The Evolution of Central Group Galaxies in Hydrodynamical Simulations

We trace the evolution of central galaxies in three ~10^13 M_sun galaxy groups simulated at high resolution in cosmological hydrodynamical simulations. The evolution in the group potential leads, at z=0, to central galaxies that are massive, gas-poor early-type systems supported by stellar velocity dispersion resembling either elliptical or S0 galaxies. Their z~2-2.5 main progenitors are massive M* ~ 3-10 x 10^10 M_sun, star forming (20-60 M_sun/yr) galaxies which host substantial reservoirs of cold gas (~5 x 10^9 M_sun) in extended gas disks. Our simulations thus show that star forming galaxies observed at z~2 are likely the main progenitors of central galaxies in galaxy groups at z=0. Their central stellar densities stay approximately constant from z~1.5 down to z=0. Instead, the galaxies grow inside-out, by acquiring a stellar envelope outside the innermost ~2 kpc. Consequently the density within the effective radius decreases by up to two orders of magnitude. Both major and minor mergers contribute to most of the mass accreted outside the effective radius and thus drive the evolution of the half-mass radii. In one of the three simulated groups the short central cooling time leads to a dramatic rejuvenation of the central group galaxy at z<1, affecting its morphology, kinematics and colors. This episode is eventually terminated by a group-group merger. Our simulations demonstrate that, in galaxy groups, the interplay between halo mass assembly, galaxy merging and gas accretion has a substantial influence on the star formation histories and z=0 morphologies of central galaxies.[Abridged]Comment: 28 pages, 23 figures, 9 tables, accepted to APJ (revised to match accepted version

CFHTLenS: Weak lensing constraints on the ellipticity of galaxy-scale matter haloes and the galaxy-halo misalignment

We present weak lensing constraints on the ellipticity of galaxy-scale matter haloes and the galaxy-halo misalignment. Using data from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), we measure the weighted-average ratio of the aligned projected ellipticity components of galaxy matter haloes and their embedded galaxies, $f_\mathrm{h}$, split by galaxy type. We then compare our observations to measurements taken from the Millennium Simulation, assuming different models of galaxy-halo misalignment. Using the Millennium Simulation we verify that the statistical estimator used removes contamination from cosmic shear. We also detect an additional signal in the simulation, which we interpret as the impact of intrinsic shape-shear alignments between the lenses and their large-scale structure environment. These alignments are likely to have caused some of the previous observational constraints on $f_\mathrm{h}$ to be biased high. From CFHTLenS we find $f_\mathrm{h}=-0.04 \pm 0.25$ for early-type galaxies, which is consistent with current models for the galaxy-halo misalignment predicting $f_\mathrm{h}\simeq 0.20$. For late-type galaxies we measure $f_\mathrm{h}=0.69_{-0.36}^{+0.37}$ from CFHTLenS. This can be compared to the simulated results which yield $f_\mathrm{h}\simeq 0.02$ for misaligned late-type models.Comment: 21 pages, 3 tables, 9 figures. This replacement matches the version accepted for publication in MNRA

The darkness that shaped the void: dark energy and cosmic voids

Aims: We assess the sensitivity of void shapes to the nature of dark energy that was pointed out in recent studies. We investigate whether or not void shapes are useable as an observational probe in galaxy redshift surveys. We focus on the evolution of the mean void ellipticity and its underlying physical cause. Methods: We analyse the morphological properties of voids in five sets of cosmological N-body simulations, each with a different nature of dark energy. Comparing voids in the dark matter distribution to those in the halo population, we address the question of whether galaxy redshift surveys yield sufficiently accurate void morphologies. Voids are identified using the parameter free Watershed Void Finder. The effect of redshift distortions is investigated as well. Results: We confirm the statistically significant sensitivity of voids in the dark matter distribution. We identify the level of clustering as measured by \sigma_8(z) as the main cause of differences in mean void shape . We find that in the halo and/or galaxy distribution it is practically unfeasible to distinguish at a statistically significant level between the various cosmologies due to the sparsity and spatial bias of the sample.Comment: 22 pages, 23 figures, 3 tables; v2: added references and short comparison of void size results; accepted for publication by MNRA

