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In the 10th February issue of The Astrophysical Journal, the results of the analysis of data taken by the Swift X-ray satellite will be published, showing that a significant fraction of active galaxies escape direct observation.
Since 2004 Swift has been mapping the X-ray emission of the whole sky at energies between 15,000 and 200,000 eV, confirming the existence of a diffuse background emission not attributable to known sources.
Although the data collected by Swift include hundreds of active galactic nuclei, and therefore super-massive black holes considered to be sources of the diffuse X-ray background, they imply the presence of many more sources that are invisible at all frequencies below low energy X-rays.
Although not directly observable, the presence of an active galactic nucleus can be detected in the infrared via the radiation emitted by dust heated by the nucleus, but unfortunately this emission is easily confused with that emitted by dust in star forming regions.
To shed light on the question, an international team of researchers, led by Davide Burlon and including Neil Gehrels (principal investigator of Swift) analysed the highest energy X-ray component of the map, finding 10 or so active galaxies whose nuclei were so heavily obscured that only the hardest X-rays escape.
Extrapolating the number of similar sources that may be present in the Universe, Burlon and colleagues found that these very obscured sources account for 20 to 30% of the active galaxies. The reason why they are so hidden is probably their inclination with respect to our line of sight, something that is also important in defining the various types of known active galaxies.
The team's findings imply that the diffuse background is from sources that had a peak of activity 7 billion years ago, and if so the cosmic X-ray background can be neatly explained, as shown in the above figure, by an appropriate sum of obscured and unobscured active galactic nuclei.
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