This illustration depicts a gas halo surrounding a quasar in the early Universe. The quasar, in orange, has two powerful jets and a supermassive black hole at its centre, which is surrounded by a dusty disc. The gas halo of glowing hydrogen gas is represented in blue. A team of astronomers surveyed 31 distant quasars, seeing them as they were more than 12.5 billion years ago, at a time when the Universe was still an infant, only about 870 million years old. They found that 12 quasars were surrounded by enormous gas reservoirs: halos of cool, dense hydrogen gas extending 100 000 light years from the central black holes and with billions of times the mass of the Sun. These gas stashes provide the perfect food source to sustain the growth of supermassive black holes in the early Universe.
Unless Einstein is wrong, a black hole is defined by three properties: mass, spin, and electric charge. The charge of a black hole should be nearly zero since the matter captured by a black hole is electrically neutral. The mass of a black hole determines the size of its event horizon, and can be measured in several ways, from the brightness of the material around it to the orbital motion of nearby stars. The spin of a black hole is much more difficult to study.
A team of astronomers from South Africa have noticed a series of supermassive black holes in distant galaxies that are all spinning in the same direction. Credit: NASA/JPL-Caltech
When it comes to the many mysteries of the Universe, a special category is reserved for black holes. Since they are invisible to the naked eye, they remain visibly undetected, and scientists are forced to rely on “seeing” the effects their intense gravity has on nearby stars and gas clouds in order to study them.
That may be about to change, thanks to a team from Cardiff University. Here, researchers have achieved a breakthrough that could help scientists discover hundreds of black holes throughout the Universe.
Led by Dr. Mark Hannam from the School of Physics and Astronomy, the researchers have built a theoretical model which aims to predict all potential gravitational-wave signals that might be found by scientists working with the Laser Interferometer Gravitational-Wave Observatory (LIGO) detectors.
These detectors, which act like microphones, are designed to search out remnants of black hole collisions. When they are switched on, the Cardiff team hope their research will act as a sort of “spotters guide” and help scientists pick up the faint ripples of collisions – known as gravitational waves – that took place millions of years ago.
X-ray/radio composite image of two supermassive black holes spiraling towards each other near the center of Abell 400 galaxy cluster. Credit: X-ray: NASA/CXC/AIfA/D.Hudson & T.Reiprich et al.; Radio: NRAO/VLA/NRL
Made up of postdoctoral researchers, PhD students, and collaborators from universities in Europe and the United States, the Cardiff team will work with scientists across the world as they attempt to unravel the origins of the Universe.
“The rapid spinning of black holes will cause the orbits to wobble, just like the last wobbles of a spinning top before it falls over,” Hannam said. “These wobbles can make the black holes trace out wild paths around each other, leading to extremely complicated gravitational-wave signals. Our model aims to predict this behavior and help scientists find the signals in the detector data.”
Already, the new model has been programmed into the computer codes that LIGO scientists all over the world are preparing to use to search for black-hole mergers when the detectors switch on.
Dr Hannam added: “Sometimes the orbits of these spinning black holes look completely tangled up, like a ball of string. But if you imagine whirling around with the black holes, then it all looks much clearer, and we can write down equations to describe what is happening. It’s like watching a kid on a high-speed spinning amusement park ride, apparently waving their hands around. From the side lines, it’s impossible to tell what they’re doing. But if you sit next to them, they might be sitting perfectly still, just giving you the thumbs up.”
Researchers crunched Einstein’s theory of general relativity on the Columbia supercomputer at the NASA Ames Research Center to create a three-dimensional simulation of merging black holes. Credit: Henze, NASA
But of course, there’s still work to do: “So far we’ve only included these precession effects while the black holes spiral towards each other,” said Dr. Hannam. “We still need to work our exactly what the spins do when the black holes collide.”
For that they need to perform large computer simulations to solve Einstein’s equations for the moments before and after the collision. They’ll also need to produce many simulations to capture enough combinations of black-hole masses and spin directions to understand the overall behavior of these complicated systems.
