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Boffins unmask hidden galactic cores

Swift helps ID new class of AGNs

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Astronomers using NASA's Swift satellite and the Japanese/US Suzaku X-ray observatory have discovered a totally new kind of active galactic nuclei (AGN).

The new class is relatively common, the researchers say, but because the galactic cores are almost entirely shrouded in gas and dust, they are hard to spot.

"This is an important discovery because it will help us better understand why some supermassive black holes shine and others don't," says astronomer and team member Jack Tueller of NASA's Goddard Space Flight Centre.

Illustration of the newly discovered type of active galactic nuclei. Credit: Aurore Simonnet, Sonoma State University.

Illustration of the newly discovered type of active galactic nuclei.

Credit: Aurore Simonnet, Sonoma State University.

The universe is home to a literally dazzling array of galaxies, many of which are powered by an accreting supermassive blackhole at their core. These are often extraordinarily luminous objects - such as quasars or Seyfert galaxies - pouring forth vast quantities of energy, often the equivalent of billions of yellow dwarf stars, all from a region roughly the size of our solar system.

But according to a paper that will appear in the 1 August issue of the Astrophysical Journal Letters, not all active galactic cores are so showy.

Most galaxies are surrounded by a torus of material, and the angle at which we observe the torus determines the characteristics we can observe. But the researchers suggest that, contrary to expectations, these galaxies are completely surrounded by a shell of obscuring material, meaning they are an entirely different animal.

This new class of AGN was only discovered thanks to the very sensitive Burst Alert Telescope (BAT) on board the Swift satellite.

The BAT is designed to respond extremely quickly to a burst of gamma rays, alerting a chain of other observatories to the detection of a gamma ray burst somewhere in the universe. It is very sensitive in the right part of the spectrum to see through the layers of dust that have hidden these AGNs from view.

The team then planned to follow up on the observations by using the Japanese/US Suzaku X-ray observatory, which is sensitive to a broader range of X-ray energies than the BAT. But observations of two galaxies revealed very few medium or low energy X-rays emanating from the galactic cores.

The team says the result implies many more such galaxies must lie hidden in the universe, accounting for perhaps 20 per cent of the point sources that make up the background X-ray radiation. This also means existing studies of AGNs have been skewed, by missing out an entire class of nucleus.

"We think these black holes have played a crucial role in controlling the formation of galaxies, and they control the flow of matter into clusters," Tueller adds. "You can't understand the universe without understanding giant black holes and what they're doing." ®

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