A team of researchers has identified a group of black holes that had previously been mistaken for a different kind of black hole, as described in our latest press release. This discovery has important implications for understanding how supermassive black holes grow and evolve over billions of years.

The misjudged black holes were found in the Chandra Deep Field-South (CDF-S), the deepest X-ray image ever taken. The main panel of the graphic shows the CDF-S, which contains over 7 million seconds of observing time from Chandra collected over many years. In this image, red, green, and blue represent the low, medium, and high-energy X-rays that Chandra can detect. Most of the points in this image are a black hole.

This latest work combines X-rays from Chandra in the CDF-S with large amounts of data at different wavelengths from other observatories, including NASA's Hubble Space Telescope and NASA's Spitzer Space Telescope. The team looked at black holes located 5 billion light years or more away from Earth in this patch of sky. At these distances, scientists had already found 67 heavily obscured, growing black holes with both X-ray and infrared data in the CDF-S. In this latest study, the authors identified another 28, highlighted by circles in a labeled version of the image. Optical and infrared images for four of these 28 are shown in a separate graphic.

Credit: NASA/CXC/Penn State/B.Luo et al

These 28 supermassive black holes were previously categorized differently — either as slowly growing black holes with low density or nonexistent cocoons, or as distant galaxies. Supermassive black holes grow by pulling in surrounding material, which is heated and produces radiation at a wide range of wavelengths including X-rays. Many astronomers think this growth includes a phase, which happened billions of years ago, when a dense cocoon of dust and gas covers most black holes. These cocoons of material, which are the fuel source that enables the black hole to grow and generate radiation, are depicted in the artist's illustration in the inset. The cocoon (red) surrounds a disk of material falling onto the black hole, plus a wind of material (blue) blowing away from the disk. A portion of the cocoon is cut out to show the heavily obscured black hole.

These results are important for theoretical models estimating the number of black holes in the universe and their growth rates, including those with different amounts of obscuration. Scientists design these models to explain a uniform glow in X-rays across the sky called the "X-ray background," first discovered in the 1960s. Individual growing black holes observed in images like the CDF-S account for most of the X-ray background.

A paper reporting the results of this study is being published in The Astrophysical Journal and a copy is available online. The other authors of the paper are Erini Lambrides (Johns Hopkins University in Baltimore, Maryland), Marco Chiaberge (Space Telescope Science Institute in Baltimore, Maryland), Roberto Gilli (National Institute of Astrophysics in Bologna, Italy), Timothy Heckman (Johns Hopkins), Fabio Vito (Pontificia Universidad Católica de Chile in Santiago), and Colin Norman (Johns Hopkins).