Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Supermassive black holes lurk inside mature galaxies, but their presence in the very early universe has been tricky to explain (Image: Image: NASA, ESA, NRAO and L. Frattare (STScI); Science: NASA/CXC/IoA/A.Fabian et al.; Radio: NRAO/VLA/G. Taylor; Optical: NASA, ESA, the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, and A. Fabian (Institute of Astronomy, University of Cambridge, UK)

Object: Moody little galaxy

Fate: The most energetic explosion in the universe

Darkness was all the young galaxy had ever known. For a time it ran with a hot-headed crowd, skulking around the early universe gathering up gas and dark matter. Then things started to change. The other galaxies cooled off and settled down, giving birth to glittering stars that blew away the fog and the gloom.

But try as it might, the moody galaxy couldn’t shake the darkness in its core. It seethed in the glow of its former friends, growing hotter and hotter – until one day, it exploded.


Such brooding protogalaxies may explain how supermassive black holes were created in the early universe, where they seeded the mature galaxies we see today. If the idea holds true, we should be able to see the unusual explosions with the next generation of telescopes.

Black holes are born when very massive stars explode and collapse into ultra-dense remnants. Stellar black holes can be up to about 10 times the mass of the sun. But we also see supermassive black holes millions to billions of times the mass of the sun at the cores of most mature galaxies.

Astronomers think that black holes grow by merging with other black holes. When two galaxies collide, their central black holes pool in the middle, making a new galaxy with an even weightier black hole at its core. However, some galaxies managed to host unusually massive black holes just a few hundred million years after the big bang. That’s not enough time for growth via mergers, leaving astronomers with a supermassive riddle.

Stripped of coolant

One possibility is that most of the mass in some protogalaxies collapsed into monster black holes, which could then merge to give rise to the earliest supermassive versions at the hearts of galaxies, says Daniel Whalen of the Los Alamos National Laboratory in New Mexico. Most early galaxies started out as clouds of atomic hydrogen that were too hot to form stars. They eventually cooled down and began forming molecular hydrogen, which helped them chill even faster. In these galaxies, dense packets of cool gas were able to collapse and ignite new stars.

However, some nearby protogalaxies were then bathed in strong ultraviolet radiation generated by the newborn stars, and this stripped away their molecular hydrogen. Without any coolant, these protogalaxies couldn’t make stars and began to heat up instead. “The gas just gets hotter and hotter and can’t collapse any further,” says Whalen. He and colleagues ran computer simulations that show hot protogalaxies can grow to be 100 million times the mass of the sun without forming a single star.

When a protogalaxy reaches that mass, gas falling in from intergalactic space gets so hot that the hydrogen atoms collide violently, moving their electrons from their lowest energy levels to the next highest rungs on the atomic energy ladder. When these electrons return to their original state, they emit a photon that carries energy away. In other words, the protogalaxy’s gas finally has a way to cool down.

Galactic seeds

The models show that the protogalaxy cools so quickly that within a few million years, its gas coalesces to form a single ball of gas 100,000 to a million times the mass of the sun.

In a separate study, researchers from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, suggested that such huge clouds of gas in the early universe might collapse directly into obese black holes millions of times the mass of the sun.

But Whalen’s simulations showed that most of the time these balls collapse into black holes only a few tens of thousands of times the mass of the sun – impressive, but hardly supermassive. In some cases, his model shows the gas will form a supermassive star that spends its short life perched at the edge of stability. “If these stars do form, they can blow up as very energetic supernova,” says Whalen. “It will be the most energetic explosion in the universe.” The remnants of such a supernova would then fall back under the gravitational grip of the galaxy’s dark matter. Most of it would collapse to form a black hole.

“And voila, you have a black hole with masses anywhere from 10,000 to a million solar masses,” says Whalen. “They’re the seeds of the supermassive black holes we see later.”

The black hole’s birth would also trigger a burst of star formation in the remaining galactic debris – the seeds of a new galaxy. The team’s simulations suggest that the supermassive supernovae, and the subsequent starbursts, will have unique radiation signatures that future observatories, such as the James Webb Space Telescope, should be able to spot.

Journal reference: arxiv.org/abs/1305.6966, submitted to The Astrophysical Journal