In 2018, an international team of astronomers proposed that around 8 billion to 10 billion years ago an unknown massive dwarf galaxy dubbed the “Sausage” –with a total mass in gas, stars and dark matter more than 10 billion times our sun– crashed head-on into our Milky Way galaxy, ripping the dwarf to shreds, leaving its stars moving in long, narrow needle-like radial orbits. This defining event in the early history of the Milky Way reshaped the structure of our galaxy into its warped, twisted shape shown above, with the Sausage debris scattered around the inner parts of the Milky Way, creating the ‘bulge’ at the galaxy’s center and the surrounding ‘stellar halo.’
Worlds in Collision
“While there have been many dwarf satellites falling onto the Milky Way over its life, this was the largest of them all,” said Sergey Koposov of Carnegie Mellon University, who has studied the kinematics of the Sausage stars and globular clusters in detail about at least eight large, spherical clumps of stars called globular clusters that were brought into the Milky Way by the Sausage galaxy. Small galaxies generally do not have globular clusters of their own, so the Sausage galaxy must have been big enough to host a collection of clusters.
Our home galaxy is not unique in its history of cataclysms –astronomers discovered that an entire medium-sized galaxy fell through the center of monster elliptical galaxy M87 that harbors the now iconic black hole the size of our solar system imaged for the first time ever by the Event Horizon Telescope (EHT) on April 10, 2019. The EHT team theorized that the M87 black hole grew to its massive size by merging with several other black holes. M87 is the largest, most massive galaxy in the nearby universe, and is thought to have been formed by the merging of 100 or so smaller galaxies.
Fast Forward to “Virgo Radial Merger”
Fast forward to less than 3 billion years ago and another dwarf galaxy –known as the Virgo Radial Merger– plunged into the center of the Milky Way and was ripped apart by the gravitational forces of the collision creating a series of telltale shell-like formations of stars in the vicinity of the Virgo constellation –the first such “shell structures” to be found in the Milky Way. The Virgo Radial Merger opens the door to greater understanding of other phenomena that we see and don’t fully understand said Heidi Jo Newberg, a physicist at the Rensselaer Polytechnic Institute.
Image below Stars identified in the research have formed “shell structures” in the aftermath of a radial merger occurring 3-billion years ago.
“Aha” Moment –The Virgo Overdensity
Two decades ago, astronomers identified an unusually high density of stars called the Virgo Overdensity. Star surveys revealed that some of these stars are moving toward us while others are moving away, which is unusual, as a cluster of stars would typically travel in concert. Based on emerging data, astrophysicists at RPI proposed in 2019 that the overdensity was the result of a radial merger, “the stellar version of a T-bone crash.”
“When we put it together, it was an ‘aha’ moment,” said Newberg, professor of physics, applied physics, and astronomy, and lead author of the The Astrophysical Journal paper detailing the discovery. “This group of stars had a whole bunch of different velocities, which was very strange. But now that we see their motion as a whole, we understand why the velocities are different, and why they are moving the way that they are.”
Umbrellas of Stars
The newly announced shell structures are planes of stars curved, like umbrellas, left behind as the dwarf galaxy was torn apart, literally bouncing up and down through the center of the galaxy as it was incorporated into the Milky Way, an event the researchers have named the “Virgo Radial Merger.”
Cycles of Shells
Each time the dwarf galaxy stars pass quickly through the galaxy center, slow down as they are pulled back by the Milky Way’s gravity until they stop at their farthest point, and then turn around to crash though the center again, another shell structure is created. Simulations that match survey data can be used to calculate how many cycles the dwarf galaxy has endured, and therefore, when the original collision occurred.
The new paper identifies two shell structures in the Virgo Overdensity and two in the Hercules Aquila Cloud region, based on data from the Sloan Digital Sky Survey, the European Space Agency’s Gaia space telescope, and the LAMOST telescope in China. Computer modeling of the shells and the motion of the stars indicates that the dwarf galaxy first passed through the galactic center of the Milky Way 2.7 billion years ago.
Most if not all of those stars appear to be “immigrants,” stars that formed in smaller galaxies that were later pulled into the Milky Way. observes Newberg, an expert on the halo of the Milky Way, a spherical cloud of stars that surrounds the spiral arms of the central disk. “As the smaller galaxies coalesce with the Milky Way,” she says. “their stars are pulled by so-called “tidal forces,” the same kind of differential forces that make tides on Earth, and they eventually form a long cord of stars moving in unison within the halo.”
“There are other galaxies, typically more spherical galaxies, that have a very pronounced shell structure, so you know that these things happen, but we’ve looked in the Milky Way and hadn’t seen really obvious, gigantic shells,” said Thomas Donlon II, a RPI graduate student and first author of the paper, who observed that they were not initially seeking evidence of such an event.
Looking at the Milky Way from the Inside
As the RPI researchers modeled the movement of the Virgo Overdensity, they began to consider a radial merger. “And then we realized that it’s the same type of merger that causes these big shells,” said Donlon. “It just looks different because, for one thing, we’re inside the Milky Way, so we have a different perspective, and also this is a disk galaxy and we don’t have as many examples of shell structures in disk galaxies.”
Older, Younger, or from the Same Event?
The finding poses potential implications for a number of other stellar phenomena, including the Gaia Sausage, says Donlon. “Previous work supported the idea that the Virgo Radial Merger and the Gaia Sausage resulted from the same event; the much lower age estimate for the Virgo Radial Merger means that either the two are different events or the Gaia Sausage is much younger and could not have caused the creation of the thick disk of the Milky Way, as previously claimed. A recently discovered spiral pattern in position and velocity data for stars close to the sun, sometimes called the Gaia Snail, and a proposed event called the Splash, may also be associated with the Virgo Radial Merger.”
The Daily Galaxy, Sam Cabot, via The Milky Way’s Shell Structure Reveals the Time of a Radial Collision and Rensselaer Polytechnic Institute
Image credit: at top of page is he first accurate 3D map of our galaxy that reveals its true shape: warped and twisted. Astronomers from Macquarie University and the Chinese Academy of Sciences used 1339 ‘standard’ stars to map the real shape of our home galaxy in a paper published in Nature Astronomy. Artist’s impression above of the warped and twisted Milky Way disk. (Chen Xiaodian)