Life at the Periphery of the Local Group: the kinematics of the Tucana
  Dwarf Galaxy
  Dwarf spheroidal galaxies in the Local Group are usually located close to the
Milky Way or M31. Currently, there are two clear exceptions to this rule, and
the Tucana dwarf galaxy is the most distant at almost 1 Mpc from the Milky Way.
Using the VLT/FORS2 spectrograph in multi-object mode we were able to measure
the velocities of 23 individual Red Giant Branch stars in and around Tucana
using the Ca triplet absorption lines. From this sample 17 reliable members
have been identified. We measured the systemic velocity and dispersion of
Tucana to be v_hel = +194.0+-4.3 km/s and sigma_l.o.s. = 15.8+4.1-3.1 km/s
respectively. These measures are derived after removing the signature of
rotation using a linear gradient of 6.5 x R/R_core+-2.9 km/s, which corresponds
to a rotation of 16 km/s at the reliable limit of our data. Our systemic
velocity corresponds to a receding velocity from the barycentre of the Local
Group of v_LG = +73.3 km/s. We also determined the mean metallicity of Tucana
to be [Fe/H] = -1.95+-0.15 with a dispersion of 0.32+-0.06 dex. Our study
firmly excludes any obvious association of Tucana with the HI emission in the
vicinity and shows that Tucana is a genuine dwarf spheroidal, with low
metallicity stars, no gaseous ISM and no recent star formation. The present
location and relatively high recession velocity are consistent with Tucana
having been an isolated Local Group galaxy for the majority of its existence.
