Subaru Deep Survey. IV. Discovery of a Large-Scale Structure at Redshift ≃5

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′ × 45′ area of the Subaru Deep Field in a narrow band (NB711, λc = 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-1 Mpc-1, Ω0 = 0.3, and λ0 = 0.7). This elongated region includes a circular region of 12 Mpc radius of higher surface overdensity (δΣ = 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.