A massive quiescent galaxy confirmed in a protocluster at z=3.09
  We report a massive quiescent galaxy at $z_{\rm
spec}=3.0922^{+0.008}_{-0.004}$ spectroscopically confirmed at a protocluster
in the SSA22 field by detecting the Balmer and Ca {\footnotesize II} absorption
features with multi-object spectrometer for infrared exploration (MOSFIRE) 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 (SFH). Both models fit
the observed SED well and confirm that this object is a massive quiescent
galaxy with the stellar mass of $\log(\rm M_{\star}/M_{\odot}) =
11.26^{+0.03}_{-0.04}$ and $11.54^{+0.03}_{-0.00}$, and star formation rate of
$\rm SFR/M_{\odot}~yr^{-1} <0.3$ and $=0.01^{+0.03}_{-0.01}$ for parametric and
nonparametric models, respectively. The SFH from the former modeling is
described as an instantaneous starburst while that of the latter modeling is
longer-lived but both models agree with a sudden quenching of the star
formation at $\sim0.6$ 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 newly find three plausible [O III]$\lambda$5007 emitters at
$3.0791\leq z_{\rm spec}\leq3.0833$ happened to be 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 strong size and stellar
mass evolution of this massive quiescent galaxy via mergers.
