We study the formation of planetesimals in protoplanetary disks from the gravitational collapse of solid over-densities generated via the streaming instability. To carry out these studies, we implement and test a particle-mesh self-gravity module for the ATHENA code that enables the simulation of aerodynamically coupled systems of gas and collisionless self-gravitating solid particles. Upon employment of our algorithm to planetesimal formation simulations, we find that (when a direct comparison is possible) the ATHENA simulations yield predicted planetesimal properties that agree well with those found in prior work using different numerical techniques. In particular, the gravitational collapse of streaming-initiated clumps leads to an initial planetesimal mass function that is well-represented by a power law, dN / dM(p) proportional to M-p(-p), with p similar or equal to 1.6 +/- 0.1, which equates to a differential size distribution of dN / dR(p) proportional to R-p(-q), with q similar or equal to 2.8 +/- 0.1. We find no significant trends with resolution from a convergence study of up to 512(3) grid zones and N-par approximate to 1.5 x 10(8) particles. Likewise, the power-law slope appears indifferent to changes in the relative strength of self-gravity and tidal shear, and to the time when (for reasons of numerical economy) self-gravity is turned on, though the strength of these claims is limited by small number statistics. For a typically assumed radial distribution of minimum mass solar nebula solids (assumed here to have dimensionless stopping time tau = 0.3), our results support the hypothesis that bodies on the scale of large asteroids or Kuiper Belt Objects could have formed as the high-mass tail of a primordial planetesimal population.

We present a complete census of all Herschel-detected sources within the six massive lensing clusters of the HST Frontier Fields (HFF). We provide a robust legacy catalogue of 263 sources with Herschel fluxes, primarily based on imaging from the Herschel Lensing Survey and PEP/HerMES Key Programmes. We optimally combine Herschel, Spitzer and WISE infrared (IR) photometry with data from HST, VLA and ground-based observatories, identifying counterparts to gain source redshifts. For each Herschel-detected source we also present magnification factor (mu), intrinsic IR luminosity and characteristic dust temperature, providing a comprehensive view of dust-obscured star formation within the HFF. We demonstrate the utility of our catalogues through an exploratory overview of the magnified population, including more than 20 background sub-LIRGs unreachable by Herschel without the assistance gravitational lensing.

Stars form when filaments and dense cores in molecular clouds fragment and collapse due to self-gravity. In the most basic analyses of gravitational stability, the competition between self-gravity and thermal pressure sets the critical (i.e. maximum stable) mass of spheres and the critical line density of cylinders. Previous work has considered additional support from magnetic fields and turbulence. Here, we consider the effects of non-ionizing radiation, specifically the inward radiation pressure force that acts on dense structures embedded in an isotropic radiation field. Using hydrostatic, isothermal models, we find that irradiation lowers the critical mass and line density for gravitational collapse, and can thus act as a trigger for star formation. For structures with moderate central densities, similar to 10(3) cm(-3), the interstellar radiation field in the Solar vicinity has an order unity effect on stability thresholds. For more evolved objects with higher central densities, a significant lowering of stability thresholds requires stronger irradiation, as can be found closer to the Galactic centre or near stellar associations. Even when strong sources of ionizing radiation are absent or extincted, our study shows that interstellar irradiation can significantly influence the star formation process.

Cosmological hydrodynamic simulations have shown that galaxies are fed by dense, cold gas streams at high redshift. However, the presence of such gas has never been observationally confirmed. Using the Horizon- MareNostrum simulation, I examined whether cold flows are detectable with low-ionisation metal absorption lines, such as C II 1334. It is concluded that due to their low metallicity and density, it is extremely difficult to prove/disprove the presence of cold flows using the metal absorption lines. Revisiting the acquisition of angular momentum in disc galaxies using high resolution simulations, I found that at the time of accretion, gas and dark matter do carry a similar amount of specific angular momentum which is systematically and significantly higher (at minimum by a factor of 2) than that of the dark matter halo as a whole. Whereas cold streams directly deposit this large amount of angular momentum within a sphere of radius r~0.1 Rvir, dark matter particles easily pass through the central region, depositing their angular momentum over a much more spatially extended region. As a result, in our simulations neither the total specific angular momentum of the baryons nor its radial profile ever follows that of the virialised dark matter halo, contrary to what is typically assumed in the standard theory of disc galaxy formation. In order to better understand the formation of disc galaxies and the missing baryon problem in a LCDM universe, continuous, collective galactic winds are implemented. It is demonstrated that stellar feedback processes are able to suppress star formation by ~30% at z=3, compared to that from the run without feedback sources, but it still produces an unrealistic central peak in the rotation curve. Although inclusion of hypernovae further suppresses star formation, it is unable to quench the formation of low-angular momentum stars enough to remove the peaked rotation curves at high redshift. Finally, feedback from active galactic nuclei turns out to be effective at suppressing star formation in massive galaxies at 1<z<2, reproducing their observed number densities in the redshift range. However, further suppression of residual star formation is required to form quiescent galaxies at z=2.

