A visualisation of gas density in the ground-breaking ANU-led simulation, highlighting the intricate gaseous turbulence in the interstellar medium.
Credit: Federrath, C. et al. The sonic scale of interstellar turbulence.
An international group of astronomers, led by ASTRO 3D Associate Investigator and ANU researcher Dr Christoph Federrath, has used a ground-breaking computer simulation to identify the “goldilocks” size for gas clouds that will form stars. These results will enable more detailed and realistic simulations to predict galaxy formation.
Most galaxies we observe in the night sky, including our own Milky Way, are made up of a few billion stars immersed in interstellar gas. While stars are known to form within clumps of gas called giant molecular clouds (GMCs), the nature of this gas and its influence on the stars it creates remains poorly understood.
Until recently, the vast range in scale from full GMCs down to individual star-forming gas nuclei (equivalent to the size of a pea in an Olympic swimming pool) had rendered these environments impossible to simulate accurately with the required resolution, even using the best supercomputers.
In a recently published Nature Astronomy paper, Dr. Federrath and colleagues utilised state-of-the-art hydrodynamical simulation software together with the computational facilities at the Leibniz Supercomputing Center (LRZ) in Germany to simulate a cube of interstellar gas roughly 60 light-years across in unprecedented detail.
Interstellar gas is known to be dynamic and turbulent. Simulating and understanding gas turbulence is an active research area in many applied sciences, including weather forecasting & aerodynamics. But unlike the gas in terrestrial applications, interstellar gas is incredibly tenuous and compressible, making it even more challenging to simulate.
High-density gas nuclei are the formation site for stars like our Sun, but these nuclei can only exist in very specific circumstances. Essentially, if gas is moving too fast and chaotically, the force of gravity cannot take over and allow it to “settle” or collapse into a sufficiently dense region. The speed of sound is a convenient reference point to determine whether a cloud can collapse: in regions of supersonic gas, where the gas speed is greater than the speed of sound, turbulence dominates over gravity, and the cloud cannot collapse. And conversely, in the subsonic case where the gas moves slower than the speed of sound, gravity can dominate over turbulence; allowing dense nuclei to form.
Of particular interest to Dr. Federrath’s team was identifying the so-called “sonic scale”, marking the transition point where gas goes from moving at supersonic to subsonic speeds. The scale predicts the size of gas nuclei that would be expected to harbour star formation – the so-called “goldilocks” zone. This makes it an incredibly important factor which underpins theories of star formation and galaxy evolution.
Modelling the gravity and fluid mechanics of an extraordinary one trillion individual gas elements made the simulation the first to accurately resolve the sonic transition scale. In their paper, the researchers found that the sonic scale is not necessarily a sharp transition point – but translates to a range of scales between roughly 0.5 and one light-year. Encouragingly, they also find their results agree well with observations of a nearby gas cloud in the Milky Way, IC5146 (“the Cocoon Nebula”, image to the right), which sits approximately 4000 light-years from Earth.
Dr. Federrath and his team state that their methodology could have far-reaching applications to understanding both Earth- and space-based gas turbulence, with the group already looking towards improving to their simulations. Among a number of additional astrophysical processes, they note that the influence of magnetic fields and gas heating and cooling mechanisms are yet to be fully explored at this scale and resolution. Future studies promise to answer a number long-standing questions about star formation, and ultimately assist in understanding how our Sun and Earth came to exist.
IC 5416, “the Cocoon Nebula”.
Credit: Marcel Dreschler, Baerenstein Observatory.
