Numerical estimation of densities

[Abridged] We present a novel technique, dubbed FiEstAS, to estimate the underlying density field from a discrete set of sample points in an arbitrary multidimensional space. FiEstAS assigns a volume to each point by means of a binary tree. Density is then computed by integrating over an adaptive kernel. As a first test, we construct several Monte Carlo realizations of a Hernquist profile and recover the particle density in both real and phase space. At a given point, Poisson noise causes the unsmoothed estimates to fluctuate by a factor ~2 regardless of the number of particles. This spread can be reduced to about 1 dex (~26 per cent) by our smoothing procedure. [...] We conclude that our algorithm accurately measure the phase-space density up to the limit where discreteness effects render the simulation itself unreliable. Computationally, FiEstAS is orders of magnitude faster than the method based on Delaunay tessellation that Arad et al. employed, making it practicable to recover smoothed density estimates for sets of 10^9 points in 6 dimensions.Comment: 12 pages, 18 figures, submitted to MNRAS. The code is available upon reques

Particle hydrodynamics with tessellation techniques

Um Galaxien, Galaxienhaufen oder noch größere Strukturen im Universum detailliert zu simulieren, benötigt man eine korrekte Simulation des in diesen Objekten vorhandenen Gases. Eine Möglichkeit zur Simulation dieses Gases bietet das etablierte Verfahren ``Smoothed Particle Hydrodynamics (SPH)''. Diese Methode empfiehlt sich besonders wegen ihrer intrinsischen geometrischen Flexibilität und ihrer adaptiven Auflösung. Neuere Untersuchungen zeigten aber, dass SPH in Situationen, in denen große Dichtesprünge auftreten, ungenau wird. Hier kann es zu einem unphysikalisch verlangsamten Wachstum von hydrodynamischen Instabilitäten kommen. Diese Probleme von SPH können vor allem auf systematisch bedingte Ungenauigkeiten in der Dichtebestimmung dieser Methode zurückgeführt werden. Um diese Probleme zu vermeiden, haben wir eine neue ``Voronoi Particle Hydrodynamics'' (VPH) genannte Methode enwickelt, um die Hydrodynamik zu simulieren. Dabei wird die Dichte der Simulationsteilchen mit Hilfe eines zusätzlichen Gitters bestimmt. Dieses Gitter ist eine Voronoi Pflasterung, die auf auf den Positionen der Teilchen basiert. Mit Hilfe dieses Prinzips können hydrodynamische Instabilitäten korrekt simuliert werden. Situationen, in denen Scherströmungen entlang großer Dichtesprünge auftreten und zu hydrodynamische Instabilitäten führen, sind besonders ungünstig für SPH, da es hier zu großen Ungenauigkeiten kommen kann. Eine Anwendung, in der solche Situationen zu erwarten sind, ist der Einfall einer Galaxie in einen Galaxienhaufen. Dabei verliert die Galaxie aufgrund des anströmenden Galaxienhaufen-Gases zunehmend Gas an den Galaxienhaufen. Da SPH aufgrund seiner Dichtebestimmung diesen Prozess nicht korrekt simuliert, ermittelt SPH einen zu geringen Verlust von Gas. Wir konnten dies mit Hilfe unserer Simulationen belegen. Wir haben diese Resultate sowohl mit Simulationen von Galaxien, die in einen Galaxienhaufen fallen, als auch mit kosmologischen Simulationen von sich bildenden Galaxienhaufen überprüft. Dort bestätigte sich, dass in SPH der Gasverlust der einfallenen Galaxien zu gering ist. Desweiteren ist der Gasverlust in den AREPO Simulationen stets am höchsten, während VPH eine mittlere Stellung einnimmt. Wir konnten ingesamt zeigen, dass VPH in Situationen mit großem Dichtekontrast eine Verbesserung zu SPH darstellt. Auch wenn unsere Resultate keine vollständige Übereinstimmung mit dem Gitter-basierten AREPO Code zeigen, stellen sie doch eine wichtige Annährung zwischen Teilchen- und Gitter-basierten hydrodynamischen Verfahren dar. VPH empfiehlt sich vor allem als eine gegenüber SPH verbesserte Methode zur Simulation von hydrodynamischen Prozesssen in kosmologischen Problemen

