The End of Reionization: Searches for Faint Galaxies and Quasars in the Deep Fields of the Hubble Space Telescope
In this thesis work, we have examined high-redshift galaxies and quasars using the available deep imaging fields, e.g., the Hubble Ultra Deep Field (HUDF) and the Great Observatories Origins Deep Survey (GOODS), by observing with the instruments ACS/WFC and WFC3/IR onboard the Hubble Space Telescope (HST). Our goal is to understand the contributions of high-redshift galaxies and quasars to the ionization of hydrogen, their evolution with cosmic time, and their distribution across the observed fields. We start with a derivation of the rest-frame ultraviolet (UV) luminosity function (LF) at redshift z ~ 6 using a new application of the maximum likelihood method and exploring the five deepest HST/ACS fields. We work on the latest improved data products, which makes our results more robust than those of previous studies. We are the first in this field to use un-binned object counts and thereby make optimal use of the information contained in the dataset. We undertake analysis to a magnitude limit where the completeness is larger than 50% to avoid possibly large errors in the faint-end slope that are difficult to quantify. We also take into account the scattering in and out of the dropout sample due to photometric errors by defining for each object a probability that an object belongs to the dropout sample (called the f-factor technique). Our z ? 6 LF is thus more reliable than that from any previous group. Using the same technique, we find the best-fit Schechter parameters of the LF at z ~ 5, 6, 7 and 8. We also study the evolution of the galaxies within the redshift window 4 < z < 9 in a unified scheme. The picture that emerges in terms of standard Schechter function parameters is: a constant phi*, a slowly steepening alpha, and a relatively rapidly dimmed M* towards higher redshifts. Our research survey continues to search for high-redshift quasars. We are among the first groups that have studied the high-redshift LF for both galaxies and quasars. We have identified ten z ~ 6 candidates in the four deep fields, much less than expected from those wide surveys at the bright end. Together with our discovery that there are more faint galaxies at z ~ 6 , we reach the conclusion that galaxies, not quasars, are the main sources for continuing the reionization of the 900-million-year universe. Based on our results for the quasar LF, we also predict the quasar number density at z ~ 7 and z ~ 10 as a guide for future searches. During our study of quasars, we found one at z ~ 5 that has also been detected in the 4 Ms Chandra Deep Field South (CDFS). The X-ray observations show a significant detection, displaying a hardness ratio of HR=0.23 +- 0.24, which, for a source at z ~ 5, is highly suggestive of Compton-thick absorption. This is the most distant obscured quasar discovered so far. In this thesis work, we have processed thousands of optical and infrared images from the HST with various PyRAF procedures, e.g., MultiDrizzle, generated source catalogs with SExtractor based on detection image, tested and modified the dropout selection criteria for high-redshift objects, inspected images visually and excluded interlopers or artifacts, developed sophisticated statistical models, and run a series of IDL/C-language programs to find numerical results.