Weird line of galaxies may have been created by a cosmic bullet

A strange line of dwarf galaxies can be the result of a cosmic collision similar to a ball.

Michael Keim at the University of Yale and his colleagues used the Keck Observatory in Hawaii to study a unique track of 12 small dwarf galaxies at around 75 million light years from the Milky Way.

The orientation and speed of the galaxies suggest that they come from a frontal collision between two of them, called NGC 1052-DF2 and NGC 1052-DF4. The collision left the gas in its wake, which finally agitated in groups of stars under gravity.

“They are very unique,” says Keim. “It is the only system like this known.”

There is a similar collection of large galaxies called the group of bullets, so Keim and his colleagues nicknamed this system the “ball dwarf”.

It is believed that the two galaxies would have crushed 350 kilometers per second compared to each other about 9 billion years ago. As they went through each other, the gas was torn from each galaxy. “It is unlikely that two stars collide,” says Keim. “But this is not true for gas clouds.”

Curiously, each of the star tufts left behind the collision is devoid of dark matter. This is very unusual because most galaxies have a large amount of dark matter, sometimes representing more than 90% of their total mass.

Keim and his team think it could be because if the gas has been torn from galaxies, dark matter does not interact with matter – or even itself – so it was not affected.

This could refute alternative ideas for dark matter which suggest that our proofs from its gravitational influence result from a bad transformation of the way the stars and the galaxies behave. “That said, dark matter is a particle, and it can separate from a galaxy,” says Keim.