Program: GS-2018B-Q-124

We request 30 hours of time to obtain deep near-infrared imaging using the new KbKr medium bands on FLAMINGOS-2 as part of our recent high-z survey, ZF2K. The primary science goal of ZF2K is to identify when the first massive galaxies appeared and how they quenched, vital to understanding the mechanisms of mass assembly and the role of feedback in the early Universe. By effectively splitting the K band, the Kb+Kr filters probes the Balmer/4000A breaks at 4<z<6, crucial to obtain reliable redshifts for galaxies in this uncharted regime. Here we propose to extend the ZF2K survey to include an extreme high-z massive galaxy identified in the recently completed UltraVISTA DR3 survey. In addition to augmenting the census of evolved, massive galaxies at z>4 by increasing the ZF2K survey volume, we will provide, for the first time, robust measurements of key galaxy properties and the environment of a unique z~5 massive target. This is a narrow window of opportunity for Gemini to make a significant impact in this fast-moving field — by leveraging the new F2 filters we will obtain the first statistical sample of massive galaxies at z>4, in time to facilitate efficient follow-up observations with the launch of JWST.