Supermassive Black Holes: No Crash Required

Contrary to what scientists have thought, colliding galaxies aren't the only way to turn massive black holes in galactic centers into superbright sources of x-rays, a new study suggests. Indeed, they may not even be the main one.

Many, if not all, large galaxies host at their center a black hole millions or even billions of times as massive as our sun. But only a few of those supermassive black holes glow fiercely enough for scientists to call them "active galactic nuclei." Until recently, most astronomers thought active nuclei were switched on when collisions between galaxies sent huge volumes of gas, including intact stars, swirling into the central black hole, says Alexis Finoguenov, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

Now, a survey of hundreds of galaxies by Finoguenov and colleagues, soon to be published in The Astrophysical Journal, hints that intergalactic smashups only occasionally play a role in turning on the active galactic nuclei. In a 5-year study, the team looked at 600 active galaxies in a well-studied patch of sky in the constellation Sextans that is about 10 times the size of the full moon as seen from Earth. Observations with another telescope indicate that the light now reaching Earth left those galaxies as much as 11 billion years ago, the researchers note.

As expected, Finoguenov says, the team found that extremely brilliant active nuclei were rare and that most were only moderately bright at x-ray wavelengths. Surprisingly, however, the vast majority of active galactic nuclei—even the oldest ones, which formed when the universe was smaller and galaxies much closer together than they are today—weren't associated with intergalactic collisions. Instead, the galactic nuclei that were the brightest sources of x-rays were found in massive galaxies that included large amounts of dark matter, the invisible material that adds mass to a galaxy and boosts its gravitational attraction.

The results indicate that a galaxy's central black hole is typically fed by processes inside the galaxy, not by collisions with another similarly large mass of stars, the researchers contend.

The new study "is a nice piece of work," says Mauricio Cisternas, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany. Earlier this year, Cisternas and colleagues used a completely different method to survey a smaller set of galaxies in the same region of the sky—140 galaxies that were as much as 8 billion years old—and reached the same conclusion as Finoguenov's team did.

Processes that switch on immense pulses of x-ray-generating activity in a galactic black hole aren't well understood, Cisternas notes. "The physics are still unknown," he adds.

In some cases, Cisternas suggests, a galaxy may gradually accrete so much mass that the orbits of its stars become unstable, sending a cascade of material into the central black hole and triggering an immense burst of x-rays. In other cases, minor collisions with small, unseen clusters of stars that orbit the main galaxy, just as the Magellanic Clouds orbit our own Milky Way, could lead to activation of the supermassive black holes at the galactic core.