This thesis attempts to test several frameworks of non-Newtonian gravity in the context of galaxies and galaxy clusters. The theory most extensively discussed was that of Modified Newtonian Dynamics (MOND) with Galileon gravity, Emergent Gravity (EG) and Modified Gravity (MOG) mentioned to a lesser extent. Specifically, the main focus of this thesis was to determine whether MOND and MOND-like theories were compatible with galaxy cluster data, without the need to include cold dark matter. To do this, the paradigms of Extended MOND (EMOND), Generalised MOND (GMOND) and superfluid dark matter were investigated. The theories were outlined and applied to galaxy cluster data. The main findings of this were that EMOND and GMOND had some success with explaining galaxy cluster mass profiles, without requiring an additional dark matter component. The superfluid paradigm also enjoyed some success in galaxy clusters, which was expected as it behaves in a similar manner to the standard cold dark matter paradigm in cluster environments. However, the superfluid paradigm may have issues in the very centre of galaxy clusters due to the theory predicting constant density cores, whereas the cold dark matter paradigm predicts density cores which are cuspier. The EMOND paradigm was also tested against ultra-diffuse galaxy (UDGs) data as they appear in cluster environments, where EMOND becomes important. It was found that EMOND can reproduce the inferred mass of the UDGs, assuming they lie on the fundamental manifold (FM). The validity of the assumptions used to model the UDGs are discussed in the text. A two-body problem was also conducted in the Galileon gravity framework. The amount of additional gravitational force, compared to Newtonian was determined for a small galaxy at the edge of a galaxy cluster.

By virtue of its proximity, the Virgo Cluster is an ideal laboratory for us to test our understanding of the formation of structure in our Universe. In this spirit, we present a dynamical study of 33 gas-poor and 34 gas-rich Virgo galaxies as part of the Spectroscopic and H-band Imaging of Virgo survey. Our final spectroscopic data set was acquired at the 3.5-m telescope at the Apache Point Observatory. Hα rotation curves for the gas-rich galaxies were modelled with a multi-parameter fit function from which various velocity measurements were inferred. Analog values were measured off of the observed rotation curves, but yielded noisier scaling relations, such as the luminosity-velocity relation (also known as the Tully-Fisher relation). Our best i-band Tully-Fisher relation has slope α=-7.2 ± 0.5 and intercept M_i(2.3)=-21.5 ± 1.1 mag, matching similar previous studies. Our study takes advantage of our own, as well as literature, data; we plan to continue expanding our compilation in order to build the largest Tully-Fisher relation for a cluster to date. Following extensive testing of the IDL routine pPXF, extended velocity dispersion profiles were extracted for our gas-poor galaxies. Considering the lack of a common standard for the measurement of a fiducial galaxy velocity dispersion in the literature, we have endeavoured to rectify this situation by determining the radius at which the measured velocity dispersion, coupled with the galaxy luminosity, yields the tightest Faber-Jackson relation. We found that radius to be 1.5 R_e, which exceeds the extent of most dispersion profiles in other works. The slope of our Faber-Jackson relation is α=-4.3 ± 0.2, which closely matches the virial value of 4. This analysis will soon be applied to a study of the Virgo Cluster Fundamental Plane. Rotation correction of our dispersion profiles will also permit the study of galaxies' velocity dispersion profile shapes in an attempt to refine our understanding of the overall manifold of galaxy structural parameters. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2012-12-22 19:09:06.192

Although exotic objects like supermassive black holes (SMBHs) and dark matter halos do not emit or interact with light, we can still detect them across the vastness of space. By observing the gravitational dance of objects we can see, astronomers are able to infer the mass of the invisible objects they orbit. This has led to the discovery that nearly every massive galaxy hosts a SMBH at its center, and has confirmed that every galaxy is embedded in an extended halo of dark matter. However, the practice of inferring mass from the motions of bright kinematics tracers has many complications, not the least of which is that we seldom observe more than the line-of-sight component of the instantaneous velocity of a star. Consequently, astronomers must build dynamical models of the galaxies they wish to study. These models often rely on overly restrictive assumptions, or are crippled by degeneracies amongst their parameters and lack predictive power. In this thesis, I introduce a significant advancement into the field of dynamical modeling. My modeling technique is based on the powerful principle of orbit superposition, also known as Schwarzschild Modeling. This technique is robust to many of the degeneracies associated with dynamical