January 20, 2014
Gaia Confirms ‘Inside-Out’ Model Of Milky Way
[ Watch the Video: Inside-out Formation Of The Milky Way ]
Brett Smith for redOrbit.com - Your Universe Online
New research from the European Space Agency's (ESA) Gaia-ESO project has confirmed previous theories on the inside-out development of the Milky Way, with the inner regions of the galaxy’s disc being the first to form.
According to their report in the astronomical database Astro-ph, the study researchers reached their conclusions using observations taken with the 8-meter VLT telescope in Chile. The team looked explicitly at stars in the Milky Way with a broad range of ages and locations to precisely determine the amount of chemical elements other than hydrogen and helium that they contained, also known as their metallicity.
The early Universe consisted almost completely of hydrogen and helium, with amounts of "contaminant metals" increasing over time. Therefore, older stars contain mostly hydrogen and helium – or have a lower metallicity.
"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 study author Gerry Gilmore, lead investigator on the Gaia-ESO Project.
Massive stars tend to have short lives with a violent end, producing huge amounts of magnesium when they go supernova. The deaths of these kinds of stars can form a neutron star or a black hole, as well as result in the formation of new stars.
Through their observations, the team found that older, 'metal-poor' stars contained within the orbit of our Sun around the center of the Milky Way have a greater chance of having high amounts of magnesium. This suggests the center of the Milky Way had more stars that lived fast and died young in the past.
The stars that lie outside this inner region, also called the Solar Circle, are mainly younger, both 'metal-rich' and 'metal-poor', and have unexpectedly low magnesium levels compared to their metallicity, the researchers found.
The team concluded that there was an abundance of very efficient and short star formations that took place inside the Solar Circle. Conversely, outside the Sun's orbit, star creation processes took much longer.
"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 study leader Maria Bergemann from Cambridge's Institute of Astronomy. "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 research team noted that their work also provides more details on the so-called "double structure" in the Milky Way's disc – which is theoretically comprised on thick and thin 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 in Barcelona, Spain. "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."
The new findings are based on some of the first observations from the Gaia-ESO survey, the ground-based expansion to the Gaia space mission that was launched by the ESA at the end of last year.