Closing In on a Monster: A black hole’s dusty environs show themselves

The first clear picture of the immediate surroundings of a supermassive black hole is confirming that these gravitational monsters hide behind thick belts of dust. The observed shroud suggests that such black holes, which weigh millions to billions of times as much as the sun, may be far more numerous than astronomers have estimated and collectively generate as much power as all the stars in the universe.

HEART OF DUST. The galaxy NGC 1068, recorded in visible light by the Hubble Space Telescope (left). The central region (middle) of the galaxy, examined at infrared wavelengths by the Very Large Telescope, shows a veil of dust (red). Model of the dusty doughnut (right) details the hottest dust in yellow. Jaffe, et al./ESO, NASA

According to a widely accepted model, the cores of all galaxies that house an active supermassive black hole have several features in common, including intense X-ray and radio emissions. The galaxy cores don’t all look the same, according to the theory, because supermassive black holes are surrounded by doughnut-shaped veils that have many orientations with respect to Earth.

If a dusty doughnut is aligned so that observers must peer through its side toward the black hole, then that central powerhouse appears muted. In a rarer alignment, an observer looks straight through the hole in the doughnut, enabling detectors to record the full array of fireworks generated by material spiraling into the black hole.

Although astronomers have had several indirect lines of evidence supporting the model, telescopes haven’t had the resolution to detect any of the proposed dust veils. