S18B0040abst

S18B0040

We propose for 2 nights of Keck/MOSFIRE observations to follow-up two high-redshift passive galaxies in UDS: SXDS-10017 at z=3.87 and SXDS-19838 at z∼4.3. We were awarded 1 night in S17B to observe SXDS-10017, but we obtained only a half of the intended data due to bad weather, resulting in insecure redshift determination. With the new observations, we aim to (1) securely confirm the redshift of SXDS-10017 to complete the original program, (2) constrain its age and formation redshift, and (3) follow-up the more promising candidate SXDS-19838 at z_phot∼4.3. The redshift PDF of the latter crucially lies above z>4, thus maximizing the chances to observe the 4000 Å break in the K-band. The break provides the most powerful way to constrain the stellar age, stellar mass to luminosity ratios, and SFHs of these early massive galaxies, opening a new window on their formation and quenching timescales. This analysis paves also the way to statistical studies of larger samples of dimmer galaxies with the advent JWST. As a byproduct, we will break the record of the highest redshift quiescent galaxy with spectroscopic confirmations.

---

This document was translated from L^AT_EX by H^EV^EA.