Stars identified in research have formed “shell structures” after a radial fusion 3 billion years ago. Photo credit: Rensselaer Polytechnic Institute
Evidence of a broadside collision with the dwarf galaxy discovered in Milky Way.
Almost 3 billion years ago, a dwarf galaxy crashed into the center of the Milky Way and was torn apart by the gravitational forces of the collision. Astrophysicists announced today that the merger produced a series of tell-tale clam-like star formations near the Virgo constellation, the first such “clam” structures in the Milky Way. The finding offers further evidence for the old event and new possible explanations for other phenomena in the galaxy.
Astronomers identified an unusually high density of stars known as Virgo Overdensity about two decades ago. Star polls found that some of these stars are moving towards us while others are moving away, which is also unusual as a group of stars would normally travel together. Based on new data, astrophysicists at the Rensselaer Polytechnic Institute suggested in 2019 that the overdensity is the result of radial fusion, the star version of a T-bone crash.
“When we put it together, it was an ‘aha’ moment,” said Heidi Jo Newberg, Rensselaer Professor of Physics, Applied Physics and Astronomy and lead author of the The astrophysical journal Paper on discovery. “This group of stars had a number of different speeds, which was very strange. But now that we see their movement as a whole, we understand why the speeds are different and why they move the way they are. ”
The newly announced shell structures are planes of stars that curve like umbrellas and are left behind when the dwarf galaxy is torn apart, literally jumping up and down through the center of the galaxy as it is built into the Milky Way, an event the researchers have named the “Virgo Radial Merger”. Each time the dwarf galaxy stars quickly pass through the galaxy’s center, when pulled back by the Milky Way’s gravity, they slow down until they stop at their farthest point, and then turn around to tumble through the center again, becoming one other shell structure generated. Simulations that match survey data can be used to calculate how many cycles the dwarf galaxy went through and 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. The computer modeling of the seashells and the movement of the stars shows that the dwarf galaxy first crossed the galactic center of the Milky Way 2.7 billion years ago.
Newberg is an expert on the halo of the Milky Way, a spherical star cloud that surrounds the spiral arms of the central disk. Most, if not all, of these stars appear to be “immigrants,” stars that formed in smaller galaxies and were later drawn into the Milky Way. As the smaller galaxies merge with the Milky Way, their stars are drawn by so-called “tidal forces,” the same differential forces that create tides on Earth, and they eventually form a long line of stars that move together in the halo. Such tidal fusions are quite common and have shaped much of Newberg’s research over the past two decades.
More violent “radial mergers” are considered to be far rarer. Thomas Donlon II, a Rensselaer graduate student and lead author of the work, said they initially did not seek evidence of such an event.
“There are other galaxies, typically more spherical galaxies, that have very distinct shell structures. So you know these things happen, but we’ve looked into the Milky Way and haven’t seen any really obvious, gigantic shells,” said Donlon, the lead author of the paper from 2019, in which the Virgo Radial Merger was proposed for the first time. When they modeled the Virgo overdensity movement, they began to consider radial fusion. “And then we found that it was the same kind of fusion that was causing these big shells. It just looks different because, on the one hand, we are in the Milky Way, so we have a different perspective, and this is also a disk galaxy, and we don’t have that many examples of shell structures in disk galaxies. ”
The finding has potential implications for a number of other stellar phenomena, including the Gaia sausage, a star formation believed to have resulted from the merger of a dwarf galaxy 8 to 11 billion years ago. 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 events are different or the Gaia sausage is much younger and could not have caused the creation of the thick slice of the Milky Way as previously claimed. A recently discovered spiral pattern in position and velocity data for stars near the Sun, sometimes called the Gaia snail, and a proposed event called a splash can also be associated with the Virgo Radial Merger.
“There are many possible links to this finding,” Newberg said. “The Virgo Radial Merger opens the door to a better understanding of other phenomena that we see and do not fully understand, and which may very well have been influenced by something that fell through the galaxy less than 3 billion years ago. ”
Reference: “The shell structure of the Milky Way shows the time of a radial collision” by Thomas Donlon II., Heidi Jo Newberg, Robyn Sanderson and Lawrence M. Widrow, October 20, 2020, The astrophysical journal.
DOI: 10.3847 / 1538-4357 / abb5f6
“The Shell Structure of the Milky Way Galaxy Shows the Time of a Radial Collision” was supported by National Science Foundation grant AST 19-08653. Contributions from the Marvin clan, Babette Josephs and Manit Limlamai; and the 2015 crowd funding campaign to support Milky Way research. Donlon and Newberg were involved in the research by Robyn Sanderson of the University of Pennsylvania and Lawrence M. Widrow of Queen’s University in Ontario. Widrow was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada.
