U.S. astrophysicists report computer simulations that seem to resolve long-standing questions about the origin and life history of spiral arms in disk galaxies.
The origin and fate of the 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 findings, accepted for publication in the Astrophysical Journal (arXiv.org version), 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 show for the first time that stellar spiral arms are not transient features, as claimed for several decades,” explained first study author Dr Elena D’Onghia of the University of Wisconsin-Madison.
“The spiral arms are self-perpetuating, persistent, and surprisingly long lived,” added co-author Dr Mark Vogelsberger from Harvard-Smithsonian Center for Astrophysics.
Dr D’Onghia said: “past theory held the arms would go away with the perturbations removed, but we see that 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 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 neighbor (a nearby dwarf galaxy, for example) get their arms as gravity from the satellite galaxy pulls on the disk of its neighbor.
“The new simulations can be used to reinterpret observational data, looking at both the high-density molecular clouds as well as gravitationally induced ‘holes’ in space as the mechanisms that drive the formation of the characteristic arms of spiral galaxies.”
Bibliographic information: Elena D’Onghia et al. Self-Perpetuating Spiral Arms in Disk Galaxies. ApJ, accepted for publication; arXiv: 1204.0513