Understanding the formation and properties of galaxies from stellar kinematics

Obwohl die Entstehung und die Eigenschaften von Galaxien schon seit fast 100 Jahren Gegenstand astrophysikalischer Forschung sind, gibt es grundlegende Aspekte, welche bis heute nicht vollständig verstanden sind. Vor dem Hintergrund der kosmologischen Strukturbildung entwickeln sich Galaxien unter dem Einfluss diverser interner und externer physikalischer Prozesse. Ein komplexes Zusammenspiel dieser Prozesse, welche auf variierenden Zeitskalen agieren, formt die Galaxien die wir heute beobachten. Es ist bekannt, dass dieses Zusammenspiel maßgeblich die orbitale Struktur von Sternen innerhalb von Galaxien beeinflusst und daher Spuren in der Phasenraumverteilung hinterlässt. Die vorliegende Dissertation beschäftigt sich mit der Frage, welche Information aus der stellaren Kinematik extrahiert werden kann, um die Entstehung und Entwicklung von Galaxien umfassender zu verstehen. Im Speziellen untersuchen wir die Geschwindigkeitsverteilung entlang der Blickachse anhand ihrer Momente. Hierfür nutzen wir die hochmoderne hydrodynamische kosmologische \textit{Magneticum} Simulation, in welcher sich auf natürliche und selbstkonsistente Weise eine realistische und statistisch aussagekräftige Galaxienpopulation bildet. Um die kinematische Struktur von Galaxien in größerem Detail zu untersuchen, werden zusätzlich hochaufgelöste idealisierte Simulationen individueller Galaxienverschmelzungen analysiert. Der erste Teil der Arbeit bezieht sich zunächst auf das Zentrum elliptischer Galaxien. Im Einklang mit Beobachtungen finden wir im Allgemeinen eine bimodale Verteilung aus schnell rotierenden und langsam rotierenden Galaxien, mit konsistenten Häufigkeiten. Die Simulation reproduziert zudem die wichtigsten beobachteten Geschwindigkeitsstrukturen, wie gegenläufig oder entkoppelt rotierende Kerne. Mit Hilfe der dreidimensionalen Information der Simulation zeigen wir, dass es möglich ist, anhand von beobachtbaren Parametern die interne Anisotropie der Sterne abzuleiten. Dies bestätigt die Validität eines theoretischen Modells, welches zur Interpretation von Beobachtungen hinsichtlich der internen Struktur von Galaxien verwendet wird. Diese Resultate zeigen, dass die Magneticum Simulation ein exzellentes Laboratorium bereitstellt, um die stellare Kinematik und deren Ursprung zu erforschen. In diesem Zusammenhang finden wir, dass die Population aus langsam rotierenden Galaxien sich graduell seit z=2 aufbaut, und ein signifikanter Anteil dieser nun langsam rotierenden Galaxien ihren Drehimpuls durch eine massive Galaxienverschmelzung verloren hat. Im zweiten Teil untersuchen wir in einer detaillierten Fallstudie Ursprung, Stabilität und Dynamik eines gegenläufig rotierenden Kerns einer elliptischen Galaxie, die durch einen einzelnen massiven Verschmelzungsprozess zweier Spiralgalaxien in einer idealisierten Simulation entstanden ist. Wir können zeigen, dass solche Kerne aus neu geformten Sternen aufgebaut sind, deren Apoapsis sich innerhalb des Kernradius befindet. Das Rotationssignal verschwindet etwa 3Gyr nach der Galaxienkollision, ausgelöst durch die gravitative Vermischung der rotierenden Sterne mit Sternen, die den Kern auf stark elliptischen Bahnen intrudieren. Während seiner Lebenszeit vollführt der Kern eine periodische globale Bewegung. Diese Bewegung ist ein Indiz für die Konservierung des orbitalen Drehimpulses in der gasförmigen Komponente, welche später durch Sternentstehung den Kern aufbaut. Im dritten Teil erweitern wir unsere Analyse auf den stellaren Halo der Galaxien. Aufgrund der niedrigen Dichten und damit verbundenen langen Relaxationszeiten dokumentiert der stellare Halo eine Vielfalt an Informationen über Verschmelzungen mit anderen Galaxien. Im Einklang mit Beobachtungen finden wir drei Arten von radialen Rotationsvariationen: stark im Zentrum und schwächer im Halo, kontinuierlich ansteigend mit hoher Amplitude und