A Massive Quiescent Galaxy Confirmed in a Protocluster at z=3.09

We report a massive quiescent galaxy at z(spec) = 3.0922(-0.004)(+0.008) absorption features with the multi-object spectrometer for infrared exploration on the Keck I telescope. This is the most distant quiescent galaxy confirmed in a protocluster to date. We fit the optical to mid-infrared photometry and spectrum simultaneously with spectral energy distribution (SED) models of parametric and nonparametric star formation histories (SFHs). Both models fit the observed SED well and confirm that this object is a massive quiescent galaxy with a stellar mass of log(M-*/M-circle dot) = 11.26(-0.04)(+0.03) and 11.54(-0.00)(+0.03), and a star formation rate of SFR/M-circle dot yr(-1) < 0.3 and =0.01(-0.01)(+0.03) for parametric and nonparametric models, respectively. The SFH from the former modeling is described as an instantaneous starburst whereas that of the latter modeling is longer-lived, but both models agree with a sudden quenching of the star formation at similar to 0.6 Gyr ago. This massive quiescent galaxy is confirmed in an extremely dense group of galaxies predicted as a progenitor of a brightest cluster galaxy formed via multiple mergers in cosmological numerical simulations. We discover three new plausible [O III]lambda 5007 emitters at 3.0791 <= z(spec) <= 3.0833 serendipitously detected around the target. Two of them just between the target and its nearest massive galaxy are possible evidence of their interactions. They suggest the future great size and stellar mass evolution of this massive quiescent galaxy via mergers.