We present Lyα luminosity function (LF), clustering measurements, and Lyα line profiles based on the largest sample, to date, of 207 Lyα emitters (LAEs) at z = 6.6 on the 1-deg 2 sky of Subaru/XMM-Newton Deep Survey (SXDS) field. Our z = 6.6 Lyα LF including cosmic variance estimates yields the best-fit Schechter parameters of φ * = 8.5 +3.0 −2.2 × 10 −4 Mpc −3 and L * Lyα = 4.4 +0.6 −0.6 × 10 42 erg s −1 with a fixed α = −1.5, and indicates a decrease from z = 5.7 at the 90% confidence level. However, this decrease is not large, only ≃ 30% in Lyα luminosity, which is too small to be identified in the previous studies. A clustering signal of z = 6.6 LAEs is detected for the first time. We obtain the correlation length of r 0 = 2 − 5 h −1 100 Mpc and bias of b = 3 − 6, and find no significant boost of clustering amplitude by reionization at z = 6.6. The average hosting dark halo mass inferred from clustering is 10 10 − 10 11 M ⊙ , and duty cycle of LAE population is roughly ∼ 1% albeit with large uncertainties. The average of our high-quality Keck/DEIMOS spectra shows an FWHM velocity width of 251 ± 16km s −1 . We find no large evolution of Lyα line profile from z = 5.7 to 6.6, and no anti-correlation between Lyα luminosity and line width at z = 6.6. The combination of various reionization models and our observational results about the LF, clustering, and line profile indicates that there would exist a small decrease of IGM's Lyα transmission owing to reionization, but that the hydrogen IGM is not highly neutral at z = 6.6. Our neutral-hydrogen fraction constraint implies that the major reionization process took place at z 7.

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We make use of deep 1.2 mm-continuum observations (12.7µJy/beam RMS) of a 1 arcmin 2 region in the Hubble Ultra Deep Field (HUDF) to probe dust-enshrouded star formation from 330 Lyman-break galaxies spanning the redshift range z = 2-10 (to ∼2-3 M ⊙ /yr at 1σ over the entire range). Given the depth and area of ASPECS, we would expect to tentatively detect 35 galaxies extrapolating the Meurer z ∼ 0 IRX-β relation to z ≥ 2 (assuming T d ∼ 35 K). However, only 6 tentative detections are found at z 2 in ASPECS, with just three at >3σ. Subdividing z = 2-10 galaxies according to stellar mass, U V luminosity, and U V -continuum slope and stacking the results, we only find a significant detection in the most massive (>10 9.75 M ⊙ ) subsample, with an infrared excess (IRX=L IR /L UV ) consistent with previous z ∼ 2 results. However, the infrared excess we measure from our large selection of sub-L * (<10 9.75 M ⊙ ) galaxies is 0.11 +0.32 −0.42 ±0.34 (bootstrap and formal uncertainties) and 0.14 +0.15 −0.14 ±0.18 at z = 2-3 and z = 4-10, respectively, lying below even an SMC IRX-β relation (95% confidence). These results demonstrate the relevance of stellar mass for predicting the IR luminosity of z 2 galaxies. We find that the evolution of the IRX-stellar mass relationship depends on the evolution of the dust temperature. If the dust temperature increases monotonically with redshift (∝ (1 + z) 0.32 ) such that T d ∼ 44-50 K at z ≥ 4, current results are suggestive of little evolution in this relationship to z ∼ 6. We use these results to revisit recent estimates of the z ≥ 3 star-formation rate density.

We investigate photometric properties of Lyman Break Galaxies (LBGs) at z = 3.5 − 5.2 based on large samples of 2,600 LBGs detected in deep (i ′ 27) and wide-field (1,200 arcmin 2 ) images taken in the Subaru Deep Field (SDF) and the Subaru/XMM Deep Field (SXDF) using broad band B, V , R, i ′ , and z ′ filters. The selection criteria for the LBG samples are examined with 85 spectroscopically identified objects, and the completeness and contamination of the samples are estimated from Monte Carlo simulations based on a photometric-redshift catalog of the Hubble Deep Field-North. We find that these LBG samples are nearly rest-frame UV magnitude-limited samples, missing systematically only 10% of red high-z galaxies (in number) which are a dusty population with E(B − V ) 0.4. We calculate luminosity functions of the LBGs with the estimated completeness and contamination, and find (i) that the number density of bright galaxies (M 1700 < −22 ; corresponding to SFR 100h −2 70 M ⊙ yr −1 with extinction correction) decreases significantly from z = 4 to 5 and (ii) that the faint-end slope of the luminosity functions of LBGs may become steeper towards higher redshifts. We estimate dust extinction of z ≃ 4 LBGs with M < M * (≃ −21) from UV-continuum slopes, and obtain E(B − V ) = 0.15 ± 0.03 as the mean value. The dust extinction remains constant with apparent luminosity, but increases with intrinsic (i.e., extinction-corrected) luminosity. We find no evolution in dust extinction between LBGs at z = 3 and 4. We investigate the evolution of UV-luminosity density by integrating the luminosity functions of LBGs, and find that the UV-luminosity density at 1700Å, ρ UV does not significantly change from z = 3 to z = 5, i.e., ρ UV (z = 4)/ρ UV (z = 3) = 1.0 ± 0.2 and ρ UV (z = 5)/ρ UV (z = 3) = 0.8 ± 0.4, thus the cosmic star-formation rate (SFR) density (with correction for dust extinction) remains constant within the error bars, or possibly a slight decline, from z = 3 to z = 5. We estimate the stellar mass density from the cosmic SFR thus obtained, and find that this stellar mass density is consistent with those derived directly from the stellar mass function at z = 0 − 1, but exceeds those at z ∼ 3 by a factor of 3. We find that the ratio of the UV-luminosity density of Lyman α emitters (LAEs) to that of LBGs is ≃ 60% at z ≃ 5, and thus about a half of star formation probably occurs in LAEs at z ≃ 5. We obtain a constraint on the escape fraction of UV-ionizing photons (i.e., UV continuum in 900Å) produced by LBGs, f esc 0.13. This implies that the escape fraction of LBGs may be larger than that of star-forming galaxies at z = 0.

When galaxy formation started in the history of the Universe remains unclear. Studies of the cosmic microwave background indicate that the Universe, after initial cooling (following the Big Bang), was reheated and reionized by hot stars in newborn galaxies at a redshift in the range 6 < z < 14 (ref. 1). Though several candidate galaxies at redshift z > 7 have been identified photometrically, galaxies with spectroscopically confirmed redshifts have been confined to z < 6.6 (refs 4-8). Here we report a spectroscopic redshift of z = 6.96 (corresponding to just 750 Myr after the Big Bang) for a galaxy whose spectrum clearly shows Lyman-alpha emission at 9,682 A, indicating active star formation at a rate of approximately 10M(o) yr(-1), where M(o) is the mass of the Sun. This demonstrates that galaxy formation was under way when the Universe was only approximately 6 per cent of its present age. The number density of galaxies at z approximately 7 seems to be only 18-36 per cent of the density at z = 6.6.