On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

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.

We report the discovery of primeval large-scale structures (LSSs) including two protoclusters in a forming phase at . We carried out extensive deep narrowband imaging in the 1 deg 2 sky of the Subaru/XMM-Newton Deep z p 5.7 Field and obtained a cosmic map of 515 Lya emitters (LAEs) in a volume with a transverse dimension of and a depth of ∼40 Mpc in comoving units. This cosmic map shows filamentary LSSs, including 180 Mpc # 180 Mpc clusters and surrounding 10-40 Mpc scale voids, similar to the present-day LSSs. Our spectroscopic follow-up observations identify overdense regions in which two dense clumps of LAEs with a sphere of 1 Mpc diameter in physical units are included. These clumps show about 130 times higher star formation rate density, mainly due to a large overdensity, ∼80, of LAEs. These clumps would be clusters in a formation phase involving a burst of galaxy formation.