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 $v_{\mathrm{T}\mathrm{u}\mathrm{c}}=216.7_{-2.8}^{+2.9}$kms${}^{-1}$, and a velocity dispersion of ${\sigma }_{\mathrm{v},\mathrm{T}\mathrm{u}\mathrm{c}}=14.4_{-2.3}^{+2.8}$kms${}^{-1}$. We also detect a rotation gradient of $\frac{d{v}_r}{d\chi }=7.6_{-4.3}^{+4.2}$ kms${}^{-1}$ kpc${}^{-1}$, which reduces the systemic velocity to $v_{\mathrm{T}\mathrm{u}\mathrm{c}}=215.2_{-2.7}^{+2.8}$ kms${}^{-1}$ and the velocity dispersion to ${\sigma }_{v,\mathrm{T}\mathrm{u}\mathrm{c}}=13.3_{-2.3}^{+2.7}$ kms${}^{-1}$. 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’ subhalos in $\mathrm{\Lambda }$CDM, and a massive dark matter halo ($\sim$${10}^{10}$,M${}_{\odot }$) 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.