The Milky Way is on a collision course — and it's not the first time - Beyond The World
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The Milky Way is on a collision course — and it’s not the first time


The fate of the Milky Way – and many others – is played out in a slow but surprisingly violent cosmic dance.

On human time scales, stars and galaxies appear to move at a glacial pace. Their dance is extremely slow, taking billions of years to complete. However, if we could see time as the stars do, the area around our Milky Way Galaxy would appear to be surprisingly active.

Galaxies move around each other, spiraling inward until they merge. Many don’t travel alone, but rather with companions, risking a dark collision that could rip some stars from their homes and scatter them across the sky. Other regions become rich in gas and dust and begin to give birth to new stars as a result of their newfound wealth. The galaxies’ dance is slow and violent, full of both life and death.

The Milky Way is responsible for the motion of the Local Group, which consists of more than 100 galaxies. Only the Andromeda Galaxy, which is roughly 125 percent more massive than the Milky Way and has a spiral shape like ours, is larger than the Milky Way. The Triangulum Galaxy, which dances around Andromeda, and the Large Magellanic Cloud (LMC), which orbits the Milky Way, are two smaller galaxies that stand out. The rest of the neighborhood is mostly made up of the pair’s satellites, smaller galaxies hovering around like adoring fans. These galaxies flit around, but they will eventually merge with their larger counterparts. It won’t be the first time our galaxy collides with another when that happens.

Ancient artifacts

Approximately 10 billion years ago, the Milky Way experienced its first major collision. It had probably had a few scrapes with smaller galaxies before that, but the dramatic collision with Gaia Enceladus left permanent scars. Those scars were hidden for a long time, and their absence perplexed astronomers. After years of hints, they were finally discovered in 2018 by the European Space Agency’s Gaia space telescope.

“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, that collided with us about 2 billion years ago. The collision would have far-reaching consequences. The Milky Way was a stubby disk with a halo created by stars flung out from it. The disk became unstable and collapsed into a bar-like structure as a result. A new, thin disk was created over time. The Milky Way was a different galaxy after the show ended.

“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.”

The Milky Way was quiet for the next few billion years, consuming the occasional satellite galaxy but leaving the larger ones alone. When the Sagittarius Galaxy made its grand entrance around 6 billion years ago, that changed.

Sagittarius is an elliptical galaxy that is one of the Milky Way’s closest neighbors, and it is nearing the end of its life as it interacts with the larger object. Sagittarius spirals around the Milky Way’s poles, a hundred to a thousand times less massive than our galaxy, and was discovered in 1994.

In 2018, researchers discovered a warp in the Milky Way’s disk. Large-scale distortions, or clusters of stars pushed together by gravity, are common in spiral galaxies, and ours travels slowly around the disk. Interactions within a galaxy can cause a warp to form, but the movement suggests an external source. “Interaction with a satellite [galaxy] is the only possible model that can explain such large precession,” says Poggio, who measured and tracked the warp.

But who’s the culprit? While it’s possible that the LMC caused the Milky Way’s warp, Poggio believes that Sagittarius’ influence is stronger, and she’s working to prove it. More simulations are needed to confirm her theory, which she is currently analyzing.

In the Milky Way, Sagittarius is also causing waves of star formation. Researchers have discovered patches of star formation that coincide with the dying galaxy’s closest approach, or pericenter. Gravitational interactions push gas and dust piles together, forming regions ripe for star formation. Tomás Ruiz-Lara, an astronomer at the Netherlands’ Kapteyn Astronomical Institute, discovered stellar formation bursts roughly 6.5 billion, 2 billion, and 1 billion years ago, and linked each one to a series of pericentric Sagittarius passes.

“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.”

sequence_of_collision
This illustrated sequence of the future future Milky Way/Andromeda collision starts with present day and unfolds over 4 billion years until our galaxy becomes warped.(NASA/ESA/Z. Levay and R. van der Marel/STSCI/T. Hallas/and A. Mellinger)
Destruction is on the way.

Sagittarius isn’t the only galaxy on the verge of colliding with the Milky Way. The LMC, the Local Group’s fourth largest object, is slowly spiraling towards us. For decades, astronomers assumed that the massive irregular galaxy and its smaller cousin, the Small Magellanic Cloud (SMC), had already circled the Milky Way several times. However, astronomers used the Hubble Space Telescope in 2007 to confirm that the pair were approaching each other for the first time.

The LMC will not merge with us for another 2 billion years, but it is already making an impact.

The Milky Way’s collision with the LMC may cause it to resemble other spiral galaxies. The Milky Way now has a supermassive black hole that is much smaller than the black holes of other galaxies of similar size. The halo of stars that surrounds the galaxy is thin and devoid of metal. The LMC is also an unusually large satellite for spirals of this type. All of these are likely signs of our galaxy’s unusual quiet period; most galaxies undergo multiple major mergers, whereas the Milky Way has only collided with Gaia Enceladus so far. “Since then, there has been hardly anything,  says Marius Cautun, an astronomer at the Netherlands’ Leiden Observatory.

