Dissertations / Theses on the topic 'Interacting and isolated galaxies'

Interactions between pairs of isolated dwarf galaxies provide a critical window into low-mass hierarchical, gas-dominated galaxy assembly and the build-up of stellar mass in low-metallicity systems. We present the first Very Large Telescope/Multi Unit Spectroscopic Explorer (VLT/MUSE) optical integral field unit (IFU) observations of the interacting dwarf pair dm1647+21 selected from the TiNy Titans survey. The Ha emission is widespread and corresponds to a total unobscured star formation rate (SFR) of 0.44 M-circle dot yr(-1), which is 2.7 times higher than the SFR inferred from Sloan Digital Sky Survey (SDSS) data. The implied specific SFR (sSFR) for the system is elevated by more than an order of magnitude above non-interacting dwarfs in the same mass range. This increase is dominated by the lower-mass galaxy, which has a sSFR enhancement of > 50. Examining the spatially resolved maps of classic optical line diagnostics, we find that the interstellar medium (ISM) excitation can be fully explained by star formation. The velocity field of the ionized gas is not consistent with simple rotation. Dynamical simulations indicate that the irregular velocity field and the stellar structure is consistent with the identification of this system as an ongoing interaction between two dwarf galaxies. The widespread, clumpy enhancements in the star formation in this system point to important differences in the effect of mergers on dwarf galaxies, compared to massive galaxies; rather than the funneling of gas to the nucleus and giving rise to a nuclear starburst, starbursts in low-mass galaxy mergers may be triggered by large-scale ISM compression, and thus may be more distributed.

Thesis (Ph.D) - Swinburne University of Technology, Faculty of Information & Communication Technologies, 2007.
A thesis presented in fulfillment of the requirements of Doctor of Philosophy, Faculty of Information and Communication Technologies, Swinburne University of Technology, 2007. Typescript. Bibliography p. 109-118.

This thesis investigates interacting galaxies using two complementary approaches: a statistical study of star formation in a large sample of close pairs of galaxies drawn from the Sloan Digital Sky Survey (SDSS) and a study of the ISM and star forming properties of a small sample of interacting pairs taken from Arp’s atlas, each comprising one early- and one late-type system (E+S pairs). The first part of this thesis investigates the effect of galaxy interaction on star formation using a large volume- and luminosity-limited sample of galaxies drawn from the SDSS. Star-formation rates were calculated from extinction and aperture corrected Hα luminosities and, for a subset of systems, IRAS data. The mean specific star-formation rate is found to be strongly enhanced for projected separations of less than 25 kpc. For late-type galaxies, the correlation extends out to projected separations of 300 kpc and is most pronounced in actively star-forming systems. The specific star-formation rate of observed to decrease with increasing recessional velocity difference, although the magnitude of this effect is small compared to that associated with the projected separation. No dependence of star formation enhancement on the morphological type or mass of the companion galaxy is observed. This second part of this thesis presents a study of the ISM and star forming properties of nine E+S pairs. Detailed case studies were undertaken for two of the pairs, Arp 140 and Arp 104, both of which display extended tidal tails in their H1 morphology. These two pairs differ markedly. Despite Arp 140’s relatively evolved interaction and weakly barred potential, NGC 275, the late-type component, has neither a molecular gas nor star formation distribution that is centrally condensed. Instead, the molecular gas and HII regions display an unusual anti-correlation.

Evolution of galaxies is one of the most important topics in astronomy to understand how the universe has been evolving. In particular, galaxy groups are important because they are the observable equivalent of dark matter (DM) haloes, and thus offer a direct insight into the physics that has occurred in the DM haloes in the Universe up to the present day. Isolated galaxies are crucial for studying intrinsic and secular processes able to affect the structure, morphology, and dynamics of galaxies for obtaining clear relationships and correlations to be confronted with the model predictions. The main goal of this work is to characterize the GAMA G3Cv1 galaxy groups catalogue and the UNAM-KIAS catalogue of isolated galaxies by one of the most important statistical studies, the galaxy Luminosity Function (LF), that helps to constrain the models of formation and evolution of galaxies. LFs have been estimated for galaxies in groups and isolated galaxies. The LF for groups has been characterized by the physical properties of the groups (mass and velocity dispersion), the photometry (colour), the morphological type and eleven wavelengths from the far infra-red to the ultra violet. The LF estimated for the isolated galaxies is characterized by morphology and the colour in the five SDSS bands. The results obtained constrain more effectively the formation and evolution models of the universe than previous samples. The differences between both catalogues are presented in the conclusions. Additionally, the galaxy morphology is one of the no well understood problems in the galaxy evolution process to support the hierarchical model of formation of large objects. In this work, a classification based on the colour and concentration of light was considered. However, due to the low resolution of the images, the confidence of this classification was only ~60%.

Thesis (Ph. D.)--University of Kentucky, 2003.
Title from document title page (viewed onJune 1, 2004). Document formatted into pages; contains ix, 106 leaves : ill. (some col.). Includes abstract and vita. Includes bibliographical references (p. 99-104).

This thesis concentrates on the numerical modelling of star formation in isolated and perturbed systems, with particular emphasis on the effects of feedback. In view of the complicated nature of star formation within an individual molecular cloud, a stochastic approach is adopted in which the timescale for star formation occurring is assumed to be a function of the cloud's mass and temperature. Feedback is included in some models by assuming that star formation may also be triggered as the result of expanding supernova remnants generating shocks in clouds. Two different models for cloud growth are investigated. The first scheme considers the growth of clouds through collisional coalescence, whilst the second assumes that clouds grow by accreting matter from the H₁ component as they orbit the galaxy. Models in which clouds grow through collisional coalescence display many properties in good agreement with those observed in galaxies. Specifically, the cluster formation rate, star formation rate and cloud mass spectrum are all within the observational limits. Models in which clouds grow through accretion were, however, inconsistent with observations on a number of points. Most notably, the cloud mass spectrum is considerably shallower and the number of very high-mass clouds considerably lower than that observed in galaxies. The behaviour of interacting galaxies is highly idiosyncratic, making them an ideal candidate for comparison with numerical simulations. The behaviour of the coalescence model when perturbed by the fly-past of a companion galaxy is therefore investigated. The star formation history of the system is found to be heavily dependent on the degree of triggered star formation present in the system. In models that include triggered star formation, the increased spatial density of clouds that arises due to the infall of gas towards the nuclear region is shown to generate a burst of triggered star formation, leading to a rapid increase in the cluster formation rate. The star formation rate in models that did not include triggered star formation was found to rise as the result of the decreased cloud collisional timescale.

