Black holes are the densest, most mysterious objects in the universe. These super-compressed objects are millions or billions of times larger than our own Sun, with gravitational pulls so great that not even light can escape.
But not all black holes are created equal. Some are larger than others and, while they all absorb the matter around them, some grow faster.
Led by researchers at the Australian National University (ANU), an international team has found the fastest-growing black hole of the last 9 billion years. The discovery was published in the Publications of the Astronomical Society of Australia.
Lead author Chris Onken, an astrophysicist at ANU, explains that we can tell how fast a black hole is growing by measuring its brightness, or luminosity. “As more stuff is falling into the black hole, that material – like a ball rolling down the hill – increases speed and there’s a lot of friction within the gas falling into the black hole. The gas then gets very hot, and shines across, in this case, more than half the universe. This black hole is eating the equivalent of 80 of our Suns every year, or an Earth every second.”
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The observations we make in the universe come in the form of light, which takes time to travel across the vast expanse of space. The light from the new black hole, SMSS J114447.77-430859.3 (J1144 for short), took 7 billion years to reach Earth.
Early in the universe’s history, fast black-hole growth rates are not unusual. But, comparing the newly discovered object with others found over the past 60 years, the team found none with comparable growth in the last 9 billion years.
The cause of J1144’s fast growth is still not clear, says Onken. “The fact that this is such an outlier that’s so much brighter than the other black holes at that age of the universe means that maybe it’s something like the collision of two big galaxies, which is suddenly able to feed the black hole and let it grow that rapidly.
“If you look at those systems much earlier in the history of the universe, that’s when there were many more mergers between galaxies. And so you have lots of growth of black holes, lots of growth of the galaxies in which they live. Looking back about 7 billion years, just past the halfway point in the universe’s life, we don’t see other black holes growing as rapidly as J1144. And the brightest ones that we know of are a few times fainter than this one.”
J1144 was found by Adrian Lucy, a PhD student at Columbia University, US, and now a postdoc in Baltimore, who was looking for close pairs of stars in our own galaxy. Using the SkyMapper Southern Sky Survey based at ANU, Lucy found several hundred candidates and one object that didn’t look like the others.
While working with Lucy, ANU professor Christian Wolf noticed the object resembled a quasar – the bright indicator of the presence of a black hole.
“The very first such quasar was found in the 1960s, when they were first able to identify that this extremely luminous object that’s at a very large distance had an energy source much more efficient than just fusion at the centers of stars,” says Onken. “The fact that this new one that hadn’t been discovered over 60 years of searching was only a little bit fainter than the very first one was discovered back in the ‘60s, and it was so much further away, must mean that it’s incredibly luminous and growing at a very fast rate.”
But why has it taken so long to find such a bright object – one a backyard astronomer with a decent telescope could see?
Onken says previous searches have avoided looking close to the plane of the Milky Way, where there’s so much stuff it can get very messy. Earlier surveys stopped at between 30 and 20 degrees from the plane of our galaxy, but J1144 sits at 18 degrees.
“That motivated us to actually go back and see how many more there might be like this that have escaped discovery so far,” says Onken. “We haven’t found any quite as bright as J1144, but our current count is up to 80 sources that have escaped previous detection but are unusually bright, so I’m working to complete that survey and find all the bright quasars that are out there.”
Onken hopes to study J1144 further in the ultraviolet spectrum.
“Getting above the atmosphere gives you access to that ultraviolet portion of the spectrum and that could tell us a lot more about the regions close to the black hole,” he says. “And it could even tell us something about our own Milky Way. You can use these bright, distant objects as headlights shining through the gas around the Milky Way to understand more about our own local system.”
Because J1144 is so bright, it is ionising – removing electrons from – a very large region of the gas around the black hole. Using telescopes in Chile, Onken believes it may be possible to see the rotation of the gas around the black hole.
The team has also used historical observations, which serendipitously captured J1144 without realising.
“It’s really been a lot of fun to look at photographic plates from 1901 that have pictures of this very black hole about as bright as we see it now. This shows it is not a transient phenomenon. It’s been going on for quite a long time. It’s been fun to build in that historical aspect to this cutting-edge research.
“We are excited to learn more about this object in particular, but also trying to complete that census for these really bright objects that will tell us more about how it is that a massive black hole can be fed at such a large rate.
“We can get a better understanding of how that may have happened early in the universe by studying these more nearby examples.”