Surprise! Our home galaxy is no pipsqueak, but it isn't nearly as massive as scientists used to think.
Astronomers often refer to the neighboring Andromeda galaxy, which is around 2.6 million light-years away from us, as the "twin galaxy" of our own Milky Way. But a new study indicates that the two galaxies are quite different when it comes to mass. The research, from an international team of scientists, shows that the Milky Way is about half as massive as Andromeda.
"We always suspected that Andromeda is more massive than the Milky Way, but weighing both galaxies simultaneously proved to be extremely challenging," researcher Dr. Jorge Peñarrubia, an astrophysics lecturer at the University of Edinburgh in Scotland, said in a written statement. "Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible."
Some previous studies indicated that the Milky Way is more massive than Andromeda, but the researchers said that these studies measured only the mass of the inner regions of our galaxy neighbor and not the outer regions.
For the new study, the researchers measured the speed, position, and motion of nearby smaller galaxies. Then they used those measurements to arrive at an estimate of the shape and mass of the Milky Way and Andromeda galaxies.
"By studying two massive galaxies that are close to each other and the galaxies that surround them, we can take what we know about gravity and pair that with what we know about expansion to get an accurate account of the mass contained in each galaxy," one of the researchers, Dr. Matthew Walker, an assistant professor of physics at Carnegie Mellon University in Pittsburgh, said in a written statement. "This is the first time we've been able to measure these two things simultaneously."
And the mass of a galaxy includes its dark matter as well as its ordinary, visible matter. Dark matter appears invisible because it doesn’t absorb, reflect, or emit light -- but scientists know it exists because they see its gravitational effects in the universe.
Dark matter makes up 90 percent of each galaxy's mass, according to the researchers. But, they say, Andromeda contains twice as much dark matter as the Milky Way.
“Most of the weight of these galaxies is present in the form of invisible dark matter,” Dr. Yin-Zhe Ma, a postdoctoral fellow at the University of British Columbia in Canada, who was also involved in the study, said in a written statement. “We don’t know much about dark matter so this discovery means we’ll get a chance to study it from within our own galaxy.”
The new study was published in the current issue of the journal Monthly Notices of the Royal Astronomical Society.