If we could somehow glance down at the entire Milky Way, our home galaxy would look like a spinning pinwheel. In it, some 100 I billion stars are sprinkled across 100,000 light years, accompanied by unfathomable amounts of gas and dust. Together, these galactic components swirl around in a stellar disc, spilling into four giant spiral arms emanating from the galaxy’s blinding centre, the home of a supermassive black hole.
We’ve come to really identify our galaxy with its spiral structure, although its exact number of galactic arms is still being debated. Yet astronomers have long puzzled over why spiral galaxies such as ours appear to be strikingly scarce in our crowded pocket of the universe. The pancake-like supergalactic plane in which the Milky Way is embedded, spanning 1.4 billion light years in width, is instead dominated by clumpy, somewhat rounded galaxies known as ellipticals. A new supercomputer simulation of the 13.8-billion-yearlong evolution of the universe suggests this scarcity is thanks to frequent naturally occurring interactions among galaxies in the supergalactic plane that smooth out any potential spiral arms, leading to formation of the elliptical galaxies we see.
Stars in spiral galaxies like the Milky Way float around their galactic centres in mostly circular orbits. However, when two galaxies of comparable mass are near each other, these ordered motions become disordered. “The stellar orbits become reshuffled or randomised,” Till Sawala, an astrophysicist at the University of Helsinki in Finland and coauthor of a study on the findings, said. “This removes any ordered features, such as the appearance of a stellar disc, and therefore also removes the appearance of spiral arms.”