A Gaia DR 2 and VLT/FLAMES search for new satellites of the LMC

A wealth of tiny galactic systems populates the surroundings of the Milky Way. However, some of these objects might actually have their origin as former satellites of the Magellanic Clouds, in particular the LMC. An example of the importance of understanding how many systems are genuine satellites of the Milky Way or the LMC are the implications that the number and luminosity/mass function of satellites around hosts of different mass have for dark matter theories and treatment of baryonic physics in simulations of structure formation. Here we aim at deriving the bulk motions and estimates of the internal velocity dispersion and metallicity properties in four recently discovered southern dwarf galaxy candidates, Columba I, Reticulum III, Phoenix II and Horologium II. We combine Gaia DR2 astrometric measurements and photometry to new FLAMES/GIRAFFE intermediate resolution spectroscopic data in the region of the nearIR Ca II triplet lines; such combination is essential for finding potential member stars in these low luminosity systems. We find very likely member stars in all four satellites and are able to determine (or place limits on) the systems bulk motions and average internal properties. The systems are found to be very metal-poor, in agreement with dwarf galaxies and dwarf galaxy candidates of similar luminosity. Among the four systems, the only one that we can place firmly in the category of dwarf galaxies is Phoenix II given its resolved large velocity dispersion ($7.1_{-1.1}^{+1.5}\,$km/s) and intrinsic metallicity spread (0.34$\,$dex). The orbital pole of Phoenix II is well constrained and close to the orbital pole of the LMC, suggesting a prior association. The uncertainty on the orbital poles of the other systems are presently very large, and in the case of Horologium II dependent on the sample of members adopted, making an association not impossible.