New insights on how spiral galaxies get their arms

Credit: Elena D'Onghia and Mark Vogelsberger and Harvard FAS Supercomputer Odyssey

Visualization: Thiago ize of the Scientific Computing and Imaging (SCI) Institute - University of Utah

"Rendering of spiral arm formation consisting of 512 time steps of 100 million particles (stars) per time step."

"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

"We visualize a 1.5TB time varying simulation of spiral arm formation. There are 512 time steps, each of which contains 100 million particles produced by a Monte Carlo N-body method."

"Traditionally, this is rendered by creating a density field from the particles and then rendering the volume. "

"However, we rendered this video by directly ray tracing the particles using the Manta Interactive Ray Tracer. Each particle is made transparent, with transparency decreasing as particles move further away from the galaxy center."

"The variable transparency allows for the spiral structure to be easily observed. The color map corresponds to particle speed."

"We show for the first time that stellar spiral arms are not transient features, as claimed for several decades," says UW-Madison astrophysicist Elena D'Onghia, who led the new research along with Harvard colleagues Mark Vogelsberger and Lars Hernquist.

"The spiral arms are self-perpetuating, persistent, and surprisingly long lived," adds Vogelsberger. The paper was publish in the Astrophysical Journal.

The origin and fate of the emblematic spiral arms in disk galaxies have been debated by astrophysicists for decades, with two theories predominating. One holds that the arms come and go over time.

A second and widely held theory is that the material that makes up the arms - stars, gas and dust - is affected by differences in gravity and jams up, like cars at rush hour, sustaining the arms for long periods.

The new results fall somewhere in between the two theories and suggest that the arms arise in the first place as a result of the influence of giant molecular clouds - star forming regions or nurseries common in galaxies.

Introduced into the simulation, the clouds act as "perturbers" and are enough to not only initiate the formation of spiral arms but to sustain them indefinitely.

"We find they are forming spiral arms," explains D'Onghia. "Past theory held the arms would go away with the perturbations removed, but we see that (once formed) the arms self-perpetuate, even when the perturbations are removed. "

"It proves that once the arms are generated through these clouds, they can exist on their own through (the influence of) gravity, even in the extreme when the perturbations are no longer there."

The new study modeled stand-alone disk galaxies, those not influenced by another nearby galaxy or object.

Some recent studies have explored the likelihood that spiral galaxies with a close neighbour (a nearby dwarf galaxy, for example) get their arms as gravity from the satellite galaxy pulls on the disk of its neighbour.

Read the full article here arxiv.org/abs/1204.0513