View source for Astronomy:Andromeda XXI

{{Short description|Dwarf spheroidal galaxy in constellation Andromeda}}
{{Infobox Galaxy
| name = Andromeda XXI
| epoch = J2000
| type = dSph
| ra = {{RA|23|54|47.7}}<ref name=SIMBAD>{{cite simbad|title=NAME Andromeda XXI|access-date=2 April 2011}}</ref>
| dec =  {{DEC|+42|28|15}}<ref name=SIMBAD/>
| dist_ly = {{convert|859|+/-|51|kpc|Mly}}<ref name=ApJ-705-1-758-765/>
| absmag_v = −9.1 ± 0.3<ref name=Collins/>
| appmag_v = 15.5 ±&nbsp;0.3<ref name=Collins/>
| size_v = 
| half_light_radius_arcminsec = {{val|4.1|+0.8|−0.4}}{{prime}}<ref name=Collins/>
| half_light_radius_pc = {{val|1005|175}} pc<ref name=Collins/>
| constellation name = [[Astronomy:Andromeda (constellation)|Andromeda]]
| notes = [[Astronomy:Satellite galaxy|Satellite galaxy]]<br>of [[Astronomy:Andromeda Galaxy|Andromeda]]
| names = Andromeda XXI, And XXI, And 21
}}
'''Andromeda XXI''' ('''And 21''', '''And XXI''') is a moderately bright [[Astronomy:Dwarf spheroidal galaxy|dwarf spheroidal galaxy]] about {{convert|859|+/-|51|kpc|Mly}} away from the [[Astronomy:Sun|Sun]] in the [[Astronomy:Constellation|constellation]] [[Astronomy:Andromeda (constellation)|Andromeda]]. It is the fourth largest [[Astronomy:Local Group|Local Group]] dwarf spheroidal galaxy. The discovery arose from the first year data of a photometric survey of the [[Astronomy:Messier 31|M31]]/[[Astronomy:Messier 33|M33]] subgroupings of the Local Group by the [[Astronomy:Pan-Andromeda Archaeological Survey|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 [[Astronomy:Satellite galaxy|satellite]] of the [[Astronomy:Andromeda Galaxy|Andromeda Galaxy]] (M31) has a half-light radius of roughly 1 kpc.<ref name=ApJ-705-1-758-765>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}};</ref>

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 [[Astronomy:Star formation|star formation]] and appears to have had 90% of its stars formed 5.8 billion years ago.<ref name=Collins>{{cite journal|doi=10.1093/mnras/stab1624|title=Andromeda XXI – a dwarf galaxy in a low-density dark matter halo|year=2021|last1=Collins|first1=Michelle L M.|last2=Read|first2=Justin I.|last3=Ibata|first3=Rodrigo A.|last4=Rich|first4=R Michael|last5=Martin|first5=Nicolas F.|last6=Peñarrubia|first6=Jorge|last7=Chapman|first7=Scott C.|last8=Tollerud|first8=Erik J.|last9=Weisz|first9=Daniel R.|journal=Monthly Notices of the Royal Astronomical Society|volume=505|issue=4|pages=5686–5701|arxiv=2102.11890}}</ref> Its central [[Astronomy:Dark matter|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 [[Astronomy:Tidal stripping|tidal stripping]] event or had experience tidal shocks from an eccentric orbit.<ref name=Collins/>

Although moderately bright (M<sub>V</sub> = −9.1 ± 0.3),<ref name=Collins/> it has low [[Astronomy:Surface brightness|surface brightness]]. This indicates that numerous relatively luminous M31 satellites remain undiscovered.

==See also==
* [[Astronomy:Low surface brightness galaxy|Low surface brightness galaxy]] (LSB galaxy)

==References==
{{reflist}}

{{Andromeda galaxy}}
{{Andromeda (constellation)}}

{{DEFAULTSORT:Andromeda XXI}}
[[Category:Dwarf spheroidal galaxies]]
[[Category:Andromeda Subgroup]]
[[Category:Andromeda (constellation)]]

{{Sourceattribution|Andromeda XXI}}