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

A strange line of dwarf galaxies may have been the result of a bullet-like cosmic collision.

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

The orientation and speed of the galaxies suggest they originated from a head-on collision between two of them, called NGC 1052-DF2 and NGC 1052-DF4. The collision left gas in its wake, which eventually clumped into groups of stars under gravity.

“They’re very unique,” says Keim. “It’s the only system like this that’s known.”

There is a similar collection of larger galaxies called the Bullet Cluster, so Keim and his colleagues have nicknamed this system the “bullet dwarf”.

The two galaxies are thought to have crashed into each other at 350 kilometres per second relative to each other about 9 billion years ago. As they passed through one another, gas was ripped from each galaxy. “It’s unlikely that two stars will collide,” says Keim. “But that’s not true for clouds of gas.”

Curiously, each of the clumps of stars left behind from the collision is devoid of dark matter. This is very unusual as most galaxies have a large amount of dark matter, sometimes accounting for more than 90 per cent of their total mass.

Keim and his team think this might be because while the gas was torn from the galaxies, dark matter does not interact with matter – or even itself – so it was unaffected.

That could refute alternative ideas for dark matter that suggest our evidence for its gravitational influence result from a mismeasurement of how stars and galaxies behave. “This is saying dark matter is a particle, and it can become separated from a galaxy,” says Keim.