Deep Space Recipe: Doughnut Cloud with Black Hole Filling

A dark doughnut-shaped ring deep in the core of a galaxy encircles what appears to be a supermassive black hole. Credit: ESA.

A group of astrophysicists studying black holes have produced more evidence for the theory that massive black holes are surrounded by a doughnut shaped gas cloud called a torus. Depending on the angle at which we are viewing the object, the torus may block the view of the black hole.

Black holes are extremely dense and have gravity so strong that even light cannot escape from them. Astronomers have known about black holes for decades but much of the actual physics of the objects are theories.

We believe there are supermassive black holes in the centre of most galaxies including our own Milky Way galaxy. The objects contain the mass of millions to billions of stars: all confined to an area no larger than our Solar System.

The group of scientists used two orbiting observatories to look at a black hole in the spiral galaxy NGC 4388 in the constellation Virgo. The object is 65 million light years away, close by astronomical distances, which means the object is fairly bright. The galaxy is a Seyfert 2 type, which refers to the type of black hole in the centre, in this case one which is enshrouded from our view. The angle that we are looking at the object allows us to view the doughnut shaped torus edge on.

This image shows two bright gamma-ray sources: NGC 4388, the nearby "Seyfert" galaxy which is the subject of this press release; and 3C 273, a very distant quasar. Credit: Volker Beckmann et al. / ESA / IBIS.

The observations produced features and data never before seen in such clarity. The scientists were able to infer the doughnut shaped structure and distance from the black hole by virtue of light that was either reflected or completely absorbed. What they don't know however, is how the doughnut forms.

"By peering right into the torus, we see the black hole phenomenon in a whole new light, or lack of light, as the case may be here," said Dr. Volker Beckmann of NASA Goddard Space Flight Centre in Greenbelt, Md., the lead author on an upcoming article in The Astrophysical Journal. "This torus is not as dense as a Krispy Kreme doughnut, but it is far hotter (up to a thousand degrees) and loaded with many more calories."

Supermassive black holes appear to be surrounded by a hot, thin disk of accreting gas and, farther out, the thick doughnut-shaped torus. Astronomers often view black holes that are aligned face-on or at a slight angle in relation to Earth, thus avoiding the dark, enshrouding torus to study the hot accretion disk.

Beckmann's group took the path less trodden and observed a black hole with a theorized torus directly in the line of sight. X-ray and gamma-ray light, as detected by XMM and INTEGRAL, respectively, partially penetrates the torus.  The new view through the haze provides valuable insight into the relationship among the black hole, its accretion disk and the doughnut.

An image of NGC 4388 in infrared wavelengths, captured by ground-based Subaru telescope. We see the entire galaxy. The black hole (and its accretion disk and doughnut ring) would be just a dot in the galaxy core.

Seyfert 2 galaxies are usually faint to optical telescopes. The torus model is one explanation. Another theory is that the central black hole, for reasons unclear, is not actively accreting gas and is therefore faint. (Accretion produces energy, or light.)

The new observation supports the torus model in several ways. Gas in the accretion disk close to the black hole reaches high speeds and temperatures (over 100 million degrees, hotter than the Sun) as it races toward the void. The gas radiates predominantly at high energies, in the X-ray wavelengths. This light, which is able to escape the black hole because it is still outside of its border, ultimately collides with matter in the torus. Some of it is absorbed; some of it is reflected at different wavelengths, like sunlight penetrating a cloud; and the very energetic gamma rays pierce through.

Beckmann's group saw how different processes around a black hole produce light at different wavelengths. For example, some of the gamma rays produced close to the black hole get absorbed by iron atoms in the torus and are reemitted at a lower energy. This in fact is how the scientists knew they were seeing "reprocessed" light farther out. Also, because of the line of sight towards NGC 4388, they knew this iron was from a torus on the same plane as the accretion disk, and not from gas clouds "above" or "below" the accretion disk.

Lower-energy X rays (below 2.5 kilo-electron volts) appear to be from a diffuse emission far away from the black hole. Higher-energy X rays (above 2.5 keV) are directly related to black hole activity. The torus itself appears to be several hundred light years from the black hole.

NGC 4261 is an elliptical galaxy, unlike NGC 4388, a spiral galaxy. However, both galaxies share a common bond -- that is, a supermassive black hole at their core. This Hubble Space Telescope image zooms into the galaxy center to reveal what appears to be a doughnut-shaped cloud around a bright core (an active black hole). Credit: NASA/HST/WFPC2.

Dr. Beckmann said the observation could not gauge the diameter of the torus, from inside to outside. Other scientists say that the doughnut shape is more intact closer to the accretion disk, but that it cannot maintain structural integrity farther away, perhaps resembling a doughnut with part of its edges eaten away.

The result marks the clearest observation of an obscured black hole in X-ray and gamma-ray "colors," a swatch of energy nearly a million times wider than the window of visible light, from red to violet. Multiwavelength studies are increasingly important to understanding black holes.  XMM-Newton was launched in December 1999, and INTEGRAL was launched in October 2002.

Dr. Beckmann is a visiting scientist at NASA Goddard through the University of Maryland, Baltimore County. His coauthors on the Astrophysical Journal article are Dr. Neil Gehrels of NASA Goddard; Pascal Favre, Dr. Roland Walter and Prof. Thierry Courvoisier of the INTEGRAL Science Data Centre in Switzerland; Dr. Pierre-Olivier Petrucci of the Laboratoire d'Astrophysique de Grenoble in France; and Dr. Julien Malzac of the Centre d'Etude Spatiale des Rayonnements in France and the Institute of Astronomy, University of Cambridge, U.K.

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