A look into the 'pantries' of black holes
The most complete survey of accreting supermassive black holes, performed by an international collaboration including ETH astronomers, provides answers to how the close environment of these black holes is shaped.
Very massive black holes, a million to billion times more heavy than our Sun, lurk at the centers of galaxies, including our own Milky Way. Some of them grow by consuming material from their environment. Such ‘accreting’ black holes emit light, caused by heating up of the dust cloud around the black hole due to friction. This light is often so intense that it can outshine the light emitted from all the stars in the galaxy combined, making it possible to observe the environment of growing black holes. And this is what an international team including astronomers at ETH Zurich has done at an unprecedented scale. They compiled the most complete survey yet of accreting supermassive black holes, and their data revealed unique insights into how the interplay between the pull exerted by the black hole and push resulting form the intense light shapes the dusty gas surrounding these black holes. These results have been published today in the journal Nature.
‘Space radiographs’ provide unique insights
The large amounts of gas and dust surrounding most luminous growing black holes are distributed in a doughnut-shaped structure, a sort of pantry that allows the black hole to keep growing. To peek inside these ‘pantries’, the team, composed of researchers from Chile, the United States, Japan and Switzerland, used X-rays. Similarly to how medical doctors obtain insights into their patients, astronomers can use X-ray observations to look inside regions in space that are blocking visible light, including the dusty regions around black holes. These ‘space radiographs’ in turn provide a basis for measuring the amount of material around a large number of black holes, and to study its evolution.
The project started as an international collaboration in 2013, says Dr Benny Trakhtenbrot, an ETH Zwicky Prize fellow. “It took us many years to collect all the data and measure the black hole masses from several ground- and space-based telescopes.” Measuring black hole masses is a challenging task, and the project involved a large number of students carrying out parts of the analysis. With all the data finally in hand, the team was able to obtain new insights into the environments of black holes. Dr Claudio Ricci, lead author of the study, explains: “The intense light emitted around the black hole pushes the material away from its vicinity, overwhelming the strong gravity pull of the black hole.” This means that the large amount of energy released by the material falling into the black hole can make the gas evaporate. If the black hole feeds too eagerly, the energy produced could evaporate the food available for the future.
Just the beginning
So far, astronomers thought that the inner layer of the torus around an accreting black hole smoothly recedes as the black hole grows faster and faster. The present study finds that this does not have to be the case. Instead, at a very specific growth rate, most of the obscuring material just disappears, leaving a nearly naked quasar, that is, a black hole without its torus. “It’s always been one of the big problems in astrophysics to understand the structure of the material around growing black holes”, says ETH-professor Kevin Schawinski. “We just don’t have the ability to resolve these systems, to take pictures and see what’s going on. Now that we better know how quasars work, we can use that to better interpret many other observations and better understand what’s going on.”
Indeed, the team is far from being done. They continue their hunt for observations of nearby growing black holes using telescopes from around the world and in space. “This is just the beginning of all our hard work paying off”, Michael Koss, a co-author on the study and a former SNSF Ambizione postdoctoral fellow at ETH Zurich, is convinced. “As we continue to observe more of these black holes I am sure nature will continue to surprise us.”
Reference
Claudio Ricci, Benny Trakhtenbrot, Michael J. Koss, Yoshihiro Ueda, Kevin Schawinski, Kyuseok Oh, Isabella Lamperti, Richard Mushotzky, Ezequiel Treister, Luis C. Ho, Anna Weigel, Franz E. Bauer, Stephane Paltani, Andrew C. Fabian, Yanxia Xie & Neil Gehrels, The close environments of accreting massive black holes are shaped by radiative feedback. Nature (advance online publication). external pagedoi:10.1038/nature23906