Andrew Yee[_1_]
January 11th 07, 05:18 PM
Gemini Observatory
Hilo, Hawaii
Science Contacts:
Dr. James Radomski
Gemini Observatory, La Serena, Chile
+56-51-205-640 (desk)
Dr. Christopher Packham
University of Florida, Gainsville, FL
(352) 392-2052 (desk)
Media Contact:
Peter Michaud
Gemini Observatory, Hilo HI, USA
(808) 974-2510 (desk)
www.gemini.edu
Embargoed until 9:20 AM (Pacific Time), Tuesday, January 9, 2007
Small Warm Doughnut Feeds Theories of Extragalactic Black Holes
Fundamental to theories describing active galactic nuclei (AGN) is the
prediction that a doughnut-shaped ring of material (a torus) surrounds a
supermassive black hole in these energetic galactic cores. Yet,
understanding an AGN's torus, or indeed seeing one directly, has proved to
be extremely frustrating, and has spurred an ongoing debate about their
extent and physical properties.
To help settle the debate, an international team of astronomers used a novel
approach to peer deeply into the energetic core of the nearby galaxy
Centaurus A. Their observations didn't resolve the theoretical torus, but
did set stringent constraints on its size and characteristics, indicating
that it is likely quite petite and clumpy.
These results were presented today at the 209th meeting of the American
Astronomical Society meeting in Seattle, Washington.
The team used observations made at mid-infrared wavelengths with the 8-meter
Gemini South telescope in Chile to study warm dust in the nucleus of the
galaxy Centaurus A. The results indicate that the theorized doughnut-shaped
torus thought to exist at the core of this nearby AGN is beyond the
resolution of the largest non-interferometric ground-based telescopes and
therefore must be smaller than many models predicted. Team members used an
innovative statistical analysis technique and high-resolution images to
establish extremely tight constraints on the size of the warm dust in the
torus. "We think that this data provides the strongest argument yet in
constraining the maximum size of the warm dusty torus in a galaxy of this
type," said Principal Investigator and Gemini Observatory astronomer James
Radomski.
The research team obtained these observations with the mid-infrared
instrument T-ReCS to help settle the ongoing debate about whether the torus
and central regions of Centaurus A were ever resolved in previous
observations. "If the torus had been resolved in previous observations, that
would mean that the extent of the torus is much larger than the latest
models predict, and it would have left many of us scratching our heads,"
said team member Chris Packham of the University of Florida.
Earlier observations made with the 10-meter Keck I telescope by D. Whysong
(now at NRAO) and R. Antonucci (University of California, Santa Barbara,
with Whysong when this work was done) initially proposed a similar
resolution-limited size for the torus in Centaurus A. However, a team using
the Magellan Telescope subsequently claimed that the nucleus was indeed
resolved and therefore indicative of a much larger structure. The new Gemini
South result clearly confirms the earlier Keck result by Whysong and
Antonucci.
"This debate definitely shows the difficulty of making these types of
observations," said Radomski. "But these observations are critical since we
cannot understand the environments around a supermassive galactic black hole
without first knowing the extent of its torus."
The new theoretical models of a galactic torus helps to settle what some
were calling the "torus size crisis." Without such a model, the observations
of Centaurus A would have deepened the size crisis. "These more recent
models predict that, far from being a huge and uniform doughnut of gas and
dust as once thought, instead the torus orbits the supermassive black hole
in clumps. The size constraints placed by the Keck and Gemini data indicate
that the clumps are primarily 'bunched up' and must orbit within a few
light-years of the black hole," explained team member Nancy Levenson of the
University of Kentucky.
Unlike with previous observations, the Gemini team cross-calibrated the
mid-infrared data many times against a reference star in order to determine
with a high degree of certainty that the nucleus of Centaurus A was
unresolved. In addition, because the object was observed for long periods
of time, these observations were deep enough to probe the very faint
emission beyond the nucleus previously observed only by space based
observatories such as the Spitzer Space Telescope. However, Gemini's larger
aperture provided higher spatial resolution to reveal details in these
regions thought to be due to star formation. It may also trace emission from
material flowing into and feeding the black hole at the center of the
galaxy.
"By working with a combination of instrument builders, observational and
theoretical astronomers, this work has pooled the knowledge of an
international group of astronomers in order to gain insights into these
objects and extract the maximum amount of knowledge from hard-won telescope
time to produce this convincing result," said team member Chris Packham of
University of Florida.
Team members (listed by institution) are James T. Radomski, James M.
DeBuizer, and Rachel Mason (Gemini Observatory), Christopher Packham,
Charles M. Telesco, Manuel Orduna (University of Florida), N.A. Levenson
(University of Kentucky), Eric Perlman (University of Maryland), Lerothodi
L. Leeuw (Rhodes University, South Africa), and Henry Matthews (Herzberg
Institute of Astrophysics, Canada).
