Supermassive black holes lurk at the centre of every galaxy, but when these black holes pair up, they can be hard to find. Astronomers have now devised a technique to seek out these black hole duos, and have discovered five new pairs.

Our Galaxy has a supermassive black hole at its centre, and while we cannot see it directly, we know it is there because of its gravitational tug on nearby stars and gas. Supermassive black holes (SMBHs) are called supermassive for a reason – the one that sits in the centre of our Galaxy is a whopping four million times more massive than our Sun.

It is thought that every galaxy boasts its own central SMBH, and some can be detected when they start sucking up material and create a bright accretion disc surrounding them, thus becoming active. Collisions of galaxies are common and as each galaxy brings its own central SMBH, there should be an imposing pair in each merger galaxy, and they should be growing rapidly.

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“When galaxies pass close to one another, their mutual gravitational forces disrupt their internal structure,” explained Sara Ellison from the University of Victoria, Canada, lead author of one of the two papers describing the discoveries.

“These internal disruptions perturb the orbits of the galaxy’s stars and can cause the gas that resides in the interstellar medium to get funnelled towards the galactic centre. This process provides a ready supply of fuel to the supermassive black hole that is located at the galaxy’s centre – it’s like turning on the tap and watching the material go down the plug hole!”

Even though theory predicts that these rapidly growing black hole pairs should be common, less than ten pairs were known prior to this new study, and these were only detected by chance. Ellison, along with Shobita Satyapal from George Mason University, Virginia, and their colleagues, boosted the number by combining observations from a suite of telescopes across a range of wavelengths.

After identifying merging galaxies with the two centres separated by less than 30,000 light years, the astronomers then used the Wide-Field Infrared Sky Explorer Survey (WISE) telescope to observe the candidates in infrared light.

“Our infrared observations enabled us for the first time to unveil a population of powerful but deeply buried actively feeding black holes,” said Satyapal, lead author of the other paper. “Infrared light can penetrate through the intervening gas and dust and allows us to ‘see’ into the centres of these colliding galaxies, where optical studies fail.”

The final step towards confirming the existence of these black hole pairs was detecting a dual X-ray source in the mergers using NASA’s Chandra X-ray Observatory. “Our observations show that the X-ray emission produced in these duals are an order of magnitude or more luminous than what you would expect from just aging stars in the galaxies, providing compelling evidence that these are actively feeding black holes,” said Satyapal.

A pair of black holes that spiral towards each other will eventually collide, sending ripples through the Universe in the form of gravitational waves. Having an effective method for rooting out these SMBH pairs is important in order to perform a census and determine how common they really are.

So far, only gravitational waves from much smaller stellar mass black hole collisions (with masses between 8 and 36 times the Sun) have been found using the LIGO and Virgo detectors. “LIGO’s ‘arms’ are only 4 km long, so are sensitive to lower mass black hole mergers,” explained Ellison. “So, even though the gravitational waves from supermassive black hole mergers will be very ‘loud’, LIGO is not designed to detect them.”

However, the hunt is on for these giant space ripples by using a different method that searches for disturbances in the regular blips of pulsars caused by a passing gravitational wave. In addition, LIGO’s bigger cousin, the Laser Interferometer Space Antenna (LISA) is planned for launch in the 2030s, and will be capable of detecting these monster collisions.

Uncloaking these new black hole pairs from their surroundings of gas and dust will pave the way towards understanding exactly how black holes grow, and also compliment the search for the loudest gravitational waves in the Universe.

The papers discussing the discoveries can be found at https://arxiv.org/abs/1707.03921 and https://arxiv.org/abs/1705.05465.

A Chandra video explains how the pairs of black holes were found.

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