The most massive black holes in the universe are often encircled by
thick, doughnut-shaped disks of gas and dust. This deep-space doughnut
material ultimately feeds and nourishes the growing black holes tucked
inside.
Until recently, telescopes weren't able to penetrate some of these doughnuts, also known as tori.
"Originally, we thought that some black holes were hidden behind
walls or screens of material that could not be seen through," said
Andrea Marinucci of the Roma Tre University in Italy, lead author of a
new Monthly Notices of the Royal Astronomical Society study describing
results from NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR,
and the European Space Agency's XMM-Newton space observatory.
With its X-ray vision, NuSTAR recently peered inside one of the
densest of these doughnuts known to surround a supermassive black hole.
This black hole lies at the center of a well-studied spiral galaxy
called NGC 1068, located 47 million light-years away in the Cetus
constellation.
The observations revealed a clumpy, cosmic doughnut.
"The rotating material is not a simple, rounded doughnut as originally thought, but clumpy," said Marinucci.
Doughnut-shaped disks of gas and dust around supermassive black holes
were first proposed in the mid-1980s to explain why some black holes
are hidden behind gas and dust, while others are not. The idea is that
the orientation of the doughnut relative to Earth affects the way we
perceive a black hole and its intense radiation. If the doughnut is
viewed edge-on, the black hole is blocked. If the doughnut is viewed
face-on, the black hole and its surrounding, blazing materials can be
detected. This idea is referred to as the unified model because it
neatly joins together the different black hole types, based solely upon
orientation.
In the past decade, astronomers have been finding hints that these
doughnuts aren't as smoothly shaped as once thought. They are more like
defective, lumpy doughnuts that a doughnut shop might throw away.
The new discovery is the first time this clumpiness has been observed
in an ultra-thick doughnut, and supports the idea that this phenomenon
may be common. The research is important for understanding the growth
and evolution of massive black holes and their host galaxies.
"We don't fully understand why some supermassive black holes are so
heavily obscured, or why the surrounding material is clumpy," said
co-author Poshak Gandhi of the University of Southampton in the United
Kingdom. "This is a subject of hot research."
Both NuSTAR and XMM-Newton observed the supermassive black hole in
NGC 1068 simultaneously on two occasions between 2014 to 2015. On one of
those occasions, in August 2014, NuSTAR observed a spike in brightness.
NuSTAR observes X-rays in a higher-energy range than XMM-Newton, and
those high-energy X-rays can uniquely pierce thick clouds around the
black hole. The scientists say the spike in high-energy X-rays was due
to a clearing in the thickness of the material entombing the
supermassive black hole.
"It's like a cloudy day, when the clouds partially move away from the sun to let more light shine through," said Marinucci.
NGC 1068 is well known to astronomers as the first black hole to give
birth to the unification idea. "But it is only with NuSTAR that we now
have a direct glimpse of its black hole through such clouds, albeit
fleeting, allowing a better test of the unification concept," said
Marinucci.
The team says that future research will address the question of what
causes the unevenness in doughnuts. The answer could come in many
flavors. It's possible that a black hole generates turbulence as it
chomps on nearby material. Or, the energy given off by young stars could
stir up turbulence, which would then percolate outward through the
doughnut. Another possibility is that the clumps may come from material
falling onto the doughnut. As galaxies form, material migrates toward
the center, where the density and gravity is greatest. The material
tends to fall in clumps, almost like a falling stream of water
condensing into droplets as it hits the ground.
"We'd like to figure out if the unevenness of the material is being
generated from outside the doughnut, or within it," said Gandhi.
"These coordinated observations with NuSTAR and XMM-Newton show yet
again the exciting science possible when these satellites work
together," said Daniel Stern, NuSTAR project scientist at NASA's Jet
Propulsion Laboratory in Pasadena, California.
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