In 2005 astronomers found a dense grouping of stars within the Virgo constellation. It sounded like a star cluster, except further surveys showed that a number of the celebs are moving towards us, and a few are moving away. That finding was unexpected and suggested the stream was no simple star cluster.
A 2019 study showed that the grouping of stars is not any star cluster at all; instead, it is the hollowed-out shell of a dwarf spheroidal galaxy that merged with the Milky Way Galaxy. It's called the Virgo Overdensity (VOD) or the Virgo Stellar Stream.
A new study involving a number of identical researchers shows how and when the merger occurred and identifies other shells from the identical merger.
The new paper's title is 'The Milky Way's Shell Structure Reveals the Time of a Radial Collision'. the primary author is Thomas Donlon II, a Rensselaer graduate student, who was also the primary author of the 2019 study. The paper is published in the Astrophysical Journal.
In their paper, the authors write "In this work, we identify shell substructure within the extragalactic nebula for the primary time, and that we argue that these shells are indeed related to the VRM and thus a radial merger event."
There are different types of mergers, and per the 2019 paper, the merger that created the VOD was what's referred to as a radial merger. These are violent merger types that a handout describes as a "stellar version of a T-bone crash".
"When we put it together, it absolutely was an 'aha' moment," said Heidi Jo Newberg, Rensselaer professor of physics, applied physics, and astronomy, and lead author of The Astrophysical Journal 2019 paper detailing the invention.
"This group of stars had an entire bunch of various velocities, which was very strange. But now that we see their motion as an entire, we understand why the velocities are different, and why they're moving the way that they're."
The new paper builds thereon work and divulges even more detail. because the dwarf galaxy collided with the Milky Way System, it left behind curved planes of stars that sort of bounce through the galactic center. They've called the event the Virgo Radial Merger (VRM).
Each time the dwarf galaxy collides with the galactic center, it barrels out the opposite side, only to be drawn back toward the middle. when it reaches the furthest point, it leaves behind a number of its stars, forming the shells.
The team created simulations using the observational data and calculated what number of times the dwarf galaxy has bounced back and forth and when it first merged with the extragalactic nebula.
The team of researchers used data from multiple sources. Using data from the Sloan Digital Sky Survey, the ESA's Gaia mission, and therefore the LAMOST telescope in China, they found two shell structures within the VOD and two more within the Hercules Aquila Cloud region.
Their computer modeling showed that the merger began when the dwarf galaxy first responded to the Milky Way's center 2.7 billion years ago.
(Rennselaer University)
Galaxy mergers don't seem to be rare. Huge galaxies just like the Milky Way System grew large by merging with much smaller galaxies. Currently, the galaxy is within the middle of two mergers. It's within the process of merging with the Sagittarius Dwarf Spheroidal Galaxy and with both the little and enormous Magellanic Clouds.
All these mergers have left their mark on the extragalactic nebula. Our galaxy's halo may be a region of stars in an exceedingly spherical shape surrounding the Milky Way's spiral arms.
The majority of these stars aren't "native" to the galaxy, but instead are "immigrants" from other galaxies that merged with the Milky Way Galaxy.
Over time, the tidal forces of the galaxy shape those immigrants into long streams of stars. Those streams move collectively with one another through the halo. Astronomers call these tidal mergers, and they are the main focus of much research.
But this merger was different. Radial mergers like this one are way more violent, and therefore the dwarf galaxy can whipsaw back and forth multiple times, leaving these shell shapes.
In a release, first author Thomas Donlon II explained that the team wasn't actively trying to find evidence of 1 of those radial mergers.
"There are other galaxies, typically more spherical galaxies, that have an awfully pronounced shell structure, so you recognize that this stuff happens, but we've looked within the Milky Way and hadn't seen really obvious gigantic shells," said Donlon, who was also the lead author on the 2019 paper that first proposed the Virgo Radial Merger.
As the team worked on their study, the whip-saw motion of the celebrities within the VOD became clearer in their modeling. That's after they had to contemplate a radial merger because of the cause.
The VOD (left) and the Hercules Aquila Cloud region. (Donlon II et al., The Astrophysical Journal, 2020)
"And then we realized that it is the same sort of merger that causes these big shells," said Donlon II.
"It just looks different because, for one thing, we're inside the Milky Way Galaxy, so we have a distinct perspective, and also this can be a disk galaxy, and that we haven't got as many samples of shell structures in disk galaxies."
This finding is additionally shedding new light on other aspects of the Milky Way's morphology, including the Gaia Sausage. The Gaia Sausage is that the remains of another dwarf galaxy that merged with the extragalactic nebula.
That merger happened between 8 and 10 billion years ago and added eight globular clusters and about 50 billion solar masses of stars, gas, and substance to the Milky Way. it's the characteristic shape of sausage because of the orbits of the celebrities.
Prior to this work, astronomers attended to think that the Virgo Radial Merger and therefore the Gaia Sausage were outcomes of the identical event. But now there is a much younger estimate for the VRM, and also the two are understood to be separate events.
If they don't seem to be separate events, then the time estimate for the Gaia Sausage needs to be younger, which suggests that the Sausage cannot be to blame for causing the Milky Way's disk to be so thick, which is one of the results attributed to the more ancient estimate for the Gaia Sausage event.
This work is potentially shedding some new light on other parts of the extragalactic nebula, too. The Gaia Snail could be a spiral-shaped group of stars near the Sun which will be connected with the VRM, and another event called the Splash may well be, too.
The Splash may be a substructure within the Milky Way's disk near the Sun. it's an outsized population of metal-rich stars moving through highly radial orbits within the inner halo. There are many questions around the origin of the Splash, but this study shows that the VRM could have caused it, and other more ancient mergers aren't needed to elucidate it.
"There are plenty of potential tie-ins to the present finding," Newberg said.
"The Virgo Radial Merger opens the door to a greater understanding of other phenomena that we see and do not fully understand, which could o.k. are stricken by something having fallen all the way through the center of the galaxy but 3 billion years ago."
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