Stellar kinematics of the Andromeda II dwarf spheroidal galaxy

Abstract

We present kinematical profiles and metallicity for the M31 dwarf spheroidal (dSph) satellite galaxy AndromedaII (AndII) based on Keck DEIMOS spectroscopy of 531 red giant branch stars. Our kinematical sample is among the largest for any M31 satellite and extends out to two effective radii (r eff = 53 = 1.1kpc). We find a mean systemic velocity of -192.4 ± 0.5kms-1 and an average velocity dispersion of σv = 7.8 ± 1.1kms-1. While the rotation velocity along the major axis of AndII is nearly zero (<1kms-1), the rotation along the minor axis is significant with a maximum rotational velocity of v max = 8.6 ± 1.8kms-1. We find a kinematical major axis, with a maximum rotational velocity of vmax = 10.9 ± 2.4kms-1, misaligned by 67° to the isophotal major axis. AndII is thus the first dwarf galaxy with evidence for nearly prolate rotation with a v max/σv = 1.1, although given its ellipticity of ε = 0.10, this object may be triaxial. We measured metallicities for a subsample of our data, finding a mean metallicity of [Fe/H] = -1.39 ± 0.03dex and an internal metallicity dispersion of 0.72 ± 0.03 dex. We find a radial metallicity gradient with metal-rich stars more centrally concentrated, but do not observe a significant difference in the dynamics of the two metallicity populations. AndII is the only known dwarf galaxy to show minor axis rotation, making it a unique system whose existence offers important clues on the processes responsible for the formation of dSphs. © 2012. The American Astronomical Society. All rights reserved.