Probing the dark ages: Observations of the high -redshift universe

This thesis attempts to describe some of the earliest phases in the collapse of galaxies from an observational standpoint. The work is composed of an assortment of projects which sample objects at very high redshift, probing the Universe 1–3 Gyr after the Big Bang.;The first section of the thesis concerns high-redshift galaxies. Search techniques for identifying distant galaxies are extensively reviewed. Radio selection was once the primary vehicle to targeting the early Universe. Keck spectroscopy of high-redshift radio galaxies from the MIT-Greenbank radio catalog (S5GHz ≳ 50 mJy) are discussed. We synthesize a composite radio galaxy spectrum, which we compare with other composite active galaxy spectra. Our data suggests a correlation between radio power and ionization state in high-redshift radio galaxies.;The following three chapters detail individual galaxies confirmed at z > 5. These galaxies are among the half-dozen most distant sources known at the close of the 20th Century. Two of the galaxies were photometrically-selected from the Hubble Deep Field (HDF 4-473.0 at z = 5.60 and HDF 3-951.0 at z = 5.34 ± 0.01). The third is TN J0924-2201, a radio galaxy at z = 5.19 selected on the basis of steep radio spectral index and faint K-band brightness. This source contains the most distant active galactic nucleus currently known, requiring early formation of supermassive blackholes within a Gyr after the Big Bang.;The second section of the thesis concerns searches for high-redshift Lyα emission, identified either from deep, narrow-band imaging surveys or deep slit spectra. We discuss in detail one faint, high equivalent width line-emitter. Conventional wisdom would suggest identifying the 9185 Å line with Lyα at z = 6.55. We argue [O II] λ3727 at z = 1.46 is the more likely identification and discuss observational tests to distinguish Lyα-emitters at high redshift from foreground (active) sources.;The final section of the thesis concerns high-redshift, radio-loud quasars. We report on a spectroscopic survey of photometrically-selected, high-redshift quasars selected from correlations between the 21 cm FIRST radio catalog and the (optical) digitized Palomar Observatory Sky Survey. Six radio-loud quasars at z ≳ 4 were identified in this survey. We also report on radio-properties of z > 4 optically-selected quasars, showing that there this is no evidence for the radio-loud fraction changing at early cosmic epoch.