Proper Motions in the Local Group

Galaxies consists of billions of stars and often cluster together in groups of galaxies - such as the Local Group the Milky Way is a member of. Under the force of gravity the galaxies are expected to revolve around each other and perform a dance on galactic scales. Such revolutions take billions of years and appear as ultra-slow motion on the sky due the enormous distances in the universe. For this reason galaxies typically appear static on the sky to the observer and their tiny motion has never been seen before.

In the 1920's the Dutch astronomer Adriaan van Maanen had electrified his colleagues with claims to have measured the angular rotation and proper motion of so called "spiral nebulae" - as galaxies were called in these days. However, shortly afterwards this was proven wrong by Edwin Hubble within the context of a historic debate on the size of the universe. He clarified that those spiral nebulae are star systems similar to the Milky Way. They are too far away from us, to show motions that would have been measurable with past telescopes.

Measuring the proper motions and geometric distances of galaxies within the Local Group is very important for our understanding of the history, present state and future of the Local Group. The problem when trying to derive the gravitational potential of the Local Group is that usually only radial velocities are known, and hence statistical approaches have to be used.

Currently, proper motion measurements using optical methods are limited only to the closest companions of the Milky Way. However, Very Long Baseline Interferometry (VLBI) provides the best angular resolution in astronomy and phase-referencing techniques yield astrometric accuracies of ~ 10 micro-arcseconds. This makes a measurement of proper motions and angular rotation rates of galaxies out to a distance of ~ 1 Mpc feasible.

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