durchgehend flach mit niedriger Amplitude. Diese Variationen in der Rotation sind Resultat des komplexen Zusammenspiels von interner Sternenstehung und der Akkretion von Sternen. Für die erste Art von Galaxien finden wir starke Indizien, dass der Halo und das Zentrum sich entkoppelt entwickeln. Während das Zentrum einer alten Scheibe entspricht die schon bei z~2 existiert und durch interne Sternenstehung wächst, baut sich der Halo durch das anisotrope Akkretieren kleiner Strukturen auf. Dieses Szenario legt nahe, dass eine Verbindung zwischen dem kinematischen Übergang und dem Übergang von insitu geformten zu akkretierten Sternen besteht. Wir demonstrieren, dass eine solche Korrelation existiert, jedoch mit einer gewissen Streuung, die durch einen vermischten Übergangsbereich verursacht wird. Abschließend wird die Form der Geschwindigkeitsverteilung genauer untersucht, indem wir die Schiefe und Wölbung der Verteilung miteinbeziehen, um die maximale Information aus der Geschwindigkeitsverteilung zu extrahieren. Dies wird erstmals für eine kosmologische Simulation durchgeführt. Wir zeigen, dass Galaxien mit ansteigender stellarer Masse eine zunehmend schmalere Geschwindigkeitsverteilung aufweisen. Diese Veränderung weist auf einen generellen Übergang in der orbitalen Konfiguration hin. Zudem können wir zeigen, dass die globalen wie auch räumlich aufgelösten höheren Momente Rückschlüsse auf die Entstehung von Galaxien zulassen. Galaxien deren lokale Schiefe stark mit der lokalen Rotationsuntestützung korreliert/anti-korreliert, weisen eine erhöhte intrinsische Sternenstehung und ein erhöhtes Reservoir an kaltem Gas auf. Zusammenfassend zeigt diese Dissertation, dass die stellare Geschwindigkeitsverteilung von Galaxien wertvolle Informationen über die Entstehung und Entwicklung von Galaxien enthält. Unsere Ergebnisse stellen aussagekräftige Interpretationen aktueller Beobachtungen dar und liefern Vorhersagen, die zum Verständnis zukünftiger Beobachtungen maßgeblich beitragen werden.Although the formation and properties of galaxies have been the subject of astrophysical research for almost 100 years, some fundamental aspects are still not fully understood. Against the background of large scale cosmological structure formation, galaxies develop under the influence of various physical processes which interplay in a complex manner to form the galaxies observed today. This includes internal as well as external processes that act on varying temporal and spatial scales. It is known that aspects of this interplay significantly influence the orbital structure of stars within galaxies, and therefore encode in the stellar phase-space distribution. This thesis addresses the question of which information can be extracted from the kinematics of stars in order to further understand the details of the formation and evolution of galaxies. In particular, we study the stellar line-of-sight velocity distribution (LOSVD) based on its moments. For this we use the state-of-the-art hydrodynamic cosmological Magneticum Pathfinder simulation, which implements a variety of relevant physical mechanisms in order to build a realistic and statically meaningful galaxy population in a self-consistent manner. In addition, high-resolution idealised simulations of individual galaxy mergers are analysed to examine the kinematic structure of galaxies in greater detail. The first part of the thesis is constrained to the centre of Early-Type Galaxies. In agreement with observations from integral field spectroscopy surveys, we find a bimodal distribution of fast and slow rotating galaxies with consistent frequencies. Furthermore, the simulation reproduces the most important observed velocity structures, such as counter-rotating or decoupled rotating cores. Exploiting the three-dimensional information provided by the simulation, we demonstrate that it is possible to infer the internal stellar anisotropy from observable parameters. This confirms the