′′ Aperture and Total Magnitudes Were Measured with Mag Aper and Mag Auto Parameters, Respectively. from the Ob

We investigate the sky distribution of z ∼ 6 Lyman break galaxies selected as i-dropouts having i − z > 1.45 down to z ≤ 26.5 in the Subaru Deep Field (SDF). We discover 37 i-dropouts clustered in a projected comoving 21.6× 21.6 Mpc region at z = 6, showing a local density excess. Carrying out follow-up spectroscopy, we identify four of them as Lyα emitters at z = 5.92, 6.01, 6.03 and 6.03 (spread over a distance of 46.6 Mpc). The number density of the cluster itself in SDF is ∼ 2.2 × 10 Mpc, smaller than those of protoclusters (i.e., forming galaxy clusters) at z ∼ 2–5.7. Also, the structure shows ∼ 4–21 times larger galaxy number density than those of z ∼ 6 galaxies in a general field. It has a mass of M ∼ 1.5 −0.5× 10 M⊙, comparable to those of z ∼ 0–5 protoclusters. Since the contamination of our sample by interlopers is estimated to be quite low, 5.9%, most of the other unconfirmed i-dropouts in the overdense region can be also z ≃ 6 galaxies. Hence, it could be a candidate forming cluster at z ≃ 6, representing a progenitor of galaxy clusters seen in the recent-day Universe. Subject headings: cosmology: observations—early universe—galaxies: evolution—galaxies: formation