Assembly Bias and Splashback in Galaxy Clusters

We use publicly available data for the Millennium Simulation to explore the implications of the recent detection of assembly bias and splashback signatures in a large sample of galaxy clusters. These were identified in the SDSS/DR8 photometric data by the redMaPPer algorithm and split into high- and low-concentration subsamples based on the projected positions of cluster members. We use simplified versions of these procedures to build cluster samples of similar size from the simulation data. These match the observed samples quite well and show similar assembly bias and splashback signals. Previous theoretical work has found the logarithmic slope of halo density profiles to have a well-defined minimum whose depth decreases and whose radius increases with halo concentration. Projected profiles for the observed and simulated cluster samples show trends with concentration which are opposite to these predictions. In addition, for high-concentration clusters the minimum slope occurs at significantly smaller radius than predicted. We show that these discrepancies all reflect confusion between splashback features and features imposed on the profiles by the cluster identification and concentration estimation procedures. The strong apparent assembly bias is not reflected in the three-dimensional distribution of matter around clusters. Rather it is a consequence of the preferential contamination of low-concentration clusters by foreground or background groups.Comment: 17 pages, 16 figures, 3 tables, accepted versio

Spatial description of large scale structure and reionisation

This thesis is the result of a PhD project that tried to investigate and find new descriptions of entities arising in large scale structure based upon their spatial configuration. For this we analyse N-body simulations of gravitational collapse in a cold dark matter universe with cosmological constant (ΛCDM) and Monte Carlo ray-tracing radiative transfer (MCRTRT) simulations of reionisation. We also use an N-body simulations to investigate possible problems with observational results connected to large scale clustering. In the first part of this thesis we develop a novel technique to characterise the density field in cosmological N-body simulations based upon a density estimate and the connectivity between particles obtained from a Voronoi tessellation of their positions. We use this estimate to find a hierarchical set of peaks in the Millennium and Millennium II simulations. This hierarchy completely decomposes the particle load of the simulations into nodes in a single tree structure we call “Tessellation Level Tree” (TLT). We investigate the properties of these peaks and concentrate on two novel aspects: the percolation of the connected set of peaks above densities of a few (6 − 7) along the cosmic web and the very strong assembly bias effect if peaks are split by saddle point density. This assembly bias effect is the strongest ever obtained from quintiles in a local property of the dark matter distribution in simulations. The second part of the thesis investigates the morphology of ionised bubbles in hydrogen and helium during reionisation. For this we use MCRTRT on regular grids and create binary representations of the ionisation fields using a threshold. We then apply techniques of mathematical morphology to extract a hierarchy of bubbles ordered by local diameter. We show the shift in the global bubble size distribution throughout reionisation and how the ionised volume is more and more unequally distributed among the bubbles as they grow and overlap. The overlap also results in a percolation process we identify at z ≳ 8 that increasingly delocalises the reionisation process. Finally, we connect the bubbles to the properties of the underlying density field. For the first time we show that the largest bubbles in the post-overlap regime are not densest in the centre are very strongly biased with respect to the large scale matter distribution. We also quantify how ionisation reaches the most underdense parts of the universe last, reconfirming the inside-out scenario of reionisation. In the final part of the thesis we test the assembly bias and splashback radius mea- surements claimed by previous publications using clusters obtained with the optical cluster finder redMaPPer. For this we develop a mock-version of the algorithm that incorporates the core aspects of the cluster identification and apply it to a semi-analytic