validity of a theoretical model, which is generally used to interpret observations regarding the internal kinematic structure of galaxies. These results show that the Magneticum Pathfinder simulation provides an excellent laboratory to study the kinematics of galaxies and to explore their origin. In this context we find that the population of slowly rotating galaxies builds up gradually since z=2, and a significant fraction loses its angular momentum due to massive galaxy merger. Furthermore, we investigate in a detailed case study the origin, stability and dynamics of a counter-rotating kinematically distinct core (KDC) in an elliptical galaxy formed in an isolated galaxy merger simulation. We show, that the KDC consists of stars that have been newly formed during the merger on orbits with an apoapsis within the core radius. The rotation signal disappears approximately 3Gyr after the galaxy merger, triggered by the gravitational mixing of the rotating stars with stars which intrude the KDC on strongly elliptical orbits. During its lifetime, the core performs a periodic global movement comparable to the precession of a gyroscope in a gravitational potential. This global motion originates from the conservation of the progenitor orbital angular momentum in the gaseous component, which later builds up the KDC through star formation. In the third part we expand the analysis to the stellar halo of galaxies, which encodes a variety of information about interactions with other galaxies for a long period of time due to the low densities and the associated long relaxation timescales in the halo. In agreement with observations we find three characteristic types of radial rotation variations: i) strong in the centre and increasingly weaker in the halo ii) continuously rising with high amplitude iii) continuously flat mostly with low amplitude. These variations in the rotation reflect the complex interplay of internal star formation and external accretion of stars. For the first type of galaxies, we find strong evidence that the halo and the centre are evolving decoupled. While the centre corresponds to an old disk which already exists at z~2 and grows through internal star formation, the halo is built up through the anisotropic accretion of small structures which are disrupted in the halo. This scenario suggests a connection between the kinematic transition and the transition from in-situ formed to accreted ex-situ stars. We demonstrate that such a correlation exists, with a certain scatter caused by a transition region where both in-situ and ex-situ stars are strongly mixed. This result represents an important interpretation of recent observations and prediction for future observations which will push the limits of radial coverage of IFS observations. Finally, to extract the maximum information from the LOSVD, its shape is examined more closely by including higher-order moments measuring the deviations from a Gaussian, i.e. the skewness and kurtosis. This is the first time this analysis is applied to galaxies from a fully cosmological simulation. We show that galaxies with higher stellar masses exhibit a more peaked LOSVD, indicating a transition in the global orbital configuration. Furthermore, we are able to extract information about the formation history of galaxies from the global as well as spatially resolved higher-order moments. Galaxies with a strong anticorrelation/correlation between the local skewness and rotational support exhibit increased insitu star formation and a larger reservoir of cold gas to prolong star formation. In summary, this thesis shows that the LOSVD of galaxies encodes valuable information about the formation and evolution of galaxies. Our findings represent meaningful interpretations of recent IFS observations and provide predictions that can be probed by future surveys which will be able to reach larger radii and redshifts and to include information about stellar populations

