Peering Through the Cosmic Fog: Spectroscopy of Galaxies During Reionization

Abstract

In the myriad observational and theoretical studies surrounding the Epoch of Reionization (EoR, z ∼ 13 − 6), one point becomes abundantly clear: we are still in the discovery phase of galaxies during this time. Our knowledge of galaxies during this epoch is still in its infancy, but what little we know hints that these galaxies emit copious UV radiation, powered by massive, metal-poor stars and significant ionization. While the Lyα emission line is the UV gold standard for finding galaxies during the EoR, the resonant line is highly sensitive to neutral Hydrogen – often pushing the redshift derived from this line hundreds of km s−1 redwards of the true redshift. However, the rest-UV spectra of galaxies contain a rich variety of nebular metal emission lines which can trace the systemic redshift of these distant sources, & contain highly valuable information about the massive stars and physical conditions in galaxies during the EoR. Such UV nebular lines are targetable at these redshifts, yet have been unexplored except in rare, individual cases.

In this work, I discuss the science that can be uncovered using Keck/MOSFIRE NIR spectroscopy of galaxies in the EoR. As part of this work, I focus on a galaxy at zsys = 7.5032 where we detect strong C iii] and C iv nebular emission. I show that this high-redshift galaxy matches well with very metal-poor (Z < 20% Solar) photoionization models that produce strong ionization (log U > −2). Such strong ionization could come from stellar populations that include binary stars and/or an IMF that extends to 300 M, or supplied by a combination of star-formation and an accreting supermassive black hole. Finally, I summarize the preliminary results of a broader Keck/MOSFIRE spectroscopic search (as PI) for rest-UV nebular metal lines in EoR galaxies (5 < z < 8) in the CANDELS fields and highlight the exciting science on the horizon using JWST.