galaxy popula- tion of the Millennium simulation. We show that the claimed concentration differences in the optically selected clusters are most likely stemming from projection effects that arise more in overdense regions, leading to a coupling between concentration and large scale clustering and therefore a false positive assembly bias detection. The claimed splashback radius identification that is inverse in connection with cluster properties compared to the results of numerical simulations is shown to be an artifact of the circular mask of the selection algorithm.Diese Arbeit ist das Ergebnis eines Dissertationsprojektes, das versuchte, neue Beschreibungen von Entitäten in großräumigen Strukturen basierend auf ihrer räumlichen Konfiguration zu finden und zu untersuchen. Dazu analysieren wir N-Körper-Simulationen des gravitativen Kollapses in einem Universum gefüllt kalter dunklen Materie mit kosmologischer Konstante (ΛCDM) und Monte-Carlo-Raytracing Strahlungstransfer (MCRTRT) Simulationen der Reionisation. Wir verwenden auch eine N-Körper-Simulation, um mögliche Probleme mit Beobachtungsergebnissen im Zusammenhang mit großräumigem Clustering zu untersuchen. Im ersten Teil dieser Arbeit entwickeln wir eine neuartige Technik zur Charakterisierung des Dichtefeldes in kosmologischen N-Körper-Simulationen, die auf einer Dichteabschätzung und der Konnektivität zwischen Teilchen basiert, die aus einer Voronoi-Tessellierung ihrer Positionen gewonnen wurden. Wir verwenden diese Schätzung, um einen hierarchischen Satz von Dichtespitzen in der Millennium- und Millennium-II-Simulationen zu finden. Diese Hierarchie zerlegt die Partikelbelastung der Simulationen vollständig in Knoten in einer einzigen Baumstruktur, die wir ”Tessellation Level Tree” (TLT) nennen. Wir untersuchen die Eigenschaften dieser Dichtespitzen und konzentrieren uns auf zwei neuartige Aspekte: die Perkolation des verbundenen Satzes von Dichtespitzen über Dichten von wenigen (6 − 7) entlang des kosmischen Netzes und den sehr starken Assembly-Bias- Effekt, wenn Dichtespitzen nach der Sattelpunktdichte getrennt werden. Dieser Assembly- Bias-Effekt ist der stärkste, der je aus Quintilen in einer lokalen Eigenschaft der Verteilung der Dunklen Materie in Simulationen erhalten wurde. Der zweite Teil der Arbeit untersucht die Morphologie ionisierter Blasen in Wasserstoff und Helium während der Reionisierung. Dazu verwenden wir MCRTRT auf regulären Gittern und erstellen binäre Darstellungen der Ionisationsfelder mithilfe eines Schwellenwerts. Wir wenden dann Techniken der mathematischen Morphologie an, um eine Hierarchie von Blasen zu extrahieren, die nach lokalem Durchmesser geordnet sind. Wir zeigen die Verschiebung der globalen Blasengrößenverteilung während der Reionisierung und wie das ionisierte Volumen immer ungleichmäßiger unter den Blasen verteilt ist, wenn sie wachsen und sich überlappen. Die Überschneidung führt auch zu einem Perko- lationsprozess, den wir bei z ≳ 8 identifizieren, der den Ionisationsprozess zunehmend delokalisiert. Schließlich verbinden wir die Blasen mit den Eigenschaften des darunter liegenden Dichtefeldes. Wir zeigen erstmals, dass die größten Blasen nach der Überlappung nicht im Zentrum am dichtesten sind und einen sehr starken Bias in Bezug auf die großräumige Materieverteilung aufweisen. Wir quantifizieren auch, wie die Reionisierung zuletzt die am wenigsten dichten Teile des Universums erreicht, und bestätigen damit das Inside-Out-Szenario der Reionisierung. Im letzten Teil der Arbeit testen wir die Montage Bias- und Rückfallradiusmessungen, die von früheren Publikationen unter Verwendung von Clustern, die mit dem optischen Clusterfinder redMaPPer erhalten wurden, beansprucht wurden. Dazu entwickeln wir eine Mock-Version des Algorithmus, der die Kernaspekte der Clusteridentifikation berücksichtigt und auf eine semi-analytische Galaxienpopulation der Millennium-Simulation anwendet. Wir zeigen, dass die behaupteten Konzentrationsunterschiede in den optisch ausgewählten Clustern höchstwahrscheinlich auf Projektionseffekte zurückzuführen sind, die eher in überdichten Regionen auftreten, was zu einer Kopplung zwischen Konzentration und großflächigem Clustering und damit zu einer falsch-positiven Verzerrung der Baugruppe führt. Die beanspruchte Rückfallradius-Identifikation, die im Zusammenhang mit Cluster-Eigenschaften im Vergleich zu den Ergebnissen numerischer Simulationen invers ist, erscheint als Artefakt der Kreismaske des Auswahlalgorithmus

