The Tucana dwarf spheroidal galaxy: not such a massive failure after all ?

摘要

Context. 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. Aims. Our aim is to explore the internal kinematic and metallicity properties of one of the three isolated LG early-type dwarf galaxies, the Tucana dwarf spheroidal. This is an intriguing system, as it has been found in the literature to have an internal rotation of up to 16 km s~(?1), a much higher velocity dispersion than dwarf spheroidals of similar luminosity, and a possible exception to the too-big-too-fail problem. Methods. We present the results of a new spectroscopic dataset that we procured from the Very Large Telescope (VLT) taken with the FORS2 instrument in the region of the Ca II triplet for 50 candidate red giant branch stars in the direction of the Tucana dwarf spheroidal. These yielded line-of-sight (l.o.s.) velocity and metallicity ([Fe/H]) measurements of 39 effective members that double 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, A&A, 499, 121), and the VLT/FLAMES one from Gregory et al. (2019, MNRAS, 485, 2010). Results. Across the various datasets analyzed, we consistently measure a l.o.s. systemic velocity of 180?±?1.3 km s~(?1)and find that a dispersion-only model is moderately favored over models that also account for internal rotation. Our best estimate of the internal l.o.s. velocity dispersion is 6.2_(?1.3)~(+1.6)km s~(?1), much smaller than the values reported in the literature and in line with similarly luminous dwarf spheroidals; this is consistent with NFW halos of circular velocities < 30 km s~(?1). Therefore, Tucana does not appear to be an exception to the too-big-to-fail problem, nor does it appear to reside 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 metallicity gradient, but more data are needed to pinpoint its presence.