2001 October 29
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.
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