Alignment of Disc Galaxies : Testing the Theory of Angular-Momentum Acquisition

This thesis is concerned with the formation of disc galaxies, a special type of galaxies characterised by ordered rotation. The main focus is on testing predictions of theories that try to explain the angular-momentum acquisition of these galaxies. A major issue concerns the proper quanti cation of galaxy morphologies. This is of vital importance for automated and reproducible classi cation of disc galaxies in large data samples. The most promising approach is to expand galaxy morphologies into orthonormal basis functions based on a well motived light pro le (the Sersic profi le). In the main part of this thesis, the angular momenta of several thousand disc galaxies from the Sloan Digital Sky Survey are analysed. It is demonstrated that previous investigations lacked a rigorous account for the relevant error sources, which lead to unjusti ed con rmations of theoretical predictions. If all relevant errors are considered, there is no statistical evidence con rming these predictions. The methods developed here are also relevant in a wider context, as they are applicable to other astrophysical investigations. This thesis concludes with a detailed discussion of potential improvements of measurements and an outline of an observational strategy that will allow for a more decisive empirical test of the theoretical predictions

Quantifying galaxy shapes: Sersiclets and beyond

Parametrising galaxy morphologies is a challenging task, e.g., in shear measurements of weak lensing or investigations of galaxy evolution. The huge variety of morphologies requires an approach that is highly flexible, e.g., accounting for azimuthal structure. We revisit the method of sersiclets, where galaxy morphologies are decomposed into basis functions based on the Sersic profile. This approach is justified by the fact that the Sersic profile is the first-order Taylor expansion of any real light profile. We show that sersiclets overcome the modelling failures of shapelets. However, sersiclets implicate an unphysical relation between the steepness of the light profile and the spatial scale of azimuthal structures, which is not obeyed by real galaxy morphologies and can therefore give rise to modelling failures. Moreover, we demonstrate that sersiclets are prone to undersampling, which restricts sersiclet modelling to highly resolved galaxy images. Analysing data from the Great08 challenge, we demonstrate that sersiclets should not be used in weak-lensing studies. We conclude that although the sersiclet approach appears very promising at first glance, it suffers from conceptual and practical problems that severly limit its usefulness. The Sersic profile can be enhanced by higher-order terms in the Taylor expansion, which can drastically improve model reconstructions of galaxy images. If orthonormalised, these higher-order profiles can overcome the problems of sersiclets while preserving their mathematical justification.Comment: 14 pages, 12 figures, 2 tables; accepted by MNRA

Methoden zur Absicherung simulationsgerechter Produktmodelle

Einleitung Immer höhere Anforderungen an die Interdisziplinarität der virtuellen Produktentwicklung (VPE) erfordern qualifizierte Produktmodelle, die eine vollständige Integration und Verknüpfung aller relevanten Teilprozesse absichern. Gleichzeitig soll dabei für den Anwender das Produktverständnis, wie auch die Qualität des Produktes und des Prozesses erhöht werden. Eine Folge daraus sind kurze Innovationszyklen und eine Erhöhung der Transparenz des Prozesses. Die Anwendung numerischer Simulationsmethoden hat sich als dritter essentieller Bestandteil neben Konstruktion und Versuch in der VPE etabliert (Pährisch et al. 2012). Eine Absicherung durch virtuelle Prototypen in einer frühen Konzeptphase unterstützt dabei den Konstruktionsprozess. Ein Nachteil ist, dass die Verwendung virtueller Prototypen noch unzureichend in die übrigen Prozessschritte integriert und damit eine Sensibilisierung für eine vorausschauende Modellerzeugung noch nicht vorhanden ist. Ebenso ergab eine Studie, dass Berechnungsingenieure durchschnittlich 50% ihrer Arbeitszeit auf Datenbeschaffung verwenden müssen und nur jeweils 10% auf die Modellaufbereitung (Sendler et al. 2011). Dies liegt u. a. an der sog. Kommunikationsbarriere zwischen der Konstruktion und Simulation beschreibt. Eine Lösung dazu ist eine tiefergehende Integration dieser beiden Disziplinen in ein Produktmodell. Ein Lösungsansatz ist die Durchführung konstruktionsbegleitender Simulationen. Diese können mit in CAD-Systemen integrierten Simulationsmodulen durchgeführt werden. Die Integrationstiefe der gegebenen Verknüpfungen ist allerdings meist sehr gering. Dieser Beitrag befasst sich mit Techniken, welche einen systematischen Aufbau eines simulationsorientierten Produktmodells absichern. Umgesetzt wird dies durch die Verwendung simulationsgerechter Komponenten, Feature und Analysen. Diese unterstützen eine automatisierte Modelltransformation im CAD-Prozess, an der Schnittstelle von Konstruktion und Simulation. Damit wird die Prozesskette Konstruktion-Simulation verkürzt. Ebenso werden auch durch die Integration tiefgehender Inferenzmechanismen fortgeschrittene Simulationstechniken, wie auch die Definition und Informationsübergabe von Rand- und Lastbedingungen und weiteren Details auf höherer Instanz ermöglicht

