Program: GS-2019A-FT-101

Our team led the upgrade of Gemini/F2 with novel (and unique) filters that split the K-band (1.9-2.4 micron) into a bluer (Kb, 1.87-2.13 um) and redder (Kr, 2.17-2.43 um) filters. These filters are designed to take advantage of the low IR backgrounds of Gemini and identify galaxies with prominent Balmer/4000Å breaks at 3 < z < 6. Since late 2017, we have been conducting the first pilot survey with these filters. We were awarded 28 hours in 2018A for two fields in COSMOS, but because of interruptions from visiting instruments on the telescope and weather, our program received less than half the allocated time. We request here 11.8 hrs of Fast Turnaround time to complete these observations. Without these data we are unable to test for (what we expect to be) strong cosmic variance in the number density of massive galaxies at z > 4. Identifying even a handful of quiescent galaxies at these redshifts will provide important constraints on modes of mass assembly and the nature of feedback in the early Universe. The requested fast-turnaround time will complete uniform coverage in Kb and Kr in a second pointing in COSMOS. This will deliver the first science result, capitalizing on the promise that these filters provide. Timing is crucial for this program to make an impact, capitalize on the investments already made, demonstrate this as a potential science niche for Gemini/F2 and provide targets for future observations with JWST and ALMA.