Kinematics and metallicity of the dwarf spheroidal galaxy Andromeda XVIII

Abstract

Andromeda XVIII is an isolated dwarf galaxy 579 kpc away from the nearest large galaxy, M31. It is a candidate "backsplash galaxy" that might have been affected by a close passage to M31. We present new Keck/DEIMOS spectroscopy of Andromeda XVIII to assess the likelihood that it is a backsplash galaxy. We estimated the velocities, metallicities ([Fe/H]), and $α$-enhancements ([$α$/Fe]) for 56 probable members. Based on the abundances of 38 stars with low errors ($δ[Fe/H] < 0.3$), parameters for the simplest chemical evolution models were estimated using the maximum likelihood coupled with a Markov Chain Monte Carlo (MCMC) method. The metallicity distribution is inconsistent with these models, due to a sharp metal-rich cutoff. We estimated Andromeda XVIII's mean heliocentric velocity, rotation velocity, position angle of the rotation axis, and velocity dispersion using the maximum likelihood coupled with an MCMC. There is no evidence for bulk rotation, though subpopulations might be rotating. The mean heliocentric velocity is -337.2 km s$^{-1}$, such that the line-of-sight velocity relative to M31 is lower than the escape velocity from M31. Together, the metallicity distribution and the mean velocity are consistent with a sudden interruption of star formation. For possible causes of this quenching, we considered gas loss due to ram pressure stripping during a close passage by M31 or due to a past major merger. However, we cannot rule out internal feedback (i.e., a terminal wind).