Astrophyscists at the French National Centre for Scientific Research (CNRS) and the Alternative Energies and Atomic Energy Commission (CEA - Commissariat à l'énergie atomique et aux énergies alternatives) have used high-performance computing simulations to analyse in detail the turbulence generated when two galaxies collide.
To obtain these results, the researchers led by Florent Renaud, used two of the most powerful supercomputers available through PRACE, the European research infrastructure, including GENCI’s Curie supercomputer and LRZ’s SuperMUC supercomputers to model an isolated galaxy, like the Milky Way, and a collision between two galaxies similar to that which gave birth to the pair of galaxies known as the 'Antennae Galaxies'.
Simulations of the Milky Way were obtained using the Curie supercomputer (using 6,080 processors) for 300,000 light-years, with a resolution of 0.1 light-year, and required the equivalent of 12 million computing hours over a period of 12 months. The Curie supercomputer, made available by GENCI (Grand Equipement National de Calcul Intensif) to European researchers within the framework of PRACE (Partnership for Advanced Computing in Europe), is housed at the CEA’s Computing Center (TGCC). Simulations of the galactic collision were obtained using the SuperMUC supercomputer housed in Leibniz-Garching, Germany using its 4,096 processors 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.
‘The type of research done by Florent Renaud and his team demands very large computing capacities; capacities so large that only PRACE can provide them in Europe,’ said Kenneth Ruud, Chair of the PRACE Scientific Steering Committee. ‘These results therefore show that Europe is at the forefront of both ground-breaking science as well as world-class HPC.’
The numerical simulations, in which the disordered motions of the gas contained in galaxies are seen at extremely small-scale resolutions, at last explain a phenomenon that astrophysicists have observed but which they have been unable to understand until now referred to as ‘starbursts’ of star formation when galaxies collide. A process of compressive turbulence helps to explain such starbursts, and why some galaxies form more stars than others. These results are published in Monthly Notices of the Royal Astronomical Society, Letters, May 2014.
Stars are formed when the gas contained in certain regions of a galaxy becomes dense enough to collapse in on itself. When two galaxies collide, a ‘starburst’ of star formation is generally observed.
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 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.
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 previously available. 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.