Galaxy’s central bulge is shaped like a peanut

The heart of our own Milky Way galaxy is shaped like a peanut, astronomers have discovered. The revelation comes after the region was mapped and studied in the best detail ever using telescopes operated by the European Southern Observatory in Chile.

Milky Way over La Silla
The centre of our galaxy appears as the richest part of the Milky Way above ESO’s 3.6-metre telescope at La Silla, Chile, in this photo. Credit: ESO/Serge Brunier

It may seem odd that we can see to the furthest reaches of the Universe but are still finding out new things about the centre of our own Galaxy. But it really is a case of not seeing the wood for the trees. Because we live inside it, the dense clouds of stars, dust and gas have worked to hide the Galaxy’s central zone from us.

This zone, the galactic bulge, is one of the most important and massive regions of a spiral galaxy like ours. It is a vast central cloud, thousands of light-years across, containing around 10,000 million stars. Ours lies about 27,000 light-years from us.

Fortunately there is a way to see through the gas and dust that obscures our view of the bulge and that is by observing at longer wavelengths of light such as infrared radiation. Step forward VISTA – the Visible and Infrared Survey Telescope for Astronomy at ESO’s Paranal site.

Galaxy's central bulge
Impression of the peanut bulge at the centre of our galaxy as it would appear from outside. Credit: ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt

VISTA data was used, along with measurements of the motions of hundreds of very faint stars, by two teams of astronomers to build a new three-dimensional map of the bulge.

Earlier observations from the 2MASS infrared sky survey had already hinted that the bulge had a strange X-shaped form. Now observations with VISTA and other ESO telescopes have provided a much clearer view of the bulge’s structure.

The first observing team, from the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, used the VVV near-infrared survey from VISTA. This new public survey can pick up stars 30 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,” said MPE’s Christopher Wegg, 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.”

Ortwin Gerhard, co-author of the first paper and leader of the Dynamics Group at MPE, added: “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.”

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 11 years apart with the MPG/ESO 2.2-metre telescope they measured 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.

VISTA by Paul Sutherland
The VISTA telescope at Paranal, Chile, photographed by the author. Credit: Paul Sutherland

Vásquez said: “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!”

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.

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By Paul Sutherland

Paul Sutherland has been a professional journalist for nearly 40 years. He writes regularly for science magazines including BBC Sky at Night magazine, BBC Focus, Astronomy Now and Popular Astronomy, plus he has authored three books on astronomy and contributed to others.

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