NGC 5557

NGC 5557 is an elliptical galaxy in the constellation Boötes. It was discovered by William Herschel on May 1, 1785.^[4] The distance to NGC 5557 is not well known, but it is estimated to be about 127 million light-years (38.8 megaparsecs) away.^[3]

NGC 5557
SDSS image of NGC 5557
Observation data (J2000 epoch)
Constellation	Boötes
Right ascension	14h 18m 25.708s[1]
Declination	+36° 29′ 37.28″[1]
Redshift	0.010764[2]
Heliocentric radial velocity	3210 ± 28 km/s[2]
Distance	127 Mly (38.8 Mpc)[3]
Apparent magnitude (V)	12.2[4]
Absolute magnitude (V)	−22.39[5]
Characteristics
Type	E1[3]
Other designations
UGC 9161, MCG +06-31-093, PGC 51104[2]

NGC 5557 is quite massive, with a K-band absolute magnitude of −24.8, and is a slow rotator,^[3] which suggests it gained mass through dry mergers (galaxy mergers involving galaxies significant amounts of gas).^[5] However, it has a faint tidal tail to its east, as well as a more complex structure to the west. This structure, if found to be connected to NGC 5557, would one of the largest around a galaxy, spanning about 1.1 million light-years (350,000 parsecs).^[3] This filamentary structure suggests that NGC 5557 may have formed from a more gas-rich galaxy merger a couple billion years ago.^[3] This implies that the galaxy merger would need to have a low impact parameter.^[5]

NGC 5557 is part of a galaxy group,^[3][4] and is the largest such galaxy in the group by far.^[3] Surrounding the galaxy near the eastern filament are several small bluish objects, which are possibly tidal dwarf galaxies.^[3]

Supernovae

Two supernovae have been observed in NGC 5557:

• SN 1996aa (type Ia, mag. 17) was discovered by Wayne Johnson on 16 May 1996.^[6][7]
• SN 2013gn (type Ia, mag. 15.3) was discovered by Kōichi Itagaki on 16 November 2013.^[8][9]