There are two main theories as to the origin and eventual demise of spiral galaxies. The first theory postulates that the arms fluctuate over time. The second theory holds that the material within the spiral arms is affected by differences in gravity and therefore ‘jams’ at times, sustaining the shape of the arms for long periods of time.
The team’s results do not conform to either theory, but fall somewhere in between. The arms appear to form initially due to the influence of giant molecular clouds, which are star forming regions within galaxies. When introduced into the simulation, the clouds disrupt the galaxy enough to set off the formation of spiral arms as well as sustain them. The researchers found that when the clouds are removed from the simulation, the arms self-perpetuate. This indicated that once the arms are formed by the molecular clouds, they persist through the influence of gravity, even if the original disruptor is removed.
The recent study modeled disk galaxies that were not influenced by nearby galaxies or objects. There have been other recent studies that investigated whether spiral galaxies with close neighbours get the formation of their arms from the gravity pull of objects nearby. All the new simulations can be used to reinterpret observational data.
Animation showing spiral arm formation consisting of 512 time steps of 100 million particles (stars) per time step: http://bit.ly/YRV6Fs. The animation was rendered using the Manta Interactive Ray Tracer by making each particle a transparent sphere with transparency decreasing with increasing galaxy radius. Colour map corresponds to particle speed.
Elena D'Onghia, Mark Vogelsberger, Lars Hernquist. Self-Perpetuating Spiral Arms in Disk Galaxies. The Astrophysical Journal, March 20, 2013http://arxiv.org/abs/
Video: Scientific Computing and Imaging (SCI) Institute - University of Utah Image: NASA/ESA/Hubble Heritage Team