Andromeda XVIII, discovered in 2008, is adwarf spheroidal galaxy (has no rings, lowluminosity, muchdark matter, little gas or dust), which is a satellite of theAndromeda 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 abacksplash 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.[2]
It was announced in 2010 that the orbiting galaxies lie close to a plane running through M31's center.[citation needed]
^abcMcConnachie, Alan W; Huxor, Avon; Martin, Nicolas F; Irwin, Mike J; Chapman, Scott C; Fahlman, Gregory; Ferguson, Annette M. N; Ibata, Rodrigo A; Lewis, Geraint F; Richer, Harvey; Tanvir, Nial R (2008). "A Trio of New Local Group Galaxies with Extreme Properties".The Astrophysical Journal.688 (2): 1009.arXiv:0806.3988.Bibcode:2008ApJ...688.1009M.doi:10.1086/591313.S2CID16950090.
^Collins, Michelle L. M; Chapman, Scott C; Rich, R. Michael; Ibata, Rodrigo A; Martin, Nicolas F; Irwin, Michael J; Bate, Nicholas F; Lewis, Geraint F; Peñarrubia, Jorge; Arimoto, Nobuo; Casey, Caitlin M; Ferguson, Annette M. N; Koch, Andreas; McConnachie, Alan W; Tanvir, Nial (2013). "A Kinematic Study of the Andromeda Dwarf Spheroidal System".The Astrophysical Journal.768 (2): 172.arXiv:1302.6590.Bibcode:2013ApJ...768..172C.doi:10.1088/0004-637X/768/2/172.S2CID67749084.
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