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17 May 2007

Merging galaxies observed in unprecedented detail

Young star clusters appear as small blue dots in this Keck image of the material surrounding the two supermassive black holes in NGC 6240

(Image: C Max, G Canalizo and W de Vries)


Two disc-shaped galaxies are thought to have brushed past one another before circling around and beginning to merge about 30 million years ago. But studying the resulting galaxy, which lies 320 million light years away and is known as NGC 6240, can be difficult because it is shrouded in patchy dust that blocks light at optical wavelengths.

Now, a team using the 10-metre Keck II telescope on Mauna Kea in Hawaii, US – the largest infrared telescope in the world – has revealed the location of the two supermassive black holes within the merging galaxies to unprecedented precision by the infrared glow of the dense dust and stars around them.


The new observations also show clumps of young stars that formed as a result of the merger (seen as aqua dots in the image). When the gas clouds from each galaxy crashed into one another, shockwaves formed that caused the gas to clump together, triggering intense star formation. “They’re very close to where the black holes are,” says team leader Claire Max, an astronomer at the University of California in Santa Cruz, US.

Get stuck

All of the material surrounding the black holes may play a role in how the black holes will ultimately merge. Some theories suggest that if they cannot lose angular momentum by flinging nearby stars and gas outwards at high speed, they may get “stuck” when they are a few light years apart and just continue spinning around one another. So having a lot of material close to the black holes might enable them to merge into one colossal black hole.

When the two black holes do merge, “it’s not going to be pretty”, says Max. When the gravitational wave detector Laser Interferometer Space Antenna (LISA) launches around 2015, it may be able to measure the ripples in space-time caused by such a massive black hole collision. LISA will not be able to witness the effects of this collision, however – it is not predicted to occur for another 50 to 150 million years.

Max says NGC 6240 is a good example of how supermassive black holes and their surrounding galaxies evolve together over time. Both grow with repeated mergers.

In 2002, the Chandra X-ray Observatory peered behind the dust shrouding the galaxy and saw the two black holes (see Black holes are double trouble for galaxy). The new Keck II observations use adaptive optics, a technique that allows the telescopes to correct for turbulence in Earth’s atmosphere, making them just as precise as if the telescope were in space.

Science Express (doi:10.1126/science.1136205)