modeling, and has enjoyed much success in measuring the SMBHs and dark matter halos of large elliptical or bulge-dominated galaxies. I use it in Chapter 2 to accurately measure the SMBH in the Sombrero Galaxy (NGC 4594) and to constrain its dark matter halo. Unfortunately, when measuring dark matter halos with Schwarzschild Modeling, the modeler is required to adopt a parameterization for the dark matter density profile. Often this is precisely the quantity one wishes to measure. To avoid this reliance on a priori parameterizations, I introduce a technique that calculates the profile non-parametrically. Armed with this powerful new technique, I set out to measure the distribution of dark matter in the halos of some of the smallest galaxies in the Universe. These dwarf spheroidal galaxies (dSphs) orbit the Milky Way as satellites, and their dark matter content has been studied extensively. However, the models used to probe their halos have been simplistic and required overly restrictive assumptions. As a result, robust conclusions about their dark matter content have remained elusive. Into this controversial and active area of study, I bring Non-Parametric Schwarzschild Modeling. The results I find offer the most robust and detailed measurements of the dark matter profiles in the dSphs to date. I begin my study with the first application of standard Schwarzschild Modeling to any dSph galaxy by using it in Chapter 3 on Fornax. This chapter details the process of re-tooling Schwarzschild Modeling for the purpose of measuring these small galaxies. In Chapter 4, I introduce the fully non-parametric technique, and apply it to Draco as proof of concept. Chapter 5 presents the main results of this thesis. Here I apply Non-Parametric Schwarzschild Modeling to Draco, Carina, Fornax, Sculptor, and Sextans. By relaxing the assumption of a parameterization for the dark matter profile, I find a variety of profile types in these five galaxies---some of which are consistent with past observations, others consistent with predictions from simulations, and still others were completely unanticipated. Finally, in Chapter 6 I describe the modeling of these galaxies in more detail. I demonstrate the accuracy of Non-Parametric Schwarzschild Modeling by recovering a known dark matter profile from artificial simulated data. I also expound upon the modeling results by presenting the detailed orbit structure of the five dSphs. Lastly, I compare my results to hydrodynamical simulations to explore the link between dark matter profile type and the baryon content of the dSphs. / text

We use Bayesian statistics and Markov chain Monte Carlo (MCMC) techniques to construct dynamical models for the spiral galaxy NGC 6503. The constraints include surface brightness profiles which display a Freeman Type II structure; HI and ionized gas rotation curves; the stellar rotation, which is nearly coincident with the ionized gas curve; and the line of sight stellar dispersion, which displays a $\sigma-$drop at the centre. The galaxy models consist of a S\'rsic bulge, an exponential disc with an optional inner truncation and a cosmologically motivated dark halo. The Bayesian/MCMC technique yields the joint posterior probability distribution function for the input parameters, allowing constraints on model parameters such as the halo cusp strength, structural parameters for the disc and bulge, and mass-to-light ratios. We examine several interpretations of the data: the Type II surface brightness profile may be due to dust extinction, to an inner truncated disc or to a ring of bright stars; and we test separate fits to the gas and stellar rotation curves to determine if the gas traces the gravitational potential. We test each of these scenarios for bar stability, ruling out dust extinction. We also find that the gas cannot trace the gravitational potential, as the asymmetric drift is then too large to reproduce the stellar rotation. The disc is well fit by an inner-truncated profile, but the possibility of ring formation by a bar to reproduce the Type II profile is also a realistic model. We further find that the halo must have a cuspy profile with $\gamma \gtrsim 1$; the bulge has a lower $M/L$ than the disc, suggesting a star forming component in the centre of the galaxy; and the bulge, as expected for this late type galaxy, has a low S\'{e}rsic index with $n_b\sim1-2$, suggesting a formation history dominated by secular evolution. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2010-01-10 00:11:41.946