We model Milky Way like isolated disk galaxies in high resolution three-dimensional hydrodynamical simulations with the adaptive mesh refinement code Enzo. The model galaxies include a dark matter halo and a disk of gas and stars. We use a simple implementation of sink particles to measure and follow collapsing gas, and simulate star formation as well as stellar feedback in some cases. We investigate two largely different realizations of star formation. Firstly, we follow the classical approach to transform cold, dense gas into stars with an fixed efficiency. These kind of simulations are known to suffer from an overestimation of star formation and we observe this behavior as well. Secondly, we use our newly developed FEARLESS approach to combine hydrodynamical simulations with a semi-analytic modeling of unresolved turbulence and use this technique to dynamically determine the star formation rate. The subgrid-scale turbulence regulated star formation simulations point towards largely smaller star formation efficiencies and henceforth more realistic overall star formation rates. More work is necessary to extend this method to account for the observed highly supersonic turbulence in molecular clouds and ultimately use the turbulence regulated algorithm to simulate observed star formation relations
In dieser Arbeit beschäftigen wir uns mit der Modellierung und Durchführung von hoch aufgelösten dreidimensionalen Simulationen von isolierten Scheibengalaxien, vergleichbar unserer Milchstraße. Wir verwenden dazu den Simulations-Code Enzo, der die Methode der adaptiven Gitterverfeinerung benutzt um die örtliche und zeitliche Auflösung der Simulationen anzupassen. Unsere Galaxienmodelle beinhalten einen Dunkle Materie Halo sowie eine galaktische Scheibe aus Gas und Sternen. Regionen besonders hoher Gasdichte werden durch Teilchen ersetzt, die fortan die Eigenschaften des Gases beziehungsweise der darin entstehenden Sterne beschreiben. Wir untersuchen zwei grundlegend verschiedene Darstellungen von Sternentstehung. Die erste Methode beschreibt die Umwandlung dichten Gases einer Molekülwolke in Sterne mit konstanter Effektivität und führt wie in früheren Simulationen zu einer Überschätzung der Sternentstehungsrate. Die zweite Methode nutzt das von unserer Gruppe neu entwickelte FEARLESS Konzept, um hydrodynamische Simulationen mit analytischen-empirischen Modellen zu verbinden und bessere Aussagen über die in einer Simulation nicht explizit aufgelösten Bereiche treffen zu können. Besonderes Augenmerk gilt in dieser Arbeit dabei der in Molekülwolken beobachteten Turbulenz. Durch die Einbeziehung dieser nicht aufgelösten Effekte sind wir in der Lage eine realistischere Aussage über die Sternentstehungsrate zu treffen. Eine zukünftige Weiterentwicklung dieser von uns entwickelten und umgesetzten Technik kann in Zukunft dafür verwendet werden, die Qualität des durch Turbulenz regulierten Sternentstehungsmodells noch weiter zu steigern

Stellar bars are a common feature in massive disc galaxies. On a theoretical ground, the response of gas to a bar is generally thought to cause nuclear starbursts and, possibly, AGN activity once the perturbed gas reaches the central super-massive black hole. By means of high resolution numerical simulations we detail the purely dynamical effects that a forming bar exerts on the gas of an isolated disc galaxy. The galaxy is initially unstable to the formation of non-axisymmetric structures, and within 1 Gyr it develops spiral arms that eventually evolve into a central stellar bar on kpc scale. A first major episode of gas inflow occurs during the formation of the spiral arms while at later times, when the stellar bar is establishing, a low density region is carved between the bar co-rotational and inner Lindblad resonance radii. The development of such "dead zone" inhibits further massive gas inflows. Indeed, the gas inflow reaches its maximum during the relatively fast bar formation phase and not, as often assumed, when the bar is fully formed. We conclude that the low efficiency of long-lived, evolved bars in driving gas toward galactic nuclei is the reason why observational studies have failed to establish an indisputable link between bars and AGNs. On the other hand, the high efficiency in driving strong gas inflows of the intrinsically transient process of bar formation suggests that the importance of bars as drivers of AGN activity in disc galaxies has been overlooked so far. We finally prove that our conclusions are robust against different numerical implementations of the hydrodynamics routinely used in galaxy evolution studies.

