Leibniz Institute for Astrophysics Potsdam (AIP) | 2016 Apr 22
An international team of astronomers led by Dr. Andrea Kunder of the Leibniz Institute for Astrophysics Potsdam (AIP) in Germany and Dr. R. Michael Rich of UCLA has discovered that the central 2000 light years within the Milky Way Galaxy hosts an ancient population of stars. These stars are more than 10 billion years old and their orbits in space preserve the early history of the formation of the Milky Way.
[c][attachment=0]Milky Ways Ancient Core.jpeg[/attachment][/c][hr][/hr]For the first time the team kinematically disentangled this ancient component from the stellar population that currently dominates the mass of the central Galaxy. The astronomers used the AAOmega spectrograph on the Anglo Australian Telescope near Siding Spring, Australia, and focused on a well-known and ancient class of stars, called RR Lyrae variables. These stars pulsate in brightness roughly once a day, which make them more challenging to study than their static counterparts, but they have the advantage of being "standard candles". RR Lyrae stars allow exact distance estimations and are found only in stellar populations more than 10 billion years old, for example, in ancient halo globular clusters. The velocities of hundreds of stars were simultaneously recorded toward the constellation of Sagittarius over an area of the sky larger than the full moon. The team therefore was able to use the age stamp on the stars to explore the conditions in the central part of our Milky Way when it was formed.
Just as London and Paris are built on more ancient Roman or even older remains, our Milky Way galaxy also has multiple generations of stars that span the time from its formation to the present. Since heavy elements, referred to by astronomers as “metals”, are brewed in stars, subsequent stellar generations become more and more metal-rich. Therefore, the most ancient components of our Milky Way are expected to be metal-poor stars. Most of our Galaxy's central regions are dominated by metal-rich stars, meaning that they have approximately the same metal content as our Sun, and are arrayed in an American football-shaped structure called the "bar". These stars in the bar were found to orbit in roughly the same direction around the Galactic Centre. Hydrogen gas in the Milky Way also follows this rotation. Hence it was widely believed that all stars in the centre would rotate in this way. But to the astronomers’ astonishment, the RR Lyrae stars do not follow football-shaped orbits, but have large random motions more consistent with their having formed at a great distance from the centre of the Milky Way. ...
Before the Bar: Kinematic Detection of A Spheroidal Metal-Poor Bulge Component - Andrea Kunder et al
- Astrophysical Journal Letters 821(2):L25 (20 Apr 2016) DOI: 10.3847/2041-8205/821/2/L25
arXiv.org > astro-ph > arXiv:1603.06578 > 21 Mar 2016