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ASTRONOMY

Monumental efficiency

Giant black holes consume less energy than thought, but drive the largest gas and radiation jets in the Universe

Jets of particles and radiation emitted by the black hole of the Centaurus A galaxy,12 million light-years from the Milky Way.

NASA / DOE / Fermi Lab Collaboration / Capella ObservatoryJets of particles and radiation emitted by the black hole of the Centaurus A galaxy,12 million light-years from the Milky Way.NASA / DOE / Fermi Lab Collaboration / Capella Observatory

The reputation of black holes as immense gluttons or vacuum cleaners on a cosmic scale—inexorably sucking in everything around them—is still valid. Normally they consume the gas in the interstellar medium, though larger black holes that swallow entire stars in one fell swoop can be found at the center of galaxies. These gigantic black holes, however, do not consume as much gas as was thought. In fact, we now know that they expel almost as much gas out of their vicinity as they suck in. Even with a lower consumption level than astrophysicists had thought until a short time ago, these black holes still have enough energy to discharge accelerated gas jets that extend millions of light-years beyond their galaxies at speeds comparable to the speed of light. These jets are the greatest and most powerful particle accelerators in the Universe (see Pesquisa FAPESP Issue No. 200). “It’s completely counterintuitive,” says Rodrigo Nemmen, a researcher at the Institute of Astronomy, Geophysics and Atmospheric Sciences of the University of São Paulo (USP). “How can the gas falling into the black hole escape it so copiously?”

Nemmen worked with astrophysicist Alexander Tchekhovskoy, of the University of California, Berkeley, to more precisely compare the quantity of energy that, in the form of hot gas, feeds giant black holes with the quantity of energy that emanates from them in the form of jets. The pair analyzed dozens of giant black holes in the centers of galaxies observed using the Chandra X-ray space telescope. The study suggests that the energy from the jets is almost always greater than that supplied by the hot gas absorbed by the black hole. In many cases, the jets are more than three times more energetic than the gas that the hole absorbs. Nemmen compares the absurdity of the situation with that of an imaginary engine that provides three times as much energy to an automobile than that contained in its fuel. “Something is wrong, since energy conservation is the most fundamental law of physics,” he says.

Only one solution to the paradox does not violate the laws of physics. The jets discharged by the black hole can only be that energetic if there is an extra source of energy much more powerful than the hot gas. Although the analysis by Nemmen and Tchekhovskoy does not definitively clarify what source this might be, the figures they calculated favor a theory that astrophysicists have been discussing since the 1970s. The idea is that the jets are created by magnetic fields molded and strengthened by energy that comes from the black hole itself.

A black hole, Nemmen explains, is a spherical region in space with an attractive gravitational force so strong that a spaceship—or any other object—would need to travel at a speed greater than the speed of light to escape it. The edge of the hole is called the event horizon. “If not even light escapes, and it is the fastest thing in the Universe, nothing can escape after the event horizon has been crossed,” he explains.

Astronomers have no idea what is inside black holes, which first appeared as a mathematical solution to one of the equations of Albert Einstein’s theory of general relativity, published in 1915. However, since the 1960s, researchers have accumulated indirect evidence that black holes of a variety of sizes are abundant in the Universe.

All-you-can-eat buffet
There is sufficient proof, for example, that there is a dark object considerably smaller than our solar system with a mass four million times greater than that of the Sun at the center of the Milky Way. “Given everything we know about gravitation and astrophysics, there must be a black hole there,” says Nemmen. “Astronomical observations, especially those from the Hubble space telescope, have established that most galaxies have a black hole at their center.”

The black hole at the center of the Milky Way, however, is different from those found at the center of other galaxies. It is tranquil most of the time—swallowing some gas or a little star every once in a while—while those located at the center of some galaxies are much more active and their brightness is thousands of times greater than all of the stars in the galaxies in which they are located.

This brightness comes from the radiation emitted by the gas from the interstellar medium that falls, copiously, into the center of the galaxy. “These black holes are like me when I am at an all-you-can-eat buffet. They eat voraciously,” says Nemmen, who was born in Passo Fundo, Rio Grande do Sul, obtained his undergraduate and PhD degrees from the Federal University of Rio Grande do Sul, and became a professor at USP in April, 2014, after post-doctoral studies at NASA.

In this buffet regime, these black holes, also known as active galactic nuclei (AGNs), do something else extraordinary. They emit a pair of brilliant, immense gas jets beyond the reaches of their galaxies.  The gas in these jets travels through intergalactic space, emitting super high-energy radiation.

Researchers are still debating the origin of these jets. One possibility involves the disk of gas that rotates near the event horizon. “The gas rotates with a lot of energy and could be diverted and channeled into jets,” says Nemmen. Another possibility, he says, involves the magnetic fields generated by the hot, electrically charged gas near the event horizon. “These fields are able to extract energy from the black hole and transfer it to the gas,” he explains.

