The Rest-Frame Ultraviolet Spectra of Galaxies in the Reionization-Era

Abstract

Recent studies have revealed detections of UV metal emission lines (CIII], OIII], CIV) in three of the first galaxies observed at z>6, providing the first spectroscopic insight into the nature of early star forming galaxies. The equivalent widths (EWs) of these lines are more than an order of magnitude stronger than what is typical at lower redshifts, potentially indicating significant differences in the ionizing spectrum of galaxies in the reionization era. The origin of the high ionization lines remains unclear, likely requiring either a low metallicity stellar population or AGN. However owing to the limited number of z>6 sources with deep spectroscopy and limited number of analogs at lower redshifts, it has proven difficult to generalize and interpret the initial results. In the first part of my thesis, I have undertaken a comprehensive Keck, LBT and VLT spectroscopic campaign to characterize the EW distribution of UV metal lines at z>6. My results present new constraints on the fraction of strong CIII] and CIV emitting galaxies at z>6 and indicate that ionizing spectrum from stellar population at z>6 could be significantly harder than systems at a lower redshift. In the second part of my thesis, I have developed a new strategy to identify and characterize analogs of the strong UV metal line emitters at lower redshift. This has resulted in a new sample of extreme CIII] emitters at z~2. Using spectroscopic tools, I have shown that large equivalent width CIII] emission lines tend to be those with moderately low gas-phase oxygen abundance, low stellar metallicity, large ionization parameter, and very little dust extinction. My results will guide surveys with current and future facilities (Spitzer, JWST, WFIRST) providing optimal strategy to target UV metal lines in the reionization epoch.