A few years ago, NASA’s Chandra X-ray Observatory captured the deepest X-ray image of the universe ever taken, containing supermassive black holes and galaxies from early in the distant cosmos. However, a fresh look at this spectacular image, coupled with further data from NASA’s Hubble and Spitzer Space Telescopes, has revealed a “case of mistaken black hole identity” to astronomers.
“With our new identifications we've found a bunch of heavily obscured black holes that had previously been missed,” lead researcher Erini Lambrides of Johns Hopkins University (JHU), Maryland, said in a statement. “We like to say we found these giant black holes, but they were really there all along.”
Billions of years ago, astronomers believe that a dense cocoon of dust and gas surrounded black holes. Whilst this provided a fuel source for black holes, it could also block some of the radiation emitted from the in falling material. These “heavily obscured” black holes have been notoriously difficult to find, with numbers falling short of predictions.
However, by looking again for black holes located 5 billion light-years or more away from Earth in the Chandra Deep Field-South survey, Lambrides and her colleagues identified a further 28 of the target type. Previously, these cosmic objects had been categorized as slowly growing black holes with low density or nonexistent cocoons, or as distant galaxies.
“This could be considered a case of mistaken black hole identity,” co-author of the study due to be published in The Astrophysical Journal, Marco Chiaberge of Space Telescope Science Institute in Baltimore, Maryland, said, “but these black holes are exceptionally good at hiding exactly what they are.”
The butterfly emerged from the cocoon when the researchers found a much lower level of X-rays than expected from the 28 sources. This implied a cocoon 10 times denser than previously estimated, which had prevented X-rays from escaping and reaching the Chandra telescope. At the re-evaluated X-ray level, the black holes had a much quicker growth rate.
Not only are these results important in helping to understand the evolution of supermassive black holes and their host galaxies, but they can also be used to refine the uniform glow of X-rays across the sky – the “X-ray background.”
“It's like the X-ray background is a blurry picture that has been slowly coming into focus for decades,” co-author Roberto Gilli from the National Institute of Astrophysics (INAF) in Bologna, Italy, explained. “Our work has involved understanding the nature of the objects that have been some of the last to be resolved.”