Subaru deep survey.: IV.: Discovery of a large-scale structure at redshift ≃51

We report the discovery of a large-scale structure of Lyalpha emitters (LAEs) at z = 4.86 based on wide-field imaging with the prime-focus camera (Suprime-Cam) on the Subaru Telescope. We observed a 25' x 45' area of the Subaru Deep Field in a narrow band (NB711, lambda(c) = Angstrom and FWHM = 73 Angstrom) together with R and i'. We isolate from these data 43 LAE candidates down to NB711 = 25.5 mag using color criteria. Follow-up spectroscopy of five candidates suggests the contamination by low-z objects to be similar to20%. We find that the LAE candidates are clustered in an elongated region on the sky of 20 Mpc in width and 50 Mpc in length at z = 4.86, which is comparable in size to present-day large-scale structures (we adopt H-0 = 70 km s(-1) Mpc(-1), Omega(0) = 0.3, and lambda(0) = 0.7). This elongated region includes a circular region of 12 Mpc radius of higher surface overdensity (delta(Sigma) = 2), which may be the progenitor of a cluster of galaxies. Assuming this circular region to be a sphere with a spatial overdensity of 2, we compare our observation with predictions by cold dark matter models. We find that an Omega(0) = 0.3 flat model with sigma(8) = 0.9 predicts the number of such spheres consistent with the observed number (one sphere in our survey volume) if the bias parameter of LAEs is b similar or equal to 6. This value suggests that the typical mass of dark halos hosting LAEs at z similar or equal to 5 is of the order of 10(12) M-circle dot. Such a large mass poses an interesting question about the nature of LAEs.