In this thesis, I analyze the physical conditions such as temperature, volume density and column density of the molecular gas in four Luminous Infrared Galaxies (LIRGs): Arp 55, NGC 1614, VV 114 and NGC 2623. LIRGs are systems where two gas-rich galaxies are in the process of merging. The goal of my thesis is to look for trends in the molecular gas properties during the merger process. I use several observations of transitions of carbon monoxide (12CO) and its isotopologue 13CO from the Submillimeter Array, Combined Array for Research in Millimeter-wave Astronomy and Atacama Large Millimeter/submillimeter Array. The high-resolution observations allow me to analyze the molecular gas at several positions inside a single galaxy. The observations are fitted to models obtained from a radiative transfer code using a Bayesian likelihood method. I find that advanced mergers such as NGC 2623 and VV 114 have warmer (≥40 K), less dense (≤ 10^3 cm^−3) molecular gas than early/intermediate stage mergers such as Arp 55 and NGC 1614. I suggest that there are mechanisms such as stellar winds, supernovae and AGN activity that dissipate the molecular gas and thus lower the density and warm the gas as the merger progresses. The molecular gas pressure of the advanced mergers is found to be lower by nearly an order of magnitude when compared to the early/intermediate stage mergers. I also find that the [12CO]/[13CO] abundance ratio in NGC 1614, VV 114 and NGC 2623 is unusually high (> 100) when compared to the interstellar medium value near the center of the Milky Way (∼ 30). Interestingly, Arp 55 does not conform to this trend with a [12CO]/[13CO] value of ∼ 30, similar to the Milky Way center. I suggest that nucleosynthesis may play a big role in enhancing the abundance ratio and/or the molecular gas from the outer radii of Arp 55 has not reached the central inner regions to drive the abundance ratio up. Nevertheless, Arp 55 is in an interesting merger stage. Finally, I measured the CO luminosity to molecular gas mass conversion factor, alpha_{CO}, across the sample in search of the transition stage from a Galactic-like alpha_{CO} to the 4-5 times lower value found in LIRGs. iii The four sources all have measured alpha_{CO} values that are consistent with the LIRG value of 0.8 M_{sol} (K km s^−1 pc^2)^−1. I suggest that we look at an even earlier merger stage such as Arp 240 to find the point of transition. With the golden age of submillimeter astronomy upon us, this is just the beginning of furthering our knowledge of the merger process and what happens to the molecular gas, the fuel for all star formation. / Thesis / Doctor of Philosophy (PhD)

No description available.

<p>Ultra/Luminous infrared galaxies (U/LIRGs) are some of the most amazing systems in the local universe exhibiting extreme star formation triggered by mergers. Since molecular gas is the fuel for star formation, studying the warm, dense gas associated with star formation is important in understanding the processes and timescales controlling star formation in mergers. We have used high resolution (∼2.3”) observations of the local LIRG Arp 299 to map out the physical properties of the molecular gas. The molecular lines 12CO J=3-2, 12CO J=2-1 and 13CO J=2-1 were observed with the Submillimeter Array and the short spacings of the 12CO J=3-2 and J=2-1 observations have been recovered using James Clerk Maxwell Telescope single dish observations. We use the radiative transfer code RADEX to measure the physical properties such as density and temperature of the different regions in this system. The RADEX solutions of the two galaxy nuclei, IC 694 and NGC 3690, show two gas components: a warm moderately dense gas with T_kin ∼ 30-500 K (up to 1000K for NGC3690) and n(H2)~0.3-3×10^3 cm^−3 and a cold dense gas with T_kin~10-30 K and n(H2) > 3 × 10^3 cm^−3. The overlap region is shown to have a well-constrained solution with T_kin ∼ 10-30 K and n(H2)~3-30 × 10^3 cm^−3. We estimate the gas masses and star formation rates of each region in order to derive molecular gas depletion times. The depletion time of each region is found to be about 2 orders of magnitude lower than that of normal spiral galaxies. This can be probably explained by a higher fraction of dense gas in Arp 299 than in normal disk galaxies.</p> / Master of Science (MSc)

I present a spectroscopic study of the molecular gas in the Whirlpool Galaxy (M51) and the Antennae Galaxies (NGC 4038/39) using data from the Herschel Space Observatory (Herschel) and the Atacama Large Millime- ter/submillimeter Array (ALMA). Using data from the Herschel Spectral and Photometric Imaging REceiver (SPIRE) Fourier Transform Spectrometer (FTS), I perform an excitation analysis to determine the physical characteris- tics (temperature, density, column density) of the cold and warm molecular gas across both systems. I do not find significant variation in the cold molecular gas across an individual system or between the systems. The warm molecular gas temperature is greater in NGC 4038/39 than in M51, while the density in both M51 and the nucleus of NGC 4038 is greater than the rest of the Anten- nae system. Both galaxies exhibit a similar fraction of warm to cold molecular gas. I compare Herschel SPIRE-FTS data to models of photon dominated regions (PDRs) to determine the strength of the background far ultraviolet field (G0) within both systems and find little variation across each system. I find that PDRs