Numerical Simulations of the Dark Universe: State of the Art and the Next Decade

We present a review of the current state of the art of cosmological dark matter simulations, with particular emphasis on the implications for dark matter detection efforts and studies of dark energy. This review is intended both for particle physicists, who may find the cosmological simulation literature opaque or confusing, and for astro-physicists, who may not be familiar with the role of simulations for observational and experimental probes of dark matter and dark energy. Our work is complementary to the contribution by M. Baldi in this issue, which focuses on the treatment of dark energy and cosmic acceleration in dedicated N-body simulations. Truly massive dark matter-only simulations are being conducted on national supercomputing centers, employing from several billion to over half a trillion particles to simulate the formation and evolution of cosmologically representative volumes (cosmic scale) or to zoom in on individual halos (cluster and galactic scale). These simulations cost millions of core-hours, require tens to hundreds of terabytes of memory, and use up to petabytes of disk storage. The field is quite internationally diverse, with top simulations having been run in China, France, Germany, Korea, Spain, and the USA. Predictions from such simulations touch on almost every aspect of dark matter and dark energy studies, and we give a comprehensive overview of this connection. We also discuss the limitations of the cold and collisionless DM-only approach, and describe in some detail efforts to include different particle physics as well as baryonic physics in cosmological galaxy formation simulations, including a discussion of recent results highlighting how the distribution of dark matter in halos may be altered. We end with an outlook for the next decade, presenting our view of how the field can be expected to progress. (abridged)Comment: 54 pages, 4 figures, 3 tables; invited contribution to the special issue "The next decade in Dark Matter and Dark Energy" of the new Open Access journal "Physics of the Dark Universe". Replaced with accepted versio

Resonant scattering of the OVII X-ray emission line in the circumgalactic medium of TNG50 galaxies

We study the impact of resonantly scattered X-ray line emission on the observability of the hot circumgalactic medium (CGM) of galaxies. We apply a Monte Carlo radiative transfer post-processing analysis to the high-resolution TNG50 cosmological magnetohydrodynamical galaxy formation simulation. This allows us to model the resonant scattering of OVII(r) X-ray photons within the complex, multi-phase, multi-scale CGM. The resonant transition of the OVII He-like triplet is one of the brightest, and most promising, X-ray emission lines for detecting the hot CGM and measuring its physical properties. We focus on galaxies with stellar masses 10 < log(M*/Msun) < 11 at z ~ 0. After constructing a model for OVII(r) emission from the central galaxy as well as from CGM gas, we forward model these intrinsic photons to derive observable surface brightness maps. We find that scattering significantly boosts the observable OVII(r) surface brightness of the extended and diffuse CGM. This enhancement can be large -- an order of magnitude on average at a distance of 200 projected kpc for high-mass M* = 10^10.7 Msun galaxies. The enhancement is larger for lower mass galaxies, and can even reach a factor of 100, across the extended CGM. Galaxies with higher star formation rates, AGN luminosities, and central OVII(r) luminosities all have larger scattering enhancements, at fixed stellar mass. Our results suggest that next-generation X-ray spectroscopic missions including XRISM, LEM, ATHENA, and HUBS -- which aim to detect the hot CGM in emission -- could specifically target halos with significant enhancements due to resonant scattering.Comment: Published in MNRAS. See https://www.lem-observatory.org/ and https://www.tng-project.org/ for more details; 2023MNRAS.522.3665

