NASA’s Swift satellite finds unseen black-hole-powered galaxies

WASHINGTON - NASA’s Swift satellite has found a largely unseen population of black-hole-powered galaxies, according to an international team of scientists.

“These heavily shrouded black holes are all around us. But before Swift, they were just too faint and too obscured for us to see,” said Neil Gehrels, the Swift principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Md.

The black holes found in the galaxies weight about 100 million times the sun’s mass. In an active galaxy, matter falling toward the supermassive black hole powers high-energy emissions is so intense that two classes of active galaxies, quasars and blazars, rank as the most luminous objects in the universe.

However, their X-ray emissions are so heavily absorbed that it led astronomers to suspect that active galaxies were undercounted. However, some of the black hole’s more energetic X-rays do penetrate the shroud, and that’s where Swift comes in.

NASA’s Swift has been in operation since 2004 and gradually building up its exposure year after year. Now the survey is now the largest, most sensitive and most complete census at these energies. It includes hundreds of active galaxies out to a distance of 650 million light-years.

“These extremely obscured active galaxies are very faint and difficult to find. Out of a sample of 199 sources, we detected only nine of them,” said Davide Burlon at Max Planck Institute for Extraterrestrial Physics in Munich.

“But even Swift’s BAT has trouble finding these highly absorbed sources, and we know that the survey undercounts them. When we factored this in, we found that these shrouded active galaxies are very numerous, making up about 20 to 30 percent of the total,” he added.

“With Swift we have now quantified exactly how many active galaxies there are around us-really, in our back yard,” said Marco Ajello at the SLAC National Accelerator Laboratory, Menlo Park, Calif.

“The number is large, and it agrees with models that say they are responsible for most of the X-ray background.”

“This is the first time we could investigate the average spectrum of heavily absorbed active galaxies,” said Ajello.

“These galaxies are responsible for the shape of the cosmic X-ray background-they create the peak of its energy.”

The findings appear in the Feb. 10 issue of The Astrophysical Journal. (ANI)