6 May 2016

An international team of astronomers has discovered that the central region of our Milky Way Galaxy hosts an ancient population of stars more than 10 billion years old.

To their surprise this newly discovered population of stars does not rotate in line with the rest of the Galaxy around the centre.

Instead, this ancient core of stars demonstrates large random motions more consistent with their formation far from the centre of the Milky Way, where they currently reside.

The team is led by Dr Andrea Kunder of the Leibniz Institute for Astrophysics Potsdam (AIP) in Germany, with Dr R. Michael Rich of UCLA and Dr Andreas Koch of Lancaster University.

Co-author Juntai Shen of the Shanghai Astronomical Observatory said: “This even more ancient population of stars appears to have a completely different origin.”

Most of the Milky Way’s centre is dominated by stars in a football-shaped structure called the "bar"; the stars in the bar orbit in roughly the same direction around the Galactic Centre. 

Dr Kunder said: "We expected to find that these stars rotate just like the rest of the bar. Our team was quite surprised by the results." 

The astronomers studied a well known and ancient class of star, called RR Lyrae variables, found only in stellar populations more than 10 billion years old, like ancient globular clusters. 

So the team used the age stamp on the stars to explore what conditions might have been like when the central Milky Way formed.

Using the AAOMEGA spectrograph on the Anglo Australian telescope near Siding Spring, Australia, the team simultaneously recorded the velocities of hundreds of stars over a relatively wide area of the sky toward the constellation of Sagittarius.

The RR Lyrae stars are "moving targets" - their pulsations results in changes in their apparent speed or velocity over the course of a day. 

The team accounted for this, and was able to show that the velocity dispersion or random motion of the RR Lyrae population was very high relative to the other stars in the Milky Way's centre, consistent with their possibly having been one of the first parts of the Milky Way to form.

Dr Andreas Koch of Lancaster University said: "Our first tests indicated that the RR Lyrae population also behaves very differently from the rest of their neighbourhood in terms of their metal content."

This will be consolidated in the next measurements which give additional clues to the history of the stars, and enhance by three or four time the number of stars studied, that presently stands at 946.

The next steps will be to measure the metal content of the RR Lyrae population, which gives additional clues to the history of the stars, and enhance by three or four time the number of stars studied, that presently stands at 946.

"This really was quite unexpected, and we look forward to confirming this result" said UCLA astronomer Michael Rich.