INVESTIGATIONS OF DARK MATTER USING COSMOLOGICAL SIMULATIONS

In the successful concordance model of cosmology, dark matter is crucial for structures to form as we observe it in the universe. Despite the overwhelming observational evidence for its existence, it is not yet directly detected, and its nature is largely unknown. Physicists propose various dark matter candidates, with masses ranging over dozens of orders of magnitude. However, both indirect and direct detection experiments for dark matter have reported no convincing results. Dark matter research is therefore critically relying on computer simulations. Using supercomputer numerical simulations, we can test the correctness of the current cosmological model, as well as obtain guidance for future detection experiments. In this dissertation, we study dark matter from several perspectives using cosmological simulations: its possible radiation, its warmth, and other related issues. A commonly accepted candidate for dark matter is the weakly interacting massive particle (WIMP). WIMPs interact with normal matter only through the weak force (as well as gravity). It is thus extremely challenging to detect these particles directly. However, depending on the type of dark matter, they can %self-annihilate annihilate with other dark matter particles, or decay into high-energy photons (i.e., $\gamma$-ray). We studied the spatial distribution of possible emission components from dark matter annihilation or decay in a large simulation of a galaxy like the Milky Way. The predicted emission components can be used as templates for observations such as those from the {\it Fermi}/LAT $\gamma$-ray instrument, to constrain for the physical properties of dark matter. Structure formation theory suggests that dark matter is ``cold'', i.e., moving non-relativistically during structure formation. However, cold dark matter predicts many more dark-matter satellites, or subhaloes, around galaxies such as the Milky Way than observed. One well-established mechanism to bring the theory in line with observations is that many of these satellites are not visible because they are too small for baryons to form stars in them. Another way is to attenuate the small-scale structure directly, positing ``warm'' dark matter. Using simulation, we propose a method of testing this possibility in a complementary environment, by measuring the density profile of cosmic voids. Our results suggest that there are sufficient differences between warm and cold dark matter to test using future observations. Furthermore, our data analyzing methods are based on sophisticated data stream algorithms and newly developed Graphic Process Unit (GPU) hardware. These tools lead to other studies of dark matter as well. For example, we studied the spin alignment of dark matter halos with its environment. We show that the spin alignments are highly related to the hierarchical levels of the cosmic web, in which the halo is located. We also studied the responses in different density variables to ``ringing'' the initial density field at different spatial frequencies (i.e. putting spikes in the power spectrum at a particular scale). The conventional wisdom is that power generally migrates from large to small comoving scales from the initial to final conditions. But in this work, we found that this conventional wisdom is only true for a density variable emphasizing dense regions, such as the usual overdensity field. In the log-density field, however, power stays about at the same scale but broadens. In the reciprocal-density field, emphasizing low-density regions, power moves to larger scales. This is an example of voids as ``cosmic magnifying glasses.'' The GPU density-estimation technique was crucial for this study, allowing the density to be estimated accurately even when modestly sampled with particles. Our results provide guidance for designing future statistic analytics for dark matter and the large-scale structure of the Universe in general

