A team of astronomers from the United States and Canada say that the Milky Way may have had a close encounter with another galaxy or large dark matter structure in recent cosmological history, about 100 million years ago.
Evidence for this claim comes from results gathered by the Sloan Digital Sky Survey. The survey uses a dedicated wide-angle optical telescope located in New Mexico to collect images of the sky. Different imaging filters can provide detailed spectrum information that allows astronomers to characterize the various objects seen through the telescope. This study focused on the movements of stars relative to the mid-plane of the galaxy, or the plane running through the center of the galaxy and dividing it into upper and lower halves. Stars normally move up and down relative to the mid-plane at a rate of 20-30 kilometers per second.
Information from the Sloan Digital Sky Survey, however, suggests that this up-and-down movement is not as uniform as was previously thought. Brian Yanny, a scientist at Fermilab, says “our part of the Milky Way is ringing like a bell.” A bell, when struck, vibrates at a particular frequency and produces sound. This study has identified a distribution and motion of stars in the Milky Way suggesting that our galaxy was struck by something and is “ringing” like a bell.
Yanny goes on to say that “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.” Susan Gardener, a physics professor from the University of Kentucky, says that whatever caused the observed distribution and motion of the stars may even be an ongoing process, not a single isolated collision in the distant past.
A number of “satellite galaxies”—more than twenty—circle the dense center of the Milky Way galaxy. One of these may have crossed the galaxy’s mid-plane, causing the observed vertical waves. A dark matter structure may also be responsible. Dark matter is a hypothetical type of matter that accounts for the large difference between the calculated total mass of the universe and the amount of observable matter in the universe. More than six times as much dark matter than ordinary matter exists in the universe, so there is a high likelihood that dark matter is responsible for the “ringing” of the galaxy.
According to computer simulations, the galaxy should stop “ringing” in about another 100 million years—assuming, of course, the galaxy is not hit again.