Andromeda XXI
Fourth largest Local Group dwarf spheroidal galaxy, satellite of M31.
Andromeda XXI (And 21, And XXI) is a moderately bright dwarf spheroidal galaxy located approximately 859 ± 51 kiloparsecs (2.80 ± 0.17 Mly) from the Sun in the constellation Andromeda. It is the fourth largest Local Group dwarf spheroidal galaxy and a large satellite of the Andromeda Galaxy (M31). Its discovery came from the first year data of a photometric survey of the M31/M33 subgroupings of the Local Group by the Pan-Andromeda Archaeological Survey (PAndAS), conducted with the Megaprime/MegaCam wide-field camera on the Canada-France-Hawaii Telescope.
Quick Facts
- Epoch
- J2000
- Ra
- 23 · 54 · 47.7
- Dec
- +42 · 28 · 15
- Dist Ly
- 859 +/-
- Absmag V
- −9.1 ± 0.3
- Appmag V
- 15.5 ± 0.3
- Half Light Radius Arcminsec
- 4.1 · +0.8 · −0.4
- Half Light Radius Pc
- 1005 · 175 pc
- Constellation Name
- Andromeda
- Notes
- Satellite galaxy / of Andromeda
- Names
- Andromeda XXI, And XXI, And 21
Facts from the source article.
Lore & Background
Andromeda XXI appears as a spatial overdensity of stars, with red giant branches at the distance of M31/M33. In a color-magnitude diagram, it follows a metal-poor abundance of [Fe/H] = −1.8. Like other dwarf spheroidal galaxies, it shows no sign of current star formation, and 90% of its stars formed 5.8 billion years ago. Its central dark matter density is lower than expected from the ΛCDM model, a discrepancy that could be explained if it lost most of its mass in a previous tidal stripping event or experienced tidal shocks from an eccentric orbit.
Reader's Guide
Andromeda XXI is notable as the fourth largest Local Group dwarf spheroidal galaxy and a moderately bright satellite of M31, with an absolute magnitude of MV = −9.1 ± 0.3. Despite its brightness, it has low surface brightness, suggesting that numerous relatively luminous M31 satellites remain undiscovered. Its discovery via the Pan-Andromeda Archaeological Survey highlights the effectiveness of wide-field photometric surveys in detecting faint stellar overdensities. The galaxy's low central dark matter density relative to ΛCDM predictions provides a test case for understanding tidal interactions and mass loss in dwarf galaxies. Its ancient stellar population, with most stars formed 5.8 billion years ago and no ongoing star formation, aligns with the quiescent nature of dwarf spheroidals. The uncertainty in its dark matter content, possibly due to tidal stripping or orbital shocks, underscores the complexity of modeling satellite galaxies in the Local Group.
More in Elliptical and Irregular Galaxies, Part 5 1-24
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