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Astrophysics

A mirrored black hole

A cone of ions reflects the image of the mysterious object at the center of the galaxy

Aurore Simonnet/Sonoma State University/NasaDrawing of a black hole with a ring of matter around itAurore Simonnet/Sonoma State University/Nasa

For almost half a century, astrophysicists have been cataloguing regions in space that are so dense and compact and that have such a powerful gravitational field that nothing can escape from them, not even light. To date, black holes (the generic name for all these points in the Universe where space and time are deformed) of various types and shapes have been identified. Focusing on only two extreme cases, only slightly more than 1% of the known stars can, at the end of their lives, turn into such sinkholes for matter and most galaxies, or perhaps all of them, hold supermassive black holes in their center with mass greater than millions or even billions of suns.

One strong indication of the presence of one more variant of these objects of a singular nature was obtained by the astrophysicist João Steiner, from the University of São Paulo (USP), along with two of his doctoral students, Tiago Ricci and Roberto Menezes, both of whom hold FAPESP grants. By using a sophisticated method to analyze three-dimensional data, which they themselves invented, the researchers found a mirrored black hole, i.e., a black hole and its image projected on a cone of hydrogen ions that behaves like a mirror –  at the center of NGC 7097, an elliptical galaxy in the austral constellation of Grus, which lies at some 105 million light-years from Earth. “It’s the first time that this phenomenon, previously forecast only in theory, has been recorded,”  says Steiner, who is about to publish an article on the finding in Astrophysical Journal Letters.

The black hole and image are so close to each other that it is almost impossible to tell them apart. The distance between them is in the order of 20% of a second of arc. A measurement used in astronomy, a second of arc equals 1/3,600 of a one-degree angle. In other words, very little lies between the actual object and its virtual image. “Our method allows one to see two points at the center of the galaxy where other techniques see only one,”  states Steiner.

By definition, black holes cannot be observed by means of any of the lengths of an electromagnetic wave. Therefore, there is no unequivocal proof of their existence, only indirect clues. A little before being swallowed by a black hole, matter becomes so hot that it releases energy in the form of radiation, such as X-rays. Mysterious or unexplained sources of radiation in certain points of the Universe, as in the center of the galaxies with a still active nucleus, are interpreted by astrophysicists as being indicative of black holes.

060-061_Buraco Negro_183IAG-USP/GEMINIIn the case of  NGC 7097, the evidence of the presence of an object with these characteristics was captured by an integral field spectrograph installed at the Gemini South Telescope at Cerro Pachon, Chile, an international project in which Brazil is one of the partners. The instrument generates extremely detailed information about the sky and the object observed in the form of a data cube, in 3D. Two dimensions of the cube are spatial: two-dimensional images that represent the height and width of the analyzed region. The third dimension, which corresponds to depth, is provided in the form of a spectrum graph of the so-called energy emission lines and obtained from the object under study. This type of spectrograph produces so much information that the extraction of relevant data for scientific studies is only partial and is not mathematically optimized.

To circumvent this limitation, Steiner and his students created, two years ago, a statistical method of analysis of the so-called main components of the data cubes (see article in Pesquisa FAPESP, issue 159, dated May 2009). Vaguely inspired in the tomography employed in medicine, the technique screens the records produced by the spectrograph, summarizing and ordering 99.9% of the data in the form of a set of five images and their respective graphs. The first picture alone contains 99.53% of the data cube information. Together, the second, third, fourth and fifth images represent the other 0.46%.

As an example, the image that revealed the mirror of the NGC 7097 black hole is equivalent to 0.02% of the information contained in the data cube about the galaxy. “At first, it took months to use the method and extract the information,” states Tiago Ricci. “Today, however, I produce the images and graphs in two days.”  Of course, interpreting the data is a different story and this work can, indeed, consume a lot of time. The technique can be useful in order to uncover subtle phenomena of the Universe. “It can reveal a weak signal that is “hidden”  within another, stronger one, as is the case in the signature of the supermassive black holes at the center of galaxies, where the light is dominated by the emission of the stars,”  states the astrophysicists Thaisa Storchi-Bergmann, from the Federal University of Rio Grande do Sul (UFRGS), who also uses the method.

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