Spiral galaxies like the Milky Way are surprisingly rare. Astronomers may finally know why.
Galaxies like our Milky Way are mysteriously hard to come by in our cosmic backyard. New supercomputer simulations have helped astronomers finally answer why.
The Milky Way sits within a galaxy cluster on the Supergalactic Plane — a billion light-year-wide sheet, or “supercluster,” upon which large galaxy clusters are pinned. But other spiral galaxies are surprisingly rare along this gigantic plane, while bright elliptical galaxies are far more common.
Now, astronomers suggest this is because a violent history of frequent galactic collisions filled our neighborhood with ellipses while somehow sparing our own galaxy. They published their findings Nov. 20 in the journal Nature Astronomy.
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“The distribution of galaxies in the Supergalactic Plane is indeed remarkable,” Carlos Frenk, a professor of physics at Durham University in the U.K. said in a statement. “It is rare but not a complete anomaly: our simulation reveals the intimate details of the formation of galaxies such as the transformation of spirals into ellipticals through galaxy mergers.”
The local supercluster is a pancake-shaped formation of several massive galaxy clusters containing thousands of galaxies. The galaxies within these clusters fall into two broad categories: elliptical galaxies filled with ancient stars and anchored by gigantic supermassive black holes; and spiral galaxies like our own, with smaller supermassive black holes at their centers and many young stars still forming along their delicate spiral arms.
Yet since the plane’s discovery by French astronomer Gérard de Vaucouleurs in the 1950s, scientists noticed the puzzling discrepancy: the plane swarmed with bright elliptical galaxies, yet spiral galaxies were noticeably rare.
To study our cosmic neighborhood’s evolution, the researchers behind the new study turned to a supercomputer simulation called Simulations Beyond the Local Universe (SIBELIUS). By rewinding the evolution of the observed galaxies 13.8 billion years to their beginning with the Big Bang, the researchers built a model that closely recreated the evolution of the plane.
In their simulation, the researchers saw that the spiral galaxies in the dense clusters of the Supergalactic Plane frequently smashed into each other in catastrophic collisions — shattering their delicate arms and smoothing them out into elliptical galaxies. This process also pushed more matter into the maw of a struck galaxy’s supermassive black hole, making the black holes even bigger.
On the other hand, spiral galaxies found in regions away from the plane were mostly left out of the cosmic bar brawl, enabling them to preserve their structures. This doesn’t stop spiral galaxies like our own from evolving in the chaotic environment of the plane, but it does mean they’re unusual in having avoided the worst of the damage — so far.