alone can explain the observed Herschel SPIRE-FTS data in both systems. Using ALMA observations of dense molecular gas tracers in NGC 4038/39, I investigate the physical processes affecting the dense molecular gas. Ratios of various molecular gas tracers suggest that the contributions of mechanical heating relative to PDR heating are similar across the entire system. The dense gas fraction in the nucleus of NGC 4038 and the nucleus of NGC 4039 is higher than in the overlap region, which I attribute to an increase in the stellar potential within the two nuclei. Furthermore, I find evidence for an increased cosmic ray rate in the overlap region of NGC 4038/39 relative to the two nuclei, which I attribute to an increased supernova rate in the overlap region. Most of the molecular gas in M51 and NGC 4038/39 is in the form of PDRs, while the increased temperature of the warm molecular gas in NGC iii 4038/39 compared to M51 is likely due to an increase in the mechanical heating from both supernova and stellar winds and the ongoing merger. Furthermore, a comparison of these results to previous studies of the interacting galaxy M82 and the late-stage merger Arp 220 suggests that mergers and interactions have a greater effect on the warm molecular gas compared to the cold molecular gas. The results from this thesis help to further our understanding of the effects of merging and interacting galaxies on molecular gas, while helping understand differences between interacting galaxies and merging galaxies. / Thesis / Doctor of Philosophy (PhD)

Le milieu interstellaire (MIS) joue un rôle important dans l'évolution des galaxies. Les radiations et vents stellaires, ainsi que les supernovae par exemple, sont à l'origine de nombreux processus ayant un impact sur les propriétés globales des galaxies. Cependant, l'efficacité des ces processus est liée aux propriétés et à la structure des différentes phases du MIS, et est souvent incertaine. Grace à la sensibilité et résolution accrues des nouveaux télescopes observant dans l'infrarouge lointain (FIR) et le submillimetrique (comme par exemple le Herschel Space Observatory, SOFIA et ALMA), il est désormais possible d'étudier en détail les interactions réciproques entre la formation stellaire et les différentes phases du MIS environnant. Ce travail est axé sur les propriétés physiques du gaz dans les Nuages de Magellan. Le Grand Nuage de Magellan et le Petit Nuage de Magellan, nos plus proches voisins, tout deux à métallicité sub-solaire, sont de bons laboratoires pour étudier les interactions entre la formation stellaire et l'environnement. La région 30 Doradus, dans le Grand Nuage de Magellan, l'une des plus massives et des plus actives régions de formation stellaire connues dans notre voisinage, est étudiée en détail. Les observations des télescopes spatiaux Herschel et Spitzer sont utilisées pour contraindre la pression, le champ de radiation ainsi que la structure tri-dimensionnelle des régions de photo-dissociation (PDR), en combinaison avec le code PDR de Meudon. Cette modélisation permet également d'estimer la fraction de gaz moléculaire qui n'est pas détectée par le traceur généralement utilisé CO. Cette méthode est ensuite appliquée à d'autres régions de formation stellaire dans les Nuages de Magellan, présentant différents environnements. Cette étude permet d'évaluer les diagnostiques clés du chauffage et du refroidissement du gaz à faible métallicité dans des régions actives de formation stellaire, avec une bonne résolution spatiale. Ceci constitue une première étape pour mieux comprendre les observations non résolues de telles régions dans des galaxies lointaines. / The interstellar medium (ISM) plays a major role in galaxy evolution. Feedback from stars, in particular, drives several processes responsible for the global properties of a galaxy. However, the efficiency of these processes is related to the properties and structure of the different gas and dust ISM phases and remains uncertain. Due to the increased sensitivity and resolution of the new far-infrared (FIR) and submillimeter facilities (such as the Herschel Space Observatory, SOFIA and ALMA, in particular), it now becomes possible to study in detail the interplay between star formation and the surrounding ISM phases. This work focuses on the physical properties of the gas in the Magellanic Clouds. The Large Magellanic Cloud and the Small Magellanic Cloud, our closest neighbors, both at subsolar metallicity, are good laboratories to study the interaction between star formation and environment.The 30 Doradus region, in the Large Magellanic Cloud, one of the most massive and active star forming region known in our neighborhood, is first studied in detail. We use the FIR and mid-infrared tracers, provided by the space telescopes Herschel and Spitzer, to bring constrains on the pressure, radiation field and 3D structure of the photo-dissociation regions (PDR) in this extreme region, using the Meudon PDR code. This modeling allows us to estimate the fraction of molecular gas not traced by CO, also known as the "CO-dark" molecular gas.We apply this method to other star forming regions of the Magellanic Clouds, which are characterized by different environmental conditions. This study allows us to evaluate key diagnostics of the gas heating and cooling of low metallicity resolved starburst regions. This is a first step toward understanding similar but unresolved regions, in high-redshift galaxies.

