Astronomers using NSF’s Karl G. Jansky Very Large Array (VLA) have caught the direct image of the thick torus surrounding the supermassive black hole at the core of Cygnus A, one of the most powerful radio galaxies in the Universe.
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, astrophysicists 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 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 Dr. Chris Carilli, an astronomer at 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.”
VLA image of Cygnus A’s central region. Image credit: Carilli et al / NRAO / AUI / NSF.
Dr. Carilli and colleagues studied Cygnus A, a powerful radio galaxy some 760 million light-years from Earth.
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 astronomers Walter Baade and Rudolf Minkowski in 1951.
The galaxy harbors a black hole at its core that is 2.5 billion times more massive than 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.
“Cygnus A is the closest example of a powerful radio-emitting galaxy — 10 times closer than any other with comparably powerful radio emission. That proximity allowed us to find the torus in a high-resolution VLA image of the galaxy’s core,” said Dr. Rick Perley, also from NRAO.
An artist’s conception of active galactic nucleus. Image credit: Bill Saxton / NRAO / AUI / NSF.
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,” Dr. Carilli said.
The results appear in the Astrophysical Journal Letters.
_____
C.L. Carilli et al. 2019. Imaging the Active Galactic Nucleus Torus in Cygnus A. ApJL 874, L32; doi: 10.3847/2041-8213/ab1019
