MOL

LONDON-Cambridge scientists have found evidence that the Milky Way was formed from the inside out. Using data from the Gaia-ESO project, scientists were able to back up predicted divisions in the chemical make-up of stars.

Their research reveals how stars in centre of the Milky Way ‘live fast and die young’, shedding light on the formation of disk galaxies. The scientists analysed levels of magnesium - which is present in older stars - to determine their age.

Using the eight metre Very Large Telescope in Chile, they discovered the inner regions of the Milky Way were the first to form, explaining its ‘flying saucer’ shape. When the Milky Way first evolved it is thought to have been made purely made of hydrogen and helium. The older the star the less it contains other contaminated elements built up over time.

But older and purer stars in the centre actually have very high levels of magnesium. Magnesium is present in more efficient stars that form quickly, but bursts of magnesium also explosively shoot out of stars as they die. This catastrophic event can form a neutron star or a black hole, and even trigger the formation of new stars.

Because of this the astronomers have now worked out that the older stars in the centre will die before those on the outer circle where the sun sits.

‘The different chemical elements of which stars - and we - are made are created at different rates - some in massive stars which live fast and die young, and others in sun-like stars with more sedate multi-billion-year lifetimes,’ said Professor Gerry Gilmore, lead investigator on the Gaia-ESO Project.

The findings are the part of the first wave of new findings from the Gaia-ESO survey, launched by the European Space Agency at the end of last year. ‘We have been able to shed new light on the timescale of chemical enrichment across the Milky Way disc, showing that outer regions of the disc take a much longer time to form,’ said Maria Bergemann from Cambridge’s Institute of Astronomy, who led the study. ‘This supports theoretical models for the formation of disc galaxies in the context of Cold Dark Matter cosmology, which predict that galaxy discs grow inside-out.’

The new research also sheds further light on another much debated ‘double structure’ in the Milky Way’s disc – the so-called ‘thin’ and ‘thick’ discs. ‘The thin disc hosts spiral arms, young stars, giant molecular clouds – all objects which are young, at least in the context of the galaxy,’ explained Aldo Serenelli from the Institute of Space Sciences , a co-author of the study. ‘But astronomers have long suspected there is another disc, which is thicker, shorter and older. This thick disc hosts many old stars that have low metallicity.’

‘From what we now know, the galaxy is not an ‘either-or’ system. You can find stars of different ages and metal content everywhere’, said Professor Bergemann. ‘There is no clear separation between the thin and thick disc. The proportion of stars with different properties is not the same in both discs - that’s how we know these two discs probably exist – but they could have very different origins.’

In theory, say astronomers, the thick disc could have emerged in a variety of ways, from massive gravitational instabilities to consuming satellite galaxies in its formative years. ‘The Milky Way has cannibalised many small galaxies during its formation. Now, with the Gaia-ESO Survey, we can study the detailed tracers of these events, essentially dissecting the belly of the beast,’ said Greg Ruchti, a researcher at Lund Observatory in Sweden, who co-leads the project.