Two international teams of astronomers have used ESO’s VISTA telescope and MPG/ESO 2.2-m telescope, both in Chile, to produce the best 3D map yet of the central bulge of our Milky Way Galaxy.

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.

Unfortunately, from our vantage point from within the galactic disc, the view of this central region — at about 27 000 light-years’ distance — is heavily obscured by dense clouds of gas and dust. Astronomers can only obtain a good view of the bulge by observing longer wavelength light, such as infrared radiation, which can penetrate the dust clouds.

Now the teams have used new data from several of telescopes to get a much clearer view of the bulge’s structure.

The first team used the VVV near-infrared survey from the VISTA telescope. This survey can pick up stars thirty times fainter than previous bulge surveys. The astronomers 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. 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,” said Dr Christopher Wegg of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who is a lead author of a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society (full paper).

“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. 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,” added co-author Dr Ortwin Gerhard, also from the Max Planck Institute for Extraterrestrial Physics.

The second team took a different approach to pin down the structure of the bulge. By comparing images taken 11 years apart 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. 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!” said Sergio Vásquez from the Pontificia Universidad Católica de Chile, Santiago, Chile, who is a lead author of the study published in the journal Astronomy & Astrophysics (full paper).

The teams 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.

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Bibliographic information: Christopher Wegg, Ortwin Gerhard. 2013. Mapping the Three-Dimensional Density of the Galactic Bulge with VVV Red Clump Stars. Monthly Notices of the Royal Astronomical Society, accepted for publication; arXiv: 1308.0593

S. Vásquez et al. 2013. 3D kinematics through the X-shaped Milky Way bulge. A&A, vol. 555, article number A91; doi: 10.1051/0004-6361/201220222