In the March 4th issue of Science, astronomers report that
they have measured the slowest ever motion of a galaxy across the plane of
the sky. This distant whirlpool of stars appears to creep along despite its
actual speed through space because it is located so far from the Earth.
Measuring this galaxy’s glacial pace of only 30 micro-arcseconds per year
stretched current radio astronomy technology to its limit.

“A snail crawling on Mars would appear to be moving across the surface more
than 100 times faster than the motion we measured for this galaxy,” said
Mark Reid (Harvard-Smithsonian Center for Astrophysics), a co-author on the

Reid and his colleagues used the National Science Foundation’s Very Long
Baseline Array (VLBA) to measure the motion across the sky of a galaxy
located nearly 2.4 million light-years from Earth. While scientists have
been measuring the motion of galaxies directly toward or away from Earth for
decades, this is the first time that the transverse motion (called proper
motion by astronomers) has been measured for a galaxy that is not a nearby
satellite of the Milky Way.

An international scientific team analyzed VLBA observations made over two
and a half years to detect minuscule shifts in the sky position of the
spiral galaxy M33. Combined with previous measurements of the galaxy’s
motion toward Earth, the new data allowed the astronomers to calculate
M33’s movement in three dimensions for the first time.

M33 is a satellite of the larger galaxy M31, the well-known Andromeda
Galaxy that is the most distant object visible to the naked eye. Both are
part of the Local Group of galaxies that includes the Milky Way.

The astronomers’ task was not simple. Not only did they have to detect an
impressively tiny amount of motion across the sky, but they also had to
separate the actual motion of M33 from the apparent motion caused by our
Solar System’s motion around the center of the Milky Way. The motion of the
Solar System and the Earth around the galactic center, some 26,000
light-years away, has been accurately measured using the VLBA over the last

“The VLBA is the only telescope system in the world that could do this
work,” Reid said. “Its extraordinary ability to resolve fine detail is
unmatched and was the absolute prerequisite to making these measurements.”

In addition to measuring the motion of M33 as a whole, the astronomers also
were able to make a direct measurement of the spiral galaxy’s rotation. Both
measurements were made by observing the changes in position of giant clouds
of molecules inside the galaxy. The water vapor in these clouds acts as a
natural maser, strengthening, or amplifying, radio emission the same way
that lasers amplify light emission. The natural masers acted as bright
radio beacons whose movement could be tracked by the ultra-sharp radio
“vision” of the VLBA.

Reid and his colleagues plan to continue measuring M33’s motion and also to
make similar measurements of M31’s motion. This will allow them to answer
important questions about the composition, history and fates of the two
galaxies as well as of the Milky Way.

“We want to determine the orbits of M31 and M33. That will help us learn
about their history, specifically, how close have they come in the past?”
Reid explained. “If they have passed very closely, then maybe M33’s small
size is a result of having material pulled off it by M31 during the close
encounter,” he added.

Accurate knowledge of the motions of both galaxies also will help determine
if there is a collision in their future. In addition, orbital analysis can
give astronomers valuable clues about the amount and distribution of dark
matter in the galaxies.

Reid worked with Andreas Brunthaler of the Max Planck Institute for
Radioastronomy in Bonn, Germany; Heino Falcke of ASTRON in the Netherlands;
Lincoln Greenhill, also of the Harvard-Smithsonian Center for Astrophysics;
and Christian Henkel, also of the Max Planck Institute in Bonn.

Note: This release is being issued jointly with the National Radio
Astronomy Observatory (NRAO).

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

Note to editors: An image to accompany this release is online at