This thesis presents observations and analysis relating to the understanding of processes that govern the formation and evolution of low mass galactic systems. In particular we have focused on separating out the contribution to the chemical and dynamical evolution of dwarf galaxies due to solely secular (internal) processes compared to external effects from the local environment a galaxy resides in. Our observational data focus on an extremely isolated dwarf galaxy, WLM, which we demonstrate has had a uniquely quiescent tidal history, thereby making it an excellent test case for such a study. With spectroscopic and photometric observations of the resolved stars and neutral gas in WLM we have been able to characterize the chemical, structural and kinematic properties of this gas rich dwarf galaxy. As WLM has not been subject to strong tidal or ram-pressure stripping of its stellar and gaseous populations, we have been able to compare the dynamical evolution and chemical history of WLM to theoretical models which are environment independent. A differential comparison of WLM to more environmentally processed dwarf galaxies in the Local Group has revealed that WLM's structural and dynamical state is far from the idealized picture of dIrrs as thin gas-rich rotating systems. The stellar component of WLM shows equal parts rotation and dispersion, and both the gaseous and stellar structural properties show an intrinsically thick axisymmetric configuration. The time evolution of the random (dispersion) component of the stellar orbital energy shows an increase with stellar age, which we show is consistent with secular processes alone - such as disk heating from giant molecular clouds and dark matter substructure. While the degree to which the thick structural and dynamically hot configuration for WLM is surprising, its chemical properties show remarkably consistent values with other galaxies of the same halo mass. Comparing the spatial chemical trends in WLM with other dwarf galaxies we identify a correlation between the strength of the radial abundance gradients and the angular momentum content of dwarf galaxies in the Local Group. Finally using a large sample of chemical abundance measurements in the literature for dwarf galaxies and star clusters, we demonstrate that their distributions of chemical elements all exhibit a binomial form, and use the statistical properties of the distributions to identify a new metric for differentiating low luminosity stellar systems. We further apply a simple binomial chemical evolution model to describe the self-enrichment and pre-enrichment in the two classes of objects, and suggest how this may be used to place constraints on the formation environments of globular clusters in particular. / Graduate

Dusty circumnuclear disks (CNDs) in luminous early-type galaxies (ETGs) show regular, dynamically cold molecular gas kinematics. For a growing number of these ETGs, Atacama Large Millimeter/sub-millimeter Array (ALMA) CO imaging and detailed gas-dynamical modeling have yielded moderate to high precision black hole (BH) mass (M_BH) determinations. To date, however, few studies have explored the effects of dust attenuation, or uncertainties in dust corrections, on recovered stellar luminosity models from high angular resolution optical/near-IR observations and M_BH measurements. Recent kinematic studies that modeled ALMA CO data sets have found that uncertainties in the intrinsic central stellar surface brightness slope due to dust may even dominate the BH mass error budgets. From the ALMA archive, we identified a subset of 26 ETGs with clean CO kinematics and good prospects for eventual MBH determination but that did not have sufficiently high angular resolution observations in the optical and near-IR. We have obtained new optical and near-IR Hubble Space Telescope (HST) images of this sample to supplement the archival HST data. Together, these new and archival HST data will enable the mitigation of dusty CND obscuration in the construction of dust-corrected stellar luminosity models, leading to both BH mass measurement and complete error analysis using existing ALMA CO imaging. Here, we present the sample properties, data analysis techniques, and dust-masked stellar surface brightness profiles and luminosity models using the multi-Gaussian expansion formalism. With estimated M_BH/M_⊙ ≳ 10^8 to few x10^9 , securing quality M_BH determinations for this sample of ETGs will significantly improve the high-mass end of the current BH census, facilitating new scrutiny of local BH mass-host galaxy scaling relationships.

Dans le domaine complexe de l’archéologie galactique, l’étude des flux stellaires s’est imposée comme une voie essentielle pour dévoiler la formation et l’évolution des galaxies, en particulier celle de la Voie lactée. Ces systèmes stellaires perturbés portent l’empreinte de l’histoire de la Voie lactée, offrant des perspectives sur l’accrétion de galaxies plus petites et de grappes d’étoiles sur des milliards d’années. Avec l’avènement de la mission Gaia et de ses données d’observation inédites, l’application d’outils d’apprentissage automatique non supervisés à la détection d’anomalies est devenue de plus en plus pertinente dans la recherche de ces énigmatiques flux stellaires. L’objectif principal de cette recherche était d’exploiter la puissance des intégrales de mouvement et des algorithmes de regroupement pour élaborer une méthodologie permettant d’identifier les flux stellaires au sein du halo de la Voie lactée. Guidés par la volonté d’affiner notre compréhension des dynamiques complexes de la galaxie, nous nous sommes efforcés de développer une méthode robuste capable de mettre au jour ces structures stellaires perturbées. Notre approche a impliqué une évaluation exhaustive