Clue Found to How Black Holes Form
The recent finding of an intermediate-mass black hole provides evidence that could support some theories of how supermassive black holes form.
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.
Black holes are some of the strangest objects in the universe, and they typically fall into one of two size extremes: "small" ones that are dozens of times more massive than the sun and other "supermassive" black holes that are billions of times larger than our nearest star. But until now, astronomers had not seen good evidence of anything in between.
A recent discovery of an intermediate-mass black hole in the nearby galaxy Messier 82 (M82) offers the best evidence yet that a class of medium-size black holes exists. The finding may provide a missing link that could explain how supermassive black holes — which are found at the centers of most, if not all, galaxies — come to be, researchers say.
"We know that supermassive black holes exist at the centers of almost every massive galaxy, but we don't know how form," said Dheeraj Pasham, an astronomy graduate student at the University of Maryland, College Park, who led the research. [The 9 Biggest Unsolved Mysteries in Physics]
A black hole is a region of space where the gravitational field is so strong that neither matter nor light can escape. Though it can't be seen directly, astronomers can infer a black hole's existence by the way its gravity tugs on nearby matter, and from the radiation it spews out as bits of material falling into the black hole rub against one another, producing friction.
Astronomers have detected stellar-mass black holes, which are 10 to 100 times the mass of the sun, and supermassive black holes, which are hundreds of thousands to billions of solar masses. But the intermediate-mass variety has proved very difficult to detect, causing some to doubt their existence.
The recently identified medium-size specimen has a mass about 400 times that of the sun (give or take 100), according to the study published Sunday (Aug. 17) in the journal Nature. Scientists had hypothesized that such intermediate black holes existed, but this is the first time that one has been measured so precisely, the researchers said.
Astronomers know how stellar-mass black holes form: A massive star collapses under its own gravity. But such a process would seem unable to explain how much larger black holes arise, because they can only gobble material up to a rate known as the Eddington limit, and the universe isn't old enough for them to have grown from stellar mass to supermassive, said Cole Miller, an astronomer also at the University of Maryland.
"If you feed matter to the black hole too fast, it produces so much radiation that it blows away the matter that's trying to ," Miller told Live Science.
Building a black hole
How, then, might supermassive black holes form? Some theories suggest these strange behemoths grew from intermediate-mass black holes — which act as "seeds" — that formed in the early stages of the universe from the collapse of giant clouds of gas.
Others say these black hole giants started out as stellar-mass black holes that somehow gobbled up material at a rate much faster than the typical limit.
Miller has theorized that maybe a dense cluster of stars merged in the early universe, "colliding with each other and sticking together like wet clay," producing a black hole that gathers mass at a rate exceeding the normal limit. "If you can evade that limit, you might be able to build bigger black holes," he said.
Priyamvada Natarajan, a theoretical physicist at Yale University in New Haven, Connecticut, and her colleagues recently developed a new theoretical concept that suggests it is possible to grow black holes from a stellar mass seed faster than the Eddington limit, if the seed is trapped in a star cluster feeding off cold, flowing gas. The research was detailed Aug. 7 in the journal Science.
The finding of an intermediate-mass black hole in a nearby galaxy is exciting because it provides a "missing piece" between stellar-mass black holes and supermassive ones, Natarajan told Live Science.
"We have very young black holes that are like the infant stage, and we have geriatric ones," Natarajan told Live Science. Intermediate mass black holes are like the teenagers, she said.
Now that Pasham's team has shown that at least one of these adolescent black holes exists, astronomers will no doubt look for more.
"There's very exciting science here," Natarajan said. "The discovery space is wide open."
Original article on Live Science.
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