We present the discovery of two new dwarf galaxies , Andromeda XXI and Andromeda XXII , located in the surroundings of the Andromeda and Triangulum galaxies ( M31 and M33 ) .
These discoveries stem from the first year data of the Pan-Andromeda Archaeological Survey ( PAndAS ) , a photometric survey of the M31/M33 group conducted with the Megaprime/MegaCam wide-field camera mounted on the Canada-France-Hawaii Telescope .
Both satellites appear as spatial overdensities of stars which , when plotted in a color-magnitude diagram , follow metal-poor , { [ Fe / H ] } = -1.8 , red giant branches at the distance of M31/M33 .
Andromeda XXI is a moderately bright dwarf galaxy ( M _ { V } = -9.9 \pm 0.6 ) , albeit with low surface brightness , emphasizing again that many relatively luminous M31 satellites still remain to be discovered .
It is also a large satellite , with a half-light radius close to 1 { kpc } , making it the fourth largest Local Group dwarf spheroidal galaxy after the recently discovered Andromeda XIX , Andromeda II and Sagittarius around the Milky Way , and supports the trend that M31 satellites are larger than their Milky Way counterparts .
Andromeda XXII is much fainter ( M _ { V } = -6.5 \pm 0.8 ) and lies a lot closer in projection to M33 than it does to M31 ( 42 vs. 224 kpc ) , suggesting that it could be the first Triangulum satellite to be discovered .
Although this is a very exciting possibility in the context of a past interaction of M33 with M31 and the fate of its satellite system , a confirmation will have to await a good distance estimate to confirm its physical proximity to M33 .
Along with the dwarf galaxies found in previous surveys of the M31 surroundings , these two new satellites bring the number of dwarf spheroidal galaxies in this region to 20 .