The Milky Way has battle scars that collide with a ghostly galaxy



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Our Milky Way, illustrated here in the concept of an artist, presents strange "ripples" in its peripheral regions. New research indicates that these ripples were caused by a collision with a dwarf galaxy called Antlia 2. (Source: ESA)

The Milky Way probably collided with a newly discovered dwarf galaxy called 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 ten years ago about how the Milky Way created a unique "wavy" pattern on her external drive. 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.

Looking back at 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 periphery of 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 in an interview." 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? "It's almost over," says Chakrabarti.

But before being certain, she and her team will have to wait for the next batch of data from the Gaia mission of the European Space Agency, which is trying to map more than a billion stars in the Milky Way. The new data will allow researchers to test their prediction "on the fly," says Chakrabarti, about how the stars of Antlia 2 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.

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