From stars to galaxies, the formation of structure in the Universe relies on a process called accretion. Accretion is the steady build-up of material, like the gradual depositing of sediment at the bottom of a stream. Stars and galaxies form by accreting material from a disk orbiting the object. The material in this disk has a property called angular momentum. This property is often demonstrated by a figure skater pulling their arms in as they spin, making them rotate faster due to the conservation of their angular momentum.
“Matter in accretion disks has to lose angular momentum to be able to accrete onto the central object,” explains Dr Ross Parkin from The Australian National University’s Research School of Astronomy and Astrophysics. “When a parcel of material in the disk loses angular momentum, it can move closer to the object in the middle of the disk. Once it loses enough angular momentum, it can drop onto that object, making it larger.” Although it sounds simple, the process of angular momentum transport is a major focus of research in astronomy due to its widespread importance.
It was originally thought that the viscosity, or ‘stickiness’, of the material in the accretion disk was enough to remove angular momentum. However, over the past two decades, it has been realised that stresses arising from turbulent, tangled, rapidly changing magnetic fields could do the job better. “Gas dynamics become very complex in turbulent environments,” says Dr Parkin. “We have to do numerical simulations with supercomputers in order to figure out what’s going on.”
Typically, simulations of small portions of accretion disks are performed to save on computer time. However, it has been questioned if these `local’ simulations are really giving the true story of turbulent accretion. “A good analogy would be trying to study the waves in a fish tank, and then using that to infer what was happening in an entire ocean,” Dr Parkin explains. “It gives you an idea of what might be going on, but you can’t be sure until you actually study the ocean.”
Recent work by Dr Parkin and Professor Geoffrey Bicknell at the Research School of Astronomy and Astrophysics used large-scale numerical simulations of entire accretion disks to study turbulent accretion. Aiming to find out if the turbulence in the large-scale disk was the same as that found in small-scale simulations, they performed demanding computations on National Computational Infrastructure (NCI) supercomputers at the ANU campus.
Dr Parkin’s simulations have shown that turbulent magnetic fields are much easier to create in large-scale disks than would be suggested by local models. “The differences in magnetic field and accretion rates across the disk as a whole means it can sustain turbulent energy much better than a small patch of disk can,” says Dr Parkin. “As the turbulent energy cascades and breaks into smaller and smaller turbulent motions, the disk can sustain that energy more effectively. The turbulence is actually being driven by gravitational energy.”
Dr Parkin aims to develop his models to discover how nature feeds stars and galaxies as they grow. “Our results are very encouraging,” he says. “However, what we’ve discovered is only the tip of the iceberg in terms of understanding turbulent accretion.”
Dr Parkin’s work has recently been published in The Astrophysical Journal, Volume 763, article id. 99. A second paper is currently under review by Monthly Notices of the Royal Astronomical Society.
Image: Artist's impression of an accretion disk at the center of a galaxy. The central supermassive black hole accretes material from the surrounding disk (orange and red). As part of this process, a jet with a velocity close to the speed of light is propelled away from the black hole (blue). Credit: NASA/JPL-Caltech.