Hundreds of black holes hidden from view by gas and dust have been unmasked by the X-rays they emit, captured instead by NASA's Chandra space telescope: they are all supermassive black holes, which contain millions or even billions of times the mass of our Sun, and which are located at distances between 550 million and 7.8 billion light-years from Earth.
The discovery, presented at the American Astronomical Society convention in Seattle, was made by a group of researchers led by the Harvard-Smithsonian Center for Astrophysics, who compared the data collected by Chandra in its first 15 years of activity, starting since 1999, with those of the Sloan Digital Sky Survey research, which uses an optical telescope of the Apache Point observatory, in New Mexico.
For about 40 years it has been known that some galaxies appear normal to observations made with optical telescopes, while they shine brightly if observed in X-rays. By comparing the two types of images, researchers led by Dong-Woo Kim have identified more than 800 of these objects, over ten times the number known before Chandra went into operation.
About half of these have been confirmed as hitherto invisible supermassive black holes.
“These results demonstrate how fundamental it is to compare optical data with X-ray data”, comments Amanda Malnati of the American Smith College of Northampton, one of the authors of the study: “The catalog collected by Chandra is an ever-growing treasure, which will be source of many discoveries in the years to come”.
The other objects identified, however, still remain mysterious: some could be galaxies grouped in clusters never discovered before, while the rest could hide other black holes located in galaxies where the optical signals are confused with the intense light of the stars.