| NGC 4650A | |
|---|---|
 AHubble Space Telescope (HST) image of NGC 4650A | |
| Observation data (J2000epoch) | |
| Constellation | Centaurus |
| Right ascension | 12h 44m 49.035s[1] |
| Declination | −40° 42′ 51.69″[1] |
| Redshift | 0.009607[2] |
| Heliocentric radial velocity | 2,908.5±3.6 km/s[3] |
| Distance | 130 Mly (39.8 Mpc)[3] |
| Group orcluster | Centaurus Cluster[4] |
| Apparent magnitude (V) | 13.3[5] |
| Apparent magnitude (B) | 14.72[3] |
| magnitude (K) | 11.02[3] |
| Characteristics | |
| Type | S0/apec[2] |
| Mass | 4.5×1010[3] M☉ |
| Apparent size (V) | 1.6′ × 0.8′[5] |
| Notable features | Polar-ring galaxy[6][7] |
| Other designations | |
| NGC 4650A,LEDA 42951,PGC 42951[8][5] | |
NGC 4650A is alenticular galaxy[5] of the rarepolar-ring[6] type, located in the southernconstellation ofCentaurus. It has anapparent visual magnitude of 13.3 and spans anangular size of1.6′ × 0.8′.[5] This galaxy is located at an estimated distance of 130 millionlight-years (39.8 Mpc) and is receding with aheliocentricradial velocity of2,908.5 km/s.[3] It should not be confused with the nearbyspiral galaxyNGC 4650; the physical distance between both galaxies is only about 6 times the optical radius of NGC 4650.[9] NGC 4650A lies in a chain of five galaxies in theCentaurus Cluster.[4]
Early studies of this galaxy beginning in 1967 identified it aspeculiar, although it was not included inHalton Arp'sAtlas of Peculiar Galaxies. It has the appearance of having two very different components. In the middle is what resembles anelliptical galaxy with anaxial ratio of around 3, which would give it amorphological classification of E6–7. Itsmajor axis is aligned along a NW-SE direction. Oriented at nearlyright angles to this central component is a fainter, S-shaped feature resembling aspiral orring galaxy seen almost edge-on. The elliptical central nucleus does not appear to be especiallyactive.[10]
During the 1990s, attempts were made to model thedark matter distribution around the galaxy, with only partial success.[4][11][12] A more extensive mapping of theneutral hydrogen distribution was made in 1997. The observed shapes and motions suggested that the polar disk most likely possessed two spiral arms, which produce the S-shape seen in images. The total flux suggested a neutral hydrogen mass of8×109 M☉.[13]
Imaging of the central S0 (lenticular) feature by theHubble Space Telescope showed a complex pattern of dust lanes extending inward toward the nucleus, while the outer portions of the S0 showed an aging stellar population that had ceased newstar formation some 3–5 billion years ago. The outer polar galaxy displays a complex shape that shows warping. Gas is accumulating to form a ring that encircles the central S0 at a radius of 3 thousand light-years (1 kpc). The inner disk is inclined about 63°, then flattens out with increasing radius, forming what appears to be twostar-forming spiral arms. Most of the light from the polar feature is coming from stars less than a billion years old.[14][15] The youngest detected stars are 6.5 Myr old.[16]
Measurements of the motions of stars in NGC 4650A showed a consistent rotation pattern in the polar ring. The rotation curve is flat, providing more support for a disk structure rather than a ring. There is no counter-rotating streams of stars. Both the structure and shape of NGC 4650A support the idea of a merger of two separate galaxies.[17] The central component appears to be an oblate spheroid supported by rotation.[18] Careful examination of the central part of the disk shows that the stellar motions extend right to the middle, with the two components co-existing with separate rotations.[19]
On April 29, 1990, Oscar Pizarro discovered asupernova in the polar ring of NGC 4650A,[20] positioned13.4″ east and44.9″ south of the galactic nucleus.[21] This identification was made from aphotographic plate taken April 27 with theESO Schmidt telescope atLa Silla Observatory.[20] Given the identifierSN 1990I, this event was later classified as aType Ib supernova. Based onlight curve data, SN 1990I was estimated to have begun April 11 and reached peak light on April 29.[22]
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