des algorithmes de regroupement, notamment Single Linkage, Ward, DBSCAN et OPTICS, afin de distinguer des groupes cohérents d’étoiles qui présentent des motifs ou des relations distincts, une caractéristique clé des flux stellaires. Les données synthétiques ont servi de terrain de test, facilitant l’évaluation des performances des algorithmes et l’étalonnage optimal des hyperparamètres. Cette exploration a couvert trois phases, englobant des flux isolés, la résilience face au bruit de fond et les complexités de dispersions de vitesse plus élevées. Alors que la méthodologie a donné des résultats favorables sur des données synthétiques, lorsque nous l’avons appliquée aux données de la Gaia Data Release 3 (DR3), nous avons constaté que les grappes étaient principalement situées autour du noyau galactique ou du Système solaire, contrairement à nos attentes de découvrir des courants stellaires dans le halo galactique. Cette divergence par rapport à nos résultats anticipés met en évidence la nature complexe de la dynamique de la Voie lactée et soulève un défi important couramment rencontré en apprentissage automatique : le problème hors distribution. La capacité des algorithmes d’apprentissage automatique à généraliser à partir des données d’entraînement vers de nouvelles données non vues peut être entravée par la présence de points de données qui tombent en dehors de la distribution sur laquelle ils ont été formés. Dans le contexte de notre recherche, ce défi souligne la nécessité de techniques avancées en détection d’anomalies et d’adapter les modèles d’apprentissage automatique pour gérer les complexités de l’observation du monde réel. / In the intricate realm of galactic archaeology, the study of stellar streams has emerged as a vital avenue for unraveling the formation and evolution of galaxies, particularly the Milky Way. These disrupted stellar systems, bear the imprint of the Milky Way’s history, offering insights into the accretion of smaller galaxies and star clusters over billions of years. With the advent of the Gaia mission and its unprecedented observational data, the application of unsupervised machine learning tools to anomaly detection has become increasingly relevant in the search for these elusive stellar streams. The primary aim of this research was to harness the power of integrals of motion and clustering algorithms to devise a methodology for identifying stellar streams within the halo of the Milky Way. Guided by the goal of refining our understanding of the galaxy’s intricate dynamics, we endeavored to develop a robust method capable of uncovering these disrupted stellar structures. Our approach involved a comprehensive evaluation of clustering algorithms, including Single Linkage, Ward, DBSCAN, and OPTICS, to distinguish cohesive groups of stars that exhibit distinct patterns or relationships—a key characteristic of stellar streams. Synthetic data served as a testing ground, facilitating algorithm performance assessment and optimal hyperparameter calibration. This exploration spanned three phases, encompassing isolated streams, resilience against background noise, and the complexities of higher velocity dispersions. While the methodology yielded favorable outcomes on synthetic data, when applied to the Gaia Data Release 3 (DR3) data, we found that clusters were predominantly located around the galactic core or the Solar System, in contrast to our expectations of discovering stellar streams in the galactic halo. This deviation from our anticipated results highlights the complex nature of the Milky Way’s dynamics and raises an important challenge commonly encountered in machine learning: the out-of-distribution problem. The ability of machine learning algorithms to generalize from training data to new, unseen data can be hindered by the presence of data points that fall outside the distribution they were trained on. In the context of our research, this challenge underscores the need for advanced techniques in anomaly detection and adapting machine learning models to handle real-world observational complexities.

In this thesis I test the modified Newtonian dynamics as an alternative to the cold dark matter hypothesis. In the Milky Way, I show that the dynamics of the dwarf galaxies are well described by the paradigm and I confirm its distant low surface brightness globular clusters provide a strong test, for which I make predictions. Through analysis of a sample of 26 X-ray bright galaxy groups and clusters I demonstrate that the three active neutrinos and their anti-particles are insufficient to reconcile modified Newtonian dynamics with the observed temperatures of the X-ray emitting gas, nor with weak-lensing measurements, in particular for the bullet cluster. To this end, I propose an 11eV sterile neutrino to serendipitously resolve the residual mass problem in X-ray bright groups and clusters, as well as matching the angular power spectrum of the Cosmic Microwave Background. With this in mind, I show that the large collision velocity of the bullet cluster and the high number of colliding clusters is more naturally reproduced in MOND than in standard dynamics.

