The central region of the Milky Way

Two team of astronomers used data from ESO’s telescopes to build the best three-dimensional map ever of the central region of the Milky Way.  They found that the internal regions take a look like a peanut, or X depending on angle of view.  This strange shape has been rebuilt using public data of the telescope for ESO’s VISTA survey. One of the most important and massive areas of the Galaxy is the Galactic bulge: this huge cloud of about 10.000 million stars extending for thousands of light-years, but its structure and its origins were never properly understood. Unfortunately, the view of the central region – about 27000 light-years away – is disturbed by dense clouds of gas and dust.


 Source: X-ray: NASA/CXC/UMass/D. Wang et al.; Optical: NASA/ESA/STScI/D.Wang et al.; IR: NASA/JPL-Caltech/SSC/S.Stolovy

Astronomers can see better the Galactic bulge observing at wavelengths more, like those of infrared radiation, which can penetrate the dust clouds. Previous observations of the sky by the infrared 2MASS survey had already left to guess that the Galactic bulge has an uncanny X-shaped structure. Two groups of scientists used new observations from several telescopes of ESO to get a better view of the structure of the bulge. The first group, from the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, has used the VVV in near-infrared survey obtained with the telescope sight at ESO’s Paranal Observatory, Chile.  This new public survey can identify a star which is 30 times weaker than the previous survey of the Galactic bulge.  The team has collected a total of 22 million stars that belong to a class of red giants. The red giants of asymptotic branch were selected for this study because they can be used as standard candles: at this stage of the life of a giant star brightness is roughly independent of age or composition. “The depth of the star catalogue of VISTA is far greater than the previous works and can reveal the entire population of these stars in all regions except the most obscured,” says Christophe Wegg (MPE), lead author of one of the two articles.  “From this distribution of stars we can construct a map with the tridimensional rendering of the Galactic bulge.  This is the first time that such a map is made without taking a model for the shape of the bulge “. “We find that the interior region of the Galaxy is shaped like a peanut in its shell when viewed from the side, while from above it appears very elongated,” says Ortwin Gerhard, coauthor of the first article and the Dynamic Group at MPE.  Peanut-like structures were observed in the central bulge of other galaxies and their drill was predicted by numerical simulations showing that the peanut shape is given by the stars in orbit that form a structure for X. “This is the first time we see so clearly in the Galaxy and simulations of our group and others have shown that this form is characteristic of spirals with a bar that started as a pure disk of stars”. The second international team, led by Chilean doctoral student Sergio Vásquez (Pontificia Universidad Católica de Chile, Santiago), chose a different approach to define the structure of the bulge.  By comparing images taken at eleven years later with the 2.2 m telescope of the MPG/ESO were able to measure tiny movements due to the motion of the stars in the bulge in heaven.



These were combined with measurements of the star motion towards the ground, approaching or moving away, measuring just over 400 stars in three dimensions.  “This is the first time that so many swiftnesses in three dimensions are measured for single stars on both sides of the Galactic bulge” says Vásquez.” Astronomers think that the Milky Way was originally a pure disk of stars formed some billion years ago.  The inner part is then curled into the peanut that you see in high resolution photos.

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