Program: GN-2008A-C-4

We investigated sky distribution of z ~ 6 Lyman break galaxies selected as i'-dropouts by i' - z' > 1.5 color down to z' = 26.5 in the Subaru Deep Field and discovered 32 i'-dropouts, including 2 extremely bright ones (z' = 21.88 and 22.16) that might be z ~ 6 quasars, are clustering in a small ~ 14Mpc x 14Mpc region in projected comoving scale at z = 6, showing a 5sigma local density excess. We confirmed four of them as Lyman-alpha emitters at z = 5.92-6.03 (comoving scale of ~ 34 Mpc). This structure is comparable to z = 5.7 and 4.8 protoclusters found to date and thus can be a z ~ 6 protocluster of galaxies physically associated to quasars. However, the rest of i'-dropouts are photometric candidates. Hence, we propose 2 nights Gemini/GMOS spectroscopy of them to (1) confirm this is a real z ~ 6 protocluster, (2) derive its size, velocity dispersion and mass and its individual galaxies' sizes, masses, spatial distribution, Lyman-alpha/UV star-formation rates and velocity distribution to compare them with those of field and (proto)cluster galaxies at z ~ 0-5 to reveal how mass assembly, star formations and environment are related to early structure/galaxy formation and (3) test if current structure formation models in hierarchical clustering can reproduce the properties of z ~ 6 protocluster and its galaxies.