Small Warm Doughnut Feeds Theories of Extragalactic Black Holes(Forwarded)

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 Comisión Nacional de
investigación Cientifica y Tecnológica (CONiCYT), the Australian Research
Council (ARC), the Argentinean Consejo Nacional de investigaciones
Científicas y Técnicas (CONiCET) and the Brazilian Conselho Nacional de
Desenvolvimento Científico e Tecnológico (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.