Andromeda XXI

Short description: Dwarf spheroidal galaxy in constellation Andromeda

Andromeda XXI

Observation data (J2000 epoch)
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
of Andromeda
Other designations
Andromeda XXI, And XXI, And 21

Andromeda XXI (And 21, And XXI) is a moderately bright dwarf spheroidal galaxy about 859 ± 51 kiloparsecs (2.80 ± 0.17 Mly) away from the Sun in the constellation Andromeda. It is the fourth largest Local Group dwarf spheroidal galaxy. The discovery arose 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). This survey was conducted with the Megaprime/MegaCam wide-field camera mounted on the Canada-France-Hawaii Telescope.

This large satellite of the Andromeda 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 current star formation and appears to have had 90% of its stars formed 5.8 billion years ago.[3] Its central dark matter density is lower than expected from the ΛCDM model, but this could be explained if it lost most of its mass in a previous tidal stripping event or had experience tidal shocks from an eccentric orbit.[3]

Although moderately bright (MV = −9.1 ± 0.3),[3] it has low surface brightness. This indicates that numerous relatively luminous M31 satellites remain undiscovered.

## See also

* Low surface brightness galaxy (LSB galaxy)

## References

1. ↑ 1.0 1.1 "NAME Andromeda XXI". SIMBAD. Centre de données astronomiques de Strasbourg. http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=NAME+Andromeda+XXI.
2. ↑ 2.0 2.1 Nicolas F. Martin, Alan W. McConnachie, Mike Irwin, Lawrence M. Widrow, Annette M. N. Ferguson, Rodrigo A. Ibata, John Dubinski, Arif Babul, Scott Chapman, Mark Fardal, Geraint F. Lewis, Julio Navarro, R. Michael Rich; "PAndAS' cubs: discovery of two new dwarf galaxies in the surroundings of the Andromeda and Triangulum galaxies"; The Astrophysical Journal, Volume 705, Issue 1, pp. 758-765 (2009); doi:10.1088/0004-637X/705/1/758; Bibcode: 2009ApJ...705..758M; arXiv:0909.0399;
3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 3.6 Collins, Michelle L M.; Read, Justin I.; Ibata, Rodrigo A.; Rich, R Michael; Martin, Nicolas F.; Peñarrubia, Jorge; Chapman, Scott C.; Tollerud, Erik J. et al. (2021). "Andromeda XXI – a dwarf galaxy in a low-density dark matter halo". Monthly Notices of the Royal Astronomical Society 505 (4): 5686–5701. doi:10.1093/mnras/stab1624.

* v
* t
* e

Andromeda Galaxy

Location|

Andromeda Galaxy → Andromeda subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Observable universe → Universe
Each  → may be read as "within" or "part of".

Andromeda Galaxy

Satellite galaxies|

* M32
* M110
* NGC 185
* NGC 147
* Andromeda I
* Andromeda II
* Andromeda III
* Andromeda IV*
* Andromeda V
* Andromeda VI
* Andromeda VII
* Andromeda VIII
* Andromeda IX
* Andromeda X
* Andromeda XI
* Andromeda XII
* Andromeda XIII
* Andromeda XIV
* Andromeda XV
* Andromeda XVI
* Andromeda XVII
* Andromeda XVIII
* Andromeda XIX
* Andromeda XX
* Andromeda XXI
* Andromeda XXII
* Andromeda XXIII
* Andromeda XXIV
* Andromeda XXV
* Andromeda XXVI
* Andromeda XXVII
* Andromeda XXVIII
* Andromeda XXIX
* Triangulum Galaxy*

*It is uncertain whether it is a companion galaxy of the Andromeda Galaxy

Catalogued stars|

* AE Andromedae
* M31-RV

Other|

* Andromeda–Milky Way collision
* S Andromedae
* Mayall II
* M32p
* NGC 206
* 3XMM J004232.1+411314

