Galaxy Protoclusters as Drivers of Cosmic Reionization

The environmental dependence of galaxy properties today requires an accelerated assembly history in high density environments. Protoclusters, the overdense regions of the universe that ultimately collapse into massive galaxy clusters, contribute a significant fraction of all cosmic star formation during the era of reionization. The galaxy overdensity may enhance the escape of ionizing photons, making protoclusters crucial in driving the timing and topology of cosmic reionization. We propose to measure the brightest rest-frame-optical emission lines from spectrosopically-confirmed galaxies comprising the largest overdensity yet identified near the midpoint of cosmic reionization, z7OD. Eight protocluster members, identified by their Lyman-alpha emission, are the primary targets for these NIRSpec MOS observations, which will measure rest-frame optical emission lines between the [OII] doublet and hydrogen Balmer alpha lines. We aim to measure the transmission of the IGM in and around an ionized, cosmic bubble, determine the physical properties of the galaxies that ionized the bubble, and compare them to field galaxies at z > 3.37 (observed simultaneously). We will measure the Lyman-alpha escape fraction and the Lyman-alpha velocity offset, information required to map spatial variation in IGM transmission. Protoclusters at this redshift are predicted to contain significant amount of cold gas, possibly triggering high specific star formation rates and accelerating the chemical evolution of the galaxies; ideas we will test by directly measuring diagnostic emission-line ratios.