How Do Supermassive Black Holes Get So Fat?
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.
Gravitational waves are the elusive ripples in spacetime that are theorized to pervade the entire Universe, generated by violent events, chronicling the evolution of some of the most massive objects in the Cosmos. Now, by using a clever technique to measure the observed variations in pulsar spin rates, astronomers may be getting close to not only observing the presence of these waves, but also understanding why some of the biggest black holes are so fat.
Supermassive black holes are known to lurk in the centers of the majority of galaxies. These are truly massive structures, weighing in at millions to billions of solar masses. The supermassive black hole in the center of the Milky Way, for example, has a mass of approximately 4 million suns; the most massive black hole observed to date can be found 308 million light-years away in the supergiant elliptical galaxy NGC 4889 (Caldwell 35) — it is estimated to be 21 billion solar masses.
Although we know there gargantuan objects exist, astronomers are having a hard time understanding how they became so big.
So, in an effort to unlock this black hole evolution conundrum, astronomers using data from the CSIRO Parkes radio telescope in eastern Australia are observing the fast-spinning husks of burnt-out stars to reveal the history of supermassive black hole growth.
Neutron stars are formed after the core collapse of massive stars as they go supernova. Some of these exotic objects manifest themselves as pulsars — powerful beams of energy blast from their poles and, should the alignment be correct, these emissions are observed as radio flashes in the dark, like cosmic lighthouses. These pulses are very precise, so should astronomers detect slight variations in the pulse times, something has changed in the environment separating us from the pulsar.
Gravitational waves, as predicted by Einstein, have yet to be directly observed, although physicists are going through great pains to detect them. Astronomers hope to measure the tiny pulse variations in the timings of 20 pulsars to put some physical constraints on gravitational waves generated via black hole mergers. This is an epic quest; they’re basically looking for the echoes of supermassive black hole collisions that reverberate throughout the Universe as a graviational wave “background.”
“This is the first time we’ve been able to use information about gravitational waves to study another aspect of the Universe — the growth of massive black holes,” said Ramesh Bhat, of the Curtin University node of the International Center for Radio Astronomy Research (ICRAR), in a news release.
“Black holes are almost impossible to observe directly, but armed with this powerful new tool we’re in for some exciting times in astronomy. One model for how black holes grow has already been discounted, and now we’re going to start looking at the others.”
The big question about the biggest black holes is whether they packed on the pounds by violently merging with other black holes, or whether some other growth process is at work. If the former is true, there should be a clue hidden in the nature of gravitational waves.
“When the black holes get close to meeting they emit gravitational waves at just the frequency that we should be able to detect,” said Bhat, who co-authored a study that has been published in the journal Science with postdoctorate researcher Ryan Shannon and PhD postgraduate Vikram Ravi.
When gravitational waves pass through spacetime they are theorized to slightly shrink or expand the distance between two objects. In the case of timing the radio pulses from pulsars, the presence of gravitational waves should slightly affect the detection times of the pulses as the radio waves will have traveled through an ocean of rippling gravitational waves. After careful analysis of the received pulse timing variations, the cacophony of gravitational waves generated by colliding black holes should be detected.
“The strength of the gravitational wave background depends on how often supermassive black holes spiral together and merge, how massive they are, and how far away they are. So if the background is low, that puts a limit on one or more of those factors,” said Bhat.
Using 20 years of data from the Parkes Pulsar Timing Array (PPTA), the researchers have drawn an early conclusion that supermassive black holes didn’t solely gain mass through mergers. While the data isn’t enough to detect gravitational waves outright, they have begun to put valuable limits on gravitational wave frequency and their possible source.
Image: Artist’s impression of the surroundings of the supermassive black hole in NGC 3783. Credit: ESO