* 19:00 03 March 2005
The nearly imperceptible "sideways" motion of a galaxy has been directly measured for the first time beyond the Milky Way and its immediate galactic neighbours. The observation will improve measurements of astronomical distances and nearby dark matter concentrations.
Astronomers study how a celestial object moves by breaking down its motion into two components - movement along the observer's line of sight, called radial motion, and movement across the sky, called proper motion. The first can be measured by studying changes in the wavelength of light, called Doppler shifts, as objects move toward or away from Earth.
But side-to-side motion does not affect an object's spectrum, so astronomers must track changes in an object's location relative to background stars. This is quite easy for objects in the solar system, such as asteroids, or even for objects in the galaxy, by "triangulating" their observed positions from two different points along Earth's orbit.
But simply tracking an object's visible light cannot reveal movement much beyond the Milky Way - because the objects are so distant, they appear motionless.
Bright young stars
Now, astronomers have overcome that problem by tracking bright spots of radio emission from the Triangulum Galaxy - also known as M33 - which the new study locates at 2.4 million light years from Earth.
The team, led by Andreas Brunthaler at the Max Planck Institute for Radio Astronomy in Bonn, Germany, measured the gas around two star-forming regions on opposite sides of the M33 galaxy. The bright, young stars heat water vapour in the gas, which in turn emits radio waves in what are called "masers".
These masers resemble the bright spots of light from laser pointers, explains team member Mark Reid of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US. So the team used them as beacons to track the galaxy's rotation and sideways motion at four intervals over the course of about three years.
They made the measurement with a system of 10 radio dishes that boasts the sharpest vision of any telescope in existence. Called the Very Long Baseline Array (VLBA), the dishes - each spanning 25 metres - are scattered from Hawaii to the Caribbean Sea.
The precision of the measurement is such, says Reid, that "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," says Reid.
The VLBA may be able to use the same technique to measure the distance and movement of galaxies even 10 times farther away than M33. But right now, the team is searching for masers in M33's much larger neighbour, the Andromeda Galaxy, which is the most distant object visible to the naked eye.
"Once we get the motion of one object relative to another, we can trace it backwards in time," says Reid, adding that a previous collision with Andromeda may have stripped stars away from M33, leaving it smaller. The same technique can tell astronomers whether the galaxies will crash into each other in the future - a possibility Reid says is likely in the next few billion years.
Philip Diamond, an astronomer at Jodrell Bank Observatory in Cheshire, UK, says the motions of galaxies in the "Local Group" - the Milky Way's neighbouring galaxies - will reveal the pull of invisible dark matter in the region.
He adds that the technique will improve measurements of astronomical distances, which build on the distances of nearby objects such as M33 to estimate the distances to other "standard candles" farther away, such as supernovae. Observations of supernovae led to the discovery in 1998 of "dark energy" - the unknown force thought to be accelerating the expansion of the universe.
"Without a reliable distance scale, the pyramid upon which all such results are based will collapse," Diamond told New Scientist. He adds that more studies like this one "will provide a key component of our attempts to understand the nature of the dark universe that has recently been revealed to us."
Journal reference: Science (vol 307, p 1440)