* Category

* v
* t
* e

Constellation of Andromeda

* Adhil
* Andromeda in Chinese astronomy
* List of Andromeda's satellite galaxies
* List of stars in Andromeda

Stars|

| Bayer|

* α
* β
* γ
* δ
* ε
* ζ
* η
* θ
* ι
* κ
* λ
* μ
* ν
* ξ
* ο
* π
* ρ
* σ
* τ
* υ
* φ
* χ
* ψ
* ω

|

Flamsteed|

* 2
* 3
* 4
* 5
* 6
* 7
* 8
* 9
* 10
* 11
* 12
* 13
* 14
* 15
* 18
* 22
* 23
* 26
* 28
* 32
* 36
* 39
* 41
* 44
* 45
* 47
* 49
* 51
* 55
* 56
* 58
* 59
* 60
* 62
* 63
* 64
* 65
* 66

Variable|

* R
* T
* U
* W
* Z
* RT
* RU
* RV
* RX
* SU
* SW
* TU
* TW
* WZ
* XZ
* AB
* AC
* AD
* AE
* AR
* BM
* BX
* CC
* CN
* DS
* DX
* EG
* ET
* EU
* FF
* GP
* GR
* GY
* HN
* KK
* KX
* KZ
* LL
* LP
* LQ
* OP
* OS
* OU
* PX
* QR
* QV
* QX
* V385
* V455
* V529

HD|

* 3
* 166
* 895
* 1185
* 1606
* 2421
* 2767
* 2942
* 3322
* 3346
* 4778
* 5608
* 5789/5788
* 6114
* 7853
* 8673
* 10307
* 13931
* 14622
* 15082
* 16004
* 16028
* 16175
* 217811
* 220105
* 221246
* 221776
* 222109
* 222399
* 223229
* 224365
* 224801
* 225218

Other|

* 3XMM J004232.1+411314
* 6 Persei
* ADS 48
* Groombridge 34
* HAT-P-6
* HR 178
* HR 8768
* M31-RV
* NGC 44
* NGC 82
* NGC 84
* NGC 91
* NGC 162
* NGC 464
* PA-99-N2
* Ross 248
* WASP-1
* WISE 0146+4234
* WISE J0005+3737
* WISE J004945.61+215120.0

| Exoplanets|

* 14 Andromedae b
* Upsilon Andromedae b
* c
* d
* e
* Gliese 15 Ab
* HD 13931 b
* HD 16175 b
* HAT-P-6b
* HAT-P-32b
* WASP-1b
* WASP-33b

|

| Star clusters|

* Mayall II
* NGC 206
* NGC 272
* NGC 752
* NGC 956
* NGC 7686

|

| Nebulae|

* NGC 7662

|

Galaxies|

| NGC|

* 5
* 11
* 13
* 19
* 21
* 20
* 27
* 39
* 43
* 48
* 49
* 51
* 67
* 68
* 69
* 70
* 71
* 72
* 74
* 76
* 79
* 80
* 81
* 83
* 85
* 86
* 90
* 93
* 94
* 96
* 97
* 108
* 109
* 112
* 140
* 149
* 160
* 169
* 181
* 183
* 184
* 214
* 218
* 226
* 228
* 229
* 233
* 243
* 252
* 258
* 260
* 262
* 280
* 304
* 317
* 389
* 393
* 404
* 425
* 431
* 477
* 478
* 512
* 513
* 523
* 528
* 536
* 668
* 679
* 687
* 700
* 703
* 704
* 705
* 708
* 709
* 710
* 712
* 714
* 717
* 732
* 753
* 759
* 891
* 1000
* 7640
* 7836

|

Numbered|

* I
* II
* III
* IV
* V
* VIII
* IX
* X
* XI
* XVIII
* XIX
* XXI

Other|

* 3C 66A
* 3C 66B
* Andromeda Galaxy
* Donatiello I
* Messier 32
* Messier 110

| Galaxy clusters|

* Abell 262
* NGC 68 group

|

| Astronomical events|

* GRB 101225A
* SN 1885A

|

Category

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