With and without spectroscopy: Gaia DR2 proper motions of seven Ultra-Faint Dwarf Galaxies

Understand

We present mean absolute proper motion measurements for seven ultra-faint dwarf galaxies orbiting the Milky Way, namely Bo\"{o}tes III, Carina II, Grus II, Reticulum II, Sagittarius II, Segue 2 and Tucana IV.

• For four of these dwarfs our proper motion estimate is the first ever provided.
• The adopted astrometric data come from the second data release of the Gaia mission.
• We determine the mean proper motion for each galaxy starting from an initial guess of likely members, based either on radial velocity measurements or using stars on the Horizontal Branch identified in the Gaia ($G_{\rm BP}$-$G_{\rm RP}$,$G$) colour-magnitude diagram in the field of view towards the UFD.