Parametrising arbitrary galaxy morphologies: potentials and pitfalls

We demonstrate that morphological observables (e.g. steepness of the radial light profile, ellipticity, asymmetry) are intertwined and cannot be measured independently of each other. We present strong arguments in favour of model-based parametrisation schemes, namely reliability assessment, disentanglement of morphological observables, and PSF modelling. Furthermore, we demonstrate that estimates of the concentration and Sersic index obtained from the Zurich Structure & Morphology catalogue are in excellent agreement with theoretical predictions. We also demonstrate that the incautious use of the concentration index for classification purposes can cause a severe loss of the discriminative information contained in a given data sample. Moreover, we show that, for poorly resolved galaxies, concentration index and M_20 suffer from strong discontinuities, i.e. similar morphologies are not necessarily mapped to neighbouring points in the parameter space. This limits the reliability of these parameters for classification purposes. Two-dimensional Sersic profiles accounting for centroid and ellipticity are identified as the currently most reliable parametrisation scheme in the regime of intermediate signal-to-noise ratios and resolutions, where asymmetries and substructures do not play an important role. We argue that basis functions provide good parametrisation schemes in the regimes of high signal-to-noise ratios and resolutions. Concerning Sersic profiles, we show that scale radii cannot be compared directly for profiles of different Sersic indices. Furthermore, we show that parameter spaces are typically highly nonlinear. This implies that significant caution is required when distance-based classificaton methods are used.Comment: 18 pages, 13 figure

Only marginal alignment of disc galaxies

Testing theories of angular-momentum acquisition of rotationally supported disc galaxies is the key to understand the formation of this type of galaxies. The tidal-torque theory tries to explain this acquisition process in a cosmological framework and predicts positive autocorrelations of angular-momentum orientation and spiral-arm handedness on distances of 1Mpc/h. This disc alignment can also cause systematic effects in weak-lensing measurements. Previous observations claimed discovering such correlations but did not account for errors in redshift, ellipticity and morphological classifications. We explain how to rigorously propagate all important errors. Analysing disc galaxies in the SDSS database, we find that positive autocorrelations of spiral-arm handedness and angular-momentum orientations on distances of 1Mpc/h are plausible but not statistically significant. This result agrees with a simple hypothesis test in the Local Group, where we find no evidence for disc alignment. Moreover, we demonstrate that ellipticity estimates based on second moments are strongly biased by galactic bulges, thereby corrupting correlation estimates and overestimating the impact of disc alignment on weak-lensing studies. Finally, we discuss the potential of future sky surveys. We argue that photometric redshifts have too large errors, i.e., PanSTARRS and LSST cannot be used. We also discuss potentials and problems of front-edge classifications of galaxy discs in order to improve estimates of angular-momentum orientation.Comment: 20 pages, 15 figures; accepted by MNRA