Le rôle de l'environnement sur l'évolution des galaxies n'est pas encore entièrement connu. Pour quantifier les rôles joués par les processus externes, on doit identifier un échantillon de galaxies isolées. Nous avons étudié 950 galaxies en provenance du Catalogue de Galaxies isolées et évalué leur isolation. Nous avons défini, comparé et discuté différents critères pour quantifier le degré d'isolation de ces galaxies, comme la densité de surface locale, l'estimation des forces de marées externes affectant chaque galaxie isolée. De plus nous avons cherché les redshifts des galaxies centrales ainsi que ceux de leurs compagnons pour avoir une image en trois dimensions de l'environnement. Enfin, nous avons appliqué nos procédures aux triplets, groupes compacts et amas de galaxies et interprété la population de galaxies isolées à la lumière de ces échantillons de contrôle. La formation d'étoiles est connue pour être affectée par l'environnement local des galaxies mais le taux de formation d'étoiles dépend aussi des caractéristiques intrinsèques du milieu interstellaire. Nous avons observé et compilé des données photométriques pour 200 galaxies spirales isolées. Ensuite, nous avons étudié l'aspect de la morphologie en H alpha des 45 galaxies les plus grandes et les moins inclinées. En utilisant les techniques de Transformation de Fourier Rapide, nous nous focalisons sur les modes des bras spiraux. Nous quantifions la force des barres et nous donnons les couples entre les étoiles nouvellement formées et la matière optique. La fréquence observée des modèles morphologiques particuliers apporte des contraintes sur la durée de vie des barres, et les temps de destruction associés
The role of environment on galaxy evolution is still not fully understood. In order to quantify and set limits on the role of nurture one must identify and study an isolated sample of galaxies. We processed 950 galaxies from the Catalogue of Isolated Galaxies and evaluated their isolation. We defined, compared and discussed various criteria to quantify the degree of isolation for these galaxies : e. G. Local surface density computations, estimation of the external tidal force affecting each isolated galaxy. Additionally, we sought for the redshifts of the primary and companion galaxies to access the radial dimension and have accurate three dimensional picture of the surroundings. Finally, we applied our pipeline to triplets, compact groups and clusters and interpret the isolated galaxy population in light of these control samples. The star formation is known to be affected by the local environment of the galaxies, but the star formation rate also highly depends on the intrinsic interstellar medium features. To address this issue, we observed and gathered photometric data for 200 isolated galaxies. We subsequently studied the H alpha morphological aspect of the 45 biggest and less inclined galaxies. Using Fast Fourier Transform techniques, we focus on the modes of the spiral arms, quantify the strength of the bars, and we give the torques between the newly formed stars and the bulk of the optical matter. We interpret the various bar and H alpha morphologies observed in terms of the secular evolution experienced by galaxies isolation. The observed frequency of particular patterns bring constraints on the lifetime of bars and their fading time-scales

We calculated the fraction of ‘clumpy’ galaxies (fclumpy) for three samples of nearby galaxies. These samples include interacting galaxies with strong tidal features, collisional ring galaxies, and normal spiral galaxies. We define a ‘clumpy’ galaxy as a galaxy that has luminous star-forming regions contributing more than 8% of the total flux for the galaxy. We calculate fclumpy for 16 different wavelengths. We find that fclumpy is highest in the ultraviolet, Hα, and 24μm, while fclumpy is the lowest in optical and near-infrared wavelengths. We also see a significant increase in fclumpy for the interacting samples compared to the normal spirals.

This thesis aims to model the effects of interaction and buckling upon pairs of micro-shells embedded within an elastic medium under far field hydrostatic pressure. This analysis is motivated by the role shell buckling plays in the nonlinear nature of the pressure relative volume curve of elastomers containing micro-shells. Current models of the effective properties of these types of composites assume shells are in a dilute distribution within the host medium, and as such assume shells will buckle at the pressure of the associated isolated embedded shell model. For composites with a high volume fraction of micro-shells, or in poorly mixed composites, the dilute distribution model may provide a first approximation to the effective properties of the composite, however, interaction between shells must be considered to find a more accurate model. We begin the process of modelling the buckling of interacting embedded shells by considering the buckling of an isolated embedded thin spherical shell. For a host medium undergoing far field hydrostatic pressure we demonstrate the parameter ranges in which Jones et al. thin shell buckling theory agrees with the thin shell buckling theory of Fok and Allwright. We then use scalings to increase the range of validity of the thin shell approximation used in the Jones et al. theory to include composites with a high contrast between medium and shell materials. This enables more accurate predictions of buckling pressures of embedded shells under far field axially symmetric pressures to also be found, as is demonstrated for an embedded shell under far field axial compression. We model the linear elastic deformation of pairs of embedded micro-shells using the Boussinesq-Papkovich stress function method, before employing the thin shell linear analysis method developed in previous chapters to calculate the critical buckling pressure and buckling patterns of the pair of embedded shells.

Interactions are crucial for galaxy formation and profoundly affect their evolution. However, our understanding of the impact of interactions on star formation and activity of the central supermassive black hole remains incomplete. In the canonical picture of the interaction process, these processes are expected to undergo a strong enhancement, but some recent studies have not found this prediction to be true in a statistically meaningful sense. This thesis uses a sample of local interactions observed from the ultraviolet to the far-infrared and a suite of N-body hydrodynamic simulations of interactions to examine the evolution of star formation, stellar mass, dust properties, and spectral energy distributions (SEDs) over the interaction sequence.
Astronomy