The Gemini Observatory provides the astronomical communities in each partner
country with state-of-the-art astronomical facilities that allocate
observing time in proportion to each country's contribution. in addition to
financial support, each country also contributes significant scientific and
technical resources. The national research agencies that form the Gemini
partnership include: the US National Science Foundation (NSF), the UK
Particle Physics and Astronomy Research Council (PPARC), the Canadian
National Research Council (NRC), the Chilean Comisi Nacional de investigaci
Cientifica y Tecnolica (CONiCYT), the Australian Research Council (ARC), the
Argentinean Consejo Nacional de investigaciones Cienticas y Tnicas (CONiCET)
and the Brazilian Conselho Nacional de Desenvolvimento Cientico e Tecnolico
(CNPq). The Observatory is managed by the Association of Universities for
Research in Astronomy, inc. (AURA) under a cooperative agreement with the
NSF. The NSF also serves as the executive agency for the international
partnership.
SIDEBAR: Centaurus A
Centaurus A is one of the nearest examples of an active galactic nucleus
(AGN) visible in the sky. This makes it ideally suited for the study of the
properties of AGN in general. Its distance from us is thought to be about
11.4 million light-years away. It is an elliptical galaxy that has undergone
a merger event with another galaxy as recently as 160 million years ago.
An AGN is a region at the center of a galaxy that emits radiation across the
entire spectrum. This radiation is emitted from material that is heated by
interactions with a supermassive black hole's intense gravitational field.
In the case of Centaurus A, the black hole is about 100 million times the
mass of the Sun and shines about 2.5 billion times brighter.
The mid-infrared observations presented here were obtained using the Thermal
Region Camera Spectrometer (T-ReCS) which was built at the University of
Florida for Gemini.
IMAGE CAPTION:
[http://www.gemini.edu/centa]
The central ~30 arcseconds of the galaxy Centaurus A (one arcsecond ~ 55
light-years). The contours are the mid-infrared emission at 8.8 microns from
the Gemini data and processed to highlight the low-level extended emission.
The color HST image shows the Paschen-alpha emission from Marconi et al.
2000, ApJ, 528, 276 which primarily traces regions of star formation. The
dotted line represents the axis of the powerful radio jet emanating from the
supermassive black hole perpendicular to the torus.
Hilo, Hawaii
Science Contacts:
Dr. James Radomski
Gemini Observatory, La Serena, Chile
+56-51-205-640 (desk)
Dr. Christopher Packham
University of Florida, Gainsville, FL
(352) 392-2052 (desk)
Media Contact:
Peter Michaud
Gemini Observatory, Hilo HI, USA
(808) 974-2510 (desk)
www.gemini.edu
Embargoed until 9:20 AM (Pacific Time), Tuesday, January 9, 2007
Small Warm Doughnut Feeds Theories of Extragalactic Black Holes
Fundamental to theories describing active galactic nuclei (AGN) is the
prediction that a doughnut-shaped ring of material (a torus) surrounds a
supermassive black hole in these energetic galactic cores. Yet,
understanding an AGN's torus, or indeed seeing one directly, has proved to
be extremely frustrating, and has spurred an ongoing debate about their
extent and physical properties.
To help settle the debate, an international team of astronomers used a novel
approach to peer deeply into the energetic core of the nearby galaxy
Centaurus A. Their observations didn't resolve the theoretical torus, but
did set stringent constraints on its size and characteristics, indicating
that it is likely quite petite and clumpy.
These results were presented today at the 209th meeting of the American
Astronomical Society meeting in Seattle, Washington.
The team used observations made at mid-infrared wavelengths with the 8-meter
Gemini South telescope in Chile to study warm dust in the nucleus of the
galaxy Centaurus A. The results indicate that the theorized doughnut-shaped
torus thought to exist at the core of this nearby AGN is beyond the
resolution of the largest non-interferometric ground-based telescopes and
therefore must be smaller than many models predicted. Team members used an
innovative statistical analysis technique and high-resolution images to
establish extremely tight constraints on the size of the warm dust in the
torus. "We think that this data provides the strongest argument yet in
constraining the maximum size of the warm dusty torus in a galaxy of this
type," said Principal Investigator and Gemini Observatory astronomer James
Radomski.
The research team obtained these observations with the mid-infrared
instrument T-ReCS to help settle the ongoing debate about whether the torus
and central regions of Centaurus A were ever resolved in previous
observations. "If the torus had been resolved in previous observations, that
would mean that the extent of the torus is much larger than the latest
models predict, and it would have left many of us scratching our heads,"
said team member Chris Packham of the University of Florida.
