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The structural and mechanical properties of a surface often play an integral part in the determination of the cell adhesion strength and design parameters for creating a biodegradable electrospun scaffold. Nanofibers composed of the globular proteins bovine serum albumin (BSA) and fibronectin were produced by electrospinning with the electrospun protein scaffold serving as an extracellular matrix to which adhesion interaction will exist with cells via cell surface integrin. This interaction is vital in regulation cell differentiation, growth and migration and cell adhesion. We will demonstrate the ability to manipulate ligand-receptor interaction, the properties of the electrospun fibers, control and the formation of focal adhesions sites in cells cultured on the fibers with the ultimate goal of developing a biomimetric scaffold to investigate how cell adhesion molecules modulate cell behavior in a 3-dimentional culture.

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I investigate the observational signatures of the formation of dark matter cores in dwarf galaxies. I adopt the paradigm where the energy from star formation feedback is injected into the orbits of dark matter particles, forming a constant density core consistent with observations of dwarf galaxies. Using physically motivated constraints I show there is ample feedback energy available given the average stellar mass of dwarf galaxies to form cores in $10^{8}$--$10^{11}$\thinspace M$_{\odot}$ halos, and predict the maximum core size as a function of stellar mass. I describe how observational features of the old stellar content of dwarf galaxies are due to this core formation paradigm. As both dark matter and stars are collisionless fluids, the stars responsible for the feedback form in the centres of dwarf galaxies and have their orbits grown by subsequent star formation. This will naturally lead to age and metallicity gradients, with the younger and more metal rich stellar population near the dwarf centres. This process also prevents the destruction of globular clusters by driving them out of the dwarf nucleus --- the decrease in central dark matter density reduces the strength of dynamical friction --- and increases the likelihood of being stripped onto the stellar halos of larger galaxies. It also offers a model for forming multiple populations in globular clusters, with the only assumption being that the source of the polluted gas resides within the dwarf progenitor. As the orbit of a globular cluster grows, it will experience multiple accretion events with each pass through the gas-rich galaxy centre. The simple accretion model exhibits two traits revealed from observations --- a short accretion timescale and a sensitive dependence on mass --- without requiring an exotic initial stellar mass function or the initial globular cluster mass function to be 10--25 times larger than at present. / Dissertation / Doctor of Philosophy (PhD)

We use the APOSTLE suite of cosmological simulations to examine the role of the cosmic ionizing background in regulating star formation (SF) in low-mass LCDM halos. In agreement with earlier work, we find that, after reionization, SF can only proceed in halos whose mass exceeds a redshift-dependent ``critical'' virial mass determined by the structure of LCDM halos and the thermal pressure of UV-heated gas. This critical mass increases from Mcrit~10^8 Msun at z~11$ to ~10^9.7 Msun at z=0, roughly following the average mass growth history of halos in that mass range. This implies that most halos above or below critical at present have remained so since early times. In particular, the halos of most galaxies today were already above-critical (and thus forming stars) at high redshift, providing a simple explanation for the ubiquitous presence of ancient stellar populations in dwarfs, regardless of luminosity. It also implies that Mcrit today represents a ``threshold'' mass below which the fraction of ``dark'' halos increases steeply. Sub-critical halos may still host luminous galaxies if they were above-critical at some point in the past. SF ceases if a halo falls into the sub-critical regime; depending on each halo's accretion history this can occur over a wide range of times, explaining why SF in many dwarfs seems to continue well past the reionization epoch. It also suggests a tantalizing explanation for the episodic nature of SF in some dwarfs, which, in this interpretation, would be linked to temporary halo excursions above and below the critical boundary. In the simulations, Mcrit(z) cleanly separates star-forming from non-star-forming systems at all redshifts, indicating that the ionizing UV background, and not stellar feedback, is what regulates the beginning and the end of SF in the faintest dwarfs. Galaxies in sub-critical halos should make up a sizable population of faint field dwarfs, distinct from those in more massive halos because of their lack of ongoing star formation. Although few such galaxies are known at present, the discovery of this population would provide strong support for our results. / Graduate

