The 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 experiment is conducted at z∼5 with medium-band filters (K2, K3) on SWIMS which can capture redshifted Balmer break feature from quiescent galaxies inbetween the two. From this pilot survey, a promising candidate at z∼4.8 (cosmic age of only 1 Gyr) has been found in a LBG-selected proto-cluster CL4 in ELAIS-N1, whose stellar mass is estimated to be 1.1×1011M⊙, already twice as big as our Milky Way galaxy. We here propose to spectroscopically confirm this intriguing object with deep K-band spectroscopy with MOSFIRE by robustly identifying the Balmer break feature at the rest-frame 3645Å which is redshifted to 2.11µm and/or Balmer absorption lines (2.2–2.4µm). If confirmed, it will serve as an excellent spectroscopic target for JWST to characterize their properties such as accurate stellar masses and ages to put strict constraints on the initial onset of star formation at and beyond the reionization epoch. Moreover, we will confirm a foreground group at z∼2.5 hosting starbursting galaxies with strong Hα emission lines falling into the K3 band.
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