The Tucana dwarf spheroidal galaxy: not such a massive failure after all
  Isolated Local Group (LG) dwarf galaxies have evolved most or all of their
life unaffected by interactions with the large LG spirals and therefore offer
the opportunity to learn about the intrinsic characteristics of this class of
objects. Here we explore the internal kinematic and metallicity properties of
one of the three isolated LG dwarf spheroidal galaxies, i.e. the Tucana dSph.
This is an intriguing system, as it has been found in the literature to have an
internal rotation of up to 16 km/s, a much higher velocity dispersion than
other dSphs of similar luminosity, and a possible exception to the
too-big-too-fail problem. We present results for a new VLT/FORS2 spectroscopic
dataset in the CaII triplet region for 50 candidate red giant branch stars in
the direction of Tucana, which yielded line-of-sight velocity and metallicity
([Fe/H]) measurements of 39 effective members. This doubles the number of
Tucana's stars with such measurements. In addition, we re-reduce and include in
our analysis the other two spectroscopic datasets presented in the literature,
the VLT/FORS2 sample by Fraternali et al. (2009) and the VLT/FLAMES one by
Gregory et al. (2019). We measure a systemic velocity of $180.0\pm1.3$ km/s,
consistently across the various datasets analyzed, and find that a
dispersion-only model is moderately favored over models accounting also for
internal rotation. Our best estimate of the internal velocity dispersion is
$6.2_{-1.3}^{+1.6}$ km/s, much smaller than the values reported in the
literature and in line with similarly luminous dSphs; this is consistent with
Tucana not being an exception to the too-big-to-fail problem, nor living in a
dark matter halo much more massive than those of its siblings. As for the
metallicity properties, we do not find anything unusual; there are hints of the
presence of a [Fe/H] gradient but more data are needed to pin its presence
down.
