• Astronomers discovered 28 previously misidentified 'heavily obscured black holes' 
  • They were once identified as either distant galaxies or another type of black holes
  • The discovery helps uncover a long-standing mystery in astrophysics

Astronomers spotted over two dozen black holes that were previously misidentified as something else. The mistaken identities were uncovered with the help of data from powerful telescopes.

Supermassive black holes contain hundreds of thousands to billions of times the mass of the Sun and, they grow to such massive sizes by devouring surrounding material. It is believed that at some point in the supermassive black holes' lives, there is a stage wherein they are covered in dense "cocoons" of thick gas and dust, which serve as their fuel source but also obscure them from sight.

These "heavily obscured black holes" are difficult to find. However, in a new study published in the Astrophysical Journal, the authors said over two dozen such black holes have previously been discovered but were misidentified as being other celestial objects.

"With our new identifications we've found a bunch of heavily obscured black holes that had previously been missed," study lead Erini Lambrides of Johns Hopkins University said in a press release. "We like to say we found these giant black holes, but they were really there all along."

'Heavily Obscured' Black Holes
Image: Illustration of 'heavily obscured' black holes. The red cocoon of materials obscure the black hole, which can be seen from the portion of the cocoon that has been cut out. NASA/CXC/M. Weiss

For the study, the researchers looked at massive amounts of data from powerful telescopes including NASA's Chandra X-ray Observatory, Hubble Space Telescope and Spitzer Space Telescope to locate black holes five billion light years away, where scientists have already found 67 heavily obscured black holes. The researchers found 28 more such black holes but, they weren't newly discovered ones. Instead, they had been previously discovered but were identified as either distant galaxies or slowly-moving black holes.

The researchers determined that the heavily obscured black holes are producing more X-rays than previously thought but, the cocoons, which they found to be 10 times denser than previously estimated, are actually preventing the X-rays from being detected by Chandra. This is possibly why they found much lower X-rays from the black hole than estimated.

The discovery of the true identities of the black holes helps solve one of the mysteries of astrophysics. Scientists use the theoretical models estimating the number of black holes in the Universe to explain a uniform glow in X-rays called "X-ray background," which has been attributed to individual growing black holes.

However, X-rays with energies that are above the threshold levels that Chandra can detect have not yet been attributed to individual sources. The discovery of the heavily obscured black holes can explain this "unresolved" component between the theories and the observations.

"It's like the X-ray background is a blurry picture that has been slowly coming into focus for decades," study co-author Roberto Gilli of the National Institute of Astrophysics said. "Our work has involved understanding the nature of the objects that have been some of the last to be resolved."