Growth and fuelling of galactic nuclei

In den letzten Jahrzehnten wurde durch Beobachtungen gezeigt dass die meisten Galaxien eine zentral gebundene Struktur aufweisen: einen zentralen Sternhaufen. Diese stellaren Systeme gehren zu den dichtesten Objekten des Universums und es wird angenommen, dass ihre Entwicklung mit der Entwicklung der Galaxie zusammenhngt. Aufgrund ihres hufigen Vorkommens, eignen sich zentrale Sternhaufen zur Untersuchung von Galaxien. Obwohl die Entstehung der zentralen Sternhaufen noch nicht wirklich verstanden ist, wer- den momentan zwei mgliche Szenarien angenommen: in-situ Entstehung, bei der sich Gas im galaktischen Zentrum anhuft und verdichtet bis Sterne darin entstehen, und das dry merger Szenario, bei dem die Sternhaufen aus der Scheibe durch dynamische Reibung ins Zentrum wandern und sich miteinander vereinen. Das Ziel dieses Projektes ist es, die Prozesse einzugrenzen, die fr die Entstehung und das Wachstum der zentralen Sternhaufen verantwortlich sind. Zu Beginn dieser Arbeit wird ein Beobachtungsbeispiel vorgestellt, in dem Nahinfrarot-Beobachtungen mit dem Instrument SINFONI von der zentralen Region der unter kleiner Inklination erscheinenden Galaxie NGC 300 gemacht wurden. Ich erklre, wie ich diese Daten mit der SINFONI Pipeline reduziert habe und beschreibe detailliert, wie die Daten analysiert wurden, um eine kinematische Karte des galaktischen Zentrums zu erstellen. Erste Ergebnisse zeigen, dass das galaktische Zentrum nicht rotiert und nur sehr geringe Geschwindigkeitsdispersionen aufweist. Aus einer theoretischen Perspektive heraus, untersuche ich die Entstehung und das Wachstum zentraler Sternhaufen-Vorgnger mit state-of-the-art hydrodynamischen Simu- lationen gasreicher Zwerggalaxien mit vorbestimmten Eigenschaften und einer rumlichen Auflsung von einigen Parsec. Ein Schlsselergebnis dieser Studie ist, dass das galaktis- che Zentrum durch sogenannte wet-merger entstehen kann, welche die Prozesse der bei- den genannten Szenarien verbindet: Ein massiver Sternhaufen entsteht in der gasreichen Scheibe, behlt seinen Gasvorrat und wchst weiter whrend er ins galaktische Zentrum wan- dert. In solchen gasreichen Umgebungen formt das induzierte stellare Feedback die Eigen- schaften des Sternhaufens und kann mglicherweise die Entstehung der zentralen Stern- haufen abndern. Vor allem der Strahlungsdruck scheint die wichtigste Rolle zu spielen, bei der Zerstrung der dichten Gasstrukturen und beim Abndern und Abschwchen der Haufe- nentstehung. Zuletzt untersuche ich die Entwicklung von Sternhaufen wenn ein Gashalo kollabiert, um eine neue Galaxie zu bilden und zu formen. In dieser Situation ist es eine Heraus- forderung, eine Population stabiler Sternhaufen in den ersten paar hundert Millionen Jahren zu bilden, da alle Haufen durch den Effekt des Strahlungsdrucks wieder zerstrt werden. Letzterer tendiert dazu, das Gas von den Haufen weg zu treiben und verursacht Lcken in der Gasdichte und fhrt zur Expansion der Scheibe. Die dominanten und zerstrerischen Effekte, die Strahlungs-Feedback auf Sternhaufen haben kann, stellt das berleben letzterer in Frage. Da Sternhaufen sowohl bei kleinen als auch bei grossen Rotverschiebungen beobachtet werden, rufen unsere vorlufigen Ergebnisse dazu auf, dass diese Art von Feedback und Ihre Wirkung auf kleinen und grossen Skalen besser verstanden werden muss.Over the last decades, observations have shown that a majority of galaxies host a bound structure at their centre: a nuclear cluster. These stellar systems are among the densest objects in the universe and it has been suggested that their evolutionary path is closely linked to that of their host. Due to their ubiquity, nuclear clusters are objects of choice to study galaxies. Although the formation of nuclear cluster is still poorly understood, the current paradigm offers two possible scenarios: “in-situ” formation where gas piles up at the galactic centre and collapses into stars, and “dry-merger ” scenario where star clusters in the disc migrate towards the galactic centre through dynamical friction and merge. The aim of this project is to constrain the fuelling and growth mechanisms at play in the formation of nuclear clusters. The thesis first presents an example of observations in the near-infrared of the nuclear region of the low-inclined galaxy NGC 300 with the SINFONI instrument. I explain how I reduced these data using the SINFONI pipeline and detail the first steps in their analysis, leading to the kinematic maps of the nucleus. Preliminary results show the apparent absence of rotation of the nucleus with low velocity dispersion. From a theoretical perspective, I study the formation and growth of nuclear cluster progenitors using state-of-the-art hydrodynamical simulations of gas-rich dwarf galaxies with predetermined properties, at parsec resolution. A key result is that a nucleus can form through a “wet-merger” which combines the processes involved in the two paradigm scenarios: a massive star cluster forms in the gas-rich disc, keeps a gas reservoir, and grows further while migrating to the centre. In such gas-rich environments, the induced stellar feedback shapes the properties of star clusters and can potentially alter the formation channel of nuclear clusters. In particular, the radiation feedback seems to play the most important role in destroying dense gas structures, and altering or quenching the subsequent cluster formation. I finally study the evolution of star clusters when a gaseous halo collapses to form and shape the galaxy from scratch. In this situation, it is challenging to form a stable star cluster population during the first few hundreds of Myr, with all clusters being destroyed by the effect of radiative feedback. The latter tends to expel the gas away from the clusters, creates gaps in the gas density and leads to the expansion of the disc. The dominant and damaging effects that radiation feedback can have on star clusters question their survivability. Since clusters are observed both at low and high redshift, our preliminary results call for a better understanding of the inner workings of this mode of feedback at small and large scales

