Milky Way
June 29, 2012

Milky Way Encountered Satellite Galaxy Recently

Lee Rannals for - Your Universe Online

Astronomers from Canada and the U.S. have found that the Milky Way galaxy may have recently had an encounter with one of its surrounding smaller satellite galaxies.

“We have found evidence that our Milky Way had an encounter with a small galaxy or massive dark matter structure perhaps as recently as 100 million years ago,” Larry Widrow, professor at Queen´s University in Canada, said in a press release. “We clearly observe unexpected differences in the Milky Way´s stellar distribution above and below the Galaxy´s midplane that have the appearance of a vertical wave -- something that nobody has seen before.”

By using observations of about 300,000 nearby Milky Way stars, the team found that the positions and motions of these stars were not quite as regular as previously thought.

“Our part of the Milky Way is ringing like a bell,” Brian Yanny, of the Department of Energy´s Fermilab, remarked in the press release. “But we have not been able to identify the celestial object that passed through the Milky Way. It could have been one of the small satellite galaxies that move around the center of our galaxy, or an invisible structure such as a dark matter halo.”

The astronomers found a small significant difference in the distribution of stars north and south of the Milky Way's midplane during their research.

For over a year, the team explored different explanations of this north-south asymmetry, such as the effect of interstellar dust on distance determinations and the way the stars surveyed were selected.

The team also explored the alternative explanation that the data was telling them something about recent events in the history of the Galaxy.

The scientists used computer simulations to explore what happened if a satellite galaxy or dark matter structure passed through the disk of the Milky Way.

The simulations indicated that over the next 100 million years, our galaxy will "stop ringing," and the north-south asymmetry created by the collision will disappear, leaving vertical motions of stars in the solar neighborhood to revert back to their equilibrium orbits.

The Milky Way is over 9 billion years old, and contains about 100 billion stars. The galaxy has a total mass of more than 300 billion times that of the Sun. Most of the mass in and around the Milky Way is in the form of dark matter.

There is six times as much dark matter in the universe as ordinary, visible matter. Astronomers' computer simulations have found that this invisible matter formed hundreds of massive structures that move around our galaxy.

Astronomers believe that invisible satellites made of dark matter may be more at risk of colliding with the Milky Way than visible satellite galaxies.

“Future astronomical programs, such as the space-based Gaia Mission, will be able to map out the vertical perturbations in our galaxy in unprecedented detail,” Widrow said in the release. “That will offer a strong test of our findings.”

The research has been published in The Astrophysical Journal Letters.