Did an insurgent dwarf galaxy create these colossal, high-energy bubbles (shown in false colour)? (Image: NASA/GSFC)

A tiny galaxy that collided with the Milky Way spawned two huge bubbles of high-energy particles that now tower over the centre of our galaxy. This new model for the birth of the mysterious bubbles also explains discrepancies in the ages of stars at the galactic middle.

In 2010, sky maps made by NASA’s Fermi Gamma-ray Space Telescope revealed two lobes of particles billowing out from the heart of the Milky Way, each one stretching 25,000 light years beyond the galactic plane.

Astronomers suspected the bubbles were inflated by a period of violence in the galactic centre about 10 million years ago, but no one could say what had triggered the outburst.


Earlier this year, Kelly Holley-Bockelmann from Vanderbilt University in Nashville, Tennessee, was discussing the problem with Tamara Bogdanović from the Georgia Institute of Technology in Atlanta.

“We pieced together all the evidence and realised they could be explained by a single catastrophic event – the collision between two black holes,” recalls Holley-Bockelmann.

Tango and crash

We know that a supermassive black hole weighing as much as 4 million suns lurks at the core of the Milky Way. We also have an array of dwarf galaxies orbiting our much larger spiral galaxy, as well as hints that past satellite dwarfs have collided with us.

According to the new theory, a small galaxy with its own central black hole dove into the Milky Way and began spiralling through our galaxy. After billions of years, the stripped-down dwarf’s black hole made it to the galactic centre.

The two black holes then performed a tight gravitational tango before finally merging. This final act produced violent forces that flung out many of the stars that were born in the Milky Way’s middle, explaining why astronomers now find far fewer old stars there than they have every right to expect.

The whirling black holes also disrupted giant clouds of gas, some of which got squeezed so much that they collapsed to form clusters of bright new stars. Much of the rest of the gas swirled into the merged black holes, getting so hot from compression that it radiated huge amounts of energy.

“We think it’s both the energy from this ‘burp’ near the black hole and the winds of gas from the starburst that inflated the Fermi bubbles,” says Holley-Bockelmann.

Round up the runaways

“This hypothesis is probably worth considering,” says Mark Morris, an authority on the galactic centre at the University of California, Los Angeles. But he cautions against making it a leading explanation without more evidence.

Holley-Bockelmann and colleagues think that the colliding dwarf galaxy was formed early in the history of the universe and consisted mainly of dark matter and the central black hole, without many stars. That would account for why we see no tell-tale trail of stars left behind as the dwarf galaxy fragmented.

Instead, the team proposes another test of their model: hunting for the old stars catapulted outwards during the black hole merger.

“It should have carved nearly 1000 stars out of the galactic centre,” says Bogdanović. “These stars should still be racing through space, about 10,000 light years from their original orbits.” The team is now searching for these runaway stars in data amassed by the Sloan Digital Sky Survey, which has observed the properties of hundreds of millions of stars in our galaxy.

Journal reference: Monthly Notices of the Royal Astronomical Society, doi.org/kqh