The gravitational forces unleashed during the T-bone collision with the Milky Way are believed to have completely obliterated the dwarf galaxy. Astronomers have now said the collision created shell-like formations of stars near the Virgo constellation. Their findings were described in the October 20 issue of The Astrophysical Journal.
The first pieces of the puzzle were found 20 years ago when astronomers have identified an unusually dense cluster of stars known as the Virgo Overdensity.
Some of these stars were found to be flying towards us, while others were found moving away from us.
This has puzzled the researchers as typically they would expect star clusters to travel in the same direction.
The data has led astrophysicists at the Rensselaer Polytechnic Institute to determine the Virgo Overdensity was the result of a radial merger – the equivalent of a broadside car crash.
Our Milky Way ripped apart a dwarf galaxy some 3 billion years ago, a study has found
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Heidi Jo Newberg, lead author and Rensselaer professor of physics, said: “When we put it together, it was an ‘aha’ moment.
“This group of stars had a whole bunch of different velocities, which was very strange.
“But now we see their motion as a whole, we understand why the velocities are different, and why they are moving the way they are.”
Astronomers have now named the galactic collision the Virgo Radial Merger.
As the dwarf galaxy was torn apart by our Milky Way, it left behind curved planes of stars – the shell structures – that bounced up and down through the centre of the Milky Way.
With each bounce, the dwarf galaxy’s stars would quickly fly through the galactic centre, before being slowed down and pulled back by the Milky Way’s gravity.
Once they reached their farthest point, the stars would pull a u-turn and barrel towards the galactic centre once more, creating another shell structure.
In their study, the astronomers have identified two of these structures in the Virgo Overdensity and another two in the Hercules Aquila Cloud region.
The discoveries were made possible by data collected by the Sloan Digital Sky Survey, the European Space Agency’s (ESA’s) Gaia space telescope and the LAMOST telescope in China.
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Galactic megers and collisions shape the evolution of galaxies
Computer modelling then showed the dwarf galaxy first crashed into the Milky Way some 2.7 billion years ago.
However, the researchers originally assumed the dwarf galaxy had been pulled towards the Milky Way through a so-called “tidal merger”.
The process is a fairly common one but this was not the case with the Virgo Merger.
And the astronomers were not looking for evidence in support of a violent collision.
Thomas Donlon II, a Rensselaer graduate student and first author on the paper, said: “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.”
But as the researchers examined the shell structures, they came to the conclusion they were the result of a somewhat unusual radial merger.
Mr Donlon said: “And then we realized that it’s the same type of merger that causes these big shells.
“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.”
The researchers now believe there is more to learn about other aspects of the Milky Way, such as the so-called Gaia Sausage – the remnant of a dwarf galaxy that merged with the Milky.
Until now, researchers have believed the Gaia Sausage and Virgo Radial Merger were the product of the same event.
But the new evidence suggests the opposite, or that the Gaia Sausage is much younger than thought.
Professor Newberg said: “There are lots of potential tie-ins to this finding.
“The Virgo Radial Merger opens the door to greater understanding of other phenomena that we see and don’t fully understand, and that could very well have been affected by something having fallen right through the middle of the galaxy less than three billion years ago.”