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The Peanut At The Center Of The Milky Way
One of the most important and massive parts of the galaxy is the galactic bulge. This huge central cloud of about 10 000 million stars spans thousands of light years, but its structure and origin were not well understood.
This artist’s impression shows how the Milky Way galaxy would look seen from almost edge on and from a very different perspective than we get from the Earth. The central bulge shows up as a peanut shaped glowing ball of stars and the spiral arms and their associated dust clouds form a narrow band.
Credit: ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt.
Earlier observations from the 2MASS infrared sky survey had already hinted that the bulge had a mysterious X-shaped structure. Now two groups of scientists have used new observations from several of ESO’s telescopes to get a much clearer view of the bulge’s structure.
The first group, from the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, used the VVV near-infrared survey from the VISTA telescope at ESO’s Paranal Observatory in Chile. This new public survey can pick up stars thirty times fainter than previous bulge surveys. The team identified a total of 22 million stars belonging to a class of red giants whose well-known properties allow their distances to be calculated.
“The depth of the VISTA star catalogue far exceeds previous work and we can detect the entire population of these stars in all but the most highly obscured regions,” explains Christopher Wegg (MPE), who is lead author of the first study. “From this star distribution we can then make a three-dimensional map of the galactic bulge. This is the first time that such a map has been made without assuming a model for the bulge’s shape.”
“We find that the inner region of our Galaxy has the shape of a peanut in its shell from the side, and of a highly elongated bar from above”, adds Ortwin Gerhard, the coauthor of the first paper and leader of the Dynamics Group at MPE. “It is the first time that we can see this clearly in our own Milky Way, and simulations in our group and by others show that this shape is characteristic of a barred galaxy that started out as a pure disc of stars.”
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 to pin down the structure of the bulge. By comparing images taken eleven years apart with the MPG/ESO 2.2-metre telescope they could measure the tiny shifts due to the motions of the bulge stars across the sky. These were combined with measurements of the motions of the same stars towards or away from the Earth to map out the motions of more than 400 stars in three dimensions.
“This is the first time that a large number of velocities in three dimensions for individual stars from both sides of the bulge been obtained,” concludes Vásquez. “The stars we have observed seem to be streaming along the arms of the X-shaped bulge as their orbits take them up and down and out of the plane of the Milky Way. It all fits very well with predictions from state-of-the-art models!”
The astronomers think that the Milky Way was originally a pure disc of stars which formed a flat bar billions of years ago. The inner part of this then buckled to form the three-dimensional peanut shape seen in the new observations.
Contacts and sources:
Richard Hook
ESO education and Public Outreach Department
Garching bei München, Germany
Hannelore Hämmerle
Max-Planck-Institut für Extraterrestrische Physik
