Kinematics of the Tucana dwarf galaxy: An unusually dense dwarf in the local group

Abstract

We present new FLAMES+GIRAFFE spectroscopy of 36 member stars in the isolated Local Group dwarf spheroidal galaxy Tucana. We measure a systemic velocity for the system of vTuc = 216.7+2.9+2.8 km s−1, and a velocity dispersion of σv,Tuc = 14.4+2.8−2.3 km s−1. We also detect a rotation gradient of ddvχr = 7.6+4.2−4.3 km s−1 kpc−1, which reduces the systemic velocity to vTuc = 215.2+2.8−2.7 km s−1 and the velocity dispersion to σvTuc = 13.3+2.7−2.3 km s−1. We perform Jeans modelling of the density profile of Tucana, using the line-of-sight velocities of the member stars. We find that it favours a high central density consistent with 'pristine' subhaloes in cold dark matter, and a massive dark matter halo (∼1010 M) consistent with expectations from abundance matching. Tucana appears to be significantly more centrally dense than other isolated Local Group dwarfs, making it an ideal laboratory for testing dark matter models.