April 2018
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.
January 2019 / 2018 Year in Review
GeminiFocus
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