GeminiFocus April 2018 | Page 5

Blair Conn and Helmut Jerjen Classifying Ultra-faint Satellite Systems in the Milky Way’s Halo By utilizing the outstanding imaging capabilities of the Gemini Multi-Object Spectrograph on Gemini South, we determine whether three newly detected ultra-faint stellar systems belong to the known population of Milky Way dwarf galaxies or its confirmed star clusters. This process, once completed for all such candidate objects (of which there are dozens), will dramatically improve our understanding of Milky Way halo objects and refine the census of known Milky Way satellites. In recent years, around 58 new Milky Way satellite (ultra-faint dwarf galaxy and star cluster) candidates have been reported. This dramatic jump in number from the 11 classical satel- lite galaxies known before 1994 is entirely due to the advent of new all-sky imaging sur- veys: thus far, the Sloan Digital Sky Survey has revealed 16 new candidate satellites, while PanSTARRS and the Dark Energy Survey (DES) combined have added 31. Once the true nature of these objects is established, they will provide crucial empirical input for testing cosmological predictions derived from detailed observations of the nearest galaxies, and in verifying scenarios of how the Milky Way formed. The majority of these discoveries are based on relatively shallow CCD photometry. There- fore, we know little of their stellar population, structural parameters (such as size, image concentration, asymmetry, and surface brightness), distance, and luminosity. Without ac- curate estimates of these criteria, we cannot properly construct a census of known Milky Way satellite galaxies and other halo objects. The only path forward then is to determine these fundamental properties with deep photometric follow-up observations. April 2018 GeminiFocus 3