Scientists Spot Doughnut-Shaped Cloud With a Black Hole Filling
Illustration of a dark doughnut-shaped ring deep in the core of a galaxy encircling what appears to be a supermassive black hole. |
An international team of scientists has found more evidence that
massive black holes are surrounded by a doughnut-shaped gas cloud
which, depending on our line of sight, blocks the view of the black
hole in the center.
Using two European Space Agency orbiting observatories, INTEGRAL and
XMM-Newton, scientists looked "edge on" into this doughnut (called a
torus by scientists) to see features never before revealed in such clarity. They
could infer the doughnut structure and its distance from the black hole
by virtue of light that was either reflected or completely absorbed.
How the doughnut forms, however, remains a mystery.
"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 Center 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."
Black holes are objects so dense and with gravity so strong that not
even light can escape from them. Scientists say that "supermassive"
black holes are located in the cores of most galaxies, including our
Milky Way galaxy, and contain the mass of millions to billions of
suns confined within a region no larger than our Solar System.
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.
The scientists observed a black hole in the spiral galaxy NGC 4388,
which is 65 million light years from Earth in the constellation
Virgo. This galaxy is a Seyfert 2, referring to the type of
black hole in the core -- that is, one that is enshrouded from our
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. Seeing galaxies in all wavelengths -- that is, with radio, infrared, optical, ultraviolet, X-ray and gamma-ray telescopes -- reveals the entire workings of the the galaxy, from star creation (birth) to black hole activity (death). |
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.) NGC
4388 is relatively close and therefore an unusually bright Seyfert
2, easy to study.
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
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 toward 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
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
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.