Reionization is the phase transition of intergalactic atoms from being neutral to

becoming fully ionized. This process began ∼400 Myr after the Big Bang, when the first

stars and black holes began emitting ionizing radiation from stellar photospheres and

accretion disks. Reionization completed when all of the neutral matter between galaxies

became ionized ∼1 Gyr after the Big Bang, and the Universe became transparent as

it is today.

Characteristics of the galaxies that drove reionization are mostly unknown. The

physical mechanisms that create ionizing radiation inside these galaxies, and the

paths for this light to escape are even more unclear. To date, only a small fraction of

the numerous searches for this escaping light have been able to detect a faint signal

from distant galaxies, and no consensus on how Reionization was completed has been

established.

In this dissertation, I discuss the evolution of the atomic matter between galaxies

from its initially ionized state, to its current re-ionized state, potential sources of

re-ionizing energy, and the theoretical and observational status of the characteristics of

these sources. I also present new constraints on what fraction of the ionizing radiation

escapes from galaxies using Hubble Space Telescope UV imaging, theoretical models

of the stellar and accretion disk radiation, and models of the absorption of ionizing

radiation by the intergalactic medium.

Ultraviolet and optical light from stars is reddened and attenuated by interstellar dust, where different sightlines across a galaxy suffer varying amounts of extinction. Tamura et al. (2009) developed an approximate method to correct for dust extinction, dubbed the “βV method,” by comparing the observed to an empirical estimate of the intrinsic flux ratio of visible and ∼3.5 μm emission. Moving beyond that empirical approach, through extensive modeling, I calibrated the βV -method for various filters spanning the visible through near infrared wavelength range, for a wide variety of simple stellar populations (SSP) and composite stellar populations (CSP). Combining Starburst99 and BC03 models, I built spectral energy distributions of SSP and CSP for various realistic star formation histories, while taking metallicity evolution into account. I convolved various 0.44–1.65 μm filter throughput curves with each model spectral energy distribution (SED) to obtain intrinsic flux ratios, βλ,0. To validate the modeling, I analyzed spatially resolved maps for the observed V- and g-band to 3.6 μm flux ratios and the inferred dust-extinction values AV for a sample of 257 nearby galaxies. Flux ratio maps are constructed using point-spread function-matched mosaics of Sloan Digitial Sky Survey g- and r-band images and Spitzer/InfraRed Array Camera 3.6μm mosaics, with all of the pixels contaminated by foreground stars or background objects masked out. Dust-extinction maps for each galaxy were created by applying the βV -method. The typical 1σ scatter in βV around the average, both within a galaxy and in each morphological type bin, is ∼20%. Combined, these result in a ∼0.4 mag scatter in AV. βV becomes insensitive to small-scale variations in stellar populations once resolution elements subtend an area larger than 10 times that of a typical giant molecular cloud. I find noticeably redder V−3.6 μm colors in the center of star-forming galaxies and galaxies with a weak AGN. The derived intrinsic V −3.6 μm colors for each Hubble type are generally consistent with the modeling. Finally, I discuss the applicability of the βV dust-correction method to more distant galaxies, for which large samples of well-matched Hubble Space Telescope rest-frame visible and James Webb Space Telescope rest-frame ∼3.5μm images will become available in the near future.

Galaxies represent a fundamental catalyst in the ``lifecycle'' of matter in the Universe, and the study of galaxy assembly and evolution provides unique insight into the physical processes governing the transformation of matter from atoms to gas to stars. With the Hubble Space Telescope, the astrophysical community is able to study the formation and evolution of galaxies, at an unrivaled spatial resolution, over more than 90% of cosmic time. Here, I present results from two complementary studies of galaxy evolution in the local and intermediate redshift Universe which used new and archival HST images. First, I use archival broad-band HST WFPC2 optical images of local (d<63 Mpc) Seyfert-type galaxies to test the observed correlation between visually-classified host galaxy dust morphology and AGN class. Using quantitative parameters for classifying galaxy morphology, I do not measure a strong correlation between the galaxy morphology and AGN class. This result could imply that the Unified Model of AGN provides a sufficient model for the observed diversity of AGN, but this result could also indicate the quantitative techniques are insufficient for characterizing the dust morphology of local galaxies. To address the latter, I develop a new automated method using an inverse unsharp masking technique coupled to Source Extractor to detect and measure dust morphology. I measure no strong trends with dust-morphology and AGN class using this method, and conclude that the Unified Model remains sufficient to explain the diversity of AGN. Second, I use new UV-optical-near IR broad-band images obtained with the HST WFC3 in the Early Release Science (ERS) program to study the evolution of massive, early-type galaxies. These galaxies were once considered to be ``red and dead'', as a class uniformly devoid of recent star formation, but observations of these galaxies in the local Universe at UV wavelengths have revealed a significant fraction (30%) of ETGs to have recently formed a small fraction (5-10%) of their stellar mass in young stars. I extend the study of recent star formation in ETGs to intermediate-redshift 0.35<1.5 with the ERS data. Comparing the mass fraction and age of young stellar populations identified in these ETGs from two-component SED analysis with the morphology of the ETG and the frequency of companions, I find that at this redshift many ETGs are likely to have experienced a minor burst of recent star formation. The mechanisms driving this recent star formation are varied, and evidence for both minor merger driven recent star formation as well as the evolution of transitioning ETGs is identified.

Most stars form in groups, and these clusters are themselves nestled within larger associations and stellar complexes. It is not yet clear, however, whether stars cluster on preferred size scales within galaxies, or if stellar groupings have a continuous size distribution. I have developed two methods to select stellar groupings across a wide range of size-scales in order to assess trends in the size distribution and other basic properties of stellar groupings. The first method uses visual inspection of color-magnitude and color-color diagrams of clustered stars to assess whether the compact sources within the potential association are coeval, and thus likely to be born from the same parentmolecular cloud. This method was developed using the stellar associations in the M51/NGC 5195 interacting galaxy system. This process is highly effective at selecting single-aged stellar associations, but in order to assess properties of stellar clustering in a larger sample of nearby galaxies, an automated method for selecting stellar groupings is needed. I have developed an automated stellar grouping selection method that is sensitive to stellar clustering on all size scales. Using the Source Extractor software package on Gaussian-blurred images of NGC 4214, and the annular surface brightness to determine the characteristic size of each cluster/association, I eliminate much of the size and density biases intrinsic to other methods. This automated method was tested in the nearby dwarf irregular galaxy NGC 4214, and can detect stellar groupings with sizes ranging from compact clusters to stellar complexes. In future work, the automatic selection method developed in this dissertation will be used to identify stellar groupings in a set of nearby galaxies to determine if the size scales for stellar clustering are uniform in the nearby universe or if it is dependent on local galactic environment. Once the stellar clusters and associations have been identified and age-dated, this information can be used to deduce disruption times from the age distribution as a function of the position of the stellar grouping within the galaxy, the size of the cluster or association, and the morphological type of the galaxy. The implications of these results for galaxy formation and evolution are discussed.