Subaru Deep Survey . Iv . Discovery of a Large - Scale Structure at Redshift ≃ 5 1

We report the discovery of a large-scale structure of Lyman α emitters (LAEs) at z = 4.86 based on widefield 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 = 7126Å and FWHM= 73Å) 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 haloes hosting LAEs at z ≃ 5 is of the order of 10M⊙. Such a large mass poses an interesting question about the nature of LAEs. Subject headings: cosmology: observations — cosmology: early universe — cosmology: large-scale structure of universe — galaxies: high-redshift — galaxies: evolution — galaxies: photometry