Machine learning in galaxy groups detection

The detection of galaxy groups and clusters is of great importance in the field of astrophysics. In particular astrophysicists are interested in the evolution and formation of these systems, as well as the interactions that occur within galaxy groups and clusters. In this thesis, we developed a probabilistic model capable of detecting galaxy groups and clusters based on the Hough transform. We called this approach probabilistic Hough transform based on adaptive local kernel (PHTALK). PHTALK was tested on a 3D realistic galaxy and mass assembly (GAMA) mock data catalogue (at close redshift z < 0:1) (mock data: contains information related to galaxies' position, redshift and other properties). We compared the performance of our PHTALK method with the performance of two versions of the standard friends-of-friends (FoF) method. As a performance measures, we used the precision versus recall curve. Furthermore, to test the efficiency of recovering the galaxy groups' and clusters' properties, we also used completeness and reliability, fragmentation and merging, velocity and mass estimation of the detected groups. The new PHTALK method outperformed the FoF methods in terms of reducing the detection of spurious agglomerations (false positives (FPs)). This smaller sensitivity to the false positive (FP) is mainly due to the clear description of the galaxy groups' model based on astrophysical prior knowledge; in particular, the fingers of god (FoG) pattern (a pattern formed by the projected velocity dispersion of galaxies, inside a galaxy group, along the line of sight). However, the FoF methods seem to outperform the PHTALK in terms of detecting galaxy groups or clusters that do not follow the FoG pattern. The main advantage of our probabilistic model is its flexibility to incorporate any prior knowledge expressed in terms of a galaxy group model

The stellar structure and outer disk kinematics of high-redshift galaxies from near-infrared observations

