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N°33 I quarterly I April 2014 Live from the Labs | Spotlight 7 w them to measure the temperature and density of the gas at various distances from the black hole. “We discovered that a mere 1% of the matter captured by the intense gravity of the black hole gets close enough to be caught,” Grosso explains. The rest is ejected before it can reach this point of no return. Researchers believe that unlike matter surrounding other black holes, this one is too hot and not dense enough to be trapped effectively. The astronomers also used radiation from a  pulsar  detected near Sagittarius A* to accurately measure the magnetic field near the black hole. “It proved to be extremely strong,” says Ismaël Cognard from the LPC2E2 in Orléans, who co-authored the study. “We think this could be the reason why Sagittarius A* is relatively inactive.” Indeed, the intense magnetic field of the surrounding gas would counter the gravitational force of the black hole, literally preventing matter from falling into it. Birth of a Giant Astrophysicists are also trying to understand how such objects formed in the first place. For stellar black holes, which are much smaller than their supermassive counterparts and of which around 20 exist in our galaxy, astronomers know that they are created when massive stars explode as supernovae. When such a violent event occurs, the outer layers of the star are blown out into space while its core collapses under its own weight, forming an extremely dense body, namely a black hole. Thus, a black hole the size of Paris would have a mass equal to that of three suns. Yet when it comes to supermassive black holes, things are slightly more complicated. Astronomers initially believed that they were the result of stellar black holes evolving over billions of years—except that black holes weighing several billion solar masses already existed shortly after the Big Bang. “Even if you assume that they expanded by absorbing both the stars and the gas present in their galaxy and by merging with other black holes when colliding with neighboring galaxies, you still don’t end up with objects as big as these giants,” says Jean-Pierre Lasota from the IAP.3 To make headway on this issue, astronomers are stepping up observations using both ground-based telescopes and 03 satellites. They have so far managed to determine a key parameter of black holes: their spin. “When matter falls into a black hole, part of its angular momentum is transferred to the latter,” Porquet explains. “A black hole’s spin value tells us how it has absorbed matter. A slow spin indicates that matter was swallowed in with random rotations, whereas a rapid one shows that matter was absorbed continuously in the same direction as the black hole’s rotation. In the middle range are black holes that probably result from numerous mergers.” Quasars have also given up some of their secrets. We now know that these emit intense winds of hot gas in the interstellar medium, and that they convert part of the matter around them into a powerful particle jet that can extend well beyond the galaxy that hosts them, over distances of up to several thousand light years. Astronomers believe that ejecting both particles and hot gas away from the black hole may limit its growth. “Such ejections of matter may even have an effect on the entire galaxy,” says Porquet, “by either hindering or encouraging star formation, depending on whether they break up the gas clouds that collapse and form stars, or alternatively supply them with matter.” Pictures coming soon? What better than a photograph to prove the existence of black holes? Such images could be obtained in a few years’ time by linking together several radio telescopes, including those of the ALMA observatory in Chile. According to Porquet, imaging their “silhouette” could even happen in less than two years thanks to Very Long Baseline Interferometry (VLBI) at short wavelengths. “Yet the only way to prove that black holes really exist is to detect the gravitational waves they emit,” says Lasota. Einstein’s theory of gravity states that when two compact objects merge or spin around each other, they create in the very fabric of space-time ripples that travel great distances at the speed of light. In the case of black holes, these ripples have a unique shape that the VIRGO and LIGO observatories (in Europe and the US, respectively) are attempting to detect. “This shouldn’t take more than five years,” enthuses Lasota. 01. Observatoire astronomique de Strasbourg (CNRS / Université de Strasbourg). 02. Laboratoire de physique et chimie de l’environnement et de l’espace (CNRS / Université d’Orléans). 03. Institut d’astrophysique de Paris (CNRS / UPMC). Contact information: OAS, Strasbourg. Delphine Porquet > delphine.porquet@astro.unistra.fr Nicolas Grosso > nicolas.grosso@astro.unistra.fr IAP, Paris. Jean-Pierre Lasota > lasota@iap.fr LPC2E, Orléans. Ismaël Cognard > ismael.cognard@cnrs-orleans.fr pulsar. A celestial object that produces a periodic signal and emits intense electromagnetic radiation. 03 Ground-based telescopes like the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile are used to detect black holes. © X-ra y: NASA/UMASS/D. WANG et al . - Infrar ed: NASA/STSCI © C. MALIN/ ESO


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