<p> A study of metal enrichment of the intergalactic medium (IGM) using a series of smooth particle hydrodynamics (SPH) simulations is presented, employing models for metal cooling and the turbulent diffusion of metals and thermal energy. An adiabatic feedback mechanism was adopted where gas cooling was prevented on the timescale of supernova bubble expansion to generate galactic winds without explicit wind particles. The simulations produced a cosmic star formation history (SFH) that is broadly consistent with observations until z ~ 0.5, and a steady universal neutral hydrogen fraction (OHI) that compares reasonably well with observations. The evolution of the mass and metallicities in stars and various gas phases was investigated. At z=O, about 40% of the baryons are in the warm-hot intergalactic medium (WHIM), but most metals (80%-90%) are locked in stars. At higher redshifts the proportion of metals in the IGM is higher due to more efficient loss from galaxies. The results also indicate that IGM metals primarily reside in the WHIM throughout cosmic history, which differs from simulations with hydrodynamically decoupled explicit winds. The metallicity of the WHIM lies between 0.01 and 0.1 solar with a slight decrease at lower redshifts. The metallicity evolution of the gas inside galaxies is broadly consistent with observations, but the diffuse IGM is under-enriched at z ~ 2.5. Metals enhance cooling which allows WHIM gas to cool onto galaxies and increases star formation. Metal diffusion allows winds to mix prior to escape, decreasing the IGM metal content in favour of gas within galactic halos and star forming gas. Diffusion significantly increases the amount of gas with low metallicities and improves the density-metallicity relation. </p> <p> The galactic wind generation mechanism and the wind properties from our simulations were investigated. It was found that: 1. Galactic winds are most efficient for halos in the intermediate mass range 10^10Mo - 10^11 Mo . These winds dominate the metal ejection at all redshifts, although towards lower redshift the contributions from larger halos become relatively more important. At the low mass end gas is prevented from accreting onto halos and has very low metallicities. At the high mass end, the fraction of halo baryons escaped as winds declines along with the decline of stellar mass fraction in these halos. The decrease in wind ejection is likely because of the decreases in star formation activity, wind mass loading and wind escape efficiency as the halo mass increases. 2. The adiabatic feedback can generate winds with mass loading factors comparable to the ones used in explicit superwind models. The mass loading factor decreases towards lower redshift, implying that smaller halos have larger mass loading. 3. Metals located at lower density were generated at earlier epochs from small halos, suggesting that the wind traveling speed can affect the metal distribution in the IGM. </p> / Thesis / Doctor of Philosophy (PhD)

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<p> Alterations in neural tissues associated with environmental variables have been studied for many years. Anatomical changes in the neocortex of rats in response to exposure to complex environments were observed and replicated in a number of studies both within and across species. These changes are not dependent on the age of the animal or on the duration of exposure, and have been demonstrated in structures outside of the cortex. Due to the undisputed involvement of both the neocortex and the hippocampus in learning and memory, researchers applied a widely used model system of a synaptic mechanism for learning, long-term potentiation (LTP), to the environmental enrichment paradigm and demonstrated significant enhancements in hippocampal field potentials in enriched rats. The present study examines whether the neocortex also showed evidence of plasticity in synaptic transmission. No effects for environmental enrichment were observed on the maximum amplitude of neocortical field responses evoked from the corpus callosum. To assess the plasticity of the chronic preparation used in the study, the animals were exposed to trains of pulses previously shown to induce electrical LTP in the cortex, but revealed only a slight, although significant, depression of the evoked response amplitude. An alteration in the stimulation parameters did not result in an enhanced response. Cortical depth measures suggested that the enriched environment was indeed sufficient to produce plastic changes in anatomy, if not in the efficacy of synaptic transmission. The importance of these findings in the neocortex leads us to question the validity of the LTP model of learning and memory.</p> / Thesis / Master of Science (MSc)