Esta tese apresenta resultados sobre a estrutura, relações de escala e cinemática para 48 galáxias em 22 grupos compactos de Hickson, sendo que a apresentação de mapas de velocidades, monocromáticos (na linha H alpha) e de dispersão de velocidades são feitos pela primeira vez para 35 galáxias em 12 dos grupos. A partir dos mapas de velocidades e imagens óticas, foi possível obter os parâmetros cinemáticos, morfológicos e as curvas de rotação das galáxias da presente amostra. Usando as velocidades máximas de rotação para cada galáxia (derivadas das curvas de rotação) e as luminosidades óticas, infravermelhas, as massas estelares e bariônicas, foram estudadas as diferentes relações de Tuly-Fisher (TF) para as galáxias dos grupos compactos. Comparando esses resultados com os apresentados por uma amostra de galáxias de campo, foi encontrado que as galáxias de grupos compactos seguem a relação de TF definida pelas galáxias em ambientes menos densos, no entanto algumas galáxias de baixa massa apresentam altas luminosidades para as suas velocidades de rotação. Surtos de formação estelar e atividade nuclear parecem ser os principais fatores que fazem com que as galáxias de baixas massas dos grupos compactos não estejam na relação de TF definida pelas galáxias do campo. Este resultado indica que as velocidades máximas de rotação não são alteradas em galáxias em interação e portato não há um stripping de massa significativo nas galáxias de grupos compactos, dentro de R(25). O uso das curvas de rotação para estudar a distribuição de massas nestas galáxias revelou que estas curvas apresentam um alto grau de assimetria, a qual seria produzida em eventos de interação galáxia-galáxia. Esses eventos, além de perturbar as curvas de rotação, conseguem expulsar parte do gás neutro das galáxias ao meio intra grupo. Usando dados ultravioleta, nesta tese foram encontradas vários sistemas estelares jovens no meio intergaláctico de grupos compactos. Esses sistemas podem se converter em galáxias satélites ou simplesmente serem dissolvidos, enriquecendo o meio intragrupo.
This thesis presents results on the kinematics, scaling relations and structures of 48 galaxies in 22 compact groups. For 35 galaxies in 12 compact groups, velocity fields, monochromatic maps (derived from H alpha observations) and velocity dispersion maps are presented for the first time. By using these data, it was possible to determine the kinematic and morphological parameters, the rotation curves and to derive the Tully-Fisher relation for the galaxies in dense environments. By using the maximum rotational velocity for each galaxy (derived from its rotation curve) and its optical and near-infrared luminosity and mass, the different Tully-Fisher relations for galaxies in compact groups were derived. Comparing these results with the results displayed by galaxies in less dense environments, it was found that galaxies in compact groups agrees with the Tully-Fisher relation defined by non-interacting galaxies. However, some of the low-mass galaxies are off the Tully-Fisher relation, having too high luminosities for their maximum rotational velocities. This scenario can be explained by a burst of star formation and/or by nuclear activity. We conclude that the maximum rotational velocities of compact groups galaxies are not affected during galaxy-galaxy interactions which implies that there is no significant mass stripping in galaxies of compact groups inside their optical radius. The mass distribution of galaxies in compact groups indicates that the rotation curves of these galaxies are highly asymmetric. The asymmetry could be produced by interactions between galaxies. These interactions, besides affecting the shape of the rotation curve, can eject some neutral gas from the disk of the interacting galaxies into the intragroup medium. By using ultraviolet data, we find several young star-forming regions in the intragroup medium of compact groups. It is still an open question wether these young stellar systems can survive and become new members of the group or if they will fall back onto their parent galaxies.

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 NGC7252. 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/H i 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/H i 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–H i relation in galaxy mergers will be complemented by highresolution Hi data for additional systems, and pushed to yet smaller spatial scales.

Two analyses searching for squarks and gluinos which decay into final states with multiple jets, an isolated electron or muon and a large missing transverse energy are presented. Both rely on data taken by the ATLAS detector in pp collisions at a center-of-mass energy of 8 TeV at the LHC during 2012. The first analysis uses a subset of 5.8 fb-1 of this dataset, the other analysis uses the full statistics of 20.3 fb-1. Both analysis share the same methods regarding the triggers and the background estimation techniques. The two dominant backgrounds are ttbar and W+jets production. The ttbar and the W+jets backgrounds are estimated in a semi-data-driven method. The minor QCD multi-jet background is estimated in an entirely data-driven method. The final background estimates in the analyses are derived in a profile-log-likelihood fit. None of the analyses sees an excess beyond Standard Model expectations. The analysis of the partial dataset derives limits in a MSUGRA/CMSSM model with parameters A_0=0, tan(beta) = 10 and mu > 0 and excludes squarks and gluinos with masses below 1.2 TeV for equal squark and gluino masses. The analysis of the full dataset derives limits in simplified models and in a MSUGRA/CMSSM model with parameters A_0=-2 m_0, tan(beta) = 30 and mu > 0. Gluinos (squarks) with masses below 1.2 TeV (750 GeV) can be excluded for vanishing LSP masses in simplified models. Gluino masses below 1.2 TeV can be excluded for every m_0 value in the MSUGRA/CMSSM model.

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

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Interacting peculiar galaxies are objects that still require further studies because they play an important role in the processes of the evolution of galaxies. The knowledge of its location and properties constitutes a great benefit to the astronomical community. In this dissertation, we present an automatic method for identifying and classifying images of interacting galaxies at low redshifts for the Southern Hemisphere, based on the properties of stellar and interstellar extinction distribution as well as using a pattern recognition software called ?Wndchrm? on images from the 2MASS survey, in the near infrared for the filters: J, H and KS. The training phase was made with images of known interacting galaxies from the Arp &Madore Catalogue, Categories 1 and 2. After training, a validation was performed using images of a region of the sky with 573 square degrees, obtaining a hit of, approximately, 73% in the identification of galaxies identified by visual inspection as interacting. This rate can reach up 88% considering the comparison with previous know galaxy pairs of Category 2 of Arp &Madore Catalogue. The procedure was performed for an area of about 17.836 square degrees of the Southern Hemisphere, finding at least several hundred galaxy pairs as yet uncatalogued.
As gal?xias peculiares interagentes s?o objetos que ainda carecem de maiores estudos. Elas desempenham um papel importante nos processos de forma??o e de evolu??o das gal?xias. O conhecimento de sua localiza??o e propriedades, constitui um grande benef?cio para a comunidade astron?mica. Nesta disserta??o, apresentamos um m?todo autom?tico para identificar e classificar imagens de gal?xias interagentes a baixos redshifts para o Hemisf?rio Sul. Para tal, estabelecemos crit?rios baseados na distribui??o de estrelas e extin??o interestelar, assim como de objetos identificados no Cat?logo de fontes extensas do 2MASS. Usamos um software de reconhecimento de padr?es chamado Wndchrm em imagens do grande levantamento 2MASS no infravermelho pr?ximo para os filtros: J, H e Ks. A fase de treinamento foi feita com imagens de gal?xias interagentes conhecidas, do Cat?logo de Arp&Madore, das categorias 1 e 2. Ap?s o treinamento foi realizada uma valida??o com imagens de uma regi?o do c?u de 573 graus quadrados, obtendo uma taxa de acerto de, aproximadamente 73% na identifica??o de gal?xias previamente identificadas de forma visual como interagentes. Essa taxa aumenta para, aproximadamente 88% levando em conta pares da Categoria 2 previamente identificados no Cat?logo de Arp&Madore como pertencentes ? essa categoria. Executamos, o procedimento para uma ?rea de, aproximadamente 17.836 graus quadrados do Hemisf?rio Sul, encontrando, ao menos, v?rias centenas de pares de gal?xias ainda n?o catalogados.

