Artist's conception of the dusty, doughnut-shaped object
surrounding the supermassive black hole, disk of material orbiting the
black hole, and jets of material ejected by the disk, at the center of a
galaxy.Credit: Bill Saxton, NRAO/AUI/NSF. Hi-Res image
Artist's conception of active galactic nucleus, with labels.
Credit: Bill Saxton, NRAO/AUI/NSF. Hi-Res image
VLA image of the central region of the powerful radio
galaxy Cygnus A, showing the doughnut-shaped torus surrounding the black
hole and accretion disk. Credit: Carilli et al., NRAO/AUI/NSF. Hi-Res image
VLA image of Cygnus A's central region, with labels.
Credit: Carilli et al., NRAO/AUI/NSF. Hi-Res image
Viewing a Galaxy's "Central Engine" From Different Angles
Video illustrates how the dusty, doughnut-shaped torus surrounding the black hole and accretion disk at the center of a powerful radio galaxy can obscure different features when viewed from different angles. This explains how the same type of "central engine" can appear different, leading to different names for objects seen from different angles.
Credit: Bill Saxton, Sophia Dagnello, NRAO/AUI/NSF. Vimeo
Astronomers used the National Science Foundation’s Karl G. Jansky
Very Large Array (VLA) to make the first direct image of a dusty,
doughnut-shaped feature surrounding the supermassive black hole at the
core of one of the most powerful radio galaxies in the Universe — a
feature first postulated by theorists nearly four decades ago as an
essential part of such objects.
The scientists studied Cygnus A, a galaxy some 760 million
light-years from Earth. The galaxy harbors a black hole at its core that
is 2.5 billion times more massive then the Sun. As the black hole’s
powerful gravitational pull draws in surrounding material, it also
propels superfast jets of material traveling outward at nearly the speed
of light, producing spectacular “lobes” of bright radio emission.
Black hole-powered “central engines” producing bright emission at
various wavelengths, and jets extending far beyond the galaxy are common
to many galaxies, but show different properties when observed. Those
differences led to a variety of names, such as quasars, blazars, or
Seyfert galaxies. To explain the differences, theorists constructed a
“unified model” with a common set of features that would show different
properties depending on the angle from which they are viewed.
The unified model includes the central black hole, a rotating disk of
infalling material surrounding the black hole, and the jets speeding
outward from the poles of the disk. In addition, to explain why the same
type of object looks different when viewed from different angles, a
thick, dusty, doughnut-shaped “torus” is included, surrounding the inner
parts. The torus obscures some features when viewed from the side,
leading to apparent differences to the observer, even for intrinsically
similar objects. Astronomers generically call this common set of
features an active galactic nucleus (AGN).
“The torus is an essential part of the AGN phenomenon, and evidence
exists for such structures in nearby AGN of lower luminosity, but we’ve
never before directly seen one in such a brightly-emitting radio
galaxy,” said Chris Carilli, of the National Radio Astronomy Observatory
(NRAO). “The torus helps explain why objects known by different names
actually are the same thing, just observed from a different
perspective,” he added.
In the 1950s, astronomers discovered objects that strongly emitted
radio waves, but appeared point-like, similar to distant stars, when
later observed with visible-light telescopes. In 1963, Maarten Schmidt
of Caltech discovered that one of these objects was extremely distant,
and more such discoveries quickly followed. To explain how these
objects, dubbed quasars, could be so bright, theorists suggested that
they must be tapping the tremendous gravitational energy of supermassive
black holes. The combination of black hole, the rotating disk, called
an accretion disk, and the jets was termed the “central engine”
responsible for the objects’ prolific outpourings of energy.
The same type of central engine also appeared to explain the output
of other types of objects, including radio galaxies, blazars, and
Seyfert Galaxies. However, each showed a different set of properties.
Theorists worked to develop a “unification scheme” to explain how the
same thing could appear differently. In 1977, obscuration by dust was
suggested as one element of that scheme. In a 1982 paper, Robert
Antonucci, of the University of California, Santa Barbara, presented a
drawing of an opaque torus — a doughnut-shaped object — surrounding the
central engine. From that point on, an obscuring torus has been a common
feature of astronomers’ unified view of all types of active galactic
nuclei.
“Cygnus A is the closest example of a powerful radio-emitting galaxy —
10 times closer than any other with comparably bright radio emission.
That proximity allowed us to find the torus in a high-resolution VLA
image of the galaxy’s core,” said Rick Perley, also of NRAO. “Doing more
work of this type on weaker and more distant objects will almost
certainly need the order-of-magnitude improvement in sensitivity and
resolution that the proposed Next Generation Very Large Array (ngVLA)
would bring,” he added.
The VLA observations directly revealed the gas in Cygnus A’s torus,
which has a radius of nearly 900 light-years. Longstanding models for
the torus suggest that the dust is in clouds embedded in the
somewhat-clumpy gas.
“It’s really great to finally see direct evidence of something that
we’ve long presumed should be there,” Carilli said. “To more accurately
determine the shape and composition of this torus, we need to do further
observing. For example, the Atacama Large Millimeter/submillimeter
Array (ALMA) can observe at the wavelengths that will directly reveal
the dust,” he added.
Carilli and Perley, with their colleagues Vivek Dhawan, also of NRAO,
and Daniel Perley of Liverpool John Moores University in the UK,
discovered the torus when following up their surprising discovery in 2016
of a new, bright object near the center of Cygnus A. That new object,
they said, is most likely a second supermassive black hole that only
recently encountered new material it could devour, causing it to produce
bright emission the same way the central black hole does. The existence
of the second black hole, they said, suggests that Cygnus A merged with
another galaxy in the astronomically recent past.
Cygnus A, so named because it is the most powerful radio-emitting
object in the constellation Cygnus, was discovered in 1946 by English
physicist and radio astronomer J.S. Hey. It was matched to a
visible-light, giant galaxy by Walter Baade and Rudolf Minkowski in
1951. It became an early target for the VLA soon after its completion in
the early 1980s. Detailed VLA images of Cygnus A published in 1984
produced major advances in astronomers’ understanding of such galaxies.
The scientists are reporting their findings in the Astrophysical Journal Letters.
The National Radio Astronomy Observatory is a facility of the
National Science Foundation, operated under cooperative agreement by
Associated Universities, Inc.
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