Les fusions sont un évènement essentiel dans la formation des grandes structures de l’Univers; elles jouent un rôle important dans l’histoire de formation et l’évolution des galaxies. Outre une transformation morphologique, les fusions induisent d’importants sursauts de formation d’étoiles. Ces sursauts sont caractérisés par des Efficacités de Formation Stellaire (EFS) et des Taux de Formation Stellaire Spécifiques (TFSS), i.e., respectivement, des Taux de Formation Stellaire (TFS) par unité de masse gazeuse et des TFS par unité de masse stellaire, plus élevés que ceux des galaxies spirales. A toutes les époques cosmiques, les galaxies à sursaut de formation d’étoiles sont des systèmes particuliers, en dehors de la séquence définie par les galaxies spirales. Nous explorons l’origine du mode de formation stellaire par sursaut, à travers trois systèmes in interaction: Arp 245, Arp 105 et NGC 7252. Nous avons combiné des observations JVLA haute résolution de la raie à 21-cm, traçant le gaz Hi diffus, avec des observations GALEX dans l’UV, traçant les jeunes régions de formation d’étoiles. Nous sommes ainsi en mesure de sonder les conditions physiques locales du Milieu InterStellaire (MIS) pour des régions de formation d’étoiles indépendantes, et d’étudier la transformation du gaz atomique en gaz dense dans différents environnements. Le rapport SFR/HI apparaît bien plus élevé dans les régions centrales que dans les régions externes, indiquant une fraction de gaz dense plus élevée (ou une fraction de gaz HI moins élevée) dans les régions centrales. Dans les régions externes des systèmes, i.e., les queues de marées, où le gaz est dans une phase principalement atomique, nous observons des rapports SFR/ HI plus élevés que dans les environnements standards dominés par le HI, i.e., les régions externes des disques de spirales et les galaxies naines. Ainsi, notre analyse révèle que les régions externes de fusions sont caractérisées par des EFS élevées, par comparaison au mode de formation stellaire standard. Observer des fractions de gaz dense élevées dans les systèmes en interaction est en accord avec les prédictions des simulations numériques; ceci résulte d’une augmentation de la turbulence du gaz durant une fusion. La fusion affecte les propriétés de formation stellaire du système probablement à toutes les échelles, depuis les grandes échelles, avec une turbulence augmentant globalement, jusqu’aux petites échelles, avec des modifications possibles de la fonction de masse initiale. A partir d’une simulation numérique haute résolution d’une fusion majeure entre deux galaxies spirales, nous analysons les effets de l’interaction des galaxies sur les propriétés du MIS à l'échelle des amas stellaires. L’accroissement de la turbulence du gaz explique probablement la formation de Super Amas Stellaire dans le système. Notre étude de la relation SFR–HI dans les fusions de galaxies sera complétée par des données HI haute résolution pour d’autres systèmes, et poussée vers des échelles spatiales encore plus petites. / Mergers are known to be essential in the formation of large-scale structures and to have a significant role in the history of galaxy formation and evolution. Besides a morphological transformation, mergers induce important bursts of star formation. These starburst are characterised by high Star Formation Efficiencies (SFEs) and Specific Star Formation Rates, i.e., high Star Formation Rates (SFR) per unit of gas mass and high SFR per unit of stellar mass, respectively, compared to spiral galaxies. At all redshifts, starburst galaxies are outliers of the sequence of star-forming galaxies defined by spiral galaxies. We have investigated the origin of the starburst-mode of star formation, in three local interacting systems: Arp 245, Arp 105 and NGC 7252. We combined high-resolution JVLA observations of the 21-cm line, tracing the HI diffuse gas, with UV GALEX observations, tracing the young star-forming regions. We probe the local physical conditions of the Inter-Stellar Medium (ISM) for independent star-forming regions and explore the atomic-to-dense gas transformation in different environments. The SFR/HI ratio is found to be much higher in central regions, compared to outer regions, showing a higher dense gas fraction (or lower HI gas fraction) in these regions. In the outer regions of the systems, i.e., the tidal tails, where the gas phase is mostly atomic, we find SFR/HI ratios higher than in standard HI-dominated environments, i.e., outer discs of spiral galaxies and dwarf galaxies. Thus, our analysis reveals that the outer regions of mergers are characterised by high SFEs, compared to the standard mode of star formation. The observation of high dense gas fractions in interacting systems is consistent with the predictions of numerical simulations; it results from the increase of the gas turbulence during a merger. The merger is likely to affect the star-forming properties of the system at all spatial scales, from large scales, with a globally enhanced turbulence, to small scales, with possible modifications of the initial mass function. From a high-resolution numerical simulation of the major merger of two spiral galaxies, we analyse the effects of the galaxy interaction on the star forming properties of the ISM at the scale of star clusters. The increase of the gas turbulence is likely able to explain the formation of Super Star Clusters in the system. Our investigation of the SFR-HI relation in galaxy mergers will be complemented by high-resolution HI data for additional systems, and pushed to yet smaller spatial scales.