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Session 9 - Dwarf Galaxies.
Display session, Monday, June 10
Great Hall,

[9.01] The Absolute Proper Motion of the Ursa Minor Dwarf Spheroidal Galaxy

A. E. Schweitzer (U. Wisconsin- Madison), K. M. Cudworth (Yerkes Obs., U. Chicago)

We have measured an absolute proper motion of the Ursa Minor dwarf spheroidal galaxy with uncertainties of \le0.01 arcsec/century in each coordinate. Ursa Minor is a companion to the Milky Way at a distance roughly 63 kpc from us. Our result is based on 39 plates spanning 42 years, and approximately 1550 stars are included in the survey.

The zero point of the absolute proper motion is set by nine background galaxies and two QSO's. Our proper motion uncertainty is substantially smaller compared to previous proper motion work on the distant satellites of our Galaxy.

The space velocity of Ursa Minor, after correction for the solar peculiar velocity and the orbital motion of the LSR, is an unprecedented 7\sigma (or better) detection, and is in a dynamically interesting direction. This result has implications for previous tidal interactions, the possibility of dwarf spheroidal streams, and for probing the Galactic potential. This work is a part of our ongoing astrometric study of the distant satellites of our Galaxy.

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