Program: GS-2018A-Q-212

We have recently upgraded FLAMINGOS2 with two custom medium-band filters that split the K–band, taking advantage of the dark K-band background of Gemini. The filters (K-blue and K-red) are sensitive to astrophysical features in the K-band that have been under-studied. Here, we propose a pilot program, the high-redshift F2 KbKr Galaxy Evolution (ZFK2) survey. We will observe deeply in two F2 field with both filters, covering 60 arcmin^2 to K=24.5 AB. ZFK2 allows us to probe the z>4 universe with the same photometric redshift accuracy as previous medium-band surveys that have been pivotal in studying galaxy evolution at lower redshifts. For the first time, the new K-band filters will allow the identification of the Balmer/4000A break for galaxies at z=3.5-6, which is required to measure precise redshifts, reject contaminants, and study the stellar population properties of massive galaxies. The main science goal of ZFK2 is to push toward the first, complete census of massive galaxies at z > 3.5, and identify the emergence of the first quiescent galaxies with old stellar populations, which places strong constraints on galaxy formation models. We will also use imaging with these filters to study galaxies with strong nebular emission lines (e.g., redshifted [O III] 5008), including intense starburst galaxies at z > 4, and perhaps metal–free nebulae ionized by the first stars and black holes. ZFK2 is very timely as it will provide objects as targets for follow-up studies from JWST, as well as ALMA, and the GSMTs. (The time for 1 F2 field is already allocated - we request time for a second field to double the sample and guard against cosmic variance).