Black holes swarm around the Milky Way

10 April 2018

Interview with

Dr Charles Hailey, Columbia Astrophysics Laboratory, Columbia University,

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This week scientists have provided the first evidence for a decades old theory that there is a swarm of black holes at the centre of our galaxy. We know a super-massive black hole sits in the middle of our milky way, but people have theorised that there are thousands more, closely packed in around it. But despite decades of searching, there was no evidence.  Until now. By examining x-ray emissions from our galaxy, a team of scientists have identified what they think are 12 smaller black holes, which could be the first evidence for this gigantic swarm. Izzie Clarke heard more from Chuck Hailey, an astrophysicist at Columbia University in the City of New York.

Chuck - What we did is we found a dozen systems that have a black hole orbiting around a normal star. Such a system emits X-rays that we can actually detect with telescopes and these 12 systems are the tip of the very big iceberg that corresponds to these black holes that were predicted to be there by the theorists. And, in fact, when we work from the few that we see, this simple dozen up to the actual number we believe to be there, we get something very similar to what the theorists predicted of order 10,000 of these black holes.

Izzie - So it’s always been predicted that these smaller black holes would be there, and it’s only now that you’ve just found the first dozen?

Chuck - That’s correct. We found the first dozen by sorting through all the different sources that emit X-rays in the galactic centre, winnowing out the uninteresting objects and trying to zero in on these really fascinating objects, which are the black holes with a star orbiting around them.

Izzie - How can you even see these?

Chuck - Black holes are called ‘black’ for a very good reason; they’re hard to see. In fact, black holes don’t do much that would allow us to detect them. When they sit all by themselves they do eat gas and dust; that is their food but they’re not very good at converting their fuel into X-rays which we can actually see. But when a black hole captures a normal star which is moving by it, the star can send gas in a swirling mass around the black hole that we call an accretion disc, and these disks are very good at emitting X-rays. So we look for these black hole, normal star systems, and we see the X-rays that come from the gas that’s glowing at it swirls around and falls into the black hole.

Izzie - So you can’t actually see the black holes themselves, but you can see this ejection of X-ray emission and that indicates that they’re there?

Chuck - That’s exactly right. We can’t find these lonesome black holes, but if we’re lucky enough to find this dozen that have a companion star, then they’ll send out an X-ray signature that we can detect and identify them as black hole systems.

Izzie - How difficult is that to do?

Chuck - It’s actually quite difficult to do. These huge numbers of black holes are all crammed into the the galactic centre in a region about 6 light years across. And there’s an enormous number of not just black holes in this region, but many other types of cosmic sources that emit X-rays. In addition, there’s a huge amount of gas and dust in the galactic centre; that gas and dust is very hot and when things get that hot, more than 10 million degrees, they tend to emit X-rays. So these black hole binary systems, as we call them. which is just a black hole orbiting a normal star, are basically hiding in plain sight. They blended in the crowd, which is emitting these X-rays and we have to kind of tease them out from all these other sources which can emit X-rays as well.

Izzie - Gosh! So how exactly can you pinpoint the ones that you were hoping to find?

Chuck - It turns out that black holes with a companion star emit X-rays that, on average, are not as energetic as the X-rays that are emitted from most of the other objects that we would confuse with black hole systems, and an analogy I like to use is divisible light. We all know that blue light generally means something is hotter, and red light means that something is colder, and, in fact, the analogy is pretty exact when we look at these X-ray emitting sources. We were looking for the reddish looking objects which would be the black hole systems, and tying to winnow out the blueish looking objects or the hotter objects, which are the uninteresting X-ray sources.

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