy NGC 2276 are used to investigate how the early phase of galactic interaction affects the ISM, and SFE across its disk. Although NGC 2276 shows a significant asymmetrically elevated SFR surface density, and asymmetric stellar distribution, it does not show an unusual gas phase metallicity gradient or shock ionization. On the other hand, we probed the SFE at sub-kpc scales (0.5 kpc) across NGC 2276’s disk to trace the origin of its elevated and asymmetric SFR and found more than two orders of magnitude variation in SFE. This is significantly larger than what is seen in nearby galaxies. We speculate hat this is caused by both tidal forces exerted from a neighboring galaxy and ram pressure affecting NGC 2276.

Supervisor:  Eva Schinnerer   (MPIA)

Anna-Christina Eilers    (Germany)                                                                                                                                         03.05.2019

Unravelling 13 billion years of cosmic history with spectroscopic studies: from the Milky Way to the epoch of reionization  (thesis pdf, 40MB)

In this thesis we trace the cosmic history from the Epoch of Reionization to the local universe by means of several spectroscopic studies. In the first part, we analyze 34 quasar spectra at 5.8 < zem < 6.5 and measure the redshift evolution of the opacity of the intergalactic medium (IGM) within the Lyα as well as the Lyβ forest to set new constraints on the timing and morphology of the reionization epoch. We find evidence for an extended reionization process down to z ∼ 5, and, while the observed scatter in the Lyα forest optical depth can be well reproduced by current state-of-the-art simulations including spatial fluctuations in the temperature field or the ultraviolet background, we find a strong mismatch between simulations and observations in the Lyβ forest opacity, suggesting an inversion of the thermal state of the post-reionization IGM.
We also measure the sizes of the quasars’ proximity zones, which are regions of enhanced ionization in the vicinity of the quasars, ionized by their own radiation. We find a dependency of the proximity zone sizes to the quasars’ lifetime, whicpresents a novel method to estimate the lifetime of individual quasars, providing unprecedented constraints on the formation and growth of supermassive black holes in the early universe. We discover three quasars with very short lifetimes, i.e. t_Q ∼ 10^4 − 10^5 yr, that pose significant challenges to all current black hole formation theories.
In the second part of this thesis we explore the structure and dynamics of the Milky Way. We develop a new data-driven model to determine precise parallaxes by combining multi-band photometry and spectroscopy to make global kinematic maps of our Galaxy from & 45, 000 luminous red giant stars with only . 10% parallax uncertainties. Our map extends to Galactocentric distances of 25 kpc, well beyond the reach of parallax estimates by the Gaia mission. Making use of these new spectrophotometric parallaxes, we determine the most precise measurement to date of the circular velocity curve of the Milky Way over a wide range of Galactocentric distances. Based on Jeans modeling in an axisymmetric gravitational potential we find that the velocity curve is gently but significantly declining.

Supervisor:  Joe Hennawi   (MPIA/UCSB)

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,  corrigendum )

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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