Andromeda XVIII

{{Short description|Galaxy in constellation Andromeda}}

{{Infobox galaxy

| name = Andromeda XVIII

| image = Andromeda XVIII.jpg

| image_size = 250px

| caption = DSS image of Andromeda XVIII

| epoch = J2000

| constellation name = Andromeda

| ra = {{RA|00|02|14.5}}{{cite journal|doi=10.1086/591313|title=A Trio of New Local Group Galaxies with Extreme Properties|journal=The Astrophysical Journal|volume=688|issue=2|pages=1009|year=2008|last1=McConnachie|first1=Alan W|last2=Huxor|first2=Avon|last3=Martin|first3=Nicolas F|last4=Irwin|first4=Mike J|last5=Chapman|first5=Scott C|last6=Fahlman|first6=Gregory|last7=Ferguson|first7=Annette M. N|last8=Ibata|first8=Rodrigo A|last9=Lewis|first9=Geraint F|last10=Richer|first10=Harvey|last11=Tanvir|first11=Nial R|arxiv=0806.3988|bibcode=2008ApJ...688.1009M|s2cid=16950090}}

| dec = {{DEC|+45|05|20}}

| z =

| h_radial_v = −337.2 km/s

| dist_ly = {{cvt|1355|±|88|kpc|kly|lk=on|order=flip}}

| group_cluster = Local Group

| type = dSph

| size =

| appmag_v = 15.50 ± 0.24{{cite journal|doi=10.1093/mnras/stw2502|title=Star formation history of And XVIII: a dwarf spheroidal galaxy in isolation|journal=Monthly Notices of the Royal Astronomical Society|volume=464|issue=2|pages=2281|year=2017|last1=Makarova|first1=L. N|last2=Makarov|first2=D. I|last3=Karachentsev|first3=I. D|last4=Tully|first4=R. B|last5=Rizzi|first5=L|doi-access=free |arxiv=1609.09706|bibcode=2017MNRAS.464.2281M|s2cid=119275751}}

| appmag_b =

| absmag_v = −10.41 ± 0.28

| mass = {{val|4.2e6}} ± {{val|0.3e6}}

| half_light_radius_pc = 325 ± 24{{cite journal|doi=10.1088/0004-637X/768/2/172|title=A Kinematic Study of the Andromeda Dwarf Spheroidal System|journal=The Astrophysical Journal|volume=768|issue=2|pages=172|year=2013|last1=Collins|first1=Michelle L. M|last2=Chapman|first2=Scott C|last3=Rich|first3=R. Michael|last4=Ibata|first4=Rodrigo A|last5=Martin|first5=Nicolas F|last6=Irwin|first6=Michael J|last7=Bate|first7=Nicholas F|last8=Lewis|first8=Geraint F|last9=Peñarrubia|first9=Jorge|last10=Arimoto|first10=Nobuo|last11=Casey|first11=Caitlin M|last12=Ferguson|first12=Annette M. N|last13=Koch|first13=Andreas|last14=McConnachie|first14=Alan W|last15=Tanvir|first15=Nial|arxiv=1302.6590|bibcode=2013ApJ...768..172C|s2cid=67749084}}

| names = PGC 5056918{{cite simbad|title=NAME Andromeda XVIII|access-date=19 September 2017}}

}}

Andromeda XVIII, discovered in 2008, is a dwarf spheroidal galaxy (has no rings, low luminosity, much dark matter, little gas or dust), which is a satellite of the Andromeda Galaxy (M31). It is one of the 14 known dwarf galaxies orbiting M31. It is relatively isolated, being about 1.8 million light-years (579 kpc) away. However, for an isolated dwarf galaxy it is also unusually quiescent. This suggests that Andromeda XVIII is a backsplash galaxy, a galaxy that once had a close orbital encounter with a more massive galaxy which stripped it of much of its star-forming matter. However, alternative hypotheses are also possible for Andromeda XVIII.{{cite journal|doi=10.3847/1538-4357/ad55f0|doi-access=free |title=Kinematics and Metallicity of the Dwarf Spheroidal Galaxy Andromeda XVIII |date=2024 |last1=Kvasova |first1=Kateryna A. |last2=Kirby |first2=Evan N. |last3=Beaton |first3=Rachael L. |journal=The Astrophysical Journal |volume=972 |issue=2 |page=180 |arxiv=2404.11804 |bibcode=2024ApJ...972..180K }}

It was announced in 2010 that the orbiting galaxies lie close to a plane running through M31's center.{{citation needed|date=September 2017}}