The turbulent birth of stars in galactic collisions

Using very high-resolution numerical simulations, astrophysicists at the CEA and CNRS, led by Florent Renaud , have, for the very first time, achieved a detailed analysis of the effects of turbulence generated when two galaxies collide. These 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: that of “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 (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[2] and LRZ’s SuperMUC supercomputers 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”.

Research modelling 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.

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,” says 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.

Video simulation of a galactic collision
The same phenomenon, in close-up

PNG - 217.4 kb
Figure 1: Simulation of the encounter between two galaxies. The simulation demonstrates how the galaxies are deformed following their first encounter (left). The high-resolution simulations make it possible to conserve the finest details: the close-up (right) shows the density of the gas. Stars form in the dense regions (in yellow and red), as a result of “compressive turbulence”. Star formation in these regions is more efficient than in the Milky Way. © CEA-SAp

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”.

“Starbursts triggered by inter-galactic tides and interstellar compressive turbulence”, Florent Renaud, Frédéric Bournaud, Katarina Kraljic & Pierre-Alain Duc, doi: 10.1093/mnrasl/slu050 Mon. Not. R. Astron. Soc. Letters (Oxford University Press), May 2014.


  1. CEA-IRFU (Institute for Research on the Fundamental Laws of the Universe)
  2. Simulations of the Milky Way were obtained using the Curie supercomputer (using 6,080 processors running in parallel) 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 Very Large Computing Center (TGCC). 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.

The Partnership for Advanced Computing in Europe (PRACE) is an international non-profit association with its seat in Brussels. The PRACE Research Infrastructure provides a persistent world-class high performance computing service for scientists and researchers from academia and industry in Europe. The computer systems and their operations accessible through PRACE are provided by 4 PRACE members (BSC representing Spain, CINECA representing Italy, GCS representing Germany and GENCI representing France). The Implementation Phase of PRACE receives funding from the EU’s Seventh Framework Programme (FP7/2007-2013) under grant agreements RI-283493 and RI-312763. For more information, see

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