A premature black hole

A supermassive black hole, called CID-947, has grown much earlier than expected while its galaxy continued to generate stars. And what emerges from a study published in Science led by researchers at ETH Zurich is that black holes and their host galaxies do not always evolve together, a fact that calls into question the current models on the formation of galaxies.
The black holes reside in the nuclei of galaxies. Most of them have a mass small when compared with that of the host galaxy. Today, a group of researchers led by colleagues at ETH Zurich have discovered a very massive black hole that has formed so quickly that its host galaxy has not been able to keep up. This challenges some previous assumptions on the processes of co-evolution of galaxies and their black holes. The results are published in Science.


Credit: B. Trakhtenbrot et al. 2015

The international team of astronomers, led by Benny Trakhtenbrot, a researcher at the Institute of Astronomy at ETH Zurich and lead author of the study, is hunting for ancient massive black holes using the Keck 10-meter telescope located in Hawaii. The first data collected made it possible to reveal a giant black hole in a galaxy that is ordinary, although distant, known by the acronym CID-947. Since the light has traveled a long way before you get to the instruments to Earth, the age to which the object is observed dates back to a period when the universe was younger than 2 billion years.
The analysis of data collected in Hawaii indicates that the black hole in the galaxy CID-947 has a mass of nearly 7 billion solar masses and is one of the most massive objects in the category identified so far. In particular, what surprised the researchers is not the value itself of the mass of the black hole, rather than the mass of the host galaxy. ‘The measures correspond to a typical value of the mass of a galaxy ordinary”, says Trakhtenbrot. “So we have a giant black hole that lies within a galaxy of normal size.” The result was so surprising that two researchers had to independently verify the mass of the galaxy. At the end, both arrived at the same conclusion.
The question that the researchers now pose is whether the physical processes occurring during periods of primordial cosmic history may be different. Most galaxies, including the Milky Way, have black holes in their nuclei whose mass can take values of a few million to several billion solar masses. “The black holes are the astrophysical objects which create a gravitational field so strong that nothing, not even light, can escape,” says Kevin Schawinski ETH and co-author of the study. “The theory of general relativity describes how the black holes deform space-time around them. Their existence can be gauged from the fact that the matter is greatly accelerated by the gravitational field and is forced to emit radiation especially energy. ”
Until now, observations have shown that there is a relationship between the galaxy and its central black hole: in other words, the higher the number of star in the host galaxy and the greater the mass of the black hole. “This is true of the local universe, reflecting in some way the situation of the past cosmic history,” says Trakhtenbrot. This kind of connection, together with other evidence, led scientists to assume the hypothesis according to which the evolution of black holes and star formation go hand in hand. “We think this is reasonable when the existence of a common reserve of cold gas has been in charge of star formation and the ‘power’ of the galactic black hole,” continues Trakhtenbrot. In addition, previous studies it has been noted that the radiation emitted during the evolutionary growth of the black holes have controlled, or even blocked, the creation of new stars, as that the intense radiation emitted has heated the gas in the course of time. The latest results, however, suggest that these processes act differently, so it is believed in the early Universe.


Credit: B. Trakhtenbrot et al. 2015

What remains of the star formation? Meanwhile, it must be said that the black hole observed by the researchers has a mass about 10 times less than its host galaxy. Local universe today, black holes typically reach a mass equal to 0.2-0.5 percent of the mass of the host galaxy. “This means that our black hole grew more efficiently than its galaxy, contradicting models that predict one evolution hand in hand,” says Trakhtenbrot. The researchers then concluded that although the black hole has reached the end of its life, the stars are still forming. So, this means that in contrast to previous assumptions, the energy and the flow of gas, supplied by the black hole, did not stop the creation of new stars.
In short, the future Galaxy could continue its evolution, but the relationship between the mass of the black hole and that of the stars remains surprisingly unusual. The authors believe that CID-947 can be a kind of precursor systems of massive and particularly extreme that we see today in the local universe, such as galaxy NGC 1277 in the constellation of Perseus is about 220 million light years. The next step will be to get more clues on the relationship between the black hole and its host galaxy planning a series of dedicated observations that will be realized with the radio telescope ALMA.
“The source in question was selected with the X-band satellite Chandra Chandra from my COSMOS Legacy Survey, a project that I got through an X-ray Visionary Program Chandra to observe the COSMOS field to 4,800,000 seconds,” he told Media INAF Francesca Civano of Yale University and Harvard Smithsonian Center for Astrophysics, a co-author of the study. “According to the theory of co-evolution, black holes and galaxies typically evolve together. CID-947, however, is a particular source. In fact, the mass of its black hole was accumulated before and now the galaxy is forming stars at last, to reach the normal system, such as is observed in other local galaxies. ”
“This discovery reveals a more complex and surprising than expected and so far observed in the relations between the host galaxy and the central black hole,” adds Angela Bongiorno, Astrophysical Observatory of Rome and co-author of the study. “In the future, with even more powerful telescopes, such as those currently under construction, we can explore comprehensively the entire population of supermassive black holes in the distant Universe, looking for specific items like the one just discovered, and thus have a clearer picture of the mutual interactions between the galaxy and its nucleus during their evolutionary history. “

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