nasa/CXC/mit/H. marshallThe path to infinity: rays of particles launched beyond the Virgo A galaxynasa/CXC/mit/H. marshall
Towards the Virgo constellation, a gathering of galaxies houses a black hole so large that it is hard to even picture its dimensions. Its mass is three billion times the size of the Sun and, if it were in the center of the Solar System, it would occupy all space up to the sixth planet, Saturn. In the last few years astrophysicists from several countries, Brazil included, analyzed images produced by the Chandra space telescope of the central region of the Virgo A galaxy, one out of two thousand in the agglomerate of Virgo and out of eight other of similar magnitude that housed black holes in their core, located between 50 and 400 million light years away from the Earth. From this deep dive into the Cosmos, we have obtained answers on how these objects that concentrate so much mass in such a small volume interact with space around them and contribute to the architecture of the Universe.
Approximately two years ago, astrophysicist Steve Allen and his team, from Stanford University in the USA obtained the initial proof that these cosmic gluttons, capable of gobbling up any matter and energy that get too close, do not consume everything they swallow. A small part is launched outside the galaxies that house them in the form of powerful electrically charged particle jets (plasma). In the case of Virgo A and the other eight galaxies, these jets are launched in opposite directions and sweep the space above and below the black hole, creating two gigantic heated gas bubbles that emit X-rays detected by the Chandra telescope and that, when observed together, look just like an hour glass.
When reexamining the data of these cosmic hour glasses, Rodrigo Nemmen da Silva, a 26 year-old astrophysicist, took another step toward understanding how black holes with a mass equal to billions of suns return to the Cosmos part of the energy they absorb. Under the guidance of Thaisa Storchi Bergmann, from the Federal University of Rio Grande do Sul (UFRGS), he created a mathematical model that allowed him to characterize the gigantic black holes more precisely. Knowing only the energy released by the plasma jets, the young astrophysicist from Rio Grande do Sul performed some reverse engineering of sorts: he worked back from the results to the cause. And it worked.
The particle jets from these black holes release an amount of energy equal to 50 times the energy produced by the Sun in one year – in other words, an amount of energy equal to what would be generated in 365 days by 250 billion electric power stations the size of Itaipu, which is the world’s largest. Rodrigo found that all this energy, enough to supply Brazil for 50 billion years, is only a minute part of everything the black hole consumes. Just like human beings, the black hole also feeds on matter to produce energy for it to grow. And, as usual, the figures are astronomical.
“Every day it absorbs the so-called accretion disc, a ring of gas and dust that surrounds it, corresponding to the mass of ten Earths,” states Rodrigo, who worked with Richard Bower, from the University of Durham in England, and Arif Babul, from the University of Victoria in Canada. His calculations show that black holes are more efficient in producing a plasma jet than Allen had initially suggested in his 2006 article for the Monthly Notices of the Royal Astronomical Society journal. When comparing the amount of gas that gets close to the black hole with the energy of the jets, Rodrigo took into account the possibility that not all the matter is captured and incorporated into its mass, thus making it grow gradually. Therefore, there would have to be an efficient mechanism generating these jets that ward off and heat the hydrogen- and helium-rich gas, opening up the hour glass shaped cavities. “Other scientists had already observed that there is a relation between the amount of matter captured by a black hole and the power of the jets, but they were not as accurate as they didn’t take into account some effects that we included in our model, such as the black hole’s rotation,” stated Thaisa.
nasa/CXC/m.WeissHour glass: jets create gas bubbles above and below black holesnasa/CXC/m.Weiss
At the limit
In reality, the UFRGS duo is only explaining the greater efficiency in jet production if the black hole is rotating very rapidly. According to Rodrigo’s calculations, the black holes observed by the Chandra telescope are rotating at extremely high speeds that range between 90% and 99.8% of the speed of light (300 thousand kilometers per second), i.e. the maximum rotation limit established by Albert Einstein’s General Relativity Theory. “At that speed, a black hole with such dimensions would rotate around its own axis in just 24 hours,” states Rodrigo, who published his results in 2007 in the Monthly Notices of the Royal Astronomical Society and presented them on January 10, 2008, at the 211th Annual Meeting of the American Astronomical Society, the most important in the area.
Rotating at almost the speed of light, the black hole becomes flattened at its extremities. As result of this ultra-fast rotation, it also drags along the internal part of the gas cloud that makes up the accretion disc. In this disc, the gas moves at faster and faster speeds as it gets closer to the horizon of events, a region corresponding to the surface of the black hole where nothing escapes from being swallowed. Although we do not know the details of how the jets form, it is believed that as the gas is spiraling towards the black hole, it drags the magnetic field along with it. And that, in turn, creates a kind of magnetic funnel that concentrates the particles in parallel rays originating the perpendicular jets to the disc.
“Knowing the rotation of a black hole is important because it allows one to understand its effect on its own environment,” says Rodrigo, who is currently training at Penn State University. Contrary to common belief, black holes are not always active, devouring the gas clouds, stars or even entire galaxies that cross its path – and emitting the powerful jets observed with the Chandra telescope. According to Thaisa, a black hole such as Virgo A, probably formed billions of years ago, captures and consumes one star every ten thousand years. Only at intervals of approximately one billion years would it be likely to consume a galaxy. In that case, the accretion disc and the jets of plasma could remain active for approximately 100 million years, changing the environment around it.
Furthermore, its effects are not limited to its surrounding areas. It was only recently, before the Hubble space telescope identified black holes in most of the galaxies, that Universe formation models become more accurate. Previously, they were more rudimentary. “They estimated that the galaxies were a lot larger than in fact they are as this effect produced by the black holes was not taken into account, namely, that they release back into the intergalactic environment part of the matter that would make the galaxies grow,” states Thaisa. “The discovery of black holes in the majority of the galaxies has allowed us to put forth propositions that are closer to reality.”
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