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At the mouth of a black hole

Researcher from UFRGS shows how matter is swallowed up by a black hole in the center of a galaxy

In recent years, land-based and space telescopes have confirmed the existence of black holes, objects that have such an intense gravitational attraction that no matter escapes from them, not even light. The mechanisms by which they absorb everything that comes close have become less mysterious, with the work concluded in April at the Institute of Physics of the Federal University of Rio Grande do Sul (UFRGS).

“We are witnessing a black hole in the center of a galaxy devouring surrounding matter”, commemorates the coordinator of the study, Thaisa Storchi-Bergmann, when finishing the article that reports the results. Thaisa’s team reconstituted what goes on in the center of NGC 1097, a spiral galaxy 60 million light-years from the Earth (one light-year is equivalent to some 9.5 trillion kilometers). And they confirmed: there is a supermassive black hole there – with its mass equivalent to at least one million Suns -, probably formed by the collapse of gas clouds or of agglomerates of millions of stars.

Focus on the effects
As a black hole cannot be detected directly, but only from its effects on close-by objects, the group from Rio Grande do Sul centered its focus on the light emitted by an accretion disc – a flattened cloud in the form of a dense ring, made of plasma (a mixture of protons and electrons) and hydrogen, which spins around a powerful matter aspirator. Using the information gathered, the team got a clear vision of the processes of birth, evolution and death of the gas disc, which occupies an area equivalent to double the Earth’s orbit around the Sun, with a diameter of close to 300 million kilometers. Preliminary calculations indicate that every second the black hole devours about 100 quadrillions of tons of gas from the disc – or two Earths a day.

Everything indicates that the disc of gas appeared when a star came very close to the black hole and was captured by it. Next, the star broke up under the action of the tidal force of the black hole: this force works with different intensity in different parts of a body – the Sun, for example, attracts more strongly the side of he Earth that is closer rather than the far side, and that is why the planet is elongated in the direction of the star. After the star broke up, the cloud of gas was left over, forming the disc around the black hole.

Hot center
It is estimated that this phenomenon occurs in a galaxy every 10,000 years. “I was lucky with NGC 1097”, recognizes Thaisa, who in 1991, when she started to investigate the emission of light from the nucleus of this galaxy, came across the typical picture of a relatively recent capture of a star. She observed the emission from the outer region of the disc formed from the captured star in the H-alpha line – the most intense line of emission of energy from the hydrogen atom – and found that the gas was spinning at 10,000 kilometers a second (km/s). So, she concluded: gas at this speed could only happen in a disc around a black hole that had a mass of one million Suns.

This was how the hypothesis was born that was to be confirmed in the following ten years. Thaisa demonstrated that there is a heating up of the central regions of the disc, which start to emit high energy radiation – like X-rays -, in a process that lasts at least a few centuries. The inner part of the disc is hotter than the periphery, because of the friction between the atomic particles: the inside temperature can reach millions of degrees Celsius (ºC), while in the periphery, where the emissions of light in the H-alpha line come from, it is around 10,000ºC.

The inside of the disc expands, because of the high temperature, and creates a toroidal structure (in the shape of a donut) around the event horizon – the imaginary surface that defines the frontier beyond which not even light escapes. This structure emits photons that excite hydrogen when they hit the outer parts of the disc and produce the emission of the H-alpha kind, observed since 1991. “The future of the matter of the disc is to go spiraling until it goes beyond the event horizon and falls into the black hole”, Thaisa reveals. The matter of the disc – made up basically of protons and electrons on the inside and of hydrogen atoms on the outside – splits when it reaches this limit: half is swallowed by the black hole, and half is expelled in jets from the inside, an emission that is observed in radio waves.

This is how the disc slowly dissolves until it disappears, at a moment estimated at 1 thousand years hence, at the least. “It is the first time that we have witnessed matter being captured in the central region of a galaxy in a very clear manner”, Thaisa says. But this is not the end of the story. As the black hole feeds on more and more matter, the event horizon expands – its radius today measures roughly 3 million kilometers, or two hundredths of the distance between the Earth and the Sun. In this space, there is a mass equivalent to a millions suns, which Thaisa had calculated in 1997. Capturing individual stars, the black hole could even double in size, but very slowly – only 1 billion years from now.

This cosmic archeology – described by Thaisa in an article recently submitted to the Astrophysical Journal , in which she has already published works, in 1993, 1995 and 1997 – is an advance over what was discovered in 1998 in the M87 galaxy, located at a distance equivalent to that of our planet. That was the year when the space telescope Hubble, orbiting the Earth at a distance of 600 kilometers in search of novelties at the boundaries of the universe, recorded images of a disc of gas spinning around the nucleus of M87.

Although the speed that the disc of M87 was moving was high, it was only a tenth of what found with NGC 1097. And the distance from the disc to the center of the galaxy was one million times the radius of the accretion disc. Therefore, the disc of gas of M87 was not the accretion disc, but a far more external structure. As the radius of this disc is very great – around 500 trillion kilometers, a million times more than NGC 1097’s -, Thaisa rules out the presence of a central black hole. What then could the disc be? A massive stellar agglomerate, for example.