CFHTLenS: weak lensing constraints on the ellipticity of galaxy-scale matter haloes and the galaxy-halo misalignment

We present weak lensing constraints on the ellipticity of galaxy-scale matter haloes and the galaxy-halo misalignment. Using data from the Canada-France-Hawaii Telescope Lensing Survey (CFHTLenS), we measure the weighted-average ratio of the aligned projected ellipticity components of galaxy matter haloes and their embedded galaxies, fh, split by galaxy type. We then compare our observations to measurements taken from the Millennium Simulation, assuming different models of galaxy-halo misalignment. Using the Millennium Simulation, we verify that the statistical estimator used removes contamination from cosmic shear. We also detect an additional signal in the simulation, which we interpret as the impact of intrinsic shape-shear alignments between the lenses and their large-scale structure environment. These alignments are likely to have caused some of the previous observational constraints on fh to be biased high. From CFHTLenS, we find fh=−0.04±0.25 for early-type galaxies, which is consistent with current models for the galaxy-halo misalignment predicting fh ≃ 0.20. For late-type galaxies we measure $f_\mathrm{h}=0.69_{-0.36}^{+0.37}$ from CFHTLenS. This can be compared to the simulated results which yield fh ≃ 0.02 for misaligned late-type model

Measuring cosmological weak lensing using the Advanced Camera for Surveys on board the Hubble Space Telescope

Following from the theory of General Relativity, light-bundles are deflected and differentially distorted while passing through the gravitational potential of matter inhomogeneities. The gravitational lensing effect caused by the large-scale matter distribution in the Universe is termed cosmological weak lensing, and provides a powerful probe of cosmology. By studying the distortions which are imprinted onto the observed shapes of distant galaxies, the statistical properties of the foreground density field can be constrained free of assumptions on the relation between luminous and dark matter. Due to the weakness of the effect, it is challenging to measure and can only be detected statistically from large ensembles of coherently lensed galaxies. In addition, careful correction for systematic effects is required, first of all for the image point-spread-function (PSF). In this PhD thesis we present a detailed cosmological weak lensing analysis using deep high-resolution images from the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). Including data from the ACS Parallel Cosmic Shear Survey, the HST/GEMS Survey, and the HST/COSMOS Survey, this data set constitutes the largest survey used to measure cosmological weak lensing from space today. In order to achieve the high accuracy required for weak lensing studies, we developed several upgrades for the data reduction pipeline including careful image registration, improved bad pixel masks, and an optimised weighting scheme. We also perform a thorough investigation of the ACS PSF and develop a new correction scheme for its spatial and temporal variations, which are caused by thermal breathing of the telescope. We present numerous tests of our shear measurement pipeline using simulated images from the STEP Programme, and demonstrate that it achieves a relative shear-measurement accuracy better than 2% for ACS-like images. We perform the analysis of the ACS data in two steps, starting with a pilot study, in which we test the capabilities of ACS for cosmological weak lensing measurements with early parallel observations and the combined GEMS and GOODS ACS mosaic of the Chandra Deep Field South (CDFS, 0.22 deg2). We perform a number of diagnostic tests indicating that the remaining level of systematics is consistent with zero for the GEMS and GOODS data confirming the success of our PSF correction scheme. For the parallel data we detect a low level of remaining systematics which we interpret to be caused by a lack of sufficient dithering of the data. Combining our shear estimate of the GEMS and GOODS observations using 96 galaxies arcmin-2 with the photometric redshift catalogue of the GOODS-MUSIC sample, we determine a local single field estimate for the mass power spectrum normalisation σ8=0.59+0.13-0.17(stat)±0.07(sys) (68% confidence assuming Gaussian sampling variance) at a fixed matter density Ωm=0.24 for a ΛCDM cosmology, where we marginalise over the uncertainty of the Hubble constant and the redshift distribution. This estimate agrees only marginally with the WMAP-3 result of σ8=0.761+0.049-0.048 (Spergel et al. 2007) and is significantly below values found by recent ground-based surveys. From this discrepancy we conclude that the CDFS is subject to strong sampling variance with a significant under-density of compact foreground structures. This is consistent with a recent study by Phleps et al. (2007), who find a strong deficiency of red galaxies in this field. In a second step we perform a preliminary cosmological weak lensing analysis of the HST/COSMOS Survey (1.64 deg2). The significantly increased statistical accuracy reveals previously undetectable residual systematic errors indicated by a significant B-mode signal. So far we have not been able to unambiguously identify their origin, but note that similar indications for remaining systematics have been found in an independent analysis of the same data by Massey et al. (2007). Using only B-mode-free scales (>1' in the shear two-point correlation function), we find σ8 = 0.71±0.09 (68% confidence) from COSMOS for a flat ΛCDM cosmology and fixed Ωm=0.24, where the error includes the uncertainties in the redshift distribution, the Hubble constant, and the shear calibration, as well as a Gaussian estimate for sampling variance. This result is in excellent agreement with the WMAP-3 constraints, but is significantly below the estimates found by Massey et al. (2007). In addition to the cosmological weak lensing analysis we present a reconstruction of the projected mass in the COSMOS field, as well as first results from a weak lensing analysis of the HST/STAGES Survey targeting the galaxy super-cluster Abell 901/902. Furthermore, we briefly summarise ACS studies of galaxy clusters, which make use of the developed data reduction and weak lensing pipeline

