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.

The Geometric Distance and Proper Motion of M33

We measured the angular rotation and proper motion of M33 with the Very Long Baseline Array by observing two H2O masers on opposite sides of the galaxy (Fig. 1). This allows a comparison of the angular rotation rate (as measured by the VLBI observations) with the known inclination and rotation speed of the HI gas disk. This gives a geometric distance of

D = 730 +/- 100 +/- 135 kpc.

The first error indicates the statistical error from the proper motion measurements while the second error is the systematic error from the rotation model. This distance is consistent, within the errors, with the most recent Cepheid distance to M33.

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Fig.1: Position of two regions of maser activity in M33. Also shown are predicted motions due to rotation of the HI disk. The image of M33 was provided by T.A.Rector (NRAO/AUI/NSF and NOAO/AURA/NSF), David Thilker (NRAO/AUI/NSF), and Robert Braun (ASTRON).

Since all position measurements were made relative to a very distant extragalactic background source, the proper motion of M33 has also been measured. This provides, after correcting for the rotation of M33 and the motion of the Sun in our own Milky Way a three dimensional velocity vector of M33, showing that this galaxy is moving with a velocity of

v = 190 +/- 59 km/s

relative to the Milky Way and towards Andromeda (M31). This indicates that M33 is on a highly eccentric orbit around M31.

More than 80 years after van Maanen's observation, we have measured the rotation and proper motion of M33. These measurements promise a new handle on dynamical models for the Local Group that can distinguish between different scenarios for the mass and dark matter halos of Andromeda and the Milky Way.

Published in:

Brunthaler A., Reid M.J., Falcke H., Greenhill, L.J., Henkel, C. 2005, Science, 307, 1440 (04 March 2005)
Brunthaler A., Reid M.J., Falcke H., Henkel C., Menten K.M., 2007, A&A 462, 101

See also:

Press release Joint Institute for VLBI in Europe (JIVE)

Press release Max-Planck-Institut für Radioastronomie (Germany)

Press release Harvard-Smithsonian Center for Astrophysics (USA)

Press release National Radio Astronomy Observatory (USA)

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Movie (14 MB, mpeg): A simulation of the Andromeda - Milky Way merger by M. Steinmetz (AIP)

Last modified: 09-March-2007