In an attempt to unearth supermassive black holes concealed under thick clouds of interstellar gas in our cosmic neighborhood, astronomers based at the University of Southampton have sought the help of X-ray vision.
Monster black holes are sometimes hidden behind dust and gas, and are not revealed when viewed through most of the telescopes. However, they are revealed automatically when high-energy X-rays are emitted from the material they feed.
These X-rays can be detected by the Nuclear Spectroscopic Telescope Array (NuSTAR) mission, developed by NASA. In the same way, two gas-enshrouded supermassive black holes positioned at the centers of neighboring galaxies have been detected by NuSTAR.
The research was headed by Peter Boorman, a Southampton-based PhD researcher, and Dr Poshak Gandhi, Associate Professor and STFC Ernest Rutherford Fellow in Southampton’s Astronomy Group. The outcomes of the research were presented this month during a press briefing at the 229th meeting of the American Astronomical Society in Grapevine, Texas, USA. At the press briefing, Ady Annuar, a graduate student at Durham University, also joined Peter Boorman.
Boorman and his collaborators from the NuSTAR active galaxies science team have reported the way in which data from NASA’s NuSTAR have been used to analyze the intrinsic behavior of a “hidden” supermassive black hole in a neighboring galaxy, i.e. IC 3639, located nearly 175 million light years from Earth (which is comparatively very close in cosmic terms) in a recent paper published in The Astrophysical Journal.
Every large galaxy in the Universe is believed to host a supermassive black hole at their center, millions of times the mass of our Sun. These systems can devour vast quantities of matter due to their extreme gravitational pull, making the black holes grow. The in-falling matter then emits radiation across the full electromagnetic spectrum. These growing supermassive black holes are called Active Galactic Nuclei (AGN). The emission can be absorbed by thick clouds of gas and dust covering the AGN. As the level of obscuration increases, only the highest energy X-rays can escape to be observed by us. X-rays are absorbed by the atmosphere so I use data from X-ray satellites located above our atmosphere—such as NuSTAR, Suzaku and Swift—to detect high energy X-ray emission throughout the Universe.
Peter Boorman, PhD Researcher, University of Southampton
“The black hole I’ve been studying is so hidden, that it requires highly sensitive observations in the highest energy X-rays to classify it as obscured. Such sensitivities are only available now with the NuSTAR satellite [launched in June 2012] which is designed to create images of the high energy X-ray sky sharper than ever before,” further stated Boorman. “By modelling the X-ray emission of supermassive black holes, we get a glimpse of their growth rates, and learn about the amount and composition of material blanketing them. IC 3639 turns out to be glowing extremely bright due to emission from hot Iron atoms whose origin is not fully understood.”
“In broader terms, we also hope to determine the distribution of obscured AGN across the Universe, and figure out how these supermassive black holes have evolved and grown over billions of years,” concluded Boorman. “For this purpose, we will soon begin a large new survey of other nearby AGN with NuSTAR, and we can expect many more discoveries in the coming years.”
Annuar analyzed the spiral galaxy NGC 1448 and found that X-ray emission observed by NuSTAR and NASA’s Chandra X-Ray Observatory from NGC 1448 indicated for the maiden time that similar to IC 3639, there should be a thick dust and gas layer concealing the active black hole located in this galaxy from the normal line of sight.
In addition, the scientists discovered a large population of young stars, only about five million year old, located in NGC 1448, which denotes that while the galaxy’s black hole feeds on dust and gas, it simultaneously produces new stars.
“These black holes are relatively close to the Milky Way, but they have remained hidden from us until now,” stated Annuar. “They’re like monsters hiding under your bed.”
Dr Gandhi is more convinced that only now have we started to properly interpret the growth of supermassive black hole in the obscured regime.
“Although X-ray astronomy was ‘born’ in the 1960’s, the field is still in its infancy in many respects,” stated Dr Gandhi. “NuSTAR is helping to change the picture of what we know about supermassive black holes, even in the local universe where many questions remain unanswered. It is really a privilege to be the first to peer into places in the universe where no one has been able to see before.”