Compact groups of galaxies provide insight into the role of low-mass, dense environments in galaxy evolution because the low velocity dispersions and close proximity of galaxy members result in frequent interactions that take place over extended time-scales. We expand the census of star formation in compact group galaxies by Tzanavaris et al. (2010) and collaborators with Swift UVOT, Spitzer IRAC and MIPS 24 mu m photometry of a sample of 183 galaxies in 46 compact groups. After correcting luminosities for the contribution from old stellar populations, we estimate the dust-unobscured star formation rate (SFRUV) using the UVOT uvw2 photometry. Similarly, we use the MIPS 24 mu m photometry to estimate the component of the SFR that is obscured by dust (SFRIR). We find that galaxies which are MIR-active (MIR-'red'), also have bluer UV colours, higher specific SFRs, and tend to lie in H I-rich groups, while galaxies that are MIR-inactive (MIR-'blue') have redder UV colours, lower specific SFRs, and tend to lie in H I-poor groups. We find the SFRs to be continuously distributed with a peak at about 1 M-circle dot yr(-1), indicating this might be the most common value in compact groups. In contrast, the specific SFR distribution is bimodal, and there is a clear distinction between star-forming and quiescent galaxies. Overall, our results suggest that the specific SFR is the best tracer of gas depletion and galaxy evolution in compact groups.

This thesis presents the results of geoarchaeological investigations of several archaeological sites along the Elm Fork of the Trinity River in north central Texas. Archaeological data recorded from mitigation excavations in Denton and Cooke counties were analyzed to understand the geologic impacts on faunal preservation resulting from site formation processes. The faunal assemblages are highly fragmented, even in settings known for good preservation. A combined approach using geoarchaeological and taphonomic techniques was implemented to examine how fragmentation, evidence of soil weathering, and differential preservation were impacted by differing geologic conditions throughout the river basin. Intrasite and intersite results of the sites show that a great deal of variability of faunal preservation is present at difference scales of analysis.

The effect of varying substituents on structure and energies of bisketenes was studied using ab initio methods. Effect of substituents on ring closing reaction of bisketenes to the corresponding cyclobutenediones was also studied using ab initio methods. One or two of the following substituents were used to study the effect of varying substituents: BH2, CH3, NH2, OH, F, AlH2, SiH3, PH2, SH, Cl. Studies were done at the Hartree-Fock (HF), Møller-Plesset (MP2), and Density Functional Theory (B3LYP) levels of theory using the 6-31G* basis set.

La formation géologique Mursi est datée à plus de 4 millions d’années, ce qui correspond à la période de l’émergence présumée des australopithèques, premiers hominines présentant une bipédie incontestable. La collection faunique Mursi (N=201) est comparée à celle du membre A de la Formation Shungura (N=300) sur une base taphonomique. La taphonomie est la science dédiée aux modes de formation des fossiles ainsi qu’aux traces laissées par ceux-ci. L’objectif principal de ce mémoire est de vérifier quelles sont les différences et ressemblances entre ces deux assemblages fossiles et d’en tirer des interprétations paléoenvironnementales. Pour chacun des agents taphonomiques (météorisation, piétinement, etc.), les fossiles sont gradés en fonction de stades prédéfinis tirés des écrits scientifiques. Finalement, malgré quelques différences (action des carnivores, intempérisation) on constate que les assemblages sont assez semblables et que, comme prédit par les reconstructions paléoenvironnementales préexistantes, leur lien à l’eau est assez bien démontré (groupes de Voorhies, abrasion). / The Mursi Formation has an age estimated at more than 4 million years, which corresponds to the emergence of the australopithecines, the first indisputably bipedal hominins. The Mursi faunal collection (N=201) is compared taphonomically to that of the Member A of the Shungura Formation (N=300). Taphonomy is the study of the processes that intervene between the death of an animal and its discovery as a fossil. This thesis aims to describe the main differences and similarities of these two collections and to make paleoenvironmental inferences. For each taphonomical agent (weathering, trampling, etc.), fossils are graded using stages established by other researchers. In spite of a few significant differences (carnivore action, weathering), it seems that the two collections are taphonomically similar and their link with water, as predicted by paleoenvironmental reconstructions, is supported (Voorhies groups, abrasion).

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.