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A file photo of a black hole. Photo: AFP
A file photo of a black hole. Photo: AFP

Two ‘monster’ black holes hiding in cosmic backyard spotted

Monster black holes lurk behind gas and dust, hiding from the gaze of most telescopes

London: Scientists, using data from National Aeronautics and Space Administration (NASA) telescopes, have spotted two supermassive black holes, located at the centres of galaxies close to our Milky Way, that were hidden behind shrouds of gas and dust until now.

Monster black holes sometimes lurk behind gas and dust, hiding from the gaze of most telescopes. However, they give themselves away when material they feed on emits high-energy X-rays that NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) mission can detect.

Both of the black holes are the central engines of what astronomers call “active galactic nuclei," a class of extremely bright objects that includes quasars and blazars. “These black holes are relatively close to the Milky Way, but they have remained hidden from us until now," said Ady Annuar, graduate student at Durham University in the UK.

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Depending on how these galactic nuclei are oriented and what sort of material surrounds them, they appear very different when examined with telescopes. Active galactic nuclei are so bright because particles in the regions around the black hole get very hot and emit radiation across the full electromagnetic spectrum—from low-energy radio waves to high-energy X-rays.

However, most active nuclei are believed to be surrounded by a doughnut-shaped region of thick gas and dust that obscures the central regions from certain lines of sight. The active galactic nuclei that NuSTAR recently studied appear to be oriented such that astronomers view them edge-on.

That means that instead of seeing the bright central regions, our telescopes primarily see the reflected X-rays from the doughnut-shaped obscuring material. “Just as we can’t see the Sun on a cloudy day, we can’t directly see how bright these active galactic nuclei really are because of all of the gas and dust surrounding the central engine," said Peter Boorman, graduate student at University of Southampton in the UK, who led the study of an active galaxy called IC 3639, which is 170 million light years away.

Researchers analysed NuSTAR data from this object and compared them with previous observations from NASA’s Chandra X-Ray Observatory and the Japan-led Suzaku satellite. The findings from NuSTAR confirm the nature of IC 3639 as an active galactic nucleus. Annuar studied the spiral galaxy NGC 1448.

The black hole in its centre was discovered in 2009, even though it is only 38 million light years away. Researchers discovered that this galaxy also has a thick column of gas hiding the central black hole, which could be part of a doughnut-shaped region.

X-ray emission from NGC 1448 suggests for the first time that there must be a thick layer of gas and dust hiding the active black hole in this galaxy from our line of sight. Researchers also found that NGC 1448 has a large population of young (just 5 million year old) stars, suggesting that the galaxy produces new stars at the same time that its black hole feeds on gas and dust.

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