Stars and galaxies move around us at a pace that seemsglacial on human time scales. Their dance is exceedingly gradual, taking place over billions of years. But if we could see time the same way the stars do, the neighborhood around our Milky Way Galaxy would appear surprisingly active.
Galaxies swing around one another, slowly spiraling together until they merge. Many don’t travel alone but bring companions with them, in a dark collision that may tear some stars from the heart of their homes and splay them across the sky. Other regions grow rich in gas and dust and begin, in their newfound opulence, to birth new stars. The dance of the galaxies is slow and violent, filled with both life and death.
The Milky Way drives the motion of the collection of more than 100 galaxies known as the Local Group. Within that group, only the Andromeda Galaxy is larger than the Milky Way — roughly 125 percent more massive — and like our galaxy, it has a spiral shape. Two smaller galaxies stand out: the Triangulum Galaxy, dancing around Andromeda, and the Large Magellanic Cloud (LMC), orbiting the Milky Way. The rest of the neighborhood is filled mostly with satellites of the pair, smaller galaxies hovering like adoring fans. These galaxies flit about, but eventually will meld with their larger companions. When that happens, it will not be the first time our galaxy has bumped into another.
Ancient artifacts
The Milky Way suffered its first major collision early in its lifetime, roughly 10 billion years ago. Prior to that, it probably had a handful of scrapes with smaller galaxies, but the dramatic crash into a galaxy referred to as Gaia Enceladus left lasting scars. For a long time, those scars were hidden, their absence puzzling astronomers. It took the European Space Agency’s Gaia space telescope to bring them to light in 2018, after years of hints.
“Before the Gaia data was released, we thought the Milky Way was a very quiet galaxy with no dramatic impact,” says Eloisa Poggio, an astronomer at the Astrophysical Observatory of Turin in Italy. “It’s more complicated than we thought before.”
Gaia Enceladus was a dwarf galaxy, slightly smaller than the Milky Way, perhaps 2 billion years old when it crashed into us. The collision would have significant ramifications. The Milky Way was a stubby disk from which stars were flung out, creating its halo. Part of the disk then became unstable and collapsed into a barlike structure. Over time, a new, thin disk was created. When the show was over, the Milky Way was a different galaxy.
“This is a key pivotal moment in the Milky Way’s life,” says Vasily Belokurov, part of one of the two teams that co-discovered the ancient artifact. “It unleashed a sequence of transformations in the Milky Way that have changed it into the Milky Way we know.”
For the next few billion years, the Milky Way was quiet, consuming the occasional satellite galaxy but leaving the larger ones alone. That changed around 6 billion years ago when the Sagittarius Galaxy made its own grand entrance.
Sagittarius is an elliptical galaxy, one of the nearest neighbors to the Milky Way, and is coming to an agonizing end as it interacts with the larger object. Discovered in 1994, Sagittarius spirals around the Milky Way’s poles, a hundred to a thousand times less massive than our galaxy.
In 2018, scientists discovered a warp in the disk of the Milky Way. Large-scale distortions — collections of stars gravitationally shoved together — are common among spiral galaxies, and ours travels relatively slowly around the disk. A warp can form due to interactions within a galaxy, but the movement suggests an external origin. “The only possible model that can explain such large precession is interaction with a satellite [galaxy],” says Poggio, who measured and tracked the warp.
But who’s the culprit? While it’s possible that the Milky Way’s warp was caused by the LMC, Poggio thinks that the influence of Sagittarius might be stronger, and she’s working to prove it. Confirming her theory requires further simulations, which she is in the process of analyzing.
Sagittarius is also triggering waves of star formation in the Milky Way. Researchers have found patches of star formation that coincide with the closest approach, or pericenter, of the dying galaxy. Gravitational interactions push together piles of gas and dust to create regions ripe for starbirth. Tomás Ruiz-Lara, an astronomer at Kapteyn Astronomical Institute, the Netherlands, found bursts of stellar formation roughly 6.5 billion, 2 billion, and 1 billion years ago, and tied each one to severa pericentric passes of Sagittarius.
“The main surprise is that something so small is able to cause all these effects,” says Ruiz-Lara. “Sagittarius is an important actor in the film of the origin and evolution of our galaxy.”