In 2005, astronomers found a dense clustering of stars in the Virgo constellation. It looked like a star cluster, except that further polls showed that some of the stars are moving towards us and others are moving away. This finding was unexpected and suggested that the stream was not a simple star cluster.
A 2019 study showed that the grouping of stars is not a star cluster at all. Instead, it's the hollowed-out shell of a spherical dwarf galaxy that has merged with the Milky Way. It is known as Virgo Overdensity (VOD) or Virgo Stellar Stream.
A new study involving some of the same researchers shows how and when the fusion occurred and identifies other shells from the same fusion.
The title of the new paper is "The shell structure of the Milky Way shows the time of a radial collision". The first author is Thomas Donlon II, a Rensselaer student who was also the first author on the 2019 study. The paper is published in the Astrophysical Journal.
In their work, the authors write: "In this work we identify the shell substructure in the Milky Way for the first time and argue that these shells are actually connected to the VRM and thus to a radial fusion event."
There are different types of mergers, and according to the 2019 paper, the merger that created the VOD was what is known as a radial merger. These are violent types of fusion that are described in a press release as "the standout 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 Astrophysical Journal 2019, which detailed the 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 new paper builds on this work and reveals even more details. When the dwarf galaxy collided with the Milky Way, it left curved planes of stars jumping through the galactic center. You named the event Virgo Radial Merger (VRM).
Every time the dwarf galaxy collides with the galactic center, it runs out on the other side and then is withdrawn to the center. Every time it reaches the farthest point, it leaves behind some of its stars and forms the shells. Using the observation data, the team created simulations and calculated how often the dwarf galaxy jumped back and forth and when it first merged with the Milky Way.
The research team used data from multiple sources. Using data from the Sloan Digital Sky Survey, ESA's Gaia mission, and the LAMOST telescope in China, they found two shell structures in the VOD and two more in the Hercules Aquila cloud region. Their computer models showed that the merger began when the dwarf galaxy crossed the center of the Milky Way 2.7 billion years ago.
Combined data and images show the position of the four shell structures in the Milky Way. Photo credit: Rennselaer University.
Galaxy fusions are not uncommon. Giant galaxies like the Milky Way became large by merging with much smaller galaxies. Currently, the Milky Way is in the midst of two mergers. It is currently merging with the spherical galaxy of the Sagittarius Dwarf and with the small and large Magellanic Clouds.
All of these mergers have left their mark on the Milky Way. The halo of our galaxy is a spherical region in the shape of a sphere that surrounds the spiral arms of the Milky Way. Most of these stars are not "native" to the galaxy, but rather "immigrants" from other galaxies that have merged with the Milky Way.
The Milky Way's elegant, stately appearance contradicts the history of sometimes violent mergers with other galaxies. Photo credit: NASA.
Over time, the tidal forces of the galaxy shape these immigrants into long streams of stars. These currents move together through the halo. Astronomers call these tidal fusions, and they are the focus of much research.
But this merger was different. Radial fusions like this one are much more violent, and the dwarf galaxy can lash back and forth multiple times, leaving those shell shapes behind.
This picture from the study shows both the VOD (left) and the Hercules Aquila Cloud regions. To better identify the shell-shaped structures, the authors mapped the positions of RRLs and BHB stars. RRLs are RR Lyrae stars, variable stars that are used to determine distance. BHBs are blue horizontal branch stars and are extremely hot. They are used as tracers in astronomy and their distances are determined spectroscopically. Photo credit: Donlon II et al., 2020.
In a press release, lead author Thomas Donlon II stated that the team was not actively looking for evidence of any of these radial fusions.
"There are other galaxies, typically more spherical galaxies, that have very distinct shell structures. So you know these things are happening, but we've looked into the Milky Way and haven't seen any really obvious gigantic shells," Donlon said. He was also the lead author of the 2019 paper that first proposed the Virgo Radial Merger.
As the team worked on their study, the whip motion of the stars in the VOD became clearer in their modeling. Then they had to consider radial merger as the cause. "And then we found that it was the same kind of fusion that caused these big grenades," said Donlon II. "It just looks different because, on the one hand, we're in the Milky Way, so we have a different perspective, and this is also a disk galaxy, and we do not have that many examples of shell structures in disk galaxies. ”
This finding also sheds new light on several other aspects of the Milky Way's morphology, including the Gaia sausage. The Gaia sausage is the remnant of another dwarf galaxy that has merged with the Milky Way. This merger took place 8 to 10 billion years ago and added eight globular clusters and about 50 billion solar masses of stars, gas and dark matter to the Milky Way. It has the characteristic shape of a sausage due to the orbits of the stars.
This is a Hubble space telescope image of the globular cluster NGC 2808. It could be the old core of the Gaia sausage. The Sausage Itself Photo Credit: From NASA, ESA, A. Sarajedini (University of Florida) and G. Piotto (University of Padua (Padua)) – http://hubblesite.org/newscenter/archive/releases/2007/2007 / 18 / image / a / (direct link), public domain, https://commons.wikimedia.org/w/index.php?curid=2371715
Prior to this work, astronomers tended to believe that the Virgo Radial Merger and the Gaia Sausage were results of the same event. But now there's a much more recent estimate for the VRM, and the two are understood as separate events. Unless they are separate events, the time estimate for the Gaia sausage must be younger, which means that the sausage cannot be responsible for the fact that the slice of the Milky Way is so thick, which is one of the results attributed to it become the oldest estimate for the Gaia sausage event.
If you look at the distribution of stellar velocities in the Milky Way, the stars of the sausage galaxy form a characteristic sausage-like shape. This unique shape is caused by the strong radial movements of the stars. Since the Sun is at the center of this huge stellar cloud, the distribution does not include the decelerated stars that are currently making an U-turn towards the center of the galaxy. Photo credit: Myeong et al., 2018.
This work may also shed new light on other parts of the Milky Way galaxy. The Gaia snail is a spiraling group of stars near the Sun that may be connected to the VRM, and another event called Splash could be too. The splash is a substructure in the Milky Way near the sun. It has a large population of metal-rich stars that move in strongly radial orbits in the inner atrium. There are many questions about the origin of the Splash, but this study shows that the VRM may have caused it and other older mergers are not required to explain it.
"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. ”