Les étoiles massives représentent un des principaux contributeurs à l'enrichissement des galaxies en éléments lourds et en poussière interstellaire. L’ultime étape de leur évolution est représentée par le stade Wolf-Rayet (WR). Les étoiles WR présentent la particularité de générer un vent stellaire radiatif dense, qui peut interagir avec celui d’un compagnon proche, donnant naissance à un environnement de poussière en forme de spirale. Les ordres de grandeur associés à ce type d’objet sont spectaculaires : avec un taux de formation de poussière équivalent à la masse de la planète Mars produite chaque année, elles rivalisent avec les producteurs historiques de la poussière que sont les étoiles de la branche asymptotique des géantes (AGB) ou les supernovæ (SN). Les étoiles WR à poussière pourraient ainsi répondre à une problématique bien connue : d’où vient la poussière observée dans les galaxies ? Le présent travail de thèse vise donc à enrichir nos connaissances sur ce problème à travers tous les aspects de la chaîne scientifique : de l’observation à l’analyse de données en employant différents niveaux de sophistication en modélisation numérique (analytique, transfert radiatif et hydrodynamique). Le premier aspect exploré par cette thèse concerne la modélisation des nébuleuses spirales de poussières. J’ai d’abord développé un modèle analytique permettant de contraindre les aspects géométriques des spirales. Ce dernier inclut différentes hypothèses physiques comme la prise en compte d’un rayon de sublimation, de différents types de structure interne, etc. J’ai ensuite inclut le transfert de rayonnement au modèle géométrique afin de relier la distribution d’intensité de l’objet (l’image) à sa distribution en densité. Ce modèle 3-D de spirale de poussière permet d’étudier les effets d’opacité et d’ombrage liés à la masse ou au type de poussière considérée. J’ai également développé un modèle 3-D axisymétrique en transfert de rayonnement afin d’assimiler la spirale à une suite d’anneaux concentriques. Il vise à reproduire la distribution d’intensité d’une spirale à un azimut donné et permet une comparaison directe aux profils radiaux d’intensité issus d’observations. Enfin, nous avons mis en place un modèle hydrodynamique 3-D de binaire à interaction de vent, afin d’avoir une idée réaliste des conditions physiques en place au niveau de la zone de nucléation des poussières. Le second aspect abordé par cette thèse se concentre sur l’étude du prototype des nébuleuses spirales de poussière, nommé WR 104. J’explore ici toutes les échelles spatiales de l’objet : des grandes échelles avec l’imageur VLT/VISIR afin de faire le lien avec milieu interstellaire, aux régions les plus internes avec l’instrument VLTI/AMBER pour sonder la zone de nucléation de poussière, en passant par l’instrument d’optique adaptative extrême, VLT/SPHERE, afin d’étudier les premiers tours de la spirale. Le troisième et dernier aspect concerne l’instrument de seconde génération à équiper l’interféromètre européen (VLTI) : MATISSE. Il est le tout premier instrument à opérer en simultané dans les bandes L, M et N en recombinant la lumière issue de quatre télescopes. MATISSE a été conçu pour étudier une variété de cas scientifiques : des disques protoplanétaires aux noyaux actifs de galaxie, en passant par les environnements circumstellaires. Afin de préparer les premiers programmes observations, j’ai développé un outil automatisé, nommé PREVIS, visant à prédire l’observabilité des objets. Dans le cadre des nébuleuses spirales, j’ai pu explorer les capacités de l’instrument en reconstruction d’image en testant différents aspects (tailles, inclinaison, couverture (u-v), etc.). Avec un pouvoir de résolution spatiale de 3 mas à 3,5 µm, MATISSE permettra d’étudier ces objets de façon unique, en résolvant pour la première fois l’épaisseur des bras spiraux, leurs structures internes ou la position exacte du bord de sublimation
Massive stars are one of the major contributors to the enrichment of galaxies in heavy elements and interstellar dust. The last stage of their evolution is represented by the Wolf-Rayet phase (WR). WR stars generate a dense radiative stellar wind, which can interact with the wind from a close companion and cause a spiral dust environment called pinwheel nebula. The orders of magnitude associated with this kind of object are spectacular: with a dust formation rate equivalent to the mass of the planet Mars produced each year, WR stars compete with the historical dust producers, like the stars of the asymptotic giant branch (AGB) or the supernovae (SN). Dusty WR stars could thus answer a well-known problem: where does the dust observed in galaxies come from? This thesis aims at enriching our knowledge about this problem using all aspects of the scientific chain: from observation to data analysis by using different levels of sophistication in numerical modelling (analytical, radiative transfer and hydrodynamics). The first aspect explored by this thesis concerns the modelling of spiral dust nebulae. I first developed an analytical model for the spiral to constrain the geometrical aspects of the spiral, including a number of physical hypothesis like the dust sublimation radius and different types of internal structure. The next step consisted to include the radiative transfer in the geometrical model in order to link the intensity distribution of the object (the image) to its density distribution. This 3-D model of spiral allow to study the opacity and shadowing effects related to the dust mass considered. Similarly, I developed a 3-D axisymmetric radiative transfer model to mimic the spiral into a series of concentric rings. This model aims to reproduce the intensity distribution of a spiral at a given azimuth and allows a direct comparison with the radial intensity profiles derived from observations. Finally, we implemented a 3-D hydrodynamic model of a wind-wind interacting binary to get a realistic idea of the physical conditions in places around the dust nucleation zone. The second aspect addressed by this thesis focuses to the study of the prototype of the pinwheel nebula, called WR104. Such object is an ideal laboratory to study the problem of dust nucleation around massive stars. I explored all spatial scales of WR 104: From the large scale with VLT/VISIR to study the link with the interstellar medium, to the internal regions with VLTI/AMBER to probe the dust nucleation zone, including intermediate angular resolution to study the pinwheel structure with extreme adaptive optics instrument VLT/SPHERE. The third and last aspect deals with the second generation of the instrument installed at the European Very Large Telescope Interferometer (VLTI): MATISSE. It is the first instrument operating simultaneously in the L, M and N bands by recombining the light coming from four telescopes. MATISSE was developed to study different scientific cases: protoplanetary disks, the circumstellar environments and the active galactic nuclei. To prepare the first observation programs, I developed an automated tool, called PREVIS, to determine the observability of objects according to their magnitude and celestial coordinate. In the context of spiral nebulae, I explored the image reconstruction capabilities of the instrument by testing different aspects: geometric (size, inclination, opening angle, etc.) and observational (coverage (u-v), sampling). The unprecedented spatial resolution of MATISSE of 3 mas at 3.5 µm will allow to study these objects in a unique way, resolving for the first time the thickness of the spiral arm, its internal structure or the exact position of the sublimation radius