The universe at redshift 1 < z < 3 represents the peak epoch of rapid galaxy mass assembly and very active star-formation in galaxies, but also poses many observational challenges. This thesis addresses the buildup of galaxy mass as well as the shut-down of star formation in galaxies (referred to as 'quenching') using state-of-the-art spatially resolved observations of galaxies at high redshift from ground- and space based near-infrared (NIR) datasets. The first part of this thesis presents an analysis of the stellar morphology of massive galaxies (M_star > 10^10 M_sun) at 0.5 < z < 2.5 on the basis of the CANDELS dataset, providing deep rest-frame Ultraviolet(UV)-to-NIR imaging from the Hubble Space Telescope (HST) at high angular resolution. This is complemented by grism spectroscopy from the 3D-HST survey used to derive accurate redshift information. Both stellar mass and rest-frame optical light distributions of 6764 galaxies are quantified by performing single Sersic fits as well as bulge-to-disk decompositions. The stellar mass distributions are reconstructed through resolved stellar population modeling on the panchromatic imaging dataset. The results show that quiescent galaxies at high redshift possess increased bulge fractions compared to their star-forming counterparts as seen in their mass distribution, previously only observed in rest-frame optical light. Moreover, the Sersic index and bulge-to-total ratio (B/T) among star- forming galaxies show an increase towards higher stellar masses (with the median B/T reaching 40-50 % above 10^11 M_sun), hinting at significant bulge growth of star-forming galaxies along the main sequence before quenching. The bulge mass of a galaxy appears to be a more reliable predictor of quiescence than total stellar mass or disk mass. These empirical results and a further comparison to state-of-the-art theoretical models support that possible quenching mechanisms are internal to galaxies and closely associated with bulge growth. \\ The second part of this work focuses on the outer disk kinematics of star-forming galaxies at high redshift on the basis of large and deep Integral-Field-Unit (IFU) datasets tracing the resolved ionized gas kinematics from H-alpha. Both the ongoing KMOS-3D survey and the subset of the SINS/zc-SINF survey observed in adaptive optics assisted mode, are exploited to build a sample of ~ 100 massive star forming disk galaxies at 0.7 < z < 2.6. Employing a novel stacking approach, a representative rotation curve reaching out to several effective radii can be robustly constrained. The stacked rotation curve exhibits a significant decrease in rotation velocity beyond the turnover. This result confirms, and extends to a larger sample, the falloff that had so far been observed in a handful of individual high-z disks with best data quality and signal-to-noise ratio. A comparison with models shows that the falling outer rotation curve can be explained by a high mass fraction of baryons in the disk relative to the dark matter halo (m_d = 0.05 -0.1) in combination with a significant level of pressure support in the outer disk (sigma_0 = 35 km/s). These findings confirm the high baryon fractions found by comparing the stellar, gas and dynamical masses of high redshift galaxies independently of assumptions on the light-to-mass conversion and Initial stellar Mass Function (IMF). The rapid falloff of the stacked rotation curve can be explained by pressure gradients, which are significant in the gas-rich, turbulent high-z disks and suggests a possible pressure-driven truncation of the outer disk. \\ Lastly, a derivation of beam smearing corrections is presented that is applicable to high-redshift IFU datasets to recover the intrinsic values of rotation velocity and velocity dispersion. The corrections are based on simulated mock datacubes to mimic real IFU observations for a wide range of various intrinsic galaxy parameters assuming exponential disks. The correction for rotation velocity only depends on the size of the galaxy versus the size of the instrumental spatial point spread function (PSF), and fitting functions for the corrections to be easily applied to large datasets are presented. The corrections for velocity dispersion depend on several additional intrinsic galaxy parameters such as the inclination angle and dynamical mass. Based on the grid of models spanning a wide range in these galaxy parameters, the correction for velocity dispersion can be applied to any observed source

Investigating the Environmental Properties of Galaxies in the SDSS-MaNGA Survey

This thesis presents a study of galaxy evolution in the local universe. I study how environments shape the structures of galaxies, and how internal and external processes affect star formation. I perform four investigations of galaxy properties: a study of the relations between size, mass and velocity dispersion of 124,524 galaxies from SDSS DR7; I estimate star formation rates using Hα and Dn4000 for galaxies in the MaNGA survey; a study of the spatial distribution of star formation in 1494 MaNGA galaxies; and finally, a study of 215 barred and 402 unbarred galaxies, to investigate how bars affect star formation. I find that environment plays a key role in the evolution of galaxies, both structurally and in terms of their star formation. Using core velocity dispersion to study the effects of minor mergers and tidal/ram pressure stripping, I find that central galaxies are up to 30% larger and more massive than satellites. I suggest that minor mergers play a crucial role in the increase in size and mass of centrals. In addition, I find that satellites have a uniform radial suppression of star formation, compared to centrals, which may be due to the strangulation of their cold gas supplies. I study the internal processes that affect star formation and find that specific star formation rate is suppressed at all radii for high mass galaxies. Massive galaxies are more likely to have suppressed star formation in their cores, which I determined is caused by a combination of morphological quenching and AGN feedback. Finally, I study the role of galaxy bars in regulating the cirumnuclear and disk star formation in late-type galaxies. I find that barred galaxies have lower star formation in their disks than unbarred galaxies, and that they are more likely to have enhanced star formation in their cores