Globular clusters are not simple stellar populations. Practically all globular clusters show multiple populations (MPs), where at the same metallicity [Fe/H], approximately half of their stars are enriched by the products of high-temperature hydrogen burning relative to the rest that show field-like abundances. The source of enrichment for forming the enriched population is an unresolved problem. Interacting massive binaries are an underexplored proposed source of enrichment. Many assessments of the theory are based on only one modelled binary. We simulate a suite of metal-intermediate, [Fe/H]=-1.44, interacting binaries with initial primary masses of 10 to 40 solar masses, with mass ratios ranging from 0.15 to 0.9, over periods ranging from about 2 to 700 days using MESA. Our simulations show that binaries at higher masses, higher mass ratios, and near our upper period limit tend to be the most enriching with ejecta showing HeNaCNOAlMg variations consistent with hot-H burning. Some binaries do not eject material, suggesting binary mass loss can contribute to the dilution of enrichment. As a realistic population, binaries within our parameter space eject about ten times as much mass as they would as single stars. Ejection occurs on timescales of about 11 Myr, consistent with observed and theoretical limits on the age spreads for MPs. Our systems are rare, making them more suited to explaining the stochastic nature of MPs but not the large fraction of enriched stars. Spreads in He, N, Na, C, and Al for our ejecta could reasonably explain the observed spreads in clusters. Reduced variation in O and Mg suggests more massive binaries should be investigated. A multi-scale approach to cluster formation with multiple types of enrichment sources is a necessary next step for validating MP formation scenarios. / Thesis / Master of Science (MSc) / The majority of stars form in star clusters. Globular clusters are the oldest and most massive type of star cluster. Formerly thought to be made of stars of the same age and chemical composition, nearly all observed globular clusters are now known to host multiple populations. About half of their stars form from similar material as isolated stars. The other half show signs of enrichment. How enriched stars get their enriching material is an open problem in cluster formation. Pairs of stars orbiting each other as binaries were proposed to eject the material needed to form these stars. We model 408 binaries to find that some systems eject large amounts of enrichment, especially when the stars are more massive. The rarity of these systems suggests binaries can explain the variations seen in multiple populations between clusters but cannot fully explain the large fraction of enriched stars seen.

Globular clusters are found in the halos of all types of galaxies, and have been shown to play major roles in the formation of stars and galaxies. The purpose of this thesis is to advance our level of understanding of the dynamical evolution of globular clusters through N-body simulations of clusters with a range of circular, eccentric, and inclined orbits. Theoretical studies have historically assumed that globular clusters experience a static tidal field, however the orbits of globular clusters are all non-circular and the tidal field of most galaxies is not symmetric. Understanding how clusters evolve in realistic potentials allows for them to be used to constrain the formation, merger history, and evolution of a host galaxy and even map out the current size, shape, and strength of a galaxy's gravitational field. We find that dense and compact clusters evolve as if they are in isolation, despite being subject to a non-static tidal field. For larger clusters, tidal shocks and heating inject energy into the cluster and significantly alter its evolution compared to previous studies. We describe how a non-static field alters the mass loss rate and relaxation time of a cluster, and propose methods for calculating a cluster's size and orbit. We then apply our work to clusters in the giant galaxies M87, NGC 1399, and NGC 5128. We consider each cluster population to be a collection of metal poor and metal rich clusters and generate models with a range of orbital distributions. From our models we constrain the orbital anisotropy profile of each galaxy, place constraints on their formation and merger histories, and explore the effects of nearby galaxies on cluster evolution. By advancing studies of globular cluster evolution to include the effects of a non-static tidal field, we have made an important step towards accurately modelling globular clusters from birth to dissolution. Our work opens the door for globular clusters to be used as tools to study galaxy formation, evolution, and structure. Future studies will explore how galaxy formation and growth via the hierarchical merger of smaller galaxies will affect cluster evolution. / Thesis / Doctor of Philosophy (PhD)

