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Mergers not to blame for awakening black holes
Posted: 13 July 2011

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Despite being crammed with galaxies in the early Universe, combined data from ESO’s Very Large Telescope and ESA’s XMM-Newton X-ray space observatory has revealed that black holes in the centres of galaxies are not activated by mergers between these giant structures, as had been previously suspected.

Most, if not all galaxies contain a supermassive black hole which are millions, or sometimes billions of times greater in mass than our own Sun. Some of these black holes, such as the one in the Milky Way galaxy, lie sleeping at the heart of these stellar collections whereas in others, particularly in the early Universe, the central monster will wreak havoc, feeding on material that happens to fall into its clutches, and belching out intense radiation.

A very deep image showing the COSMOS field imaged by the Canada France Hawaii Telescope (CFHT). Large numbers of very faint galaxies are visible and some of the active galaxies with supermassive black holes at their centres, that were used in the new study, are marked with red crosses in this image. Image: CFHT/IAP/Terapix/CNRS/ESO.

One unsolved mystery that has left astronomers puzzled is where the material comes from to disturb a black hole from its slumber, triggering angry outbursts as it is awoken and becoming an active galactic nucleus. Up until this point, experts believed that these active nuclei were irritated when two galaxies merge, or even when they merely pass close to each other, disrupting the material which becomes fuel for the supermassive black hole. However, new results presented by a collaboration between the Max-Planck-Institut fur Plasmaphysik and Extraterrestrische Physik, Germany, and the Cosmological Evolution Survey (COSMOS), have blown this idea out of the water.

The science teams have studied more than 600 of these active galaxies in a patch of sky named the COSMOS field and, as expected, the collaboration discovered that extremely brilliant active nuclei were rare, whereas the bulk of active galaxies in the past 11 billion years were only moderately bright. This led to a surprise that the scientists were not bargaining on: the majority of these easy-to-find, dimmer active galaxies were not triggered by mergers between galaxies, even when looking back far into the past. This new finding will appear in The Astrophysical Journal.

X-rays, which were detected by ESA’s XMM-Newton space observatory, are emitted from around the black hole making the monster’s presence known to the astronomers. The galaxies were also subsequently observed using ESO’s Very Large Telescope, which was able to measure the distances to the galaxies. When the data was combined, the observations allowed the team to produce a three-dimensional map showing where the active galaxies rest.

“It took more than five years, but we were able to provide one of the largest and most complete inventories of active galaxies in the X-ray sky,” says Marcella Brusa, of the Max-Planck Institute and who is one of the authors of the study.

An artist’s conception illustrating a supermassive black hole at the core of a young, star-rich galaxy. Image: NASA/JPL-Caltech.

With the new map, the astronomers were able to find out how active galaxies were distributed, comparing their findings with predications from theory as well as investigating how the distribution changed as the Universe aged from around 11 billion years to almost the present day. “Even in the distant past, up to almost 11 billion years ago, galaxy collisions can account for a small percentage of the moderately bright active galaxies,” says Alexis Finoguenov of the Max-Planck-Institut fur Extraterrestrische Physik and who also supervised the work. “At that time galaxies were closer so mergers were expected to be more frequent than in the more recent past, so the new results are all the more surprising.”

Indeed, the team had uncovered that active galactic nuclei are mostly residing in large massive galaxies with a notable amount of mysterious dark matter. If most active nuclei were a consequence of mergers and collisions between galaxies, it was expected that they would be found in moderate masses of around a trillion times the mass of the Sun. However, the results presented an inconsistency that meant that most active nuclei reside in galaxies with masses around 20 times larger than predicted by the so-called merger theory.

“New theories need to be brought up to explain the ability of the gas to penetrate towards the black hole,” says Finoguenov. “One popular suggestion is based on the Toomre stability criterium. High gas surface density in massive spiral galaxies – a factor of 20 more massive than our Milky Way – can result in perturbations that drive angular momentum out and the mass in, fueling the black hole.”

“These new results give us a new insight into how supermassive black holes start their meals,” adds Viola Allevato, also at the Max-Planck-Institut fur Plasmaphysik and who is also the lead author of the new paper. “They indicate that black holes are usually fed by processes within the galaxy, such as disc instabilities and starbursts, as opposed to galaxy collisions.”