he existence of massive quiescent galaxies at high redshifts can put strong constraints on galaxy formation theories, as there is little cosmic time available for such matured systems to develop by the observed epoch. Such an experiment is conducted at z∼5 with medium band filters (K1, K2 and K3) on SWIMS which can capture redshifted Balmer break feature from quiescent galaxies in between the two. From this pilot survey, promising massive candidate with [OII] emission line at z∼5.34 (M∗=1.1×1011 M⊙, SFR=16 M⊙/yr) and massive quiescent candidate at z∼4.6 (M∗=3.2×1011 M⊙, SFR=5 M⊙/yr) have been found in a LBG-selected proto-cluster CL4 in ELAIS-N1. We here propose to spectroscopically confirm these objects with deep K-band spectroscopy with MOSFIRE by robustly identifying [OII] emission line and/or Balmer break at the cosmic age of only ∼ 1 Gyr. By combining deep spectra and existing photometric data, we can derive their star formation history and stellar mass accurately. Moreover, with LBG confirmation in this overdensity, we will discuss the mechanisms of accelerated formation and quenching at the maximum redshift of massive quiescent galaxies.
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