3 mins read 05 Mar 2020

The Milky Way Galaxy: Reverse Engineered

Star clusters used to identify the origins of the Milky Way’s galactic evolution – as it consumed five smaller satellite galaxies in the ancient past.

New research from Swinburne University of Technology has reverse-engineered the construction process of our Milky Way Galaxy to determine how it has reached its current size and state.

The research – submitted by Swinburne’s Professor Duncan Forbes looked at the ages, metallicities, alpha-elements, and integrals of motions of the Milky Way’s surrounding Globular Clusters (GCs) to determine the injection of new stars into the young Milky Way, between 8 – 11 billion years ago. The findings paint a picture of our Galaxy, rapidly (in astronomical terms) cannibalising the smaller satellites as they strayed too close.

“Although our Milky Way may have undergone a tumultuous past, as it grew by accreting and disrupting other small galaxies, the star clusters known as globular clusters are extremely robust and they largely survived intact to the present day,” Professor Forbes says.  

“It is these globular clusters that can be used to re-trace, or reverse engineer, the assembly history of our own galaxy going back billions of years. 

“When you look up at the night sky, some of the individual stars and star clusters that you can see were actually formed outside of our galaxy – alien objects if you like, but now part of the Milky Way galaxy as we know it,” says Professor Forbes.

Five original satellite galaxies have been identified in the research – all of which have now been disrupted, consumed and integrated with the Milky Way, leaving nothing of their former shapes to be viewed. However, GCs from these original galaxies still remain associated with the Milky Way – providing the evidence for Prof. Forbes to trace back their origin progenitors.

One of the satellites contained star clusters on low energy orbits, which Professor Forbes has dubbed ‘Koala’, after the Australian animal that sleeps for more than 18 hours each day. 

The research has also indicated that the total stellar mass added to the Milky Way from the five accreted satellite galaxies is roughly around 1 billion Suns’, in addition to adding over 50% of the current GCs that orbit the Milky Way.

GCs are important structures to study for astronomers, as they can be used to calculate both the centre of the galaxy and our (the Solar System) position within it. In addition to this, GCs contain mostly very old stars, some 11-12 billion years old, so their chemical composition is of interest to scientists to determine how stars formed during the earlier epochs of the Universe.

The paper appears in the Journal Monthly Notices of the Royal Astronomical Society