碩士
國立清華大學
物理學系
88
In this article, we select 22 interacting galaxies for studying their physical difference and similarities. I organise this article as following parts. Part. I, I describe their optical, near-infrared, and CO(1-0) properties. Part. II, the main point is their properties of 20 cm radio continuum. Part. III, I discusse the typical interacting galaxies, the Antennae, of its kinematics of molecular clouds in interacting region. And, the descriptions of interacting galaxies are in Part. IV. Part. I. We have studied optical, near-infrared, and CO properties of the 22 luminous infrared galaxies in a merging sequence. We find no systematic variations of nuclear color, spectral type, equivalance of stellar absorption lines as a function of nuclear separation. However, we find a weak correlation of the line width and star formation rate as a function of nuclear separation. These results suggest that star formation activity initiates at any moment of tidal interaction with relatively short period of time and this activity can occur recursively since we find no systematic change of starburst age as a function of nuclear separation. Part. II. Star formation rate ($SFR$), especially in the nucleus region is enhanced by the merging process of two galaxies. The $SFR$ in interacting galaxies with massive bulge would be enhanced by 20-150 times compared to that in the isolated galaxies (Mihos \& Hernquist; 1994, 1996). In order to study $SFR$ in interacting galaxies, we have selected 22 galaxies based on the infrared luminosity. For these galaxies, we have observed CO(1-0) line emission with the $BIMA$ b, c array and 20 cm radio continuum with the $VLA$ telescope. We find that there''s no clear trends that $SFR$ correlates with the projected nuclear separation. The $SFR$ correlates with the mass of molecular hydrogen but not with the dust temperature. Part. III. The non-starburst galaxy, the Antennae, is active in the overlap region of two disks. Previous work concluded that the active star formation activity is due to the collision of these two disks. The near-infrared line spectroscopy conflicts with this claim. We conclude that these two molecular clouds are leaving from each other rather than approaching to each other, therefore there must be other mechanisms for this active star formation activity.

In this dissertation I develop a three dimensional model of the dynamics of gas clouds in interacting galaxies. The gas clouds move under the combined gravitational influence of two galaxies passing close to each other. By performing a multipole expansion of the gravitational field I am able to include the effects of self-gravity within a galaxy. This also allows me to model the case in which the two galaxies merge. The gas clouds are allowed to interact with one another by colliding. They either coalesce to form a larger cloud or are disrupted, depending on their relative kinetic energy as compared to the total gravitational binding energy of the two-cloud system. Various cases are considered in this dissertation by varying such parameters as impact parameter, inclination of the gaseous disk of a galaxy to the orbital plane of the two, interacting galaxies, relative velocity of the galaxies, the mass ratio of the galaxies, and the presence of gas in the second galaxy. As the strength of the interaction increases the more disturbed the interstellar medium becomes. The clouds collide at an increased rate and with larger velocities so that the fraction of collisions which disrupt the clouds rises as the strength of the interaction increases. The region of the galaxy where increased rates of collision are induced also becomes more and more concentrated toward the center of the galaxy. Since interacting galaxies are observed to have elevated star formation rates, I conclude that the star formation induced by the interaction of two galaxies is related to the high velocity, disruptive cloud-cloud collisions. Monitoring the amount of gas mass involved in such collisions allows me to estimate the star formation rate and the luminosity produced by these stars. Considering parameters such as inclination, bound and unbound orbits, the mass of the perturbing galaxy, and the possible presence of gas in both galaxies, I find that the scatter in observations of the infrared luminosity to gas mass ratio can be explained.

