At the heart of most galaxies lies a supermassive black hole. And in some galaxies, the black hole is bigger and badder than usual. These raging overachievers, called active galactic nuclei, can be some of the brightest objects in space, sweeping up a huge amount of material from their local areas and emitting enough energy to outshine the galaxies around them. The question is, where do they get all the stuff to swallow? Not where scientists had expected, according to a new study.
An obvious answer—and the one that for years has seemed likeliest—is that these hyperactive black holes arise from the merger of galaxies. All the gas that comes together during a two-galaxy crash could feed a supermassive black hole, turning it from docile to brilliant. But there’s a problem.
“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. [Wired]
For a study coming out in the Astrophysical Journal, Jahnke and others tried to put the galaxy merger hypotheses through a true controlled test, and they found no solid evidence to back it up.
To get around the bias in some previous observations and studies, the team first needed to observe a large swath of the sky.
The new study relies on a single, massive survey called COSMOS. The same area of the sky was imaged by a number of space-based instruments (Chandra, GALEX, Hubble, Spitzer, and XMM-Newton) as well as a set of ground-based observatories. The authors use X-ray data (primarily from XMM-Newton) to identify active galactic nuclei in the field. They then match them to visible data on the galaxies taken by the Hubble’s Advanced Camera for Surveys. This helped provide their experimental population. [Ars Technica]
Those telescopes provided the team with 1,264 galaxies, including 140 with active nuclei. The task then became to compare those results to the shapes of the galaxies, to find out whether the ones with active nuclei truly matched up with the ones that went through mergers. (Galaxies that have undergone mergers may have tails or weird warped shapes.) That meant getting a little creative.
Recognizing galaxy shapes is the task for seasoned astronomers with an experienced eye; it’s sort of like CAPTCHA in that humans are better at recognition than computers. But the team didn’t want a galaxy’s brightness to effect the interpretation—they worried that astronomers would see a galaxy with a bright active nuclei and think, “must be a merger.” So when the researchers brought in 10 astronomers to interpret the galaxies’ shapes, they blacked out the bright spots in galaxies with active nuclei.
None of the experts’ findings established a significant link between a galaxy’s activity and its involvement in a major merger. The researchers concluded that the cause of at least three-quarters and possibly all of active galactic nucleus activity over the last 8 billion years must have a different explanation. “We do not rule out that mergers actually might cause AGN activity in some cases,” Jahnke told SPACE.com. “But they do not dominate the buildup of black hole mass over the last 8 billion years.” [MSNBC]
This line of questioning may even grow more complicated as scientists figure out how to look further back in time. Jahnke’s team picked out a population of galaxies that were no more than 8 billion years old, but he says that if it’s possible to study older ones—say, those that date from 10 billion years ago or so—results could be different because many more black holes formed at that time.
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