Why Galaxies Spiral
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. As the shapes of galaxies go, the spiral disk — with its characteristic pinwheel profile — is by far the most pedestrian. Our own Milky Way, astronomers believe, is a spiral. Our solar system and Earth reside somewhere near one of its swept-back arms. And nearly 70 percent of the galaxies closest to the Milky Way are spirals, suggesting they have taken the most ordinary of galactic forms in a universe with billions of galaxies. Despite their common morphology, how galaxies like ours get into a spiral shape and maintain their characteristic arms has proved to be an enduring puzzle in astrophysics. How do the arms of spiral galaxies arise?
Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very loose arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable.
The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem.
Since the 1960s, there have been two leading hypotheses or models for the spiral structures of galaxies:
Star formation caused by density waves in the galactic disk of the galaxy.
The SSPSF model — star formation caused by shock waves in the interstellar medium.
The answers to these and other questions are now coming into focus as researchers capitalize on powerful new computer simulations to follow the motions of as many as 100 million stellar particles as gravity and other astrophysical forces sculpt them into familiar galactic shapes. Writing April 1 in The Astrophysical Journal, a team of researchers from the University of Wisconsin-Madison and Harvard-Smithsonian Center for Astrophysics report simulations that seem to resolve longstanding questions about the origin and life history of spiral arms in disk galaxies.
"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-Smithsonian Center for Astrophysics colleagues Mark Vogelsberger and Lars Hernquist. "They are self-perpetuating, persistent and surprisingly long lived."
The origin and fate of the emblematic spiral arms in 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, says D’Onghia, a UW-Madison professor of astronomy, 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 neighbor — a nearby dwarf galaxy, for example — get their arms as gravity from the satellite galaxy pulls on the disk of its neighbor.
For further information see Why a Spiral.
Spiral image by UW Professor Elena D'Onghia.