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Researchers reveal new insights on how spiral galaxies get their arms

One of the most enduring mysterious in astrophysics is the question of how spiral galaxies like our own Milky Way get and maintain their characteristic arms. Astronomers have long been interested in solving this mystery, as nearly 70 percent of the galaxies nearest to the Milky Way are spirals.

Researchers are using powerful new computer simulations to seek the answer to this question and many others. A team of researchers from the University of Wisconsin at Madison and the Harvard-Smithsonian Center for Astrophysics used the computer simulations to observe the motions of as many as 100 million “stellar particles” as gravity and other forces mold them into well-known cosmic shapes. They report that the simulations appear to answer enduring questions about the origin of spiral arms in disk galaxies.

According to UW-Madison astrophysicist Elena D’Onghia, the computer simulations revealed “for the first time that stellar spiral arms are not transient features, as claimed for several decades.”

They “are self-perpetuating, persistent, and surprisingly long lived,” added Mark Vogelsberger, of the Harvard-Smithsonian Center for Astrophysics.

Two theories have dominated discussion on the origin of spiral arms in disk galaxies: One says that the arms come and go over time, while a second theory says that the materials that composes that arms is impacted by variations in gravity and “jams up,” maintaining the arms for long periods of time.

According to the researchers, their findings suggest that the answer is somewhere in between the two theories. The results reveal that spiral galaxies get their arms from the influence of large molecular clouds (star forming regions). When introduced into the computer simulation, the clouds start the formation of spiral arms and maintain them indefinitely.

“We find they are forming spiral arms,” said D’Onghia. Even when the large molecular clouds are removed from the simulation, the spiral arms self-perpetuate.


Several recent studies have looked into the chance that spiral galaxies with a nearby galaxy or space object obtain their arms as gravity from the close neighbor tugs on the disk of its neighbor.

According to the researchers, the new computers simulations can be utilized to reinterpret observational data, studying large molecular clouds and gravitationally induced “holes” in space as the mechanisms behind the formation of spiral arms.