Kinematics of the Tucana Dwarf Galaxy: an unusually dense dwarf in the Local Group.

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 |$v_{\mathrm{Tuc}} = 216.7_{-2.8}^{+2.9}$|  km s−1, and a velocity dispersion of |$\sigma _{\mathrm{v,Tuc}} = 14.4_{-2.3}^{+2.8}$|  km s−1. We also detect a rotation gradient of |$\frac{{\rm d}v_{r}}{{\rm d}\chi } = 7.6^{+4.2}_{-4.3}$| km s−1 kpc−1, which reduces the systemic velocity to |$v_{\mathrm{Tuc}} = 215.2_{-2.7}^{+2.8}$| km s−1 and the velocity dispersion to |$\sigma _{v,\mathrm{Tuc}} = 13.3_{-2.3}^{+2.7}$| 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 Λ 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. [ABSTRACT FROM AUTHOR]