Digital transformation - life cycle assessment of digital services, multifunctional devices and cloud computing

The substitution of material-based services with digital services - for example films or music - alters the environmental impact of our everyday activities and shifts visible material use to less visible digital services. A holistic Life Cycle Assessment (LCA) of digital transformation requires the assessment of information and communication technology (ICT) with its wide range of multifunctional devices as well as substitution and summation effects caused by new services enabled by modern ICT. The 73rd Discussion Forum on Life Cycle Assessment (DF LCA) was held on 21 November 2019 to discuss the environmental implications of digital transformation in view of the effective as well as the potential environmental impacts. While currently being a small, but already relevant contributor to the overall greenhouse gas emissions, the relevance of the ICT sector is predicted to grow rapidly within the next five to ten years. This one-day forum offered the possibility to discuss current as well as potential future implications of devices as well as digital services on the environment. The 73rd DF LCA was opened with a welcome address given by René Itten (Zurich University of Applied Sciences, Switzerland). The program comprised five distinct sessions in order to tackle just as much different aspects of environmental impacts caused by digital transformation as well as relevant challenges for the LCA methodology

The Pristine survey -- XXIII. Data Release 1 and an all-sky metallicity catalogue based on Gaia DR3 BP/RP spectro-photometry

We use the spectro-photometric information of ~219 million stars from Gaia's DR3 to calculate synthetic, narrow-band, metallicity-sensitive CaHK magnitudes that mimic the observations of the Pristine survey, a survey of photometric metallicities of Milky Way stars that has been mapping more than 6,500 deg^2 of the northern sky with the CFHT since 2015. These synthetic magnitudes are used for an absolute re-calibration of the deeper Pristine photometry and, combined with broadband Gaia information, synthetic and Pristine CaHK magnitudes are used to estimate photometric metallicities over the whole sky. The resulting metallicity catalogue is accurate down to [Fe/H]~-3.5 and is particularly suited for the exploration of the metal-poor Milky Way ([Fe/H]<-1.0). We make available here the catalogue of synthetic CaHK_syn magnitudes for all stars with BP/RP information in Gaia DR3, as well as an associated catalogue of more than ~30 million photometric metallicities for high S/N FGK stars. This paper further provides the first public DR of the Pristine catalogue in the form of higher quality recalibrated Pristine CaHK magnitudes and photometric metallicities for all stars in common with the BP/RP information in Gaia DR3. We demonstrate that, when available, the much deeper Pristine data greatly enhances the quality of the derived metallicities, in particular at the faint end of the catalogue (G_BP>16). Combined, both catalogues include more than 2 million metal-poor star candidates as well as more than 200,000 and ~8,000 very and extremely metal-poor candidates. Finally, we show that these metallicity catalogues can be used efficiently, among other applications, for Galactic archaeology, to hunt for the most metal-poor stars, and to study how the structure of the Milky Way varies with metallicity, from the flat distribution of disk stars to the spheroid-shaped metal-poor halo. (Shortened)Comment: 30 pages, 24 figures, submitted to A&A. First two authors are co-first author. The CaHK photometry catalogue and the two photometric metallicity catalogues are available, before acceptance, as large compressed csv files at: https://seafile.unistra.fr/d/ee0c0f05719d4368bcbb

Wissensintegration im Simulationsumfeld von Creo Simulate durch Anwendungsprogrammierung

Vorgestellt werden verschiedene Programmierschnittstellen von Creo. Des Weiteren werden Anwendungsmöglichkeiten im Bereich der konstruktionsbegleitenden Simulation (Creo Simulate) vorgestellt. Ziel ist die Entlastung des Konstrukteurs/Berechners von Routineaufgaben und die Integration von Wissen in das Simulationsmodell