Internal Kinematics of the Andromeda II Dwarf Spheroidal Galaxy

Abstract

The High Resolution Echelle Spectrometer on the Keck I telescope has been used to measure nine radial velocities having a median precision of 2 km/s for seven red giants belonging to the isolated dwarf spheroidal galaxy Andromeda II. We find a weighted mean radial velocity of -188+/-3 km/s and a central velocity dispersion of 9.3+/-2.7 km/s. There may be evidence for a radial velocity gradient across the face of the galaxy, although the significance of this result is low due to the small number of stars having measured velocities. Our best estimate for the global mass-to-light ratio of And II is M/L_V = 20.9^{+13.9}_{-10.1} in solar units. This value is similar to those of several Galactic dwarf spheroidal galaxies and is consistent with the presence of a massive dark halo in Andromeda II.