The Formation of the massive galaxies in the SSA22 z=3.1 protocluster
  The properties of K-band selected galaxies (K_AB<24) in the z = 3.09 SSA22
protocluster field are studied. 430 galaxies at 2.6 < z_phot < 3.6 are selected
as potential protocluster members in a 112 arcmin^2 area based on their
photometric redshifts. We find that \approx 20% of the massive galaxies with
stellar masses >10^11 M_sun at z_phot \sim 3.1 have colors consistent with
those of quiescent galaxies with ages > 0.5 Gyr. This fraction increases to
\approx 50% after correcting for unrelated foreground/background objects. We
also find that 30% of the massive galaxies are heavily reddened dusty
star-forming galaxies. Few such quiescent galaxies at similar redshifts are
seen in typical survey fields. An excess surface density of 24\mu m sources at
z_phot \sim 3.1 is also observed, implying the presence of dusty star-formation
activity in the protocluster. Cross-correlation with the X-ray data indicates
that the fraction of K-band selected protocluster galaxies hosting active
galactic nuclei (AGN) is also high compared with the field. The sky
distribution of the quiescent galaxies, the 24\mu m sources, and the X-ray AGNs
show clustering around a density peak of z=3.1 Ly\alpha emitters (LAEs). A
significant fraction of the massive galaxies have already become quiescent,
while the dusty star-formation is still active in the SSA22 protocluster. These
findings indicate that we are witnessing the formation epoch of massive
early-type galaxies at the center of predecessors to present-day rich galaxy
clusters.
