Strangers in the Night: Is AndXII Just Passing Through the Local Group?

Abstract

In ΛCDM cosmologies, the formation and evolution of cosmological structure is an ongoing process, and we should expect a present day drizzle of dwarf galaxies into the Local Group from the nearby filaments. Such dwarfs would not have been shaken and stirred by the tidal fields of massive galaxies, and hence their populations may be more representative of the initial building blocks from which the Milky Way was formed (c.f. [7]). Clearly, the identification of such new members of the Local Group would provide an important test of our models of structure formation. Newly accreted dwarfs should be identifiable through their kinematics, which should appear anomalous when compared to the overall dwarf population. Earlier this year, it was suggested that a newly discovered dwarf, AndXIV [4], is such a system, appearing close in projection to M31 with a relative velocity of ∼200 km/s, making it potentially unbound. However, an analysis of its RGB properties indicates that AndXIV lies on the nearside of M31, with respect to the Milky Way, and it has been suggested AndXIV used to be a tightly bound satellite and that its anomalously high velocity is due its ejection in a strong, three-body interaction [6]. The subject of this paper is another newly discovered dwarf galaxy, AndXII, whose location and kinematic properties make it an ideal candidate for being a newly accreted member of the Local Group. If it does not interact strongly with multiple members of the Local Group population, it will simply be passing by, like a stranger in the night.