Smashing display (Image: Hubble Legacy Archive, ESA, NASA; Processing - Martin Pugh)

Looks like something left the lights on. The supermassive black holes at the cores of some galaxies give off a tremendous amount of light. It comes from superheated discs of gas that slowly spiral into the maws of the black holes, but where all that gas came from has been debated for decades. Now, observations reveal tell-tale signs of debris that suggest ancient galactic crashes flipped the light switch in the brightest of these “active galaxies”.

One of the main hypotheses is that the gas could be funnelled towards black holes when colliding galaxies send vast amounts of it toward their shared centre as they merge. But an alternate theory has gained traction recently, especially to explain fainter galaxies. This theory holds that the structure of galaxies becomes unstable over time and causes a self-implosion.

To test which process is more frequent, Jueun Hong and Myungshin Im of Seoul National University in South Korea imaged 39 of these bright active galaxies, with telescopes in Chile, Texas and Uzbekistan. They spent up to four hours on each galaxy to capture fine, faint details. They inspected the images for signs of a past or ongoing merger – wispy trails of stars flung violently into space during an intergalactic pile-up.


Gassy galaxies

The team found that 17 of the galaxies had such features, which was between four and eight times more than they saw in a sample of images of non-active galaxies.

Im says the results show that although both mechanisms could trigger active galaxies, mergers are the main cause for the brightest ones. “You have to have a great amount of gas supply to fuel them,” he says.

The study is “a new piece of the puzzle”, says Nico Cappelluti of the National Institute of Astrophysics in Bologna, Italy. In 2011, Cappelluti and his colleagues published a study of fainter, more distant active galaxies, and found no signs of collisions. “In this paper they study much brighter sources than ours and in this case, the merger scenario works,” he says.

Journal reference: Astrophysical Journal, DOI: 10.1088/0004-637X/804/1/34