It is precisely the definition of the process for the emission of light from the disc of NGC 1097, apparently far more interesting scientifically than the one seen in M87, that is the reason for the team’s contentment. Thaisa worked with Michael Eracleous, from the University of Pennsylvania, United States, in collecting data from three sources: the 4-meter telescope of the Cerro Tololo Inter-American Observatory; the Eso New Technology Telescope (NTT), of 3.6 meters, both in Chile; and the 8 meter telescope of the Keck II Observatory, in Hawaii. And in the analysis of the results, she was helped by Fausto Kuhn Berenguer Barbosa, who was studying for his master’s degree, and Rodrigo Nemmen da Silva, who has a grant for scientific initiation.

Together, they converted the wavelength of the light (or radiation) into speed of the gas, and built up a graph that showed a line with two peaks – one for the maximum speed of the approach of the light in relation to the Earth, and another for the maximum distance. In technical terms, this is the double peak profile of the H-alpha line of emission, also called the disc’s kinematic signature – clear proof of the existence of the accretion disc and of the transformations it is undergoing. “It is believed that there may also be an accretion disc in M87, although still without a kinematic signature”, the researcher comments. Hubble registers other profiles in similar H-alphas, none of them with a double peak structure so clear as in NGC 1097.”

Thaisa manages to see the emission of H-alpha that arrives from the outermost and coldest part of the disc. “It is possible that not all galaxies have this outermost part, as may be the case with M87”, Thaisa notes. Over the years, she has noticed that the emission has migrated to regions that spin at increasingly higher speeds – as the disc shows Keplerian movement (like the planets), the innermost regions and nearest to the central black hole move at a higher speed that the edges. The displacement of the focus of emission of light happens because the inner part loses energy and cools down, so that the radiation is now not so intense and reaches increasingly smaller distances.

The process is similar to what happens with the light emitted by a flashlight that illuminates the area around it and weakens with time. When the batteries are fully charged, the light is more intense and it illuminates a relatively large area around the flashlight. As the battery loses power, the light loses its range and just lights up the closer regions.

In 1991, the light emitted by the outermost parts of the disc had a speed of 3,000km/s – a mere one hundredth of the speed of light, but which would allow a particle to go from Porto Alegre to Salvador in just one second. Then at the beginning of this year, as the source of photons was getting weaker and covered shorter distances in the disc, it was possible to register the light that came from the innermost parts, with a speed of 6,000 km/s. It is estimated that on the inner edge of the H-alpha emitting disc the speed of the particles may reach 15,000 km/s, while on the brink of the black hole the particles spin at 300,000 km/s, the speed of light.

“The fact that we are finding such high speeds is a sign of a supermassive structure at the center of the galaxy”, says the researcher. The maximum speeds registered for gas discs in rotation at the center of galaxies without black holes are only 250 to 300 km/s. Speeds of rotation in the order of those detected, thousands of kilometers per second, can only, according to her, be produced by the interaction with millions or billions of solar masses concentrated into a very small space – that is, a supermassive black hole. Reinforcement for this conclusion is the conversion of matter into energy by means of its absorption by a black hole, which is much more efficient than the nuclear reactions in the stars: 10% of the mass swallowed is converted into energy, while in nuclear reactions the limit is 0.7%. “This conversion may explain the great emission of energy from the nucleuses of active galaxies like NGC 1097 and M87”, explains Thaisa.

With one million solar masses, the black hole of NGC 1097 impresses, but it is not one of the largest ever found. At the end of March, the physicists who are working with the Chandra X-ray telescope reported the discovery of a much more massive structure: a black hole with around 10 billion solar masses at the center of the most distant quasars – galaxies being formed – ever found, 13 billion light years from Earth.

Milky Way
This year, the team from UFRGS intends to get data about the emission of X- and ultraviolet rays from NGC 1097, through observations with Chandra and Hubble, and to throw light on what happens in the centers of the hundreds of galaxies already identified. “It is believed that the black holes may have been formed together with the galaxies themselves, since the mass estimated for the black holes, based on observations from Hubble, is proportional to the bulges – the spherical central region – of the galaxies where they are to be found”, says Thaisa. Work done with Hubble indicates the presence of black holes in the majority of elliptical and spiral galaxies.

Even at the center of the Milky Way, 30,000 light-years from Earth, it is supposed that there exists a supermassive structure – in the order of 3 million solar masses. For the time being, its existence can only be deduced from the movement of stars close to the nucleus or of intense emissions of X-rays, like the one recently recorded by Chandra: the discharge of radiation varied in hours, something extremely rare, which was probably the result of a black hole swallowing gaseous or stellar matter.

There are still no signs that there is an accretion disc at the center of our galaxy, a testimony of the power of black holes, even at the expense of their own existence. “Perhaps there is not enough matter being swallowed by the black hole to form a disc”, Thaisa ponders. But the picture may change. If a star is captured, something like what one is seeing today in a galaxy as far away as NGC 1097 may happen.