In addition, time is somewhat limited for the Cardiff team. Once the detectors are switched on, it will only be a matter of time before the first gravitational wave-detections are made. The calculations that Dr. Hannam and his colleagues are producing will have to ready in time if they hope to make the most of them.
But Dr. Hannam is optimistic. “For years we were stumped on how to untangle the black-hole motion,” he said. “Now that we’ve solved that, we know what to do next.”
A team of astronomers from South Africa have noticed a series of supermassive black holes in distant galaxies that are all spinning in the same direction. Credit: NASA/JPL-Caltech
Checking out the spin rate on a supermassive black hole is a great way for astronomers to test Einstein’s theory under extreme conditions – and take a close look at how intense gravity distorts the fabric of space-time. Now, imagine a monster … one that has a mass of about 2 million times that of our Sun, measures 2 million miles in diameter and rotating so fast that it’s nearly breaking the speed of light.
A fantasy? Not hardly. It’s a supermassive black hole located at the center of spiral galaxy NGC 1365 – and it is about to teach us a whole lot more about how black holes and galaxies mature.
What makes researchers so confident they have finally taken definitive calculations of such an incredible spin rate in a distant galaxy? Thanks to data taken by the Nuclear Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency’s XMM-Newton X-ray satellites, the team of scientists has peered into the heart of NGC 1365 with x-ray eyes – taking note of the location of the event horizon – the edge of the spinning hole where surrounding space begins to be dragged into the mouth of the beast.
“We can trace matter as it swirls into a black hole using X-rays emitted from regions very close to the black hole,” said the coauthor of a new study, NuSTAR principal investigator Fiona Harrison of the California Institute of Technology in Pasadena. “The radiation we see is warped and distorted by the motions of particles and the black hole’s incredibly strong gravity.”
However, the studies didn’t stop there, they advanced to the inner edge to encompass the location of the accretion disk. Here is the “Innermost Stable Circular Orbit” – the proverbial point of no return. This region is directly related to a black hole’s spin rate. Because space-time is distorted in this area, some of it can get even closer to the ISCO before being pulled in. What makes the current data so compelling is to see deeper into the black hole through a broader range of x-rays, allowing astronomers to see beyond veiling clouds of dust which only confused past readings. These new findings show us it isn’t the dust that distorts the x-rays – but the crushing gravity.
Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. Image credit: NASA/JPL-Caltech
“This is the first time anyone has accurately measured the spin of a supermassive black hole,” said lead author Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics (CfA) and INAF — Arcetri Observatory.
“If I could have added one instrument to XMM-Newton, it would have been a telescope like NuSTAR,” said Norbert Schartel, XMM-Newton Project Scientist at the European Space Astronomy Center in Madrid. “The high-energy X-rays provided an essential missing puzzle piece for solving this problem.”
Even though the central black hole in NGC 1365 is a monster now, it didn’t begin as one. Like all things, including the galaxy itself, it evolved with time. Over millions of years it gained in girth as it consumed stars and gas – possibly even merging with other black holes along the way.
“The black hole’s spin is a memory, a record, of the past history of the galaxy as a whole,” explained Risaliti.
“These monsters, with masses from millions to billions of times that of the sun, are formed as small seeds in the early universe and grow by swallowing stars and gas in their host galaxies, merging with other giant black holes when galaxies collide, or both,” said the study’s lead author, Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and the Italian National Institute for Astrophysics.
This new spin on black holes has shown us that a monster can emerge from “ordered accretion” – and not simply random multiple events. The team will continue their studies to see how factors other than black hole spin changes over time and continue to observe several other supermassive black holes with NuSTAR and XMM-Newton.
“This is hugely important to the field of black hole science,” said Lou Kaluzienski, NuSTAR program scientist at NASA Headquarters in Washington, D.C. “NASA and ESA telescopes tackled this problem together. In tandem with the lower-energy X-ray observations carried out with XMM-Newton, NuSTAR’s unprecedented capabilities for measuring the higher energy X-rays provided an essential, missing puzzle piece for unraveling this problem.”