Spatial description of large scale structure and reionisation

This thesis is the result of a PhD project that tried to investigate and find new descriptions of entities arising in large scale structure based upon their spatial configuration. For this we analyse N-body simulations of gravitational collapse in a cold dark matter universe with cosmological constant (ΛCDM) and Monte Carlo ray-tracing radiative transfer (MCRTRT) simulations of reionisation. We also use an N-body simulations to investigate possible problems with observational results connected to large scale clustering. In the first part of this thesis we develop a novel technique to characterise the density field in cosmological N-body simulations based upon a density estimate and the connectivity between particles obtained from a Voronoi tessellation of their positions. We use this estimate to find a hierarchical set of peaks in the Millennium and Millennium II simulations. This hierarchy completely decomposes the particle load of the simulations into nodes in a single tree structure we call “Tessellation Level Tree” (TLT). We investigate the properties of these peaks and concentrate on two novel aspects: the percolation of the connected set of peaks above densities of a few (6 − 7) along the cosmic web and the very strong assembly bias effect if peaks are split by saddle point density. This assembly bias effect is the strongest ever obtained from quintiles in a local property of the dark matter distribution in simulations. The second part of the thesis investigates the morphology of ionised bubbles in hydrogen and helium during reionisation. For this we use MCRTRT on regular grids and create binary representations of the ionisation fields using a threshold. We then apply techniques of mathematical morphology to extract a hierarchy of bubbles ordered by local diameter. We show the shift in the global bubble size distribution throughout reionisation and how the ionised volume is more and more unequally distributed among the bubbles as they grow and overlap. The overlap also results in a percolation process we identify at z ≳ 8 that increasingly delocalises the reionisation process. Finally, we connect the bubbles to the properties of the underlying density field. For the first time we show that the largest bubbles in the post-overlap regime are not densest in the centre are very strongly biased with respect to the large scale matter distribution. We also quantify how ionisation reaches the most underdense parts of the universe last, reconfirming the inside-out scenario of reionisation. In the final part of the thesis we test the assembly bias and splashback radius mea- surements claimed by previous publications using clusters obtained with the optical cluster finder redMaPPer. For this we develop a mock-version of the algorithm that incorporates the core aspects of the cluster identification and apply it to a semi-analytic galaxy popula- tion of the Millennium simulation. We show that the claimed concentration differences in the optically selected clusters are most likely stemming from projection effects that arise more in overdense regions, leading to a coupling between concentration and large scale clustering and therefore a false positive assembly bias detection. The claimed splashback radius identification that is inverse in connection with cluster properties compared to the results of numerical simulations is shown to be an artifact of the circular mask of the selection algorithm.Diese Arbeit ist das Ergebnis eines Dissertationsprojektes, das versuchte, neue Beschreibungen von Entitäten in großräumigen Strukturen basierend auf ihrer räumlichen Konfiguration zu finden und zu untersuchen. Dazu