A new study has found no significant link between cosmic collisions and growth of black holes for at least the past eight billion years, meaning other, less dramatic phenomena are responsible.

Until now, the leading theory has been that mergers between galaxies were responsible for stimulating black hole growth by injecting matter into them, making them grow.

The latest study has obtained unexpected new insight into the feeding habits of the giant black holes, which are responsible for the emissions of some of the brightest objects in the universe: active galactic nuclei (AGN).

Black holes form whenever a big enough mass is compressed in to a small enough space. The material becomes so dense that its gravity pulls in everything -- even light. The radiation from AGN is believed to be a result of accretion of mass by the supermassive black hole at the centre of the host galaxy.

In the new study, the astronomers from France, Germany, Italy, Japan, Switzerland and the US compared 140 AGN with a control group of more than 1,200 inactive galaxies.

They found no strong connection between AGN activity and galactic mergers for at least the past eight billion years. Other phenomena such as instabilities within galaxies, collisions of molecular clouds or tidal disruption by other galaxies flying are responsible for black hole growth in at least 75 percent of, cases, if not all of them, astronomers say.

According to the astronomers, if major mergers are an important factor in transporting matter towards galaxies' central black holes, which makes the black hole shine out brightly as an AGN, then distortions -- the tell-tale traces of such mergers -- should be more frequent for active galaxies than inactive ones.

You can usually tell when galaxies have been involved in a merger, says Knud Jahnke from Max-Planck-Institute for Astronomy. Instead of the neat, geometric spiral or smooth elliptical shapes you usually see in Hubble images, colliding galaxies typically look distorted and warped. We planned to find out whether these misshapen galaxies were more likely than regular ones to host active nuclei.

The astronomers are getting ready to address the next question, which is whether there could still be a causal connection between mergers and activity in the more distant past.

The new study will be published in the Astrophysical Journal on January 10.

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