In the deepest ground-based mid-infrared image ever, the Gemini North
Telescope reveals that the mysterious environment around one of the
most massive black holes in the Universe is missing a key feature
predicted by astronomical theory.
“Contrary to what most theories predict, our Gemini observations show
that the giant elliptical galaxy M87 either lacks a torus around its
central black hole, or else this doughnut-shaped ring of material is
extremely faint. With the unparalleled resolution and depth of the
Gemini mid-infrared observations, the torus should have been easy to
detect,” says Principal Investigator Dr. Eric Perlman of the
University of Maryland, Baltimore County.
The Gemini data elaborate on earlier high-resolution optical
observations by the Hubble Space Telescope, which hinted at the unique
conditions inside this famous galaxy. “This will definitely cause
some head-scratching among theorists about how much we really
understand regarding the cores of active galaxies like M87,” adds
Perlman, lead author of a paper published in the November 1, 2001
issue of the Astrophysical Journal Letters.
“In other, similar objects, we see large quantities of warm dust
surrounding the nucleus, which we think may be associated with the
flow of fuel toward the central black hole,” says theorist Dr. Julian
Krolik of the Johns Hopkins University, in response to this finding.
“Now that we know M87 is missing this, we’ll have to look for other
ways that its activity may be powered.”
The galaxy commonly known as M87 is a popular target for astronomers,
and most famous for the high-energy jet of material that extends from
its central region. Although the galaxy is 50 million light-years
from Earth, it is one of the closest galaxies of its type and hence
lends itself extremely well to detailed studies. Hubble Space
Telescope observations have shown that at its heart is a black hole,
containing the mass of about three billion stars compressed into a
region approximately the size of our Solar system.
Such a massive black hole (sometimes called a super-massive black
hole) at the center of a galaxy can spark a huge outpouring of energy,
driven by gas and dust that the black hole slowly devours. The sheer
amount of energy released by this process staggers even astronomers
who have been studying these galaxies for years, as the energy
released can outshine the billions of stars making up the entire
galaxy! A galaxy with such extreme nuclear emission is called an
active galactic nucleus (AGN). Ironically, a black hole in a galaxy
often reveals its presence by the very bright and concentrated optical
and infrared emission in the region around the black hole.
Astronomers have long postulated that the extreme infrared emission in
the center of these active galaxies must be produced by a substantial
doughnut-shaped torus of dusty material surrounding the black hole.
The dusty torus absorbs high-energy radiation – from material that is
heated to extremely high temperatures immediately before falling into
the black hole – and re-emits it at infrared wavelengths.
Earlier, high-resolution optical images by the Hubble Space Telescope
had revealed a disk of hot gas, rotating around the black hole, and
thin, filamentary dust in M87’s nuclear regions, but not massive
obscuration, as hinted at in observations of other AGN.
To search for the torus, astronomers have used the latest in infrared
detector technology to observe light that the human eye cannot see.
Now, with these technologies on large telescopes like Gemini, it has
become possible to peer into the nuclear cauldron at mid-infrared
wavelengths with enough clarity to conclusively test for the existence
of the torus.
The Gemini observations of M87 were about a factor of 10 times deeper
than had previously been published by any other ground-based telescope
at mid-infrared wavelengths.
Due to its small size, the torus region of an AGN has never before
been resolved, so that its real shape and geometry are not fully
understood. The combination of M87’s extremely massive black hole
and its proximity to Earth led theorists to predict that the torus
could be seen at mid-infrared wavelengths at radii of between
0.3-3 arcseconds – within the grasp of the new telescopes such as
Gemini North. To put this in perspective, one arcsecond corresponds
to the angular size of a golf ball on the ground as seen by a passenger
in a commercial airliner travelling at an altitude of 30,000 feet.
“Counter to our expectations, we did not see the torus structure
nor could we detect bright thermal emission, which would have given away
its presence.” Perlman explains. “These data show that the torus in M87
is at least a thousand times fainter compared to its radio jet than in
other well-known radio galaxies, where bright mid-infrared radiation
has been observed.”
Other co-authors on the paper are W. B. Sparks and J. A. Biretta
(from the Space Telescope Science Institute), J. T. Radomski, C. Packham
and R. K. Piña (from the University of Florida), and R. S. Fisher
(from the Gemini Observatory).
The observations of M87 were made in May 2001 using the Gemini North
Telescope and the University of Florida’s OSCIR mid-infrared
imager/spectrometer, which was built with funding from the
National Science Foundation and NASA.
“Making mid-infrared observations such as these is an extremely
challenging endeavor,” says the University of Florida’s Dr. Charles Telesco,
who led the effort to build the instrument OSCIR that was
used to make these observations.
“The power of observing in the mid-infrared is that we can peer through
much of the gas and dust that obscures our view at shorter wavelength
radiation,” Telesco adds. “When combined with Gemini, OSCIR can
do things that have never been possible before in the mid-infrared.
I expect that this is just the beginning of many new discoveries made
possible by these cutting edge technologies combined with Gemini.”
In addition to the mid-infrared observations of the central regions of
the galaxy, the new Gemini data also contains images of the galaxy’s jet
that help to paint a more complete picture of this particle accelerator,
which extends for thousands of light years.
The Gemini images of the jet also complement recently released X-ray
observations with the Chandra X-ray observatory and earlier optical
observations with Hubble and radio observations by the VLA.
A detailed summary article, complete set of multi-wavelength jet
images, and full resolution reproducible original Jon Lomberg artwork
can be found at the M87 image
page.
The Gemini Observatory is an international collaboration that has
built two identical 8-meter telescopes. The telescopes are located at
Mauna Kea, Hawaii (Gemini North) and Cerro Pachón in central Chile
(Gemini South), and hence provide full coverage of both hemispheres of
the sky. Both telescopes incorporate new technologies that allow
large, relatively thin mirrors under active control to collect and
focus both optical and infrared radiation from space. Gemini North has
begun science operations and Gemini South is scheduled to begin
scientific operations in late 2001.
The Gemini Observatory provides the astronomical communities in each
partner country with state-of-the-art astronomical facilities that
allocates 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.
Media Contacts:
Peter Michaud, Gemini PIO Office
Telephone: (808) 974-2510; e-mail: pmichaud@gemini.edu
Doug Isbell, NOAO
Telephone: (520) 318-8214; e-mail: disbell@noao.edu
Science Contact:
Dr. Eric Perlman, University of Maryland – Baltimore County
Telephone: (410) 455-1982; e-mail: perlman@jca.umbc.edu
