Two teams of astronomers have used data obtained with telescopes from the European Southern Observatory to create the best three-dimensional map ever made of the central areas of the Milky Way . And thus they have discovered that, from some angles, the inner regions appear to be peanut-shaped or X-shaped.
One of the most important and massive areas of the galaxy is the galactic bulge, which contains about 10 billion stars and stretches for thousands of light years. But its structure and origin are still not very well understood. Unfortunately, from our location within the galactic disk, the view of that central region, some 27,000 light years away from Earth, is greatly obscured by dense clouds of gas and dust. Astronomers can only get a useful view of the bulb by observing at longer wavelengths, such as infrared radiation, which can penetrate dust clouds.
A team from the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching (Germany) used the VVV near-infrared survey of the VISTA Telescope at ESO’s Paranal Observatory in Chile to study the galactic bulb, taking advantage of the fact that it allows stars to be captured thirty times weaker than those captured by previous probes of the bulb. The team identified a total of22 million stars belonging to a type of giant red stars whose well-known properties allow us to calculate their distances with great precision. From this stellar catalog they have been able to make a three-dimensional map of the galactic bulb. “We discovered that the inner area of our galaxy is shaped like a peanut shell if we look at it from the side, and if we look at it from above it would have a very elongated bar shape,” says Ortwin Gerhard, co-author of the study.
The second international team, led by Chilean PhD student Sergio Vásquez (Pontificia Universidad Católica de Chile, Santiago, Chile; and ESO, Santiago, Chile), took a different approach when defining the structure of the bulb. By comparing images obtained eleven years apart by the MPG / ESO 2.2-meter telescope, they were able to measure thesmall displacements caused by the movement of the bulb stars in the sky. This information was combined with measurements of the movement of the same stars approaching or moving away from Earth in order to calculate the movements of more than 400 stars in three dimensions.
“This is the first time that such a large number of velocities have been obtained in three dimensions for individual stars on both sides of the bulb,” concludes Vásquez. “The stars we have observed appear to be moving alongthe X-shaped bulb armsas their orbits go up and down and out of the plane of the Milky Way. Everything fits perfectly with the predictions of the latest models! ”.
Astronomers believe that originally billions of years ago the Milky Way was just a disk of stars that formed a flat bar. Then the inner part collapsed, generating that three-dimensional peanut-shaped shape that we see today in the new observations.