Des percées récentes ont été faites dans l’étude des trous noirs supermassifs (SMBH), grâce en grande partie à l’équipe du télescope de l’horizon des évènements (EHT). Cependant, déterminer la masse de ces entités colossales à des décalages vers le rouge élevés reste un défi de taille pour les astronomes. Il existe diverses méthodes directes et indirectes pour mesurer la masse de SMBHs. La méthode directe la plus précise consiste à résoudre la cinématique du gaz moléculaire, un traceur froid, dans la sphère d’influence (SOI) du SMBH. La SOI est définie comme la région où le potentiel gravitationnel du SMBH domine sur celui de la galaxie hôte. Par contre, puisque la masse d’un SMBH est négligeable face à la masse d’une galaxie, la SOI est, d’un point de vue astronomique, très petite, typiquement de quelques dizaines de parsecs. Par conséquent, il faut une très haute résolution spatiale pour étudier la SOI d’un SMBH et pouvoir adéquatement mesurer sa masse. C’est cette nécessité d’une haute résolution spatiale qui limite la mesure de masse de SMBHs à de plus grandes distances. Pour briser cette barrière, il nous faut donc trouver une manière d’améliorer la résolution spatiale d’objets observés à un plus au décalage vers le rouge. Le phénomène des lentilles gravitationnelles fortes survient lorsqu’une source lumineuse en arrière-plan se trouve alignée avec un objet massif en avant-plan, le long de la ligne de visée d’un observateur. Cette disposition a pour conséquence de distordre l’image observée de la source en arrière-plan. Puisque cette distorsion est inconnue et non-linéaire, l’analyse de la source devient nettement plus complexe. Cependant, ce phénomène a également pour effet d’étirer, d’agrandir et d’amplifier l’image de la source, permettant ainsi de reconstituer la source avec une résolution spatiale considérablement améliorée, compte tenu de sa distance initiale par rapport à l’observateur. L’objectif de ce projet consiste à développer une chaîne de simulations visant à étudier la faisabilité de la mesure de la masse d’un trou noir supermassif (SMBH) par cinéma- tique du gaz moléculaire à un décalage vers le rouge plus élevé, en utilisant l’apprentissage automatique pour tirer parti du grossissement généré par la distorsion d’une forte lentille gravitationnelle. Pour ce faire, nous générons de manière réaliste des observations du gaz moléculaire obtenues par le Grand Réseau d’Antennes Millimétrique/Submillimétrique de l’Atacama (ALMA). Ces données sont produites à partir de la suite de simulations hydrody- namiques Rétroaction dans des Environnements Réalistes (FIRE). Dans chaque simulation, l’effet cinématique du SMBH est intégré, en supposant le gaz moléculaire virialisé. Ensuite, le flux d’émission du gaz moléculaire est calculé en fonction de sa vitesse, température, densité, fraction de H2, décalage vers le rouge et taille dans le ciel. Le cube ALMA est généré en tenant compte de la résolution spatiale et spectrale, qui dépendent du nombre d’antennes, de leur configuration et du temps d’exposition. Finalement, l’effet de la forte lentille gravi- tationnelle est introduit par la rétro-propagation du faisceau lumineux en fonction du profil de masse de l’ellipsoïde isotherme singulière (SIE). L’exploitation de ces données ALMA simulées est testée dans le cadre d’un problème de régression directe. Nous entraînons un réseau de neurones à convolution (CNN) à apprendre à prédire la masse d’un SMBH à partir des données simulées, sans prendre en compte l’effet de la lentille. Le réseau prédit la masse du SMBH ainsi que son incertitude, en supposant une distribution a posteriori gaussienne. Les résultats sont convaincants : plus la masse du SMBH est grande, plus la prédiction du réseau est précise et exacte. Tout comme avec les méthodes conventionnelles, le réseau est uniquement capable de prédire la masse du SMBH tant que la résolution spatiale des données permet de résoudre la SOI. De plus, les cartes de saillance du réseau confirment que celui-ci utilise l’information contenue dans la SOI pour prédire la