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The turbulent birth of stars in galactic collisions
Stars are formed when the gas contained in certain regions of a galaxy becomes dense enough to collapse in on itself (usually due to gravity). When two galaxies collide, a "starburst" of star formation is generally observed, for reasons hitherto unknown.
A galactic collision increases the disordered motion of the gas, and the vortices of turbulence thus generated should prevent the gas from condensing due to gravity. One would therefore expect that this turbulence would slow down, and even prevent star formation, whereas in fact the opposite is observed.
The very high-resolution simulations demonstrate that, in reality, the collision has changed the very nature of the turbulence at a very small scale: the vortex effect is replaced by a gas compressive mode. Contrary to all expectation, turbulence thus contributes to the collapse of the gas by compressing it. Thus, when two galaxies clash into one another, it is this compressive turbulence effect that triggers an excess of dense gas and, thereby, a starburst of star formation, in regions that cover a large volume of the galaxies, and not only in their central regions. This process now appears to play a crucial role in triggering star formation.
To obtain these results, the researchers used two of the most powerful supercomputers available through PRACE, the European research infrastructure, including GENCI's Curie supercomputer , to model an isolated galaxy, like the Milky Way, and a collision between two galaxies such as that which gave birth to the pair of galaxies known as the "Antennae Galaxies".
Simulations of the galactic collision were obtained using the SuperMUC supercomputer (Leibniz-Garching, Germany – 4,096 processors running in parallel) in a cube of 600,000 light-years, with resolution of 3 light-years, and required the equivalent of 8 million computing hours over a period of 8 months.
Research modeling these two well-known galaxies has resulted in the development of the most realistic simulations to date of the objects observed.
These new simulations have achieved a level of precision never seen before, making it possible to resolve structures with a mass 1,000 times smaller than ever before. This has enabled the astrophysicists to track the evolution of the galaxies over hundreds of thousands of light-years, and to explore a mere fraction of a light-year in detail. Thanks to this decisive advantage, new physical effects emerged, revealing the complex nature of turbulence.
What is a “starburst” of star formation
Our knowledge of galaxies is based on the light emitted by stars within them, especially in the case of young stars. Stars form when the gas in a galaxy condenses. This makes them emit particularly intense ultraviolet and infrared light. When two galaxies collide, a great many stars form very rapidly, and astronomers then observe a peak in the emission of this type of light, known as a "starburst".