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Cosmic collisions not guilty of powering active galaxies
Posted: 06 January 2011

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Using data from the Cosmological Evolution Survey (COSMOS), astronomers have found that collisions and merging events are not necessarily responsible for the violent black hole activity observed in the centres of some galaxies.

Until recently, the leading theory for the sustenance of active galactic nuclei (AGN) – the black holes in the hearts of galaxies that suck in matter from its surrounds, heating it up to shine brightly – was collisions between galaxies. While collisions and merger events may account for a small percentage of AGN, the new COSMOS study finds no significant link, meaning other phenomena such as local instabilities within galaxies, interaction with molecular clouds, or tidal disruption by passing galaxies must play a key role instead.

Analysing 1,400 galaxies from the COSMOS survey found that galaxies with active nuclei (left) and those with inactive nuclei (right) showed no statistically significant difference in the proportion that had undergone merging events. Image: NASA, ESA, M. Cisternas (Max-Planck Institute for Astronomy).

The study, lead by Mauricio Cisternas from Germany's Max Planck Institute for Astronomy and discussed in the 10 January issue of the Astrophysical Journal, uses around 1,400 galaxies from the COSMOS survey, which hosts information on some two million galaxies provided by major space- and ground-based telescopes. For each of the 140 documented active galaxies selected for the study, the team also identified a control group of nine inactive galaxies located at roughly the same distances and therefore at the same stage of cosmic evolution, from the same Hubble Space Telescopes images, to test the relationship between AGN and mergers.

Mergers are usually easy to identify because they look distorted compared with a typically neat spiral or smooth elliptical galaxy, but the process can be subjective, so the team set up a blind test whereby the bright spots of the AGN were removed from images and given to ten galaxy experts based at eight different institutions to independently assess whether each galaxy was distorted, without knowing which had AGN. The exercise did not reveal any statistically significant correlation between distortion in the galaxy and activity in the core.

The result of the survey shows that galaxy mergers are not the most dominant mechanism for feeding black holes, with at least 75 percent of AGN activity over the last eight billion years relying on other processes. This supports a recent SWIFT study that found AGN present in just a quarter of galaxy collisions (read more about that study here). One alternative theory for AGN mooted by astronomers earlier this year suggests that giant clouds of hydrogen gas, millions of times more massive than the Sun falling into the centres of galaxies, likely feed some black holes to generate AGN (see our report here for more information).

"We cannot say exactly what method, among the many alternative ones, triggers the activity of a given AGN," Cisternas tells Astronomy Now. "Collisions with gas clouds offers another interesting potential mechanism to deliver relevant amounts of gas, but we cannot assure that it should account for a given fraction of the activity. We would need to know in detail the physics that govern the central regions next to the black hole to be able to make predictions of its efficiency."

Other methods of transporting material into the central black hole include instabilities within a spiral galaxy's bar, or the movement of one galaxy past another that does not lead to a merger. The team hope that data from ongoing Hubble Space Telescope projects, as well as from the forthcoming James Webb Space Telescope, will provide the missing details.

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