analysieren wir N-Körper-Simulationen des gravitativen Kollapses in einem Universum gefüllt kalter dunklen Materie mit kosmologischer Konstante (ΛCDM) und Monte-Carlo-Raytracing Strahlungstransfer (MCRTRT) Simulationen der Reionisation. Wir verwenden auch eine N-Körper-Simulation, um mögliche Probleme mit Beobachtungsergebnissen im Zusammenhang mit großräumigem Clustering zu untersuchen. Im ersten Teil dieser Arbeit entwickeln wir eine neuartige Technik zur Charakterisierung des Dichtefeldes in kosmologischen N-Körper-Simulationen, die auf einer Dichteabschätzung und der Konnektivität zwischen Teilchen basiert, die aus einer Voronoi-Tessellierung ihrer Positionen gewonnen wurden. Wir verwenden diese Schätzung, um einen hierarchischen Satz von Dichtespitzen in der Millennium- und Millennium-II-Simulationen zu finden. Diese Hierarchie zerlegt die Partikelbelastung der Simulationen vollständig in Knoten in einer einzigen Baumstruktur, die wir ”Tessellation Level Tree” (TLT) nennen. Wir untersuchen die Eigenschaften dieser Dichtespitzen und konzentrieren uns auf zwei neuartige Aspekte: die Perkolation des verbundenen Satzes von Dichtespitzen über Dichten von wenigen (6 − 7) entlang des kosmischen Netzes und den sehr starken Assembly-Bias- Effekt, wenn Dichtespitzen nach der Sattelpunktdichte getrennt werden. Dieser Assembly- Bias-Effekt ist der stärkste, der je aus Quintilen in einer lokalen Eigenschaft der Verteilung der Dunklen Materie in Simulationen erhalten wurde. Der zweite Teil der Arbeit untersucht die Morphologie ionisierter Blasen in Wasserstoff und Helium während der Reionisierung. Dazu verwenden wir MCRTRT auf regulären Gittern und erstellen binäre Darstellungen der Ionisationsfelder mithilfe eines Schwellenwerts. Wir wenden dann Techniken der mathematischen Morphologie an, um eine Hierarchie von Blasen zu extrahieren, die nach lokalem Durchmesser geordnet sind. Wir zeigen die Verschiebung der globalen Blasengrößenverteilung während der Reionisierung und wie das ionisierte Volumen immer ungleichmäßiger unter den Blasen verteilt ist, wenn sie wachsen und sich überlappen. Die Überschneidung führt auch zu einem Perko- lationsprozess, den wir bei z ≳ 8 identifizieren, der den Ionisationsprozess zunehmend delokalisiert. Schließlich verbinden wir die Blasen mit den Eigenschaften des darunter liegenden Dichtefeldes. Wir zeigen erstmals, dass die größten Blasen nach der Überlappung nicht im Zentrum am dichtesten sind und einen sehr starken Bias in Bezug auf die großräumige Materieverteilung aufweisen. Wir quantifizieren auch, wie die Reionisierung zuletzt die am wenigsten dichten Teile des Universums erreicht, und bestätigen damit das Inside-Out-Szenario der Reionisierung. Im letzten Teil der Arbeit testen wir die Montage Bias- und Rückfallradiusmessungen, die von früheren Publikationen unter Verwendung von Clustern, die mit dem optischen Clusterfinder redMaPPer erhalten wurden, beansprucht wurden. Dazu entwickeln wir eine Mock-Version des Algorithmus, der die Kernaspekte der Clusteridentifikation berücksichtigt und auf eine semi-analytische Galaxienpopulation der Millennium-Simulation anwendet. Wir zeigen, dass die behaupteten Konzentrationsunterschiede in den optisch ausgewählten Clustern höchstwahrscheinlich auf Projektionseffekte zurückzuführen sind, die eher in überdichten Regionen auftreten, was zu einer Kopplung zwischen Konzentration und großflächigem Clustering und damit zu einer falsch-positiven Verzerrung der Baugruppe führt. Die beanspruchte Rückfallradius-Identifikation, die im Zusammenhang mit Cluster-Eigenschaften im Vergleich zu den Ergebnissen numerischer Simulationen invers ist, erscheint als Artefakt der Kreismaske des Auswahlalgorithmus

