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Two supercomputers, 8 months of analysis, 18.6 million particles to which to apply the laws of gravity and fluid dynamics. All this to simulate, starting from an initial mass of 790 billion solar masses, the birth and evolution of our galaxy. To get some idea of the complexity of the calculations carried out by the Cray XT5 "Monte Rosa" of ETH Zurich’s Swiss National Supercomputing Center (CSCS) and the NASA Advanced Supercomputer Division’s "Pleiades", it would take a normal PC 570 years to carry out the same calculation.
For at least 20 years researchers have tried to simulate the birth of the Galaxy, starting from a protogalactic cloud, in such a way that the result was consistent with the observed parameters of the Milky Way. However, previous efforts typically resulted in virtual galaxies with too many stars in the central regions or total masses greater than that observed.
These problems have been overcome thanks to a more sophisticated astrophysical model conceived by astrophysicists at the University of Zurich and University of California, that, through the simulation, has produced an excellent animation (see
http://www.cscs.ch/newsroom/science/ 2011/milky_way_birth/index.html) that describes the formation of the Milky Way better than any written description.
The simulation, the results of which have been published in The Astrophysical Journal, show better than ever before the role of supernovae in primordial galaxies. The shock waves from these explosions are essential to remove a significant fraction of the central mass, resulting in a final central stellar density consistent with that observed. The role of dark matter is also fundamental, as always, but this simulation suggests that in the outer regions the ratio of ordinary matter to dark matter is 1:9 rather than 1:6 as thought until now.
Further, the simulation implies that the Galactic halo extends out to 600,000 light years from the Galactic centre, and therefore that there are numerous stars belonging to our galaxy that have not yet been identified, due to instrumental constraints. In the images above, the final result of the simulation is shown on the left, while on the right is an image of the nearby spiral galaxy M74, considered to be very similar to the Milky Way (gas in magenta, stars in blue).
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