Active galactic nuclei, found at the center of some galaxies, are the brightest objects in the universe. But their origins remain mysterious. Now we know the poetic truth: They come from the cosmic rain from giant gas clouds.


Active galactic nuclei, or AGN, are thought to be the result of matter accumulating around super-massive black holes. However, only a relatively small number of galaxies have AGNs compared to the huge number of galaxies with such black holes at their center. So why do some galaxies have active nuclei while others don't?

Barry McKernan, an astrophysicist at the American Museum of Natural History, thinks he has the answer:

"For a while, people have known that gas clouds are falling onto galaxies, and they've also known that active galactic nuclei are powered by gas falling onto supermassive black holes. But no one put the two ideas together until now and said, 'Hey, maybe one is causing the other!'"


Wandering gas clouds, each millions of times the mass of the sun, travel throughout the universe, and sometimes they find galaxies in their way. When that does happen, McKernan and his colleagues believe these gas clouds provide the galactic centers they encounter the needed energy to create hundreds of stars. This process lights up the center and feeds the super-massive black hole, creating an AGN.

This is explains why only some galaxies have these active nuclei, because only galaxies that recently (in cosmic terms, we're talking in the last ten million years) came into contact with these gas clouds would be lit up in this way. K. E. Saavik Ford, another astrophysicist at the museum, points out the delightful simplicity of this explanation:

"It's interesting that only some galaxies are active, even though we think every galaxy contains a supermassive black hole. The cloud bombardment idea provides an explanation: it's just random luck."

[Astrophysical Journal Letters]