<p>Globular clusters have linear sizes (tidal radii) which are theoretically de- termined by their mass and by the gravitational potential of their host galaxy. However observationally, cluster sizes are simply a determination of where the cluster’s surface brightness profile becomes zero. This distance is also known as the limiting radius. While it is commonly assumed that the tidal radius and the limiting radius of a globular cluster are the same thing, it has yet to be validated. The purpose of this thesis is to explore the assumption that cluster tidal radii and limiting radii are equal by comparing the tidal radii of an observed and simulated globular cluster population.</p> <p>An established link between cluster tidal radii and limiting radii will yield new methods of utilizing globular clusters as tools for studying galaxies. If cluster sizes are truly imposed by the tidal field of the host galaxy, then tidal radii measurements can be used to trace the mass distribution within a galaxy, including the dark matter halo. Additionally, as we will demonstrate in this thesis, cluster sizes can also be used a tracer for the orbital anisotropy profile of a galaxy.</p> <p>To explore the assumption that tidal radii and limiting radii are equal, we utilize the globular cluster population of the Virgo giant M87. Unusually deep, high signal-to-noise images of M87 are used to determine the radius for approximately 2000 globular clusters. To compare with these observations, we simulate a globular cluster population that has the same characteristics to the observed M87 cluster population. These characteristics include cluster radial distribution, mass distribution, central concentration distribution and line of sight velocity dispersion. Placing these simulated clusters in the well-studied tidal field of M87, the orbit of each cluster is solved and the theoretical tidal radius of each cluster is determined. We compare the predicted relationship between cluster size and projected galactocentric distance found in our sim- ulation to observations in order to test whether a cluster’s tidal radius and limiting radius are equal. We find that for an isotropic distribution of cluster velocities, theoretical tidal radii are approximately equal to observed limiting radii. The simulation predicts the observed increase in cluster size with galac- tocentric distance, which is expected if tidal radii are dependent on the tidal field. Additionally, simulated cluster sizes are of the same order of magnitude as observed cluster sizes. However the simulation does underestimate cluster sizes in the inner regions of M87. To minimize the discrepancy between theory and observations, we further explore the effects of orbital anisotropy on cluster sizes, and suggest a possible orbital anisotropy profile for M87 which yields the best fit between theory and observations. Finally, we suggest multiple future studies which will aid in our understanding of tidal theory and in establishing a stronger link between tidal radii and limiting radii.</p> / Master of Science (MSc)

<p>The currently-favoured cosmological paradigm, Lamda-CDM, predicts that galaxies are built up from smaller galaxies in a bottom-up process known as hierarchical merging. Lambda-CDM is extremely successful for large-scale structures, but is less so for the detailed features of individual galaxies. We can study these features - the galaxies’ foundations and the remnants of the smaller components that built them - only in the closest galaxies in which we can resolve individual stars. In this thesis, we use data from the Canada-France-Hawaii Telescope (CFHT)/MegaCam as part of the Pan-Andromeda Archaeological Survey (PAndAS) to observe M33 (the Triangulum Galaxy) and the detailed features of its old stellar population. The study of these details is vital for our understanding of galaxy formation and evolution. We search for two types of components within the old stellar population: globular star clusters and the faint, diffuse stellar halo. We find only one new unambiguous outer halo star cluster, in addition to the five previously known in the M33 outer halo (r=10-50 kpc). A further 2440 cluster candidates are identified, which we analyse using two different types of simulated clusters. We are able to describe the type of clusters that are likely to remain hidden from our searches. Our study of a population of red giant branch (RGB) stars far from the M33 disk reveals a low-luminosity, centrally concentrated component which we interpret as the discovery of M33’s halo. It is everywhere in our data fainter than mu_V ~ 33 mag arcsec^−2, with scale length r_exp ~ 20 kpc, an overall luminosity not more than a few percent of the total luminosity of M33, and is possibly also not azimuthally symmetric. For M33 to have so few outer halo clusters compared to M31 and to have such a low-luminosity halo, with the possible asymmetry that we see, suggests tidal stripping of M33’s halo components by M31 - a view that is also favoured by the morphology of the disk substructure and recent modelling.</p> / Doctor of Philosophy (PhD)