Opening Reionization: Quantitative Morphology of the Epoch of Reionization and Its Connection to the Cosmic Density Field

We introduce a versatile and spatially resolved morphological characterisation of binary fields, rooted in the opening transform of mathematical morphology. We subsequently apply it to the thresholded ionization field in simulations of cosmic reionization and study the morphology of ionized regions. We find that an ionized volume element typically resides in an ionized region with radius $\sim8\,h^{-1}\mathrm{cMpc}$ at the midpoint of reionization ($z\approx7.5$) and follow the bubble size distribution even beyond the overlap phase. We find that percolation of the fully ionized component sets in when 25% of the universe is ionized and that the resulting infinite cluster incorporates all ionized regions above $\sim8\,h^{-1}\mathrm{cMpc}$. We also quantify the clustering of ionized regions of varying radius with respect to matter and on small scales detect the formation of superbubbles in the overlap phase. On large scales we quantify the bias values of the centres of ionized and neutral regions of different sizes and not only show that the largest ones at the high-point of reionization can reach $b\approx 30$, but also that early small ionized regions are positively correlated with matter and large neutral regions and late small ionized regions are heavily anti-biased with respect to matter, down to $b\lesssim-20$.Comment: 18 pages, 15 figure, as accepted for publication by MNRA

A CRASH simulation of the contribution of binary stars to the epoch of reionization

We use a set of 3D radiative transfer simulations to study the effect that a large fraction of binary stars in galaxies during the epoch of reionization has on the physical properties of the intergalactic medium (i.e. the gas temperature and the ionization state of hydrogen and helium), on the topology of the ionized bubbles and on the 21 cm power spectra. Consistently to previous literature, we find that the inclusion of binary stars can speed up the reionization process of HI and HeI, while HeII reionization is still dominated by more energetic sources, especially accreting black holes. The earlier ionization attained with binary stars allows for more time for cooling and recombination, so that gas fully ionized by binary stars is typically colder than that ionized by single stars at any given redshift. With the same volume averaged ionization fraction, the inclusion of binary stars results in fewer small ionized bubbles and more large ones, with visible effects also on the large scales of the 21 cm power spectrum.Comment: 14 pages, 11 figures, MNRAS accepte

Neuromuscular Control During Stair Descent and Artificial Tibial Translation After Acute ACL Rupture.

Background Anterior cruciate ligament (ACL) rupture has direct effect on passive and active knee stability and, specifically, stretch-reflex excitability. Purpose/Hypothesis The purpose of this study was to investigate neuromuscular activity in patients with an acute ACL deficit (ACL-D group) compared with a matched control group with an intact ACL (ACL-I group) during stair descent and artificially induced anterior tibial translation. It was hypothesized that neuromuscular control would be impaired in the ACL-D group. Study Design Cross-sectional study; Level of evidence, 3. Methods Surface electromyographic (EMG) activity of the vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF), and semitendinosus (ST) muscles was recorded bilaterally in 15 patients with ACL-D (mean, 13.8 days [range, 7-21 days] since injury) and 15 controls with ACL-I during stair descent and artificially induced anterior tibial translation. The movements of stair descent were divided into preactivity, weight acceptance, and push-off phases. Reflex activity during anterior tibial translation was split into preactivity and short, medium, and late latency responses. Walking on a treadmill was used for submaximal EMG normalization. Kruskal-Wallis test and post hoc analyses with Dunn-Bonferroni correction were used to compare normalized root mean square values for each muscle, limb, movement, and reflex phase between the ACL-D and ACL-I groups. Results During the preactivity phase of stair descent, the hamstrings of the involved leg of the ACL-D group showed 33% to 51% less activity compared with the matched leg and contralateral leg of the ACL-I group (P < .05). During the weight acceptance and push-off phases, the VL revealed a significant reduction (approximately 40%) in the involved leg of the ACL-D group compared with the ACL-I group. At short latency, the BF and ST of the involved leg of the ACL-D group showed a significant increase in EMG activity compared with the uninvolved leg of the ACL-I group, by a factor of 2.2 to 4.6. Conclusion In the acute phase after an ACL rupture, neuromuscular alterations were found mainly in the hamstrings of both limbs during stair descent and reflex activity. The potential role of prehabilitation needs to be further studied

