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.

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Recently, the optical counterpart of a gravitational wave source GW170817 has been identified in NGC 4993 galaxy. Together with evidence from observations in electromagnetic waves, the event has been suggested as a result of a merger of two neutron stars. We analyze the multi-wavelength data to characterize the host galaxy property and its distance to examine if the properties of NGC 4993 are consistent with this picture. Our analysis shows that NGC 4993 is a bulge-dominated galaxy with r eff ∼ 2 − 3 kpc and the Sérsic index of n = 3 − 4 for the bulge component. The spectral energy distribution from 0.15 to 24 µm indicates that this galaxy has no significant ongoing star formation, the mean stellar mass of (0.3 − 1.2) × 10 11 M , the mean stellar age greater than ∼3 Gyr, and the metallicity of about 20% to 100% of solar abundance. Optical images reveal dust lanes and extended features that suggest a past merging activity. Overall, NGC 4993 has characteristics of normal, but slightly disturbed elliptical galaxies. Furthermore, we derive the distance to NGC 4993 with the fundamental plane relation using 17 parameter sets of 7 different filters and the central stellar velocity dispersion from literature, finding an angular diameter distance of 37.7±8.7 Mpc. NGC 4993 is similar to some host galaxies of short gamma-ray bursts but much different from those of long gamma-ray bursts, supporting the picture of GW170817 as a result of a merger of two NSs.

Faint z ∼ 5 quasars with M 1450 ∼ −23 mag are known to be potentially important contributors to the ultraviolet ionizing background in the postreionization era. However, their number density has not been well determined, making it difficult to assess their role in the early ionization of the intergalactic medium (IGM). In this work, we present the updated results of our z ∼ 5 quasar survey using the Infrared Medium-deep Survey (IMS), a near-infrared imaging survey covering an area of 85 deg2. From our spectroscopic observations with the Gemini Multi-Object Spectrograph on the Gemini-South 8 m telescope, we discovered eight new quasars at z ∼ 5 with  −26.1 ≤ M 1450 ≤ −23.3. Combining our IMS faint quasars (M 1450 > −27 mag) with the brighter Sloan Digital Sky Survey quasars (M 1450 < −27 mag), we derive the z ∼ 5 quasar luminosity function (QLF) without any fixed parameters down to the magnitude limit of M 1450 = −23 mag. We find that the faint-end slope of the QLF is very flat ( ), with a characteristic luminosity of mag. The number density of z ∼ 5 quasars from the QLF gives an ionizing emissivity at 912 Å of ϵ 912 = (3.7–7.1) × 1023 erg s−1 Hz−1 Mpc−3 and an ionizing photon density of Mpc−3 s−1. These results imply that quasars are responsible for only 10%–20% (up to 50% even in the extreme case) of the photons required to completely ionize the IGM at z ∼ 5, disfavoring the idea that quasars alone could have ionized the IGM at z ∼ 5.

Previous studies suggest that compact young early-type galaxies (ETGs) were formed by recent mergers. However, it has not yet been revealed whether tidal features that are direct evidence of recent mergers are detected frequently around compact young ETGs. Here, we investigate how the fraction of ETGs having tidal features (f T ) depends on age and internal structure (compactness, color gradient, and dust lanes) of ETGs, using 650 ETGs with M r  ≤ −19.5 in 0.015 ≤ z ≤ 0.055 that are in deep coadded images of the Stripe 82 region of the Sloan Digital Sky Survey. We find that tidal features are more frequent in younger ETGs and more compact ETGs, so that compact young ETGs with ages  ≲6 Gyr have high f T of ∼0.7 compared to their less compact or old counterparts with ages  ≳ 9 Gyr that have f T  ≲ 0.1. Among compact young ETGs, those with blue cores have ∼3 times higher f T than those with red cores. In addition, ETGs with dust lanes have ∼4 times higher f T than those without dust lanes. Our results provide direct evidence that compact young ETGs especially with blue cores and ETGs with dust lanes are involved in recent mergers. Based on our results and several additional assumptions, we roughly estimate the typical visible time of tidal features after a merger, which is ∼3 Gyr in the depth of the Stripe 82 coadded images.