NGC 4993

NGC 4993, also listed as NGC 4994 in the New General Catalogue, is a lenticular galaxy roughly 140 million light-years from Earth in the constellation Hydra. William Herschel discovered it on 26 March 1789, and it belongs to the NGC 4993 Group. The galaxy is best known as the home of GW170817, a neutron star collision that marked the first time astronomers detected an event in both electromagnetic and gravitational radiation. *Science* magazine named this discovery the 2017 Breakthrough of the Year.

The galaxy features several concentric shells of stars and a large, S-shaped dust lane, about a few kiloparsecs across, surrounding its nucleus. This dust lane appears linked to a small dust ring roughly 330 light-years (0.1 kiloparsecs) in diameter. These structures may have resulted from a merger with a gaseous late-type galaxy around 400 million years ago, though Palmese and colleagues suggest the merging galaxy was gas-poor.

NGC 4993’s dark matter halo has an estimated mass of 193.9 × 10¹⁰ solar masses. The galaxy hosts roughly 250 globular clusters, and its luminosity suggests the cluster system is likely dominated by metal-poor globular clusters. At its center lies a supermassive black hole with an estimated mass between 80 and 100 million solar masses (8 × 10⁷ M☉). Weak emission lines of O III, NII, and SII in the nucleus, along with a relatively high [NII]λ6583/Hα ratio, indicate NGC 4993 is a low-luminosity active galactic nucleus (LLAGN). This activity may have been triggered by gas from the merging late-type galaxy.

On 17 August 2017, rumors spread that NASA’s Fermi and ESA’s INTEGRAL had detected GRB 170817A, a short gamma-ray burst (sGRB) thought to arise from two colliding neutron stars. On 16 October 2017, the LIGO and Virgo collaborations announced that, 1.7 seconds before the GRB signal, they had detected GW170817—a gravitational wave event matching predictions for such a merger. The gravitational wave signal lasted about 100 seconds and was later confirmed as the first gravitational wave detection of a neutron star merger. The simultaneous detection of the GW and sGRB signals suggested they came from the same event, prompting astronomers worldwide to combine data and narrow down the sky location. Hundreds of researchers scrambled, pre-empting telescope schedules to scan the region constrained by the GW and sGRB data.