Many previous mappings of the Milky Way have focused on either the inner galaxy or the outer galaxy, and as a result, different surveys have found different numbers of spiral arms - two in the inner galaxy and four in the outer. Now, Iowa State University has completed the first map of the entire system of galactic spiral arms, which shows two arms at the center branching into four on the outside.
"For the first time these arms are mapped over the entire Milky Way," said Martin Pohl, an Iowa State associate professor of physics and astronomy. "The branching of two of the arms may explain why previous studies – using mainly the inner or mainly the outer galaxy – have found conflicting numbers of spiral arms."
You Are Here: A new galactic map confirms the existence of an additional minor arm and the splitting of two major arms into four. Photo Credit: University of Calgary's Center for Radio Astronomy
Much of the galaxy is obscured from Earth's point of view by interstellar dust, which blocks out visible light. What can be seen by conventional telescopes is only a large disk with a bulge at the middle and large areas of darkness near the center. Luckily, NASA's Cosmic Background Explorer (COBE) satellite has the capability to image in the infrared, which can see nearly perfectly through interstellar dust.
Peter Englmaier of the University of Zurich in Switzerland and Nicolai Bissantz of Ruhr-University in Bochum, Germany, used images from COBE to develop a kinematic model of gas flow in the inner galaxy, which Pohl used to determine the distribution of the gas. Determining this distribution led the researchers to discover the locations of the spiral arms.
The Milky Way, recently found to be a barred spiral galaxy rather than a pure spiral galaxy, contains a thick bar of stars and dust near the nucleus, which branches out into two spiral arms as the distance from the center increases. The two arms then each split into two, creating four major arms that spiral outward. Additionally, there are two minor arms in the inner galaxy that extend to about 10,000 light years from the center. One of these arms had been observed before, and the other (which is symmetrical) was found using the gas data.
The existence of the second inner arm was a great relief for Englmaier, who had previously supported the theory that the inner galaxy is highly symmetrical but was unable to verify it as such, since the other inner arm was unobservable due to dust. "Finally it is clear that our model assumption of symmetry was correct," he said. "The inner galaxy is indeed quite symmetric in structure."
In addition, the survey answered questions about the erratic orbit of the minor arms. Researchers expected their orbits to be nearly circular, but observations showed large deviations from circular motion. The new galactic map shows that the arms' orbit is altered by the galaxy's central bar.
Unfortunately for science fiction buffs, the new galactic map won't be installed on any starships for the purpose of exploring other parts of the Milky Way, at least not yet. But the insights gained will help science as a whole, and who knows? Maybe someday we'll have a different kind of map...