Merging Black Holes Trapped in Galactic Death Spiral
Artist's impression of a spinning supermassive black hole with a surrounding accretion disk and relativistic jets.
How to measure the spin of a black hole: This chart illustrates the basic model for determining the spin rates of black holes. The three artist's concepts represent the different types of spin: retrograde rotation, where the disk of matter falling onto the hole, called an accretion disk, moves in the opposite direction of the black hole; no spin; and prograde rotation, where the disk spins in the same direction as the black hole.
Two models of black hole spin: Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. The light comes from accretion disks that swirl around black holes, as shown in both of the artist's concepts. They use X-ray space telescopes to study these colors, and, in particular, look for a "fingerprint" of iron -- the peak shown in both graphs, or spectra -- to see how sharp it is. Prior to observations with NASA's Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM- Newton telescope, there were two competing models to explain why this peak might not appear to be sharp. The "rotation" model shown at top held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole. The alternate model held that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.
This chart depicts the electromagnetic spectrum, highlighting the X-ray portion. NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton telescope complement each other by seeing different colors of X-ray light. XMM-Newton sees X-rays with energies between 0.1 and 10 kiloelectron volts (keV), the "red" part of the spectrum, while NuSTAR sees the highest-energy, or "bluest," X- ray light, with energies between 3 and 70 keV.
This image taken by the ultraviolet-light monitoring camera on the European Space Agency's (ESA's) XMM-Newton telescope shows the beautiful spiral arms of the galaxy NGC1365. Copious high-energy X-ray emission is emitted by the host galaxy, and by many background sources. The large regions observed by previous satellites contain so much of this background emission that the radiation from the central black hole is mixed and diluted into it. NuSTAR, NASA's newest X-ray observatory, is able to isolate the emission from the black hole, allowing a far more precise analysis of its properties.
What XMM-Newton saw: The solid lines show two theoretical models that explain the low-energy X-ray emission seen from the galaxy NGC 1365 by the European Space Agency's XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the measurements from XMM-Newton, which are explained equally well by both models.
Two X-ray observatories are better than one: NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has helped to show, for the first time, that the spin rates of black holes can be measured conclusively. It did this, together with the European Space Agency's XMM-Newton, by ruling out the possibility that obscuring clouds were partially blocking X-ray right coming from black holes. The solid lines show two theoretical models that explain low-energy X-ray emission seen previously from the spiral galaxy NGC 1365 by XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.
We now know that the vast majority of galaxies have supermassive black hole behemoths living in their cores. We also suspect that they grow when their host galaxies merge with other galaxies, eventually spawning the mother of all growth spurts: black hole mergers — when bigger black holes with masses millions or even billions of times the mass of the sun are created. Now, with the help of NASA’s Wide-field Infrared Survey Explorer (WISE), two supermassive black holes have been discovered, caught in the middle of a merging dance in the center of a galaxy 3.8 billion light-years away.
WISE’s primary mission came to an end in 2011 after its coolant ran dry, but a new batch of data has just been released and an oddity was discovered. Assumed to be a lively star-forming region, astronomers soon realized the infrared object known as WISE J233237.05-505643.5 had some weird properties. With the help of followup studies by the Australian Telescope Compact Array (ATCA) near Narrabri, Australia, and the Gemini South observatory in Chile, the real nature of the object was unraveled.
“At first we thought this galaxy’s unusual properties seen by WISE might mean it was forming new stars at a furious rate,” said WISE project manager Peter Eisenhardt, at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “But on closer inspection, it looks more like the death spiral of merging giant black holes.”
When active black holes “feed” on surrounding matter in galactic hubs, a superheated plasma forms around the black hole’s event horizon. Through complex physics in the intense relativistic environment that are not yet fully understood, jets blast from the poles of the spinning black hole, generating powerful emissions. In the case of WISE J233237.05-505643.5, it appears that two black holes are orbiting one another, separated by only a few light-years (which is very close considering the gigantic masses of these huge multi-million solar mass black holes), and one of the black hole’s jets are being “wiggled” by the gravitational interactions with its merging partner.
“We think the jet of one black hole is being wiggled by the other, like a dance with ribbons,” said Chao-Wei Tsai, of NASA’s Jet Propulsion Laboratory, lead author of this research set to appear in a paper to be published in the Dec. 10 issue of Astrophysical Journal. “If so, it is likely the two black holes are fairly close and gravitationally entwined.”
There’s only one way this extreme cosmic dance will end — both black holes will eventually spiral together, losing energy and momentum to gravitational waves rippling from their spacetime warping, collide and merge to form an even bigger black hole.
Black hole mergers are a rare event to observe and only a few candidates have been found. Some black hole merger candidates have been identified, but this example is the most distant discovered to date. Following the WISE discovery of the black hole pair, radio emission data from the ATCA spotted the strange zig-zag pattern one of the black holes’ jets seemed to be producing. Then, from infrared/optical data gathered by Gemini South, the anomalous jet wiggle was confirmed. Clumpiness in material surrounding one of the suspected black holes also indicates the local region is being perturbed by the gravitational presence of another black hole.
According to a JPL news release, astronomers are not sure how close the merging black holes are, only that this event will inevitably provide a privileged window into supermassive black hole evolution at a time just before their masses catastrophically combine.
“We note some caution in interpreting this mysterious system,” said JPL’s Daniel Stern, a co-author of the study. “There are several extremely unusual properties to this system, from the multiple radio jets to the Gemini data, which indicate a highly perturbed disk of accreting material around the black hole, or holes. Two merging black holes, which should be a common event in the universe, would appear to be simplest explanation to explain all the current observations.”
Image credit: NASA/JPL-Caltech