This energy transfer would be possible because, in addition to the force of attraction towards the interior of the event horizon, black holes contain a large amount of rotation energy, which forces everything around them to rotate in the same direction. This spatial whirlwind could drag the magnetic field lines of the gas around the event horizon like a skein of yarn. In 1977, the calculations of astrophysicists Roger Blandford and Roman Znajek suggested that the energy of these lines could sculpt and drive the jets. Since then, computer simulations of AGNs, some performed by Tchekhovskoy, Nemmen’s colleague, have confirmed that the Blandford-Znajek mechanism is the most probable source of energy for the jets.

“Theoretical studies have strongly suggested this, but observations have still not tested these ideas very well,” says Nemmen. To better compare observations with theory, he and Tchekhovskoy decided to model AGNs as machines. “Imagine an engine whose internal workings we cannot examine,” Nemmen describes. “We can try to understand how the machine works by measuring its output, comparing how much fuel it uses with the energy that it puts out.”

Imaginary sphere
Searching data obtained from the Chandra space telescope, the pair selected 27 AGNs that had been observed in enough detail to determine how much energy enters an imaginary sphere with a radius of about 1 light-year around the black hole and compare it with how much energy leaves the sphere. To estimate the amount of energy that feeds this machine, they calculated how much gas enters this region, its speed and its temperature. Not all of the gas entering this sphere falls into the black hole. The gas is so hot and so turbulent, and it spins so fast that a good part of it has enough energy to escape before it is too late. “Prior studies estimated this energy input inadequately,” states Nemmen. “Observations of the center of the Milky Way and of the NGC3115 galaxy made during the last two years have shown an enormous loss of gas.”

The astrophysicists measured the energy escaping from the machine by observing how the X-rays emitted by the jets inflate two enormous cavities of hot gas above and below the galaxies (see Pesquisa FAPESP Issue No. 144). “Much more energy leaves than enters,” concludes Nemmen. “Doing the calculations, we were able to explain this yield by assuming that the extra energy must come from the rotation of the black hole.”

The conclusion coincides with that of another article Tchekhovskoy published in the June, 2014 issue of the journal Nature. He and his colleagues discovered a relationship between the magnetic fields of the jets and the light emitted by the gas disks. This relationship only makes sense if the jets had been created by magnetic field lines fed by the black hole. Nemmen points out, however, that there are uncertainties in the observations. More consistent data would require more precise measurements of the jets. “In order to observe the formation of the jets directly,” he explains, “we would need an X-ray telescope with a resolution thousands of times greater than that of the Chandra Observatory.”

Two or three ways to be killed by a black hole

Falling into a black hole is fatal. Once the event horizon is passed, there is no way to escape or call for help. The fate of a person who crosses this region is to be disintegrated by gravitational forces before reaching the center of the black hole, a singularity that physicists still do not fully understand.

What many people do not realize, though, is that black holes can be mortal even at a distance of several light-years. “The most energetic phenomena in the Universe occur in the neighborhood of black holes,” explains astrophysicist Rodrigo Nemmen, of USP, to the audience of his popular science talks. At his 2014 presentations in São Paulo, Nemmen used the different ways to be killed by a black hole as a way to introduce the public to the astronomy and physics of these objects, whose existence has only been inferred through indirect observations. “It is fatalism didactics,” recounts Nemmen.

Being fried by radiation is one of the ways to be killed by a black hole. When a star with a mass hundreds of times greater than that of the Sun collapses, its nucleus turns into a black hole. This black hole feeds on the remaining material in such an explosive manner that it expels a jet of particles and radiation known as a gamma-ray explosion, capable of incinerating everything in its way.

Black holes do not create radiation storms at just the beginning of their lives. Both large and small black holes tend to attract gas clouds, and these form accretion disks. The rotation of the disk heats up the gas so much that it emits radiation levels that would cause cancer in someone several light-years away.

Equally dangerous would be to stray into the path of a jet of gas and particles expelled by the black hole of an active galactic nucleus. In 2007, astronomers observed a galaxy being hit head-on by the jet of a neighboring galaxy, which is now being called the death-star galaxy. The planets in the galaxy struck must have suffered a radiation shower.

Even if a person near a black hole survived the radiation, he would run a serious risk of an Italian-style death that physicists call spaghettification—having his body stretched like a strand of spaghetti. In the neighborhood of the event horizon, the difference in the gravitational force between the feet and head of a person falling towards the black hole could be enough to stretch and compress his body in such a way as to transform it into spaghetti.

The closer to the event horizon, the greater the probability that the whirlwind created by the black hole’s rotation would also spaghettify the body of anyone in the vicinity, making it spin in a manner than Nemmen compares to a medieval torture wheel.

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