NGC 4993 (also catalogued as NGC 4994 in the New General Catalogue) is a lenticular galaxy[5] located about 140 million light-years away[2] in the constellation Hydra.[6] It was discovered on 26 March 1789[7]byWilliam Herschel[6][7] and is a member of the NGC 4993 Group.[3]
| NGC 4993 | |
|---|---|
NGC 4993 and GRB 170817A afterglow as taken by Hubble Space Telescope[1]
| |
| Observation data (J2000 epoch) | |
| Constellation | Hydra |
| Right ascension | 13h09m 47.7s[2] |
| Declination | −23° 23′ 02″[2] |
| Redshift | 0.009727[2] |
| Heliocentric radial velocity | 2916 km/s[2] |
| Distance | 44.1 Mpc (144 Mly)[2] |
| Grouporcluster | NGC 4993 Group[3] |
| Apparent magnitude (V) | 13.32[2] |
| Characteristics | |
| Type | (R')SAB0^-(rs)[2] |
| Size | ~55,000 ly (17 kpc) (estimated)[2] |
| Apparent size (V) | 1.3 x 1.1[2] |
| Notable features | Host of neutron star merger detected as gravitational wave GW170817 and gamma-ray burst GRB 170817A |
| Other designations | |
| NGC 4994, ESO 508-18, AM 1307-230, MCG -4-31-39, PGC 45657, WH III 766[4] | |
NGC 4993 is the site of GW170817, the first astronomical event detected in both electromagnetic and gravitational radiation, the collision of two neutron stars, a discovery given the Breakthrough of the Year award for 2017 by the journal Science.[8][9] Detecting a gravitational wave event associated with the gamma-ray burst provided direct confirmation that binary neutron star collisions produce short gamma-ray bursts.[10]
NGC 4993 has several concentric shells of stars and large dust lane with diameter of approximately a few kiloparsecs which surrounds the nucleus and is stretched out into an "s" shape. The dust lane appears to be connected to a small dust ring with a diameter of ~330 ly (0.1 kpc).[11] These features in NGC 4993 may be the result[12] of a recent merger with a gaseous late-type galaxy that occurred about 400 million years ago.[13] However, Palmese et al. suggested that the galaxy involved in the merger was a gas-poor galaxy.[14]
NGC 4993 has a dark matter halo with an estimated mass of 193.9×1010 M☉.[13]
NGC 4993 has an estimated population of 250 globular clusters.[5]
The luminosity of NGC 4993 indicates that the globular cluster system surrounding the galaxy may be dominated by metal-poor globular clusters.[15]
NGC 4993 has a supermassive black hole with an estimated mass of roughly 80 to 100 million solar masses (8×107 M☉).[16]
The presence of weak O III, NII and SII emission lines in the nucleus of NGC 4993 and the relatively high ratio of [NII]λ6583/Hα suggest that NGC 4993 is a low-luminosity AGN (LLAGN).[16] The activity may have been triggered by gas from the late-type galaxy as it merged with NGC 4993.[13]
In August 2017, rumors circulated[17] regarding a short gamma-ray burst designated GRB 170817A, of the type conjectured to be emitted in the collision of two neutron stars.[18] On 16 October 2017, the LIGO and Virgo collaborations announced that they had detected a gravitational wave event, designated GW170817. The gravitational wave signal matched prediction for the merger of two neutron stars, two seconds before the gamma-ray burst. The gravitational wave signal, which had a duration of about 100 seconds, was the first gravitational wave detection of the merger of two neutron stars.[1][19][20][21][22]
Anoptical transient, AT 2017gfo (also known as SSS 17a), was detected in NGC 4993 11 hours after the gravitational wave and gamma-ray signals, allowing the location of the merger to be determined. The optical emission is thought to be due to a kilonova. The discovery of AT 2017gfo was the first observation (and first localisation) of an electromagnetic counterpart to a gravitational wave source.[19][21][22][23][24]
GRB 170817A was a gamma-ray burst (GRB) detected by NASA's Fermi and ESA's INTEGRAL on 17 August 2017.[17][25][26][27] Although only localized to a large area of the sky, it is believed to correspond to the other two observations,[23] in part due to its arrival time 1.7 seconds after the GW event.