I present a multiwavelength study of a sample of peculiar galaxies in order to constraint the physics of interacting objects, to study how the physical processes affect the structure of galaxies, and to derive some hints on the formation and evolution history of such galaxies. One of the major open issues in modern cosmology is to understand how galaxies formed and evolved. Its likely that the formation of galaxies was dominated by two processes: the assembly of luminous and dark matter through accretion and merger, and the conversion of baryonic and non-baryonic matter into stars. This is the reason why the study of galaxy interactions has received an increasing attention both on the observational and the theoretical sides. Several theoretical works based on numerical simulations, have tested different plausible scenarios for the origin of peculiar galaxies, such as ``Tidal accretion'', ``Cold accretion'' and ``Merging''. For this reason, the sample of galaxies to use was selected in order to reproduce all these mechanisms. The first peculiar object that I studied is the minor axis dust lane galaxy NGC1947. I performed a detailed study of the main properties of this galaxy (Spavone et al 2009). In NGC1947 are present components with different angular momentum, infact gas and dust rotate along the minor axis while stars rotate along the major one. This is a clear evidence that it cannot be the result of a single protogalactic cloud collapse, but rather the result of an interaction event. I performed a detailed study of the main properties of this galaxy and compared them with the prediction of simulations. Putting together all this evidences it was difficult to disentangle in a non ambiguous way the two possible scenarios, even though some aspects can help us in understanding. First of all, the galaxy does not present clear signs of interaction, such as tidal tails and so on, and this leads to the conclusion that the merger occurred about 10 Gyrs ago, a fact which is not consistent with the fact that, according to my estimate, the last burst of star formation occurred 1 Gyr ago. So I can say that the accretion scenario is favoured. The second object in my sample is the Polar Ring Galaxy (PRG) NGC4650A. I used high resolution NIR and optical spectroscopy along the North and South side of the polar disk, to measure the metallicity of the HII regions in the polar disk of this galaxy because, if it formed from the accretion of external cold gas from cosmic web filaments, we expect metallicities similar to those of late type galaxies, while if the metallicities are similar to those of early type galaxies, the accretion from a gas rich donor is favoured. I estimated the metallicity by using both direct and empirical methods, the Stellar Formation Rate (SFR), and the metallicity gradient along the disk. The average metallicity for the polar disk of NGC4650A turned out to be $Z = 0.2 Z_{\odot}$, which is lower than the typical values found in spiral galaxies, and is instead consistent with the metallicities predicted for the formation of disks by cold accretion processes ($Z \sim\ 1/10 Z_{\odot}$), due to the accretion of pristine gas in the cold streams. Moreover, also the absence of any metallicity gradient is consistent with the infall of metal-poor gas from outside which is still forming the disk (see Spavone et al 2010 for details). As a follow up of this work, I obtained observing time at the TNG telescope to observe the PRGs UGC7576 and UGC9796. I performed the study of the chemical abundances also for these galaxies in order to constrain their formation history (Spavone et al. submitted). Both PRGs have metallicities (respectively 0.4 $Z_{\odot}$ and 0.1 $Z_{\odot}$) lower than that observed in spiral galaxies of the same total luminosity and, given their present star formation rate, this values is again consistent with the predictions of the cold accretion mechanism for disk formation. UGC7576 is an isolated galaxy and the absence of close companions led to exclude both the tidal accretion from a donor galaxy and the merging with another galaxy. UGC9796 is instead in a group and has 5 close companions with an amount of HI gas comparable with that of UGC9796. The merging scenario is however ruled out because in order to produce a massive polar disk such as those observed in UGC9796 is required a merging with high mass ratios (7:1 or 10:1) and this would destroy the ordered motion of the central galaxy, transforming it into an elliptical-like, not rotationally supported galaxy. In conclusion, for this object, both the tidal accretion and the cold accretion seem to be plausible scenarios. Finally, to analyze also another type of merging process, I am studying the pair of interacting galaxies known as CSL-1. By using high resolution spectroscopy (FORS1@VLT) and imaging (HST) I am studying the morphology, light distribution and structural parameters of this system, to test the dry-merger scenario (cf. ``A prototype dry-merger caught in the act'', M. Paolillo , G. Covone, C. Nipoti, M. Spavone, M. Capaccioli, G. Longo, A.Cimatti, L. Ciotti, in preparation). In order to investigate also the minor merging processes I also studied the photometric and kinematical properties of a compact group of galaxies belonging to the Hickson's catalogue. The group analyzed in this work was HCG62, one of the nearest group in the celestial Southern hemisphere, which was selected by cross correlating the available X-ray and optical data. Galaxies in compact groups are in a very dense configuration in the sky, having a mean separation comparable with their dimensions and a very low velocity dispersion. Taking into account that theories on formation and evolution of galaxies predict that the intensity and frequency of interactions strongly depend on the density of the environment, compact groups may be considered the ideal place where to test interaction processes, such as dynamical friction, tidal interaction, collisions, merging and so on. The main goal of this part of my work was to derive some hints on the formation and evolutionary history of compact groups, in order to address the possible scenario for the formation of structure in the Universe, and to determine the evolutionary status of the studied objects. To this aim, I performed a detailed study of the kinematical properties of HCG62, that revealed the presence of many peculiarities in the dominant galaxy of the group, NGC4778, such as the presence of a kinematically decoupled and counter-rotating core (KDC), or kinematical profiles strongly perturbed, also in the outer regions of this galaxy. Moreover, I also performed an analysis of the photometric properties of the whole group, to look for correlations between kinematical and photometric peculiarities. The absence of such correlations in HCG62, can be explained by stating that weak interactions do not perturb the rotation curves but produce morphological deformations in the outer regions, while the so called minor mergers perturb the rotation curves in the inner regions, without producing morphological peculiarities. The results obtained in this work are in good agreement with similar studies performed on the same group and with its observed X-ray properties (Spavone et al 2006). I was also Co-Investigator in two accepted proposals to observe, with the TNG telescope, a sample of Shakhbazian galaxy groups, with the aim of building a larger statistical sample and obtain redshift informations which are lacking for most Shakhbazian groups, and are needed to establish on firm grounds their physical nature. Groups of galaxies have been extensively studied in the past decades. Despite this effort, their evolution is yet not well understood. Loose groups are almost certainly still collapsing and are therefore crucial to uncover the formation processes shaping cosmic structures. As it was already mentioned, in compact groups a few member galaxies are compressed in a small volume of space with low relative velocities. Early theoretical studies suggested that in such high density environments the low velocity dispersion of compact galaxy groups would favor strong interactions and mergers, leading to rapid evolution (within $\sim 10^9$ yrs) into a single massive merger remnant. The best studied sample of compact groups to date is the one included in the Hickson catalogue; this sample however, is biased towards extremely high values of matter density and therefore it allows to investigate only the ``close-to-final'' stage of the complex dynamical evolution of groups. The density range bridging the field to these almost coalesced structures is still poorly explored, mainly due to the difficulties encountered in constructing reliable samples of `physically bound', low multiplicity groups. Shakhbazian Groups of galaxies (SHKGs) in spite of having been originally selected as ``compact groups of compact galaxies'', have been shown to sample a large range of spatial densities. To properly characterize the properties of these groups and their evolutionary path, I obtained more accurate redshift determination for a sample of 10 SHK, so almost doubling the sample of SHK groups, with detailed spectroscopic data. Main goals of this work are: i) obtain redshift estimates for groups without literature data, sampling the different sub-populations; ii) confirm the galaxy membership of the groups, which is currently based on photometric estimates for most of the objects; iii) study the dynamical status of the group and derive dynamical mass estimates; iv) study the stellar population of the member galaxies through comparison with population synthesis models, and the degree of activity from emission line measurements.

