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Discovery of Andromeda XIV: A Dwarf Spheroidal Dynamical Rogue in the Local Group?
Majewski, S R, Beaton, R L, Patterson, R J, Singh-Kalirai, J, Geha, M C, Muñoz, R R, Seigar, M S, Guhathakurta, P, Bullock, J, Rich, R M, Gilbert, K M, David B. Reitzel
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Abstract
In the course of our survey of the outer halo of the Andromeda Galaxy we have discovered a remote, possible satellite of that system at a projected 162 kpc (11.7 degrees) radius. If mass follows light, the fairly elongated (0.31 +/- 0.09 ellipticity) dwarf has an estimated 3.3 (+3.1,-1.8) 10^7 Solar masses (d/784 kpc) mass, M/L of 165 (+156/-92} M/L_solar (d/784 kpc), and 1.07 kpc (d/784 kpc) limiting radius, where the satellite distance, d, is estimated at 630-850 kpc from the tip of the red giant branch. The newfound galaxy, ``Andromeda XIV'' (``AndXIV''), distinguishes itself from other Local Group galaxies by its extreme dynamics: Keck/DEIMOS spectroscopy reveals it to have a large heliocentric radial velocity (-478 km/s), or -203 km/s velocity relative to M31. Even at its projected radius AndXIV already is at the M31 escape velocity based on the latest M31 mass models. If AndXIV is bound to M31, then recent models with reduced M31 virial masses need revision upward. If not bound to M31, then AndXIV is just now falling into the Local Group for the first time and represents a dwarf galaxy that formed and spent almost its entire life in isolation.