Earlier observations made with the 10-meter Keck I telescope by D. Whysong
(now at NRAO) and R. Antonucci (University of California, Santa Barbara,
with Whysong when this work was done) initially proposed a similar
resolution-limited size for the torus in Centaurus A. However, a team using
the Magellan Telescope subsequently claimed that the nucleus was indeed
resolved and therefore indicative of a much larger structure. The new Gemini
South result clearly confirms the earlier Keck result by Whysong and
Antonucci.
"This debate definitely shows the difficulty of making these types of
observations," said Radomski. "But these observations are critical since we
cannot understand the environments around a supermassive galactic black hole
without first knowing the extent of its torus."
The new theoretical models of a galactic torus helps to settle what some
were calling the "torus size crisis." Without such a model, the observations
of Centaurus A would have deepened the size crisis. "These more recent
models predict that, far from being a huge and uniform doughnut of gas and
dust as once thought, instead the torus orbits the supermassive black hole
in clumps. The size constraints placed by the Keck and Gemini data indicate
that the clumps are primarily 'bunched up' and must orbit within a few
light-years of the black hole," explained team member Nancy Levenson of the
University of Kentucky.
Unlike with previous observations, the Gemini team cross-calibrated the
mid-infrared data many times against a reference star in order to determine
with a high degree of certainty that the nucleus of Centaurus A was
unresolved. In addition, because the object was observed for long periods
of time, these observations were deep enough to probe the very faint
emission beyond the nucleus previously observed only by space based
observatories such as the Spitzer Space Telescope. However, Gemini's larger
aperture provided higher spatial resolution to reveal details in these
regions thought to be due to star formation. It may also trace emission from
material flowing into and feeding the black hole at the center of the
galaxy.
"By working with a combination of instrument builders, observational and
theoretical astronomers, this work has pooled the knowledge of an
international group of astronomers in order to gain insights into these
objects and extract the maximum amount of knowledge from hard-won telescope
time to produce this convincing result," said team member Chris Packham of
University of Florida.
Team members (listed by institution) are James T. Radomski, James M.
DeBuizer, and Rachel Mason (Gemini Observatory), Christopher Packham,
Charles M. Telesco, Manuel Orduna (University of Florida), N.A. Levenson
(University of Kentucky), Eric Perlman (University of Maryland), Lerothodi
L. Leeuw (Rhodes University, South Africa), and Henry Matthews (Herzberg
Institute of Astrophysics, Canada).
The Gemini Observatory provides the astronomical communities in each partner
country with state-of-the-art astronomical facilities that allocate
observing time in proportion to each country's contribution. in addition to
financial support, each country also contributes significant scientific and
technical resources. The national research agencies that form the Gemini
partnership include: the US National Science Foundation (NSF), the UK
Particle Physics and Astronomy Research Council (PPARC), the Canadian
National Research Council (NRC), the Chilean Comisi Nacional de investigaci
Cientifica y Tecnolica (CONiCYT), the Australian Research Council (ARC), the
Argentinean Consejo Nacional de investigaciones Cienticas y Tnicas (CONiCET)
and the Brazilian Conselho Nacional de Desenvolvimento Cientico e Tecnolico
(CNPq). The Observatory is managed by the Association of Universities for
Research in Astronomy, inc. (AURA) under a cooperative agreement with the
NSF. The NSF also serves as the executive agency for the international
partnership.
SIDEBAR: Centaurus A
Centaurus A is one of the nearest examples of an active galactic nucleus
(AGN) visible in the sky. This makes it ideally suited for the study of the
properties of AGN in general. Its distance from us is thought to be about
11.4 million light-years away. It is an elliptical galaxy that has undergone
a merger event with another galaxy as recently as 160 million years ago.
An AGN is a region at the center of a galaxy that emits radiation across the
entire spectrum. This radiation is emitted from material that is heated by
interactions with a supermassive black hole's intense gravitational field.
In the case of Centaurus A, the black hole is about 100 million times the
mass of the Sun and shines about 2.5 billion times brighter.
The mid-infrared observations presented here were obtained using the Thermal
Region Camera Spectrometer (T-ReCS) which was built at the University of
Florida for Gemini.
IMAGE CAPTION:
[http://www.gemini.edu/centa]
The central ~30 arcseconds of the galaxy Centaurus A (one arcsecond ~ 55
light-years). The contours are the mid-infrared emission at 8.8 microns from
the Gemini data and processed to highlight the low-level extended emission.
The color HST image shows the Paschen-alpha emission from Marconi et al.
2000, ApJ, 528, 276 which primarily traces regions of star formation. The
dotted line represents the axis of the powerful radio jet emanating from the
supermassive black hole perpendicular to the torus.