| Andromeda XXI | |
|---|---|
| Observation data (J2000epoch) | |
| Constellation | Andromeda |
| Right ascension | 23h 54m 47.7s[1] |
| Declination | +42° 28′ 15″[1] |
| Distance | 859 ± 51 kiloparsecs (2.80 ± 0.17 Mly)[2] |
| Apparent magnitude (V) | 15.5 ± 0.3[3] |
| Absolute magnitude (V) | −9.1 ± 0.3[3] |
| Characteristics | |
| Type | dSph |
| Half-light radius (physical) | 1005±175 pc[3] |
| Half-light radius (apparent) | 4.1+0.8 −0.4′[3] |
| Notable features | Satellite galaxy ofAndromeda |
| Other designations | |
| Andromeda XXI, And XXI, And 21 | |
Andromeda XXI (And 21,And XXI) is a moderately brightdwarf spheroidal galaxy about 859 ± 51 kiloparsecs (2.80 ± 0.17 Mly) away from theSun in theconstellationAndromeda. It is the fourth largestLocal Group dwarf spheroidal galaxy. The discovery arose from the first year data of aphotometric survey of theM31/M33 subgroupings of the Local Group by thePan-Andromeda Archaeological Survey (PAndAS). This survey was conducted with the Megaprime/MegaCam wide-field camera mounted on theCanada-France-Hawaii Telescope.
This largesatellite of theAndromeda Galaxy (M31) has a half-light radius of roughly 1 kpc.[2]
Andromeda XXI appears as a spatial overdensity of stars. It has red giant branches at the distance of M31/M33, and follows metal-poor, [Fe/H]=-1.8 when plotted in a color-magnitude diagram.
Like other dwarf spheroidal galaxies, Andromeda XXI shows no sign of currentstar formation and appears to have had 90% of its stars formed 5.8 billion years ago.[3] Its centraldark matter density is lower than expected from theΛCDM model, but this could be explained if it lost most of its mass in a previoustidal stripping event or had experience tidal shocks from an eccentric orbit.[3]
Although moderately bright (MV = −9.1 ± 0.3),[3] it has lowsurface brightness. This indicates that numerous relatively luminous M31 satellites remain undiscovered.
| Location | Andromeda Galaxy → Andromeda subgroup →Local Group →Local Sheet →Local Volume →Virgo Supercluster →Laniakea Supercluster →Pisces–Cetus Supercluster Complex →Local Hole →Observable universe →Universe Each → may be read as "within" or "part of". | .jpg) |
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| Satellite galaxies |
*It is uncertain whether these are companion galaxies of the Andromeda Galaxy | |
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