UGC 12591

UGC 12591 is the third most massive known spiral galaxy, after ISOHDFS 27 and J2345-0449.^{[citation needed]} It is located about 400 million light-years away from the Earth in the constellation Pegasus. In addition, it is the spiral galaxy with the highest known rotational speed^[2] of about 500 km/s, almost twice that of our galaxy, the Milky Way. The high rotational speed means the galaxy must be very massive at the center; the galaxy has a mass estimated at 4 times that of the Milky Way,^[3] making it the third of the most massive spiral galaxies known to date.

UGC 12591
UGC 12591, taken by the Hubble Space Telescope
Observation data (J2000 epoch)
Constellation	Pegasus
Right ascension	23h 25m 21.7s[1]
Declination	28° 29′ 43″[1]
Redshift	0.023179[1]
Heliocentric radial velocity	6949 ± 10[1]
Distance	394.26 ± 133.84 Mly (120.880 ± 41.036 Mpc)[1]
Apparent magnitude (B)	13.90[1]
Characteristics
Type	S0/a[1]
Mass	1.9×1012[2] M☉
Apparent size (V)	1.7′ × 0.7′[1]
Other designations
MCG +05-55-015, PGC 71392

UGC 12591 is relatively isolated; the nearest galaxy to it is 3.55 million light-years (1.09 Mpc) away. However, its morphology suggests a merger or accretion event in its past: it is somewhat lenticular-like, with a central bulge and dust lanes reminiscent of the Sombrero Galaxy.^[4]