Reionization from Z to A: Lessons from z∼6−7 Lyman-α emitters and local analogs of high-redshift galaxies
2018-08
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Reionization from Z to A: Lessons from z∼6−7 Lyman-α emitters and local analogs of high-redshift galaxies
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2018-08
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The dark ages that followed recombination ended with the appearance of metal-free stars and the subsequent formation of numerous low-mass, metal-poor galaxies. The collective ionizing background from these newly-forming galaxies is thought to be responsible for the reionization of the diffuse hydrogen in the intergalactic medium (IGM) between redshifts 10 and 6.5. The progression of the reionization history of the universe depends on the nature of these first sources — their number densities, luminosities, clustering, and production rates of ionizing photons — which is currently the subject of considerable observational and theoretical efforts. In this thesis, we combine both local and distant observations in a two-part approach to studying the Epoch of Reionization: exploring the progression of the transition and investigating the sources responsible for initiating and maintaining it. In the first part, we present the results of a systematic search for Lyman-α emitting galaxies (LAEs) at redshifts greater than 6. Lyα photons are produced by a resonant transition and so are sensitive to the presence of even a small fraction of neutral hydrogen. The evolving neutral hydrogen fraction in the IGM can therefore be constrained using observations of the Lyα output from galaxies around the end of reionization. We select high-redshift LAE candidates using imaging and spectroscopic data from the the HST WFC3 Infrared Spectroscopic Parallel (WISP) Survey. The sample selection combines an emission line search with a cut on galaxy colors chosen to identify a Lyman break. This approach, when combined with the broad spectral coverage of the WISP Survey, eliminates almost all lower-redshift contaminants. We find two LAEs at z = 6.38 and 6.44 with line luminosities of L_Lyα ∼ 4.7 × 10^43 erg/s, putting them among the brightest LAEs discovered at these redshifts. The WISP LAEs have a high number density of 7.7 × 10^−6 Mpc^−3. We argue that the LAEs reside in megaparsec-scale ionized bubbles that allow the Lyα photons to redshift out of resonance before encountering the neutral IGM. In the context of recent detections of similarly bright, high-redshift LAEs, we discuss the implications of the high number densities for a non-homogeneous or “patchy” reionization process. After considering possible ionizing sources for the WISP bubbles, we conclude that the observed LAEs alone are not sufficient. In the second part of this thesis, we present an analysis of a sample of local galaxies that are potential analogs to those responsible for the reionization of the IGM. Observational studies at z>7 are limited to the brightest sources, and the more common, fainter, lower-mass galaxies that are expected to be responsible for reionization remain largely inaccessible. Additionally, emission at λ<1216 Å from galaxies at these redshifts is absorbed by the neutral hydrogen in the IGM. Analogs of these high-redshift galaxies are therefore required in order to study the properties of the galaxies that contributed to reionization. Selected from the Sloan Digital Sky Survey for their large [O III]/[O II] ratios and high Hα equivalent widths, the local galaxies have very low masses and high-ionization lines present in their spectra. We perform a careful two-dimensional reduction and calibration that preserves the spatial information in both the continuum and emission lines. We present preliminary results and describe the next steps of this project, including fitting the spectrum of each galaxy with ionization models to estimate galactic properties such as gas temperature, elemental abundance, and the production rate of ionizing radiation. Both the emission lines and continua of the spectra are spatially extended, allowing for the spatial mapping of these properties. We compare these measurements with those of similar local analogs in the literature to show that these galaxies are candidate Lyman continuum leakers, a necessity for ionizing the hydrogen in the IGM at high redshift. Finally, in anticipation of future slitless cosmological surveys, we use the WISP and 3D-HST HST grism surveys to provide constraints on the observed distributions and properties of emission line galaxies at redshifts z∼1−2. We calculate the number densities of Hα+[N II] and [O III]λ5007-emitters, and the equivalent width and emission size distributions that ESA’s Euclid mission can expect to detect. Such observations are crucial for calibrating the simulations and mock catalogs used to test the survey design and expected results.
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University of Minnesota Ph.D. dissertation.August 2018. Major: Astrophysics. Advisor: Claudia Scarlata. 1 computer file (PDF); xii, 150 pages.
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