Artist’s conception of the core of Cygnus A,
including the dusty donut-shaped surroundings, called a torus, and jets
launching from its center. Magnetic fields are illustrated trapping the
dust in the torus. These magnetic fields could be helping power the
black hole hidden in the galaxy’s core by confining the dust in the
torus and keeping it close enough to be gobbled up by the hungry black
hole. Credits: NASA/SOFIA/Lynette Cook
Two images of Cygnus A layered over each other to
show the galaxy’s jets glowing with radio radiation (shown in red).
Quiescent galaxies, like our own Milky Way, do not have jets like this,
which may be related to magnetic fields. The yellow image shows
background stars and the center of the galaxy shrouded in dust when
observed with visible light. The area SOFIA observed is inside the small
red dot in the center. Credits: Optical Image: NASA/STSiC Radio Image: NSF/NRAO/AUI/VLA
Collimated jets provide astronomers with some of the most powerful evidence that a supermassive black hole
lurks in the heart of most galaxies. Some of these black holes appear
to be active, gobbling up material from their surroundings and launching
jets at ultra-high speeds, while others are quiescent, even dormant.
Why are some black holes feasting and others starving? Recent
observations from the Stratospheric Observatory for Infrared Astronomy,
or SOFIA, are shedding light on this question.
SOFIA data indicate that magnetic fields are trapping and confining dust near the center of the active galaxy, Cygnus A, and feeding material onto the supermassive black hole at its center.
The unified model,
which attempts to explain the different properties of active galaxies,
states that the core is surrounded by a donut-shaped dust cloud, called
a torus.
How this obscuring structure is created and sustained has never been
clear, but these new results from SOFIA indicate that magnetic fields
may be responsible for keeping the dust close enough to be devoured by
the hungry black hole. In fact, one of the fundamental differences
between active galaxies like Cygnus A and their less active cousins,
like our own Milky Way, may be the presence or absence of a strong
magnetic field around the black hole.
Although celestial magnetic fields are notoriously difficult to observe, astronomers have used polarized
light — optical light from scattering and radio light from accelerating
electrons — to study magnetic fields in galaxies. But optical
wavelengths are too short and the radio wavelengths are too long to
observe the torus directly. The infrared wavelengths observed by SOFIA
are just right, allowing scientists, for the first time, to target and
isolate the dusty torus.
SOFIA’s new instrument, the High-resolution Airborne Wideband Camera-plus (HAWC+),
is especially sensitive to the infrared emission from aligned dust
grains. This has proven to be a powerful technique to study magnetic
fields and test a fundamental prediction of the unified model: the role
of the dusty torus in the active-galaxy phenomena.
“It’s always exciting to discover something completely new,” noted
Enrique Lopez-Rodriguez, a scientist at the SOFIA Science Center, and
the lead author on the report of this new discovery. “These observations
from HAWC+ are unique. They show us how infrared polarization can
contribute to the study of galaxies.”
Recent observations of the heart of Cygnus A made with HAWC+ show
infrared radiation dominated by a well-aligned dusty structure.
Combining these results with archival data from the Herschel Space Observatory, the Hubble Space Telescope
and the Gran Telescopio Canarias, the research team found that this
powerful active galaxy, with its iconic large-scale jets, is able to
confine the obscuring torus that feeds the supermassive black hole using
a strong magnetic field.
The results of this study were published in the July 10th issue of The Astrophysical Journal Letters.
Cygnus A is in the perfect location to learn about the role magnetic
fields play in confining the dusty torus and channeling material onto
the supermassive black hole because it is the closest and most powerful
active galaxy. More observations of different types of galaxies are
necessary to get the full picture of how magnetic fields affect the
evolution of the environment surrounding supermassive black holes. If,
for example, HAWC+ reveals highly polarized infrared emission from the
centers of active galaxies but not from quiescent galaxies, it would
support the idea that magnetic fields regulate black hole feeding and
reinforce astronomers’ confidence in the unified model of active
galaxies.
SOFIA is a Boeing 747SP jetliner modified to carry a 106-inch
diameter telescope. It is a joint project of NASA and the German
Aerospace Center, DLR. NASA’s Ames Research Center in California’s
Silicon Valley manages the SOFIA program, science and mission operations
in cooperation with the Universities Space Research Association
headquartered in Columbia, Maryland, and the German SOFIA Institute
(DSI) at the University of Stuttgart. The aircraft is maintained and
operated from NASA’s Armstrong Flight Research Center Hangar 703, in
Palmdale, California.
Editor: Kassandra Bell
