An article published in “The Astrophysical Journal” reports the discovery of 28 heavily obscured supermassive black holes. A team led by Erini Lambrides of Johns Hopkins University combined over 80 days of observations of NASA’s Chandra space telescope in the survey known as Chandra Deep Field-South (CDF-S) with the ones of other telescopes that include Hubble and Spitzer to identify active galactic nuclei whose emissions at many wavelengths were blocked by the huge cocoon of materials that surrounds them. The heavily obscured supermassive black holes are among the most sought after because understanding their growth mechanisms helps to understand the evolution of these extreme objects that can have masses even billions of times the Sun’s.
The mechanisms of origin of supermassive black holes are still under study, and a large part of the theoretical models has yet to be verified with data deriving from observations and measurements. According to one of these models, there’s a growth phase in which they devour a huge cocoon of gas dust that surrounds them. When these materials approach the black holes, they get heated by their powerful gravity force to the point of generating very strong electromagnetic emissions in what are called active galactic nuclei.
Quasars, a common type of active galactic nucleus, are the brightest objects in the universe, to the point that even those that existed when the universe was young are visible billions of light-years away. In some cases, however, the active galactic nuclei are surrounded by such an amount of materials that they block almost all electromagnetic emissions, and for this reason, astronomers call them heavily obscured supermassive black holes. The bottom image (NASA/CXC/M.Weiss) shows an artistic representation of the cocoon that surrounds and obscures a supermassive black hole, shown in the lower half where the cocoon is cut out.
The CDF-S survey enabled to create the deepest X-ray image of part of the sky ever. Supermassive black holes emit X-rays when they devour materials, and active galactic nuclei emit large amounts of X-rays. The problem is that an active galactic nucleus may not be recognized when X-rays are mostly filtered by a large cocoon of dust and gas. To find cases of incorrect identification, Erini Lambrides’ team also used other observations because other emissions, in particular infrared and radio, can pass at least in part through a large cocoon helping to identify their source.
In the past, 67 heavily obscured supermassive black holes were discovered more than 5 billion light-years away by analyzing their X-ray and infrared emissions in the CDF-S. In this new research, Erini Lambrides’ team examined a number of supermassive black holes calculating the amount of X-rays they should emit based on their emissions at other wavelengths. They discovered 28 sources that emit significantly less than expected X-rays, which indicate the presence of a much denser cocoon than was estimated, one capable of filtering those emissions. The fact that they’re active galactic nuclei indicates that these black holes are growing more than astronomers thought.
The top image (X-ray: NASA/CXC/Penn State/B.Luo et al; Optical/IR: NASA/STScI/JHU/E. Lambrides et al) shows the product of the CDF-S survey with the 28 active galactic nuclei discovered in this study circled in green. Four of them, H289, H321, H412, H459, are shown in the panels in optical wavelength observations.
This study will help improve models about existing supermassive black hole types and their growth. Actually, the models already predicted a presence of growing supermassive black holes greater than those observed, and the fact that some of them were incorrectly identified explains why the models disagreed with the observations. One of the current subjects of astronomical research is the influence of supermassive black holes on the evolution of the galaxies that host them, therefore correctly identifying their nature is crucial to understand their activity.