The Milky Way still bears signs of collision with a ghostly galaxy

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The Milky Way probably collided with the new dwarf galaxy Antlia 2 less than a billion years ago, according to a new study presented Wednesday at the 234th meeting of the American Astronomical Society.

The research, conducted by Sukanya Chakrabarti of the Rochester Institute of Technology, corroborates a prediction she made about ten years ago about how the Milky Way got a unique "wavy" pattern on her outer disc. Once confirmed, the researchers believe that this discovery will make Antila 2 an ideal natural laboratory for studying the elusive substance called dark matter. The work was submitted for publication at Letters from the Astrophyical Journal; a pre-print is currently available.

Cosmic Whodunit

In 2006, researchers revealed that the Milky Way had a strange series of ripples that seeped into its outer gas disk. In 2009, Chakrabarti published a study analyzing these ripples, showing that a collision between a dwarf galaxy dominated by dark matter and the Milky Way could explain their formation.

To find the culprit, his team first assembled the usual suspects: satellites known to the Milky Way, such as the Magellanic Clouds and the dwarf galaxy of Sagittarius. However, the magellanic clouds are too far away and the sagittal dwarf has too little weight to explain the fingerprints left in our galaxy. This led Chakrabarti to predict that another dwarf galaxy – one that was not found at the time – was responsible for the galactic leak.

Returning to last year, when researchers using the second Gaia satellite data publication (called Gaia DR-2) discovered a weak and previously unknown dwarf galaxy, orbiting the Milky Way: Antlia 2. The galaxy, which is located about 400,000 light-years away and is about as wide as the Great Magellanic Cloud, is also fundamentally invisible. In fact, according to Chakrabarti, it is currently the galaxy with the lowest known surface luminosity (measure of the light of a galaxy by zone of the sky that it covers).

Chakrabarti sought to know if this newly discovered galaxy could truly be the elusive dark matter-dominated dwarf that she had predicted nearly ten years ago. To test her theory, she calculated the past trajectory of Antlia 2 according to her displacement and position. And now, Antlia 2 seems to have crashed into the Milky Way in the past.

"The orbit of Antlia 2, derived from Gaia DR-2 data, brings it into the [about 32,000 light-years] of the galactic center, "said Chakrabarti to astronomy." The outer parts of the Milky Way then show the disturbances [ripples] for about 500 million years.

But how does the true ripple compare to those produced by a simulated collision between our galaxy and Antlia 2? As said Chakrabarti Astronomy, "He's almost dead."

Before Chakrabarti and his team can be certain, they will have to wait for the next batch of data from the Gaia mission, which is working on mapping more than a billion stars of the Milky Way. The new data will allow researchers to test their "manual" prediction, says Chakrabarti, on how the Antlia 2 stars should move. If the movements align with the forecasts, it should convince the case of Antlia 2 to be the cause of the ripples of our galaxy.


This simulation shows the collision between Antlia 2 and the Milky Way, which began 3 billion years ago and continues until today. The right side shows the galaxies with their stars and the left side shows only the gas (Ant 2 has no visible gas). According to the author, on the left you can see that the collision reproduces the ripples observed in the gas disk of the Milky Way "almost flat".

Sukanya Chakrabarti / RIT

Probe dark matter

If their theory is confirmed and Altlia 2 turns out to be the galaxy that disrupted the gas of the Milky Way, it could open the door to using Antlia 2 to help us study dark matter.

"If Antlia 2 is the dwarf galaxy we predicted, you know what its orbit should be. You know that he had to get close to the galactic disk. This imposes rigorous constraints not only on the mass, but also on its density profile [the distribution of matter, both normal and dark, throughout the galaxy]Chakrabarti said in a press release. "That means you could use Antlia 2 as a unique laboratory to learn more about the nature of dark matter."

In other words, by examining the results of its crash in our galaxy, researchers could determine how dark matter is spreading in Antlia 2. And with this information, Chakrabarti said Astronomy"You can now start to differentiate between different models of dark matter." This would certainly bring astronomers closer to understanding the elusive nature of dark matter – one of the biggest unresolved mysteries on the ground today. 39; hui.

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