Absolute proper motions of stars and star clusters
play a key role in many aspects of galactic kinematics
and dynamics. Therefore, the measurement of absolute proper
motions is one of the most important tasks of modern astrometry.
However, the construction of an absolute reference system
(be it locally or globally) with reasonable accuracy (e.g. below
1mas/a) is a difficult subject.
While the classical approach to an absolute system was based on
the dynamics of the solar system, the more favoured approach
today is to link the observations to distant extragalactic objects,
i.e. galaxies or quasars. This is usually done by means
of earth-bound photographic astrometry. The work reported here
is part of the Bonn extragalactic link program, which is dedicated
to an extragalactic calibration of the proper motion system of the
Hipparcos catalogue. A recent summary of this program is given
by Tucholke et al. (1996) and Geffert et al. (1996b).
The present paper focuses on special
efforts to determine absolute proper motions with respect to
bright galaxies beyond the local group. The use of bright galaxies
as reference objects is motivated by the fact that for some of these
objects a very long history of photographic observations exists. This
offers the advantage of measuring proper motions with high individual
accuracy. On the other hand, the extended and structured appearance
of such galaxies causes difficulties in the derivation of
stable reference positions. In an earlier paper
(Odenkirchen & Brosche 1995, Paper I) it was shown that the problem
of complicated source structure in principle can be overcome with
the method of image cross-correlation.
We first applied this method to extragalactic
sources in and around M51.
Here we describe the continuation of this work. We present an
application of the cross-correlation method to images of the spiral
galaxy M81 and the irregular galaxies M82 and NGC3077
and discuss the results.