masse du SMBH. Dans les travaux à venir, l’effet des lentilles gravitationnelles fortes sera introduit dans les données pour évaluer s’il devient possible de mesurer la masse de ces mêmes SMBHs, mais à un décalage vers le rouge plus élevé. / Recent breakthroughs have been made in the study of supermassive black holes (SMBHs), thanks largely to the Event Horizon Telescope (EHT) team. However, determining the mass of these colossal entities at high redshifts remains a major challenge for astronomers. There are various direct and indirect methods for measuring the mass of SMBHs. The most accurate direct method involves resolving the kinematics of the molecular gas, a cold tracer, in the SMBH’s sphere of influence (SOI). The SOI is defined as the region where the gravitational potential of the SMBH dominates that of the host galaxy. However, since the mass of a SMBH is negligible compared to the mass of a galaxy, the SOI is, from an astronomical point of view, very small, typically a few tens of parsecs. As a result, very high spatial resolution is required to study the SOI of a SMBH and adequately measure its mass. It is this need for high spatial resolution that limits mass measurements of SMBHs at larger distances. To break this barrier, we need to find a way to improve the spatial resolution of objects observed at higher redshifts. The phenomenon of strong gravitational lensing occurs when a light source in the back- ground is aligned with a massive object in the foreground, along an observer’s line of sight. This arrangement distorts the observed image of the background source. Since this distor- tion is unknown and non-linear, analysis of the source becomes considerably more complex. However, this phenomenon also has the effect of stretching, enlarging and amplifying the image of the source, enabling the source to be reconstructed with considerably improved spatial resolution, given its initial distance from the observer. The aim of this project is to develop a chain of simulations to study the feasibility of measuring the mass of a supermassive black hole (SMBH) by kinematics of molecular gas at higher redshift, using machine learning to take advantage of the magnification generated by the distortion of a strong gravitational lens. To this end, we realistically generate observations of molecular gas obtained by the Atacama Large Millimeter/Submillimeter Antenna Array (ALMA). These data are generated from the Feedback in Realistic Environments (FIRE) suite of hydrodynamic simulations. In each simulation, the kinematic effect of the SMBH is integrated, assuming virialized molecular gas. Next, the emission flux of the molecular gas is calculated as a function of its velocity, temperature, density, H2 fraction, redshift and sky size. The ALMA cube is generated taking into account spatial and spectral resolution, which depend on the number of antennas, their configuration and exposure time. Finally, the effect of strong gravitational lensing is introduced by back-propagating the light beam according to the mass profile of the singular isothermal ellipsoid (SIE). The exploitation of these simulated ALMA data is tested in a direct regression problem. We train a convolution neural network (CNN) to learn to predict the mass of an SMBH from the simulated data, without taking into account the effect of the lens. The network predicts the mass of the SMBH as well as its uncertainty, assuming a Gaussian a posteriori distribution. The results are convincing: the greater the mass of the SMBH, the more precise and accurate the network’s prediction. As with conventional methods, the network is only able to predict the mass of the SMBH as long as the spatial resolution of the data allows the SOI to be resolved. Furthermore, the network’s saliency maps confirm that it uses the information contained in the SOI to predict the mass of the SMBH. In future work, the effect of strong gravitational lensing will be introduced into the data to assess whether it becomes possible to measure the mass of these same SMBHs, but at a higher redshift.