Advanced Image Analysis for Modeling the Aging Brain

Both normal aging and neurodegenerative diseases such as Alzheimer’s disease (AD) cause morphological changes of the brain due to neurodegeneration. As neurodegeneration due to disease may be difficult to distinguish from that of normal aging, interpretation of magnetic resonance (MR) brain images in the context of diagnosis of neurodegenerative diseases is challenging, especially in the early stages of the disease. This thesis presented comprehensive models of the aging brain and novel computer-aided diagnosis methods, based on advanced, quantitative analysis of brain MR images, facilitating the differentiation between normal and abnormal neurodegeneration. I aimed to evaluate and develop methods for clinical decision support using features derived from MR brain images: I evaluated a classification method to predict global cognitive decline in the general population, evaluated five brain segmentation methods and developed a spatio-temporal model of morphological differences in the brain due to normal aging. To create this model I developed two novel techniques that allow performing non-rigid groupwise image registration on large imaging datasets. The novel aging brain models and computer-aided diagnosis methods facilitate the differentiation between normal and abnormal neurodegeneration. This will help in establishing more accurate diagnoses of patients, and in identifying patients at risk of developing neurodegenerative disease before symptoms emerge. In the future, the method’s performance and efficacy should be evaluated in clinical practice

Thermo-hydro-mechanical simulation of a generic geological disposal facility for radioactive waste

Geological disposal is required for the safe and long-term disposal of legacy radioactive waste. High level waste and spent fuel generate significant heat that will cause thermo-hydro-mechanical coupled processes in the rock mass. The thermal expansion of the fluid will be greater than the grains causing a decrease in mean effective stress with the low permeability restricting Darcy flow and excess pore pressure equilibration. A decrease in mean effective stress can reduce material strength in granular materials, which may be significant near excavations where differential stress is increased. Microseismic monitoring provides cost effective, non-intrusive and three-dimensional data that can be calibrated with the stress and strain behaviour of a rock mass. However, there is no precedent for the microseismic monitoring of heat-producing radioactive waste. Generic concepts, analogue materials and data from in situ experiments are used to demonstrate the potential for the microseismic monitoring of heat-producing radioactive waste in lower strength sedimentary rocks. A mechanism for early post-closure microseismicity is demonstrated, whereby excess pore pressure decreases the mean effective stress towards yielding in shear. The rock and fluid property uncertainties are ranked according to their contribution to the excess pore pressure. Permeability is found to be important as expected, however, Biot's coefficient is demonstrably more important and yet often overlooked. Furthermore, the microseismic event locations, timings and pseudo scalar seismic moments are shown to have statistically significant relationships with the engineered backfill swelling pressure. Therefore, early post-emplacement microseismic monitoring could provide constraints for the engineered backfill swelling pressure and rock property uncertainties whilst the facility is still operational. Insights could prove timely for adapting the engineering designs, if they are not behaving as expected, in further high level waste and spent fuel tunnels