LMC drawn to the Milky Way, but the Milky Way itself is also pulled towards the LMC. The Milky Way, which is a collection of material rather than a solid body, reacts to the interaction in different ways in different parts of the galaxy. The LMC pulls on the inner halo and stellar disk, but the outer halo material does not move much. “It’s a complete disaster,” Belokurov declares.

The LMC, unlike Sagittarius, isn’t alone. The LMC appears to be bringing its own satellite galaxies in addition to the SMC. Ekta Patel, an astronomer at the University of California, Berkeley, says, “Those satellites are coming along for the ride.” Patel has investigated several of the Local Group’s smaller galaxies and discovered six that appear to be linked to the Magellanic Clouds— for now. The future collision between the Milky Way and the LMC may cause them to disconnect from the LMC. At that point, they may fall into the Milky Way along with the LMC, or they could wind up flying off on a new orbit.

Cautun and his colleagues simulated how the Milky Way will change as a result of the upcoming collision with the LMC in a paper published in 2019. Our galaxy’s black hole should grow up to eight times its current size. The stellar halo and metallicity should be boosted by stars from the infalling galaxy as well as those pulled from the Milky Way’s disk. The Milky Way should be less unique and more comparable to other galaxies by the end of the crash.

The Milky Way’s neighborhood has yet to witness the most well-known collision. In about 4 billion years, the massive Andromeda Galaxy, also known as M31, will collide with us. Although this event has long been suspected, scientists didn’t prove it until 2012, when they used NASA’s Hubble Space Telescope to measure the sideways motion of Andromeda. Scientists concluded that Andromeda was moving straight towards us based on previous observations. According to more precise measurements taken with the Gaia telescope, the collision will be slightly off-axis.

“Whether it’s fully a head-on collision or more of a glancing blow doesn’t really affect the end result,” says Roeland van der Marel, an astronomer at the Space Telescope Science Institute who led the teams that took both measurements. As a result, the two spiral galaxies will merge into a single spheroidal elliptical galaxy, devoid of nearly all of its star-forming gas. The supermassive black holes will spiral together, eventually merging into a single monster in the new galaxy’s core.

milkomedapath
The future collision path of Andromeda and the Milky Way. Triangulum may also be part of the impending crash.(NASA/ESA/Z. Levay and R. van der Marel /STSCI)
Bang! Crash!

The LMC and SMC are engaged in their own tug-of-war as they orbit the Milky Way. Gurtina Besla of the University of Arizona was a member of a team that modeled the effects of a collision between the LMC and the SMC in 2012. Another team’s observations of massive, hot young stars inside the pair two years ago backed up their theory that the two had collided only a few hundred million years ago.

Andromeda is also in the midst of a collision. The third-largest galaxy in the neighborhood, the Triangulum Galaxy, is falling into Andromeda in the same way that the LMC is falling into the Milky Way. Although radio telescopes have done a good job of measuring the motion of the smaller galaxy, Gaia was able to confirm and add to those measurements.

“It’s a little surprising, but it appears that the most consistent scenario with the observations is that Triangulum is falling into Andromeda for the very first time,” says van der Marel, who used Gaia to measure the motion of the smaller galaxy.

Only a few satellites in the Milky Way’s neighborhood were known to astronomers prior to the 2000s. With the introduction of digital sky surveys, this changed. We now know about dozens of other, smaller satellite galaxies that are connected to our own, many of which are very faint. The second data release from the Gaia telescope allowed us to track the movement of these satellites. “Gaia is amazing because it allows us to track the motions of stars in the faintest known galaxies,” Patel says. “It’s a huge game changer.”
The tiny satellites are controlled by the stronger gravity of the larger galaxy because their masses are less than 10% that of the Milky Way. As stars are ripped from their galaxies, stellar streams, or ribbons of stars, form.

The connection between Triangulum and the LMC, which both make their first approach towards galaxies roughly 10 times larger than themselves, isn’t as surprising as it appears, according to van der Marel. Smaller objects collide with larger ones to build up galaxies, according to current theories. He believes that finding something relatively massive that is just beginning to merge with its parent galaxy is more likely than orbiting it several times over billions of years. Because large galaxies are more likely to be swallowed up over a few cycles, this is the case.

The Milky Way and its neighbors will eventually merge into a single collection of stars, despite the fact that the dance of galaxies will take billions of years to complete. Andromeda, the Milky Way, and all of their satellites will have lost their spiral and their young stars when the music stops. All that will be left will be a group of aging stars in a dustless galaxy, savoring the peace of their golden years.

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