Photons from a gravitational wave event: Two neutron stars merging together generate a gravitational wave signal and have also been predicted to emit electromagnetic radiation. When the gravitational wave event GW170817 was detected, astronomers rushed to search for the source using conventional telescopes (see the Introduction by Smith). Coulteret al.describe how the One-Meter Two-Hemispheres (1M2H) collaboration was the first to locate the electromagnetic source. Droutet al.present the 1M2H measurements of its optical and infrared brightness, and Shappeeet al.report their spectroscopy of the event, which is unlike previously detected astronomical transient sources. Kilpatricket al.show how these observations can be explained by an explosion known as a kilonova, which produces large quantities of heavy elements in nuclear reactions.Science, this issue p.1556, p.1570, p.1574, p.1583; see also p.1554

Photons from a gravitational wave event: Two neutron stars merging together generate a gravitational wave signal and have also been predicted to emit electromagnetic radiation. When the gravitational wave event GW170817 was detected, astronomers rushed to search for the source using conventional telescopes (see the Introduction by Smith). Coulteret al.describe how the One-Meter Two-Hemispheres (1M2H) collaboration was the first to locate the electromagnetic source. Droutet al.present the 1M2H measurements of its optical and infrared brightness, and Shappeeet al.report their spectroscopy of the event, which is unlike previously detected astronomical transient sources. Kilpatricket al.show how these observations can be explained by an explosion known as a kilonova, which produces large quantities of heavy elements in nuclear reactions.Science, this issue p.1556, p.1570, p.1574, p.1583; see also p.1554

The morphology of galaxies gives essential constraints on the models of galaxy evolution. The morphology of the features in the low-surface-brightness (LSB) regions of galaxies has not been fully explored yet because of observational difficulties. Here we present the results of our visual inspections of very deep images of a large volume-limited sample of 177 nearby massive early-type galaxies from the MATLAS survey. The images reach a surface-brightness limit of 28.5–29 mag arcsec−2 in the g′ band. Using a dedicated navigation tool and questionnaire, we looked for structures at the outskirts of the galaxies such as tidal shells, streams, tails, disturbed outer isophotes, or peripheral star-forming discs, and simultaneously noted the presence of contaminating sources, such as Galactic cirrus. We also inspected internal substructures such as bars and dust lanes. We discuss the reliability of this visual classification investigating the variety of answers made by the participants. We present the incidence of these structures and the trends of the incidence with the mass of the host galaxy and the density of its environment. We find an incidence of shells, stream, and tails of approximately 15 per cent, about the same for each category. For galaxies with masses over 1011 M⊙, the incidence of shells and streams increases about 1.7 times. We also note a strong unexpected anticorrelation of the incidence of Galactic cirrus with the environment density of the target galaxy. Correlations with other properties of the galaxies, and comparisons to model predictions, will be presented in future papers.