A galaxy’s evolution is quite sensitive to the impact of external influences. In this thesis, the impact of external environment from both large and small scale effects is investigated, along with a study of how the HI gas fraction of a galaxy can modulate a galaxy’s response to perturbations by galaxy–galaxy interactions. This thesis makes use of the statistical power of the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) to assemble a large spectroscopic sample of galaxies, select samples of interest, and select control samples of galaxies matched to each galaxy within the sample of interest in mass, redshift, and (if applicable) local density. It is possible to trace a galaxy’s internal gas motions which mark its disturbance by using the metrics of star formation rate (SFR) and gas-phase metallicity. To investigate the influence of large scale environment, a sample of star forming galaxies in a locally dense environment, but relatively isolated from larger scale structure, is constructed. This sample is further divided into groups which are truly isolated from any large scale structure (no cluster potential within 1 Mpc), and those which, in spite of their relative local isolation, are embedded within a larger cluster structure (within 1 Mpc of a cluster). As the local galaxy density is identical between isolated and embedded group structures, a fair comparison between the star forming properties of the galaxies within those group structures can be made. Star forming galaxies whose groups are embedded within a larger structure are found to show statistically lower SFRs than those galaxies whose groups are truly isolated from any larger cluster potential. The impact of local galaxy–galaxy interactions is subsequently considered. Using a sample of star-forming galaxies in pairs from the SDSS DR7, the enhancement in SFRs and the suppression of metallicities is traced as a function of projected separation (rp). The metallicity dilution as a function of rp is presented for the first time. Galaxies in pairs are found to have SFRs and metallicity values which are offset from a carefully selected control sample to separations of at least 80 kpc/h. Using a suite of simulations developed for the purposes of comparison with these observational results, a new interpretive framework is developed for enhancements as a function of rp. To investigate the role that gas fraction plays in moderating the strength of interaction triggered starbursts, new data is obtained from the Jansky Very Large Array (VLA). The VLA data supplements the existing SDSS data with HI gas masses for a subsample of resolvable galaxy pairs at small rp(in kpc/h). HI masses are obtained and gas fractions are calculated for a sample of 34 paired galaxies. A positive correlation is detected at > 2σ between the gas fraction of a galaxy and the SFR enhancement of that galaxy. The work presented in this thesis has expanded the understanding of physical variables, both internal and external, which can change the star forming properties of a galaxy through an examination of tracers of internal gas flows in those galaxies.
Graduate
0606
jillian.scudder@gmail.com

碩士
國立中央大學
天文研究所
92
The scientific operations of space telescopes and ground-based facilities worldwide have produced a flood of astronomical data waiting to be analyzed. Thus the development of fast and efficient system is in urgent demand for the purpose of data mining. The discovery of gravitational lensing events and interacting galaxies are very important in the study of cosmology. However, both types of structures are relatively rare and often hidden in the mountain of images. For these reasons, we have developed an automatic system to identify these objects from image archives by shape analysis. First, candidates are selected with the shape parameter defined by our method and a line and an arc are then fitted to these potential candidates. From error analysis the best shape can be identified. The algorithm developed in this work has been tested on two of the gravitational lensing events found in the RCS and proved to be successful. Furthermore, it has also been applied to a portion of the RCS data set, which consists of 210 images and dozens of interacting galaxies have been found.

The motive for this work was to investigate whether small, isolated gas-rich galaxies show evidence of chemical evolution, by studying their nebular metallicities. I have identified a sample of 83 objects chosen for low luminosity and mass, the presence of active star formation, and isolation from other galaxies and galaxy clusters that might generate tidal effects or enrich the intergalactic medium. From these I have measured the spectra of 35 objects, using theWiFeS IFU spectrograph on the ANU 2.3m telescope at Siding Spring. In analysing spectra extracted from the WiFeS data cubes, I found that standard ‘strong line’ methods using emission line ratios to measure atomic abundances, gave either erratic or no results. I found that for those galaxies showing the [O iii] 4363Å auroral line, the metallicities determined using the standard ‘electron temperature’ methodwere inconsistent with previous published work. This led me to investigate the conventional assumption that electrons in Hii regions are in thermal equilibrium. I show that the non-equilibrium ‘ ’ electron energy distribution, found almost universally in solar system plasmas, can explain the long recognised ‘abundance discrepancy’ between recombination line and collisional line abundance calculations in nebular metallicity measurements. This has added an important new dimension to the analysis of nebular spectra. Using the extensively revised Mappings photoionisation modelling code and new atomic data to analyse the spectra of two exceptionally isolated dwarf galaxies, I find that they exhibit metallicities similar to galaxies in more crowded environments, and appear to have evolved quite normally, through periodic star formation and subsequent enrichment of their interstellar media. I present a new approach for calculating total oxygen abundance using electron temperatures that appears to give more consistent results than earlier methods. I apply this to my measured spectra, together with the revised Mappings photoionisation modelling code, to explore the physical parameters affecting the measurement of nebular metallicities. In particular, I find strong evidence for several of the observed nebulae being—in part—optically thin. I use the models to show that nebular optical depth affects measured abundances and temperatures, and that electron densities also have an important role. I develop models that give a very good match to the observations. I conclude that the measurement of abundances and temperatures in Hii regions is a more complex question than had generally been assumed, and important physical parameters affecting the measurement processes have in the past not been taken fully into account.