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(Tuc) = 216.7(-2.8)(+2.9) km s(-1), and a velocity dispersion of sigma(v),(Tuc) = 14.4(-2.3)(+2.8) km s(-1). We also detect a rotation gradient of dvr/d chi = 7.6(-4.3)(+4.2) km s(-1) kpc(-1), which reduces the systemic velocity to v(Tuc) = 215.2(-2.7)(+2.8) km s(-1) and the velocity dispersion to sigma(v),(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 Lambda cold dark matter, and a massive dark matter halo (similar to 10(10)M(circle dot)) 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.