Gemini Probes Milky Way’s Dark Matter Halo

Many properties of the Milky Way halo, like its mass, are not well known, and not all the mass is even ordinary matter. Direct measurements of the proper motions of objects in the halo can disentangle properties that are otherwise undistinguished, and these observations of dwarf galaxies and globular clusters also provide insight into the origin of the stars in the halo of the Milky Way.

Tobias Fritz from the University of Virginia explains further in his abstract for the Future and Science of Gemini Meeting:

“With the advent of GeMS/GSAOI on Gemini-South, it is now possible to obtain ground-based meaningful proper motions in the outer halo with a time baseline of only two years that are competitive with space-based techniques.

“In our Gemini large program we target  stars in the Sagittarius stream, five globular clusters and four dwarf galaxies. We have already analyzed  the first epoch Adaptive Optics imaging, concentrating on the astrometric errors, both statistical and systematic in origin. The results of this analysis are useful for any future project using GeMS/GSAOI to determine absolute proper motions.

“Further, we separate our objects into foreground stars, target object stars, and background galaxies using morphology and colors. To demonstrate the usefulness of proper motions for dynamics we present the case of the globular cluster Palomar 5. We have already determined its proper motion from large field seeing limited images and have achieved an accuracy of 0.20 milliarcsecond/year over a 15 year time baseline.”

Distribution of targets on the sky.