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Astrophysics

Holes in the Cosmos

More numerous than previously used to be thought, black holes may be present in all the galaxies

NASA/CXC / JOSÉ ROBERTO MEDDASeductive and mysterious, the trajectory of the black holes is like the famous false striptease performed by Rita Hayworth in the film Gilda, of 1946. Like the character embodied by the American actress, who bares herself minimally, just without gloves, in her magnetic performance, these celestial bodies never show themselves entirely. At the most, they insinuate themselves from time to time. Capable like nothing else in the Universe of attracting matter, so much so that not even light manages to escape from their gravitational field, black holes do not reveal their silhouette clearly and explicitly.

Strictly speaking, they are invisible both to the human eye and to the lenses of the most powerful telescopes, whether installed in the confines of the Universe or here in our galaxy, the Milky Way. Describing them is only possibly indirectly. When the instruments of celestial observation record colossal accelerations in the orbital velocity of a star or in the center of a galaxy, the majority of astronomers explain this phenomenon as due to the influence of a colossal gravitational force acting on this region of space. Force of such magnitude can only be derived from an extremely compact object, of an enormous mass, which, however, is not to be seen: a black hole, near to which, if Albert Einstein’s Theory of General Relativity is right, time and space must be curved.

In spite of all the difficulties and limitations in observation, astronomers and astrophysicists, with the help of powerful terrestrial and space telescopes, have produced a small revolution in the knowledge about the nature and the role of black holes in the last five years. From exotic and rare objects, found only in given regions and situations of the universe, they are now looked on as a celestial occurrence that is much more frequent and important for the comprehension of the Cosmos.

“Black holes have come to be seen as a part of the galaxies, influencing and being influenced by their evolution”, explains astrophysicist Thaisa Storchi Bergmann, from the Federal University of Rio Grande do Sul (UFRGS), who is studying how these great matter eaters feed themselves on everything around them. Between the 29th of this month and March 5, some 300 astronomers from Brazil and abroad will be meeting at Gramado, in the hills of the state of Rio Grande do Sul, to discuss precisely the interrelation between the black holes and the elements that constitute the galaxies – the stars and the interstellar medium – during a symposium of the International Astronomical Union organized by Thaisa. There is no lack of burning issues when the subject is black holes. A series of recent observations has raised some interesting points relating to this kind of celestial object:

Black holes are more numerous than previously thought, and they must be present, if not in all, in at least the majority of galaxies. It used to be believed before that they were only in the center of the so-called Active Galaxy Nuclei (AGN) – which emit great quantities of energy, above all in their central portion, and represent less than 10% of the total of galaxies in the Universe – in double star systems, and in the interior of quasars, extremely distant very luminous objects, which look like a star, but which in fact are extremely active nucleuses of galaxies.

Today, it is believed that almost all galaxies, even those that are not classified as active, have in their nucleus, albeit in a dormant state, this obscure object that sucks up matter. The Milky Way, our galaxy, which is not classified as active, seems to have a black hole that is not very big, which sporadically manifests itself. In a rare snapshot, at the end of 2002, Nasa’s X-ray telescope, Chandra, registered a galaxy, NGC 6240, with two supermassive black holes in its central region, which are apparently on the way to merging into a single, even more supermassive black hole.

Their diversity in size is greater than used to be supposed. Forecast in theory since the end of the 18th century, and called as such since the 1960’s, black holes are always gigantic. But researchers used to split them into only two different forms: there were the supermassive ones, with a mass millions or even billions greater than the Sun’s, which arise in the center of some galaxies, and the stellar ones, whose mass exceeds that of the Sun by about ten times, located in the body of the galaxies, generally in double star systems. New indications suggest the existence of black holes of a third size, medium.

This seems to be the case, amongst others, of the black hole located by the Hubble space telescope, also Nasa’s, in a cluster of stars called G1, located in the interior of Andromeda, the galaxy closest to the Milky Way. Estimated at the beginning of 2002, the hole had a mass hundreds of times greater than the Sun’s.The center of galaxies, in particular in those with an active nucleus, is the classic region for finding supermassive black holes. But, recently, this kind of object has been detected at points in space at which, up until a years ago, their presence was held to be improbable, such as on the periphery of galaxies.

