1. They’re made in galaxy collisions
It’s common belief that galaxies have a supermassive black hole nestled at their centres but what happens to them when one galaxy is on a collision course with another? It turns out that these monstrously hungry and hefty objects have the opportunity to share the same galactic centre, roughly separated by – at most – a few tens of light years.
2. They spew strong gravitational waves
Double black holes are thought to be the strongest source of the very much sought-after gravitational waves. This gravitational radiation ripples through the fabric of space-time, taking the form of waves and have never been observed directly. They’re made when one of the duo spirals towards the other, when the pair’s orbit has decayed, eventually combining as one.
3. Their event horizons make duckbill shapes
What becomes of a black hole’s event horizon – the point of no return – when two of these exotic objects get up close? Scientists believe that as they approach, their event horizons protrude as duckbill shapes towards each other, extending longer and narrower until they meet.
4. They move at incredible speed
Despite their masses, supermassive black holes are not sluggish by any means. Astronomers are able to figure out – with the help of Doppler shift – that binary supermassive black holes should generally orbit each other at speeds of around 3,800 kilometres per second – that’s 8.5 million miles per hour!
5. More have been suspected than found
It’s no secret that binary black holes are hard to come by, despite the Universe being littered with galactic smash-ups. Astronomers think that over 30 examples probably exist but have only pegged a few including one in the double nucleus of NGC 6240, a remnant of a merger between two smaller galaxies.
Image Credit: P. Marenfeld and NOAO/AURA/NSF