2003ApJ...586L.111S

We report the discovery of a large-scale structure of Lyα 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'x45' area of the Subaru Deep Field in a narrow band (NB711, λ_c=7126 Å and FWHM=73 Å) 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 ∼20%. 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 H₀=70 km/s/Mpc, Ω₀=0.3, and λ₀=0.7). This elongated region includes a circular region of 12 Mpc radius of higher surface overdensity (δ_Σ=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 Ω₀=0.3 flat model with σ₈=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≃6. This value suggests that the typical mass of dark halos hosting LAEs at z≃5 is of the order of 10¹² M_☉. Such a large mass poses an interesting question about the nature of LAEs.