Galaxies in Motion

Radio astronomers measure for first time the galactic dance of a neighboring galaxy

Using an array of radio telescopes, an international team of astronomers has fulfilled an 80 year old dream, measuring for the first time the motion of an entire galaxy in the sky. The astronomers hope to predict with these measurements the future fate of our own galaxy, the Milky Way, and determine the dark matter content in the local universe. The team reporting the results in this week's issue of the magazine "Science", was led by the young radio astronomer Andreas Brunthaler who made the observations during his time at the Max Planck Institute for Radio Astronomy in Bonn (MPIfR) as a member of the "International Max Planck Research School for Radio and Infrared Astronomy" (IMPRS).

Galaxies consist of billions of stars and often cluster together in groups of galaxies - such as the Local Group, to which the Milky Way belongs. Under the force of gravity 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 motions have never been seen before.

In the 1920's the Dutch astronomer Adriaan van Maanen electrified his colleagues with claims to have measured the angular rotation and proper motion of so called "spiral nebulae" - as galaxies were called in those days. Shortly afterwards, however, Edwin Hubble proved, within the context of a historic debate on the size of the universe, this to be incorrect. He clarified that 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.

An international team of scientists has now succeeded in making very precise radio observations. German members of the team were Andreas Brunthaler, Heino Falcke, who is now professor for Radio Astronomy in the Netherlands, and Christian Henkel, staff astronomer at the MPIfR. Also involved are Mark Reid and Lincoln Greenhill from the Harvard-Smithsonian Center for Astrophysics in the USA. To make the observations, the team followed the motion of water vapor clouds in the nearby galaxy M33 over a period of three years. The water vapor acts like a natural laser, which emits radio waves and can be precisely located.

The researchers were able to show that the galaxy moves 100 times slower than van Maanen claimed and is roughly at the distance that Hubble predicted. "More than 80 years later, van Maanen's dream became reality - but quite differently than he had hoped!" says Andreas Brunthaler. The measurements show that the water vapor moves together with the galaxy in one year by only 30 microarcseconds on the sky. The accuracy of the measurement is 5 microarcseconds per year. By comparison: With this accuracy one could measure a motion of 0.01 mm per year from a distance of 500 km.

"With this level of accuracy we could see from Bonn if something moves in Berlin by a hair's breadth." says Prof. Heino Falcke, who supervised the work in Bonn. Highly accurate results were achieved by combining radio telescopes that are separated by thousands of kilometers into one giant radio telescope by a technique called "Very Long Baseline Interferometry".

The results show that the galaxy M33 moves at a speed of 190 km/s relative to our Milky Way towards the Andromeda galaxy. With the observations, the scientists hope to get deeper insights into the history and future evolution of the Milky Way. It might be possible that the Milky Way will collide and merge with the Andromeda galaxy within the next few billion years. Besides the motion, the astronomers were also able to measure the distance to M33 using basic geometric principles. According to the new measurements, M33 is at a distance of roughly 2.4 million light years from the Earth.

Accurate distance measurements are a fundamental problem in astronomy. Since one cannot use a simple ruler to measure the universe, astronomers have to use complicated distance estimators which are often affected by unknown systematic errors. Hence, direct geometric distances are an important corner stone for getting the basic size and scale of the universe right.

These observations allow an independent recalibration of the extragalactic distance scale. With more proper-motion observations in the future, the accuracy of the distance determination is expected to increase. In addition, accurate distances and velocities are used in astronomy to determine the amount of matter within a certain region. Past observations have shown that most of the mass in universe is in the form of mysterious dark matter. With more galaxy proper-motion observations becoming available soon the astronomers expect to precisely weight the Milky Way and its companion galaxies. This will tell us how much dark matter resides in our local universe.

"It is always most rewarding if students are already able to achieve such fundamental results in the early stages of their scientific careers. Achievements like this will improve the prestige and attractiveness of our Research School." says Dr. Anton Zensus, executive director at the MPIfR and speaker of the IMPRS in Bonn.