Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Thesis (M.S.)--University of Delaware, 2006. / Principal faculty advisor: John E. Gizis, Dept. of Physics & Astronomy. Includes bibliographical references.

The abundance of satellite galaxies is a critical small-scale test of the standard cosmological model. From comparing the predictions of structure formation in simulations with observations of Local Group dwarf galaxies there is a clear mismatch in the abundance, leading to the so-called “missing satellites” problem. The comparison between simulation and observation have, however, suffered from a limited sample of satellite galaxies, with the only reasonably complete sample being from the most local galaxy groups. It is unknown whether the observed abundance of dwarf satellite galaxies of the nearest groups is statistically representative of the abundance of dwarf satellites in the greater universe. We construct a volume-limited sample of galaxies down to a well-defined stellar mass limit (M★ ≥ 6 × 109 Msun ) using the Atlas3D parent sample of spiral and ellipsoidal galaxies by Cappellari et al. 2011. In order to statistically identify bound satellites around galaxies in our primary catalogue, we apply cuts on the background based on the properties of known dwarf satellites of the Local Group using both the Sloan Digital Sky Survey Data Release 8 (SDSS DR8) (http://www.sdss3.org/dr8/) spectroscopic and photometric galaxy catalogues. We detect an over-density of faint objects at projected separations of < 500 kpc at S/N ∼ 8, corresponding to an average of 4.8 ± 0.65 satellite detections per primary after stacking these systems and subtracting the background. We further find that the over-density of faint objects strongly depends on primary morphology and magnitude. While the Milky Way seems to be unusual in its number of bright satellites, our faint end satellite abundances are in agreement from the ∆m luminosity function for primaries in our sample as bright as the Milky Way. Our work has extended the work of previous authors by several magnitudes further down the faint end of the luminosity function.

Thesis (Ph. D.)--University of Texas at Austin, 1998. / Vita. Includes bibliographical references (leaves 188-196). Available also in a digital version from Dissertation Abstracts.

Thesis (M. Sc.)--York University, 1997. Graduate Programme in Physics and Astronomy. / Typescript. Includes bibliographical references (leaves 121-128). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL:http://wwwlib.umi.com/cr/yorku/fullcit?pMQ27361.

White dwarfs are particularly interesting due to their broad application to the field of astronomy (cosmochronology, SN Ia progenitors, asteroseismology). Examining distributions of white dwarf masses and temperatures, it is evident that there is some flaw in our ability to make physical atmosphere models. Tremblay and Bergeron (2009) used an {\it ad hoc} treatment of line broadening and derived significantly different surface gravity and temperatures for white dwarfs, demonstrating the importance of the line broadneing treatment in determining stellar parameters for high surface gravity stars. This thesis presents a new line broadening program, Xenomorph, based on simulation techniques. Xenomorph is used to examine various approximations used in line broadening calculations used in white dwarf atmospheres. Some approximations, like including fine structure and lower state perturbations, have small, if detectable effects. Ion motions during a transition can make features commonly seen in Stark profiles less pronounced and leads to an increase in the FWHM of the lines. Including a more complete basis set at higher densities will result in extra features, including asymmetries that has been observed in many experiments. / text

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An outstanding question in galaxy evolution research is whether the star formation histories of low mass systems are dominated by global starbursts or modes that are more quiescent and continuous. In this thesis, we quantify the prevalence of global starbursts in dwarf galaxies at the present epoch, and attempt to infer their characteristic durations, frequencies and amplitudes in the past. Our approach is to directly tally the number of bursting dwarfs in a complete local sample, and to compute the fraction of star formation that is concentrated in these systems. The resulting starburst number and mass fractions are then combined with B-V colors from the literature, the H-alpha EWs presented here, and stellar evolutionary synthesis models in order to place constraints on the average starburst duty cycle. The primary dataset used has been put together by the 11 Mpc H-alpha UV Galaxy Survey, who have collected data on an approximately volume-limited, statistical sample of star-forming galaxies within 11 Mpc of the Milky Way.Our main observational results, along with the accumulation of star formation studies of dwarf galaxies over the past three decades, paint a consistent picture where systems that are currently experiencing a massive global burst are just the 6% +/- 3% tip of a low-mass galaxy iceberg. Moreover, bursts are responsible for 22% +/- 10% of the total star formation in the overall dwarf galaxy population, so the majority of stars in low-mass systems do not appear to be formed in this mode today.Over their lifetimes, however, a greater fraction of the stellar mass of a dwarf may be formed in the burst mode. Synthesis modeling suggests that bursts cycles appear to be necessary in order to simultaneously explain the present-day observed blue B-V colors and modest H-alpha EWs of TYPICAL, CURRENTLY NON-BURSTING dwarf irregulars, unless non-standard assumptions concerning the IMF and the escape fractions of Lyman continuum photons are made. The starburst cycle that we converge upon involves burst durations of 50-100 Myrs, cycle frequencies of 1 to 3 per Gyr, and elevated burst SFRs that are a factor of 6-10 higher than the rate in the quiescent state. Galaxies characterized by such a SFH would spend ~10% of their lives in the burst state, and form ~50% of their stellar mass during this time.

This thesis examines the structure and heating requirements of the outer atmospheres (the lower transition region, upper transition region and corona) of the five dwarfs χ¹ Ori (G0V), ϵ Eri (K2V), ξ Boo A (G8V), α Cen A (G2V) and α Cen B (K0V), and the sub-giant Procyon (F5 IV-V). Alternative fits to X-ray spectra of ten late-type dwarfs from ROSAT are made using the latest available versions of the radiative power loss codes of Raymond and Smith, Landini and Monsignori Fossi, and Mewe et al. Differences between these codes are found to arise from the choice of atomic physics and the number of transitions included. The resulting coronal temperatures and emission measures are found to follow correlations proposed by Montesinos and Jordan (1993). The lower transition region is modelled using observations made with the International Ultraviolet Explorer and up-to-date atomic data. Models of the upper transition region and corona are derived assuming a balance between the radiative and conductive losses. Wave pressure is included in these models for the first time. Recent observations with the Extreme Ultraviolet Explorer aid the choice of starting parameters. The calculations are carried out in plane parallel and spherically symmetric geometries. The models of the lower and upper transition region are then combined; the spherically symmetric models of the upper transition region fit more smoothly on to those of the lower transition region than do the plane parallel models. The new model of Procyon and new measurements of N_e resolve a previous discrepancy which led to suggestions that emission in the transition region is restricted in area. The heating requirements are examined. In the five dwarfs, but not in Procyon, the conductive fluxes at the base of the upper transition region are found to exceed the radiative energy losses for the layers immediately below. In Procyon, it is shown that acoustic wave heating is a viable mechanism for heating the lower transition region and corona, while the correlation of coronal properties with the dynamo Rossby number suggest that magnetic heating models are more likely for the five dwarfs.