The image above doesn’t look like much at a glance, does it?
Look again. What you’re seeing are thousands of black holes. Thousands.
That image is a part of the Chandra Deep Field South, the result of a series of very long exposures of one small section of the sky using the space-based Chandra X-Ray Observatory. Astronomers combined images taken over the 18-year period from 1999 to 2016, creating a stacked image that’s the equivalent of a single 7,016,500 second exposure. That’s more than 81 days.
So yeah, it’s a deep image. The entire Deep Field image covers roughly the area of the full Moon on the sky using multiple pointings of the telescope. The center of the field has the most observations and is therefore the most sensitive; the image above shows that inner portion of it.
Everything you see in that image is a source of high-energy X-rays—a form of light like the kind we see but with far, far higher energy. Each dot represents the X-rays from an entire galaxy, some more than 12 billion light-years away! The light we see from those most distant galaxies left them when the Universe itself was only a little more than 1 billion years old.
Only very powerful astronomical objects can generate strong X-ray emission, and the X-rays from the galaxies in the Deep Field are coming from one or both of two very luminous sources: high-mass X-ray binary stars and supermassive black holes.
The binaries are pretty cool. Many very massive stars are born in pairs, which orbit each other. After a short time, one of them can explode as a supernova, and its core collapses to become an ultra-compact neutron star or a black hole. This compact object can feed off material from the “normal” star; as that stuff falls down into the tiny companion’s ferocious gravity, it can heat up to millions of degrees and emit X-rays.
One important part is that these binary systems are young. These stars are so massive they use up their nuclear fuel in the blink of a cosmic eye, perhaps a few million years. That’s critical, because these stars are born in gigantic gas clouds that form lots of stars. By adding up all the X-ray emission we see from high-mass binaries, we can calculate how many there are in a galaxy, and from that extrapolate how many stars are being born in total. That tells us a lot about the conditions in galaxies, and in really distant galaxies we can then see what they were like when they were very young.
Our galaxy was very young once, but we only see it now, after it’s more than 10 billion years old. By looking at distant galaxies we can better understand how our own was formed.
But that’s only half the story. In the center of every big galaxy today we think there lurks a supermassive black hole, a beast with millions or even billions of times the mass of the Sun. That’s still small compared with the host galaxy (the Milky Way has a mass of hundreds of billions of times the Sun), but that supermassive black hole is important. We’ve found that the mass of the central supermassive black hole in the galaxy is correlated with galactic characteristics like the total mass, luminosity (how much energy it emits), and rotation. These are hard to measure directly in distant galaxies, so by looking at their central black holes we can learn more about the galaxies themselves.
The Milky Way’s black hole isn’t currently feeding, so it’s relatively quiet. But in other galaxies the black holes are eating, and when they do that, matter piles up in a disk and can reach temperatures of millions of degrees due to friction and other forces. That’s hot enough to blast out X-rays, which is why we can see them in the Chandra image.
That’s why this deep observation is so important! By examining the X-rays from each source we learn a lot about the galaxy that emits them, far more than simply how much X-ray light they’re blasting out.
Remarkably, astronomers were able to see X-rays from galaxies more than 12.5 billion light-years away, the farthest ever reliably detected. Also, they did not see X-rays from galaxies even a bit farther than that (about 12.6 billion light-years), suggesting either those sources are too faint, or that it was around that time (1.2 billion years after the birth of the Universe) that these sources started turning on, or that they are so obscured by dust in the host galaxy we can’t see them.
The scientists estimate that roughly 70 percent of the objects in that image are supermassive black holes, and in the whole image there are about 5,000 sources. Imagine: Thousands of black holes in just that one tiny part of the sky! Extrapolating to the whole sky, astronomers estimate there must be more than 1 billion supermassive black holes out in the deep Universe that Chandra could see. A billion.
That’s a lot of black holes. And it’s actually only a tiny percentage of what’s out there; there are hundreds of billions or even trillions of galaxies in the Universe. Each may have its own central black hole, but we just don’t see them (because they’re quiet, or feeding but still too faint to see at large distances).
And that’s just the supermassive black holes. Ones with lower mass, formed when stars explode, probably number in the many millions per galaxy. Extrapolating that means there are quadrillions of black holes in the visible Universe. More.
Holy cow. The good news is they’re far away, and don’t pose any sort of threat to us. And in reality, instead of being scared, you should be thankful: Galaxies and black holes form together, so the Milky Way being here the way it is today is due to its central supermassive black hole. And massive stars exploding seed the Universe with heavy elements like iron, calcium, and other ingredients necessary for life to form. They may leave behind a black hole after the supernova, but they also made it possible for us to be here at all.
It’s a weird Universe indeed where we owe our existence to these cosmic devourers. But, literally, that’s where we are. And that’s why I love this Chandra image and research so much. It tells us so much about ourselves and how we came to be.