Probing the high-z IGM with the hyperfine transition of 3^3He+^+

The hyperfine transition of $^3$He$^+$ at 3.5cm has been thought as a probe of the high-z IGM since it offers a unique insight into the evolution of the helium component of the gas, as well as potentially give an independent constraint on the 21cm signal from neutral hydrogen. In this paper, we use radiative transfer simulations of reionization driven by sources such as stars, X-ray binaries, accreting black holes and shock heated interstellar medium, and simulations of a high-z quasar to characterize the signal and analyze its prospects of detection. We find that the peak of the signal lies in the range 1-50 $\mu$K for both environments, but while around the quasar it is always in emission, in the case of cosmic reionization a brief period of absorption is expected. As the evolution of HeII is determined by stars, we find that it is not possible to distinguish reionization histories driven by more energetic sources. On the other hand, while a bright QSO produces a signal in 21cm that is very similar to the one from a large collection of galaxies, its signature in 3.5cm is very peculiar and could be a powerful probe to identify the presence of the QSO. We analyze the prospects of the signal's detectability using SKA1-mid as our reference telescope. We find that the noise power spectrum dominates over the power spectrum of the signal, although a modest S/N ratio can be obtained when the wavenumber bin width and the survey volume are sufficiently large.Comment: 10 pages, 13 figures, accepted for publication in MNRA

Large scale simulations of H and He reionization and heating driven by stars and more energetic sources

We present simulations of cosmic reionization and reheating from $z=18$ to $z=5$, investigating the role of stars (emitting soft UV-photons), nuclear black holes (BHs, with power-law spectra), X-ray binaries (XRBs, with hard X-ray dominated spectra), and the supernova-associated thermal bremsstrahlung of the diffuse interstellar medium (ISM, with soft X-ray spectra). We post-process the hydrodynamical simulation Massive-Black II (MBII) with multifrequency ionizing radiative transfer. The source properties are directly derived from the physical environment of MBII, and our only real free parameter is the ionizing escape fraction $f_{\rm esc}$. We find that, among the models explored here, the one with an escape fraction that decreases with decreasing redshift yields results most in line with observations, such as of the neutral hydrogen fraction and the Thomson scattering optical depth. Stars are the main driver of hydrogen reionization and consequently of the thermal history of the intergalactic medium (IGM). We obtain $\langle x_{\rm HII} \rangle = 0.99998$ at $z=6$ for all source types, with volume averaged temperatures $\langle T \rangle \sim 20,000~{\rm K}$. BHs are rare and negligible to hydrogen reionization, but conversely they are the only sources which can fully ionize helium, increasing local temperatures by $\sim 10^4~{\rm K}$. The thermal and ionization state of the neutral and lowly ionized hydrogen differs significantly with different source combinations, with ISM and (to a lesser extent) XRBs, playing a significant role and, as a consequence, determining the transition from absorption to emission of the 21 cm signal from neutral hydrogen.Comment: 17 pages, 19 figures, accepted for publication in MNRA

Observing the Redshifted 21 cm Signal around a Bright QSO at z ∼ 10

We use hydrodynamics and radiative transfer simulations to study the 21 cm signal around a bright QSO at z ~ 10. Due to its powerful UV and X-ray radiation, the QSO quickly increases the extent of the fully ionized bubble produced by the pre-existing stellar type sources, in addition to partially ionizing and heating the surrounding gas. As expected, a longer QSO lifetime, t QSO, results in a 21 cm signal in emission located at increasingly larger angular radii, θ, and covering a wider range of θ. Similar features can be obtained with a higher galactic emissivity efficiency, f UV, such that determining the origin of a large ionized bubble (i.e., QSO versus stars) is not straightforward. Such degeneracy could be reduced by taking advantage of the finite light travel time effect, which is expected to affect an H ii region produced by a QSO differently from one created by stellar type sources. From an observational point of view, we find that the 21 cm signal around a QSO at various QSO could be detected by Square Kilometre Array1-low instrument with a high signal-to-noise ratio (S/N). As a reference, for t QSO = 10 Myr, a S/N ~ 8 is expected assuming that no pre-heating of the intergalactic medium has taken place due to high-z energetic sources, while it can reach values above 10 in cases of pre-heating. Observations of the 21 cm signal from the environment of a high-z bright QSO could then be used to set constraints on its lifetime, as well as to reduce the degeneracy between f UV and t QSO