Vol. 570
In section 4. Extragalactic astronomy

A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members

by M. Fabrizio, G. Raimondo, E. Brocato, et al. A&A 570, A61


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Ultra-faint dwarfs appear to be the least luminous, most dark-matter dominated, and most metal-poor galaxies known today. They consist of very sparse groups of stars that are superimposed upon a background of Milky Way stars. The main difficulties in accurately constraining the properties of these ultra-faint galaxies is to separate these overlapping stellar populations. The most accurate method is to measure the proper motions of all the stars in the field. Hercules is the prototype of this type of galaxy, and to remove the foreground (and background) contaminants, a proper-motion study was carried out using imaging from the Large Binocular Telescope (LBT). The datasets span a time baseline of about five years which allowed the authors to measure relative stellar proper motions of 528 sources to a precision of better than 5 mas yr−1 and distinguish a significant fraction (>90%) of Milky Way contaminants. The authors then compared the resulting cleaned color-magnitude diagram with stellar models, and confirmed that Hercules contains an old population (t =12±2 Gyr) with a wide spread in metallicity (−3.3<[Fe/H]<−1.8).