Program: GS-2023A-Q-318

JWST is poised to revolutionize our understanding of the physical conditions in galaxies in the reionization era, offering unprecedented measurements of the metallicity and ionization states of sources at z>7. However, due to absorption by the intergalactic medium, we will never measure their escaping Lyman continuum (LyC) radiation and will never know how much an individual galaxy at these redshifts contributed to the reionization of the universe. The identification of a robust sample of local analogs to the JWST sources, for which LyC emission can be measured directly, is thus a pressing problem. We are assembling a large photometric catalog of extreme [O III]-emitters at 0.30 < z < 0.37, using the narrowband Census of the Local Universe, which covers the entire northern sky above declinations of -20 degrees. A spectroscopic pilot study using the 4-m CTIO Blanco/COSMOS spectrograph confirms that our narrowband selection criteria successfully identify [O III]-emitters at z~0.3, including extreme sources with high [OIII]/[OII] ratios. However, the weaker lines that are key to detailed studies are not detected in most cases. We here request 22.88 hours of Gemini-S GMOS spectroscopy of 15 additional candidates with the aims of refining our selection criteria to focus on the most extreme sources, constructing a robust comparison sample for JWST-observed z>6 sources, conducting detailed studies of the physical conditions of the targets, and identifying the most promising candidates for followup LyC and integral field observations.