INQUISITION NEWS: Galactic Smashups Leave Giant Black Holes Hungry

Galactic Smashups Leave Giant Black Holes Hungry

By Lisa Grossman

Colliding galaxies don’t shake up enough food to feed enormous black holes lurking in their centers. A new study suggests that less violent events, like gravitational disturbances within the galaxy, are probably black holes’ primary source of fuel.

Almost every galaxy is centered on a supermassive black hole hundreds of millions of times more massive than the sun. Some of them, like the black hole at the center of the Milky Way, are relatively docile. But others draw a huge, hot disk of gas that glows white-hot just before it vanishes forever. These disks, called active galactic nuclei, can outshine the rest of the host galaxy.

Astronomers have suspected for decades that these ravenous black holes get their fuel from major mergers between two large galaxies. In all the chaos of a galactic pile-up, plenty of gas should get funneled toward the center and gobbled up, astronomers reasoned. Simulations have found that black holes and galaxies grow together. Some observations even suggested that galaxies with active galactic nuclei were slightly distorted, a sign of a recent collision.

“It’s totally intuitive,” said astrophysicist Knud Jahnke of the Max-Planck Institute for Astrophysics in Germany, a coauthor of the new study. “But it was a gut-feeling idea. In court you would say there was some circumstantial evidence for it, but no proof.”

Earlier studies looked only at galaxies with the brightest active nuclei, which could have biased their results, Jahnke said. They also didn’t compare active galaxies to those with quiet black holes.

In a paper to appear in the Jan. 10 Astrophysical Journal, Jahnke and colleagues selected galaxies from the COSMOS dataset, the largest continuous galaxy survey the Hubble Space Telescope has ever completed. The survey covered an area of the sky 10 times the size of the full moon, and found hundreds of thousands of galaxies.

The team chose 140 active galactic nuclei, or AGNs, by selecting galaxies that emit a lot of X-rays, which can pierce the galactic gas and dust that might otherwise block the nuclei from view. They then chose more than 1,200 inactive galaxies at the same distance and brightness levels to ensure all images were of the same quality.

Next, the astronomers checked the galaxies for the subtle signs of a recent galactic collision, such as a warped disk or a trailing tail of stars. Jahnke and nine other astronomers examined each galaxy by eye, a low-tech but reliable method of picking out galaxy shapes.

“There’s no magic algorithm that will tell you if a galaxy is merging,” said Mauricio Cisternas, a graduate student at the Max-Planck Institute and lead author of the paper. “The human brain is much better at these things than any algorithm you could write.”

The human galaxy sorters didn’t always agree on which galaxies were merger survivors, but they all found merging galaxies are no more likely to aggressively feed black holes at their hearts than ordinary galaxies. At least 75 percent of the active galaxies get their fuel somewhere else.

“We don’t observe more mergers in the AGN host galaxies,” Cisternas said. “From there it’s straightforward to infer that mergers are not triggering AGNs, and are not responsible for black-hole fueling.”

Instead, black holes could be fed by smaller mergers, like a large galaxy gobbing a smaller one. Or perhaps a series of gravitational disturbances, which Jahnke calls an “angular momentum transport chain,” could ferry fuel to the black hole over great distances.

“This is a significant step forward,” said galaxy astronomer Romeel Davé of the University of Arizona, who was not involved in the new study. “The fact that we can do this now is unique and new. This represents a significant addition to the literature for sure.”

There are two caveats, though. One is that black holes could start feeding long after a merger, when the galaxy’s shape has smoothed out. Cisternas and Jahnke said that’s unlikely, however, because so many of their galaxies had prominent spiral shapes, and mergers tend to destroy a galaxy’s spiral forever. But Davé is not so sure.

“Just because one sees spiral structure does not mean that you don’t have merger activity,” he said.

The second is that the new study used galaxies whose light is 8 billion years old when it reaches Earth. Galaxies farther away than that are too fuzzy to see, but black holes were growing fastest about 10 billion years ago. That’s when mergers may still have been an important source of food, Davé said.

An ongoing survey with Hubble’s new Wide-Field Camera 3 could see further and resolve the question.

“That sort of information will be coming down the line fairly soon,” Davé said.

Image: A sample of the galaxies studied; sorted based on whether they show no signs of a recent merger (top row), minor signs (middle) or major disruptions (bottom). The black spot in the center of each galaxy ensured that the test was blind. The team blocked out the bright nuclei from active galaxies and added a spot to galaxies with dim centers, so the sorters couldn’t tell which was which. Credit: NASA, ESA, M. Cisternas (Max-Planck Institute for Astronomy)