Astronomers have discovered one type of growing supermassive black hole masquerading as another, thanks to a set of telescopes, including NASA’s Chandra X-ray Observatory. The true identity of these black holes helps solve a long-standing mystery in astrophysics.
The misidentified black holes are from a survey known as Chandra Deep Field-South (CDF-S), the deepest X-ray image ever taken.
Supermassive black holes grow by pulling on the surrounding material, which heats up and produces radiation over a wide range of wavelengths, including X-rays. Many astronomers think that this growth includes a phase, which happened billions of years ago, when A dense cocoon of dust and gas covers most of the black holes. These cocoons of material are the fuel source that allows the black hole to grow and generate radiation.
Based on the current image of astronomers, there should be many black holes embedded in such a cocoon (known as “very dark” black holes). However, this growing type of black hole is notoriously difficult to find, and so far the observed number has been below predictions, even in the deepest images such as CDF-S.
“With our new identifications we have found a lot of very darkened black holes that had previously been lost,” said Erini Lambrides of Johns Hopkins University (JHU) in Baltimore, Maryland, who led the study. “We like to say that we found these giant black holes, but they really were there the whole time.”
The latest study combined more than 80 days of Chandra observation time on 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 observed black holes located 5 billion light-years or more from Earth. At these distances, the scientists had already found 67 very darkened and growing black holes with X-ray and infrared data on the CDF-S. In this latest study, the authors identified 28 others.
These 28 supermassive black holes were previously classified differently, either as slow-growing black holes with low-density or non-existent cocoons, or as distant galaxies.
“This could be considered a case of mistaken black hole identity,” said co-author Marco Chiaberge of the Space Telescope Science Institute in Baltimore, Maryland, “but these black holes are exceptionally good at hiding exactly what they are.”
Lambrides and colleagues compared their data to expectations for a typical growing black hole. Using data from all wavelengths except X-rays, they predicted the amount of X-rays they should detect from each black hole. The researchers found a much lower level of X-rays than they expected from 28 sources, implying that the cocoon around them is approximately ten times denser than scientists previously estimated for these objects.
Considering the higher density of the cocoon, the team showed that misidentified black holes are producing more X-rays than previously thought, but the denser cocoon prevents most of these X-rays from escaping and reaching the Chandra telescope. This implies that they are growing faster.
The previous groups did not apply the analysis technique adopted by Lambrides and his team, nor did they use the full set of data available for the CDF-S, giving them little information on cocoon density.
These results are important to theoretical models that estimate the number of black holes in the universe and their growth rates, including those with different amounts of darkening (in other words, how dense are their cocoons). Scientists design these models to explain a uniform X-ray brightness across the sky called the “X-ray background,” first discovered in the 1960s. The individual growing black holes seen in images like the CDF-S represent most of the x-ray background.
The X-ray background that is not currently resolved in individual sources is dominated by X-rays with energies above the threshold that Chandra can detect. Highly darkened black holes are a natural explanation for this unsolved component because low-energy X-rays are absorbed by the cocoon more than high-energy X-rays, and are therefore less detectable. The additional highly obscured black holes reported here help to reconcile past differences between theoretical models and observations.
“It is as if the X-ray background is a blurry image that has been slowly focusing for decades,” said co-author Roberto Gilli of the National Institute of Astrophysics (INAF) in Bologna, Italy. “Our job has been to understand the nature of objects that have been some of the last to be resolved.”
In addition to helping explain the X-ray background, these results are important for understanding the evolution of supermassive black holes and their host galaxies. The masses of galaxies and their supermassive black holes are correlated with each other, which means that the more massive the galaxy is, the more massive the black hole is.
An article reporting the results of this study is being published in The Astrophysical magazine. The other authors of the article are Timothy Heckman of JHU; Fabio Vito of the Pontificia Universidad Católica de Chile, in Santiago, Chile; and Colin Norman of JHU.
NASA’s Marshall Space Flight Center administers the Chandra program. The Chandra X-ray Center at the Smithsonian Astrophysical Observatory controls scientific and flight operations from Cambridge and Burlington, Massachusetts.
Hungry for black holes among the most massive in the universe
A large population of obscured AGNs disguised as low-light AGNs at Chandra Deep Field South, arxiv.org/abs/2002.00955
Provided by Chandra X-ray Center
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