A medium-sized black hole, whose presence in the M82 galaxy was detected by Chandra, is 600 light years far from the heart of this galaxy. It is outside the nucleus of the galaxy. Equivalent to the distance that light, with its speed of constant movement at 300,000 kilometers per second, covers in the course of 365 days, a single light-year is equivalent to some 9.5 trillion kilometers.

These new clues about the quantity, the size, and the location of black holes seem to lead to a fundamental question: today, it is almost impossible to study the evolution of the galaxies – agglomerates formed by a vast number of stars (somewhere between millions and billions of them), gas and dust, all this kept cohesive by the action of gravity – without, at some moment, analyzing the role of these mysterious eaters of matter. Many astronomers believe that the mass of the black hole is proportional to that of a concentration of stars, called bulge, present in the central part of the majority of galaxies – more or less as if both, the hole and the bulge, had been formed and/or grown up together.

“Possibly, all galaxies have a black hole in their center”, explains researcher Laerte Sodré Júnior, from the Astronomy, Geophysics and Atmospheric Sciences Institute at the University of São Paulo (IAG-USP). “But not all the holes are active.”Perhaps most black holes, above all those located in the “calmer” galaxies, like the Milky Way and so many others in which there is no great emission of energy, work in a regime reminiscent of some terrestrial volcanoes: for the most part of time, they show no signs of life, and, suddenly, start a phase of intense activity, to the surprise of their observers.

The hypothesis has not been discarded that, at some moment, all galaxies may have been active and shown the almost incontestable form of a supermassive – and hungry for matter – black hole in its nucleus. With time, due to the reduced availability of “food” – basically, dust and gas – in its surroundings, or for whatever reason, they would have calmed down. This does not mean that the black hole in their centers has vanished. For lack of food, it could just have fallen silent.

“Every galaxy may have been a quasar in a distant past”, comments astrophysicist Marcio Maia, from Valongo Observatory, of the Federal University of Rio de Janeiro (UFRJ), who is studying galaxies with an active nucleus. Nowadays, in an almost paradoxical way, the luminous quasars are looked on as the celestial objects endowed with the most active supermassive black holes ever to have been heard of. The problem is that, in the case of the black holes, the time of inactivity may have been millions or billions of years – and, depending on the wavelength used to observe the galaxies at a given moment, the clues that denounce the existence of a great devourer of matter cease to exist or pass unperceived. For practical effects, it is as if there were no (longer) a black hole there, since it, in itself, is invisible.

Schizophrenic galaxy
The history of NGC 6221, distant 60 million light-years from the Earth, illustrates the difficulty of descrying black holes in the Universe and simply labeling galaxies as active or “normal”, whether they have or not the great devourer of matter. Images obtained of the NGC 6221 in the wavelength of visible light and in near infrared suggest that it is a galaxy of the “Starburst” kind, a nursery of (young) stars. This kind of galaxy is not usually associated with the presence of black holes, although this concept is being reviewed today. However, the same NGC 6221, if seen with the assistance of X-rays, displays characteristics that are typical of the Seyfert kind, with a very active nucleus, where there should be a supermassive black hole. “I like to joke that this galaxy is schizophrenic”, says astronomer Roberto Cid Fernandes, from the Federal University of Santa Catarina (UFSC), who studied this galaxy with Thaisa and American researchers. “It is a hybrid. Instead of stars or a black hole in its nucleus, it has the two things.”

A question difficult to address is the cosmological version of the classic question of the chicken and the egg. After all, which appeared first: the galaxy or its black hole? Apparently, these two structures compete for the dust and gas available in space. Hence the query: are the great star agglomerates are formed around preexisting black holes, or is it these objects without any visible light that derive from the presence, earlier in time, of the galaxies. There are defenders of the most varied points of view.

Including of the idea that this is a false question. After examining 120,000 galaxies in the Sloan Digital Sky Survey project, an international team of researchers coordinated by Tim Heckman, from the John Hopkins University, concluded that stars and black holes are formed and grow at the same speed. To translate: there is no way of knowing which came first. Heckman is going to be in Gramado in March, at the meeting of the International Astronomical Union, debating this and other data about black holes and galaxies.

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