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Press release
A black hole at the center of our Galaxy | |||
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Paris, October 21, 2002 |
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| An international team made up of astronomers from the "Laboratoire d'études spatiales et d'instrumentation en astrophysique" (Space Research and Astrophysics Instrumentation Laboratory, CNRS-Observatoire de Paris-CNES, French National Space Research Center) and from the "Laboratoire d'astrophysique de l'observatoire des sciences de l'univers de Grenoble" (Astrophysics Laboratory of the Observatory of Sciences of the Universe of Grenoble, CNRS - Observatoire de Grenoble), have recently produced almost certain evidence that a black hole exists at the center of our Galaxy. Using the Nasmyth adaptive optics system (NAOS) and the infrared camera CONICA that equip one of the telescopes of the Very Large Telescope of the European Southern Observatory, they have determined the motion of a star around the central object. The star takes 15.2 years to orbit a black hole whose mass is 2.6 million times the mass of the Sun, and as it goes round, it approaches the black hole at distances of about 124 astronomical units (AU), i.e. three times the distance between the Sun and Pluto. At the center of our Galaxy, an intense radio source known as "Sagittarius A*" or "SgrA*" exists. It could be a black hole of from 2.6 million to 3.3 million solar masses because past observations have shown that the closer the stars come to this central object, the faster they travel. An international team (1) has recently observed a star very close to the central object using the Very Large Telescope of the European Southern Observatory (ESO) in Chile. The researchers used the adaptive optics system NAOS (2) equipped with the infrared camera CONICA installed on the telescope whose diameter is 8.2 meters. On the basis of observations made from 1992 to 2001 with the New Technology Telescope (3.6 m in diameter) of the ESO at La Silla, Chile, and of the data obtained with NAOS and CONICA as the star (S2) was passing closest to SgrA*, they have been able to reconstruct its trajectory. The trajectory is an ellipse of 11 light-days by 5.5 light-days, and, when closest to SgrA*, the star is at a distance of 17 light-hours, i.e. 124 astronomical units (three times the distance between the Sun and Pluto; 1 AU = 150 million kilometers). The star completes its orbit in 15.2 years. Analysis of this trajectory would imply that the central object is a 2.6 (±0.2) million solar mass object. The data obtained as the star S2 was passing closest to the central object reveal that the central object is concentrated within a very small radius for such a considerable mass, which means that it can only be an extremely dense object. If it were a cluster of stars of the neutron star or stellar black hole type, the object should collapse in less than 100,000 years, which is inconceivable in view of the observations and the theoretical models. Moreover, a central system made up of strange particles of the axion or gluon ball type, depending on the size and mass of the central object, would imply an orbital period for the star of at least 37 years, which is incompatible with observations. We are therefore almost certain to be in the presence of a supermassive black hole. Such findings have been made possible because the NAOS adaptive optics system is the highest performance in its category, and also because it is equipped with a turbulence-correcting analyzer that operates in the infrared domain. This system is absolutely unique and has been developed by the team from the Observatoire de Paris. The stars at the center of our Galaxy are hidden by enormous quantities of dust, and they can only be observed effectively in the infrared domain. In addition, it was possible to use a star very close to SgrA* as an infrared reference for the adaptive optics correction. This result marks a first and very important step in observing the center of our Galaxy and its central object. Analysis of the orbits of the stars around SgrA* enables the definition of physical constraints, which helps to improve theoretical models concerning the black hole at the center of our Galaxy. Measurement of direct emission from the source SgrA* in the infrared domain is in progress and will improve understanding of the very high energy mechanisms at work around such objects. In addition, with the commissioning of the Very Large Telescope Interferometer of the ESO, astronomers will be able to obtain images of very high resolution (of the order of a few light-hours), and thus to have information on the behavior of matter in the environment very close to the black hole, of 10 to 100 times the radius of collapse of the black hole. Understanding the physics in the environment surrounding a black hole is important and what we learn can also be applied to other galaxies, in particular active-nuclei galaxies which have supermassive black holes at their centers. Reference: Published in Nature, October 17, 2002 (1) Laboratoire d'Etudes Spatiales et d'instrumentation en Astrophysique (CNRS-Observatoire de Paris-CNES); Laboratoire d'astrophysique de l'observatoire des sciences de l'univers de Grenoble (CNRS - Observatoire de Grenoble); Max-Planck-Institute, Germany; University of Cologne, Germany; Weizmann Institute of Science, Israel; Harvard-Smithsonian Center for Astrophysics, USA; and ESO. (2) For further information about NAOS, please consult the CNRS Web Site: http://www.cnrs.fr/cw/fr/pres/dyncom/communique.php?theme=6&;article=18&nbrttl=19&page=1 et : http://www.cnrs.fr/cw/fr/pres/dyncom/communique.php?theme=6&;article=21&nbrttl=19&page=1 Researcher
contact: CNRS-INSU contact: Press contact
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