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We present a new model of the extragalactic background light (EBL) and corresponding γγ opacity for intergalactic gamma-ray absorption from z = 0 up to z = 10, based on a semi-analytical model of hierarchical galaxy formation that reproduces key observed properties of galaxies at various redshifts. Including the potential contribution from Population III stars and following the cosmic reionization history in a simplified way, the model is also broadly consistent with available data concerning reionization, particularly the Thomson scattering optical depth constraints from WMAP. In comparison with previous EBL studies up to z ∼ 3-5, our predicted γγ opacity is in general agreement for observed gamma-ray energy below 400/(1 + z) GeV, whereas it is a factor of ∼ 2 lower above this energy because of a correspondingly lower cosmic star formation rate, even though the observed UV luminosity is well reproduced by virtue of our improved treatment of dust obscuration and direct estimation of star formation rate. The horizon energy at which the gamma-ray opacity is unity does not evolve strongly beyond z ∼ 4 and approaches ∼ 20 GeV. The contribution of Population III stars is a minor fraction of the EBL at z = 0, and is also difficult to distinguish through gamma-ray absorption in high-z objects, even at the highest levels allowed by the WMAP constraints. Nevertheless, the attenuation due to Population II stars should be observable in high-z gamma-ray sources by telescopes such as Fermi or CTA and provide a valuable probe of the evolving EBL in the rest-frame UV. The detailed results of our model are publicly available in numerical form at the URL http://www.slac.stanford.edu/%7eyinoue/Download.html.

We present the Lyα luminosity functions (LFs) at z = 5.7 and 6.6 derived from a new large sample of 1266 Lyα emitters (LAEs) identified in total areas of 14 and 21 deg2, respectively, based on the early narrowband data of the Subaru/Hyper Suprime-Cam survey. Together with careful Monte Carlo simulations that account for the incompleteness of the LAE selection and the flux estimate systematics in the narrowband imaging, we have determined the Lyα LFs with unprecedentedly small statistical and systematic uncertainties in a wide Lyα luminosity range of 1042.8–43.8 erg s−1. We obtain best-fit Schechter parameters of $L^{*}_{\mathrm{Ly}\alpha } = 1.6^{+2.2}_{-0.6} \ (1.7^{+0.3}_{-0.7}) \times 10^{43}\:\mathrm{erg}\:\mathrm{s}^{-1}$, $\phi ^{*}_{\mathrm{Ly}\alpha } = 0.85^{+1.87}_{-0.77} \ (0.47^{+1.44}_{-0.44}) \times 10^{-4}\:\mathrm{Mpc}^{-3}$, and $\alpha = -2.6^{+0.6}_{-0.4} \ (-2.5^{+0.5}_{-0.5})$ at z = 5.7 (6.6). We confirm that our best-estimate Lyα LFs are consistent with the majority of the previous studies, but find that our Lyα LFs do not agree with the high number densities of LAEs recently claimed by Matthee/Santos et al.’s studies that may overcorrect the incompleteness and the flux systematics. Our Lyα LFs at z = 5.7 and 6.6 show an indication that the faint-end slope is very steep (α ≃ −2.5), although it is also possible that the bright-end LF results are enhanced by systematic effects such as the contribution from AGNs, blended merging galaxies, and/or large ionized bubbles around bright LAEs. Comparing our Lyα LF measurements with four independent reionization models, we estimate the neutral hydrogen fraction of the intergalactic medium to be $x_\mathrm{H\,{\small I}} = 0.3 \pm 0.2$ at z = 6.6, which is consistent with the small Thomson scattering optical depth obtained by Planck 2016.

We present the SILVERRUSH program strategy and clustering properties investigated with ∼ 2, 000 Lyα emitters at z = 5.7 and 6.6 found in the early data of the Hyper Suprime-Cam (HSC) Subaru Strategic Program survey exploiting the carefully designed narrowband filters. We derive angular correlation functions with the unprecedentedly large samples of LAEs at z = 6 − 7 over the large total area of 14 − 21 deg 2 corresponding to 0.3 − 0.5 comoving Gpc 2 . We obtain the average large-scale bias values of b avg = 4.1 ± 0.2 (4.5 ± 0.6) at z = 5.7 (z = 6.6) for > ∼ L * LAEs, indicating the weak evolution of LAE clustering from z = 5.7 to 6.6. We compare the LAE clustering results with two independent theoretical models that suggest an increase of an LAE clustering signal by the patchy ionized bubbles at the epoch of reionization (EoR), and estimate the neutral hydrogen fraction to be x HI = 0.15 +0.15 −0.15 at z = 6.6. Based on the halo occupation distribution models, we find that the > ∼ L * LAEs are hosted by the dark-matter halos with the average mass of log( M h /M ⊙ ) = 11.1 +0.2 −0.4 (10.8 +0.3 −0.5 ) at z = 5.7 (6.6) with a Lyα duty cycle of 1 % or less, where the results of z = 6.6 LAEs may be slightly biased, due to the increase of the clustering signal at the EoR. Our clustering analysis reveals the low-mass nature of > ∼ L * LAEs at z = 6 − 7, and that these LAEs probably evolve into massive super-L * galaxies in the present-day universe.

We conducted a deep narrowband NB973 (FWHM = 200Å centered at 9755Å) survey of z = 7 Lyα emitters (LAEs) in the Subaru/XMM-Newton Deep Survey Field, using the fully depleted CCDs newly installed on the Subaru Telescope Suprime-Cam, which is twice more sensitive to z = 7 Lyα at ∼ 1µm than the previous CCDs. Reaching the depth 0.5 magnitude deeper than our previous survey in the Subaru Deep Field that led to the discovery of a z = 6.96 LAE, we detected three probable z = 7 LAE candidates. Even if all the candidates are real, the Lyα luminosity function (LF) at z = 7 shows a significant deficit from the LF at z = 5.7 determined by previous surveys. The LAE number and Lyα luminosity densities at z = 7 is ∼ 7.7-54% and ∼ 5.5-39% of those at z = 5.7 to the Lyα line luminosity limit of L(Lyα) 9.2 × 10 42 erg s −1 . This could be due to evolution of the LAE population at these epochs as a recent galaxy evolution model predicts that the LAE modestly evolves from z = 5.7 to 7. However, even after correcting for this effect of galaxy evolution on the decrease in LAE number density, the z = 7 Lyα LF still shows a deficit from z = 5.7 LF. This might reflect the attenuation of Lyα emission by neutral hydrogen remaining at the epoch of reionization and suggests that reionization of the universe might not be complete yet at z = 7. If we attribute the density deficit to reionization, the intergalactic medium (IGM) transmission for Lyα photons at z = 7 would be 0.4 ≤ T IGM Lyα ≤ 1, supporting the possible higher neutral fraction at the earlier epochs at z > 6 suggested by the previous surveys of z = 5.7-7 LAEs, z ∼ 6 quasars and z > 6 gamma-ray bursts.