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. We will conduct this experiment at the maximum redshift of z=5.3 that can be probed by ground-based observations, using our new purpose-made medium-band filters K3 and K4 on MOIRCS. We propose to search for such massive galaxies associated to a proto-cluster COSMOS-AzTEC3 at z=5.3. By targeting this most overdense region where galaxies are expected to form and grow earlier and faster in a biased manner, we can maximize the probability of finding such massive monsters with M*>6×1010M⊙ at this high redshift. We can also quantify the bias/acceleration of galaxy formation in overdense regions in the early Universe. Massive galaxy candidates, if discovered, will serve as excellent spectroscopic targets for JWST to characterize their properties such as accurate stellar masses and ages to put strong constraints on the initial onset of star formation beyond the reionization epoch. This is an extension of our partly conducted RubyRush program (4.6<z<5.1; S21A-S22A) to even higher redshift (z=5.3), and is complimentary to the narrow, general field JWST programs.
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