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 $v_{\mathrm{Tuc}}=216.7_{-2.8}^{+2.9}$kms${}^{-1}$, and a velocity dispersion of ${\sigma }_{\mathrm{v},\mathrm{Tuc}}=14.4_{-2.3}^{+2.8}$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 $v_{\mathrm{Tuc}}=215.2_{-2.7}^{+2.8}$ kms${}^{-1}$ and the velocity dispersion to ${\sigma }_{v,\mathrm{Tuc}}=13.3_{-2.3}^{+2.7}$ 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 $\Lambda$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.

Accepted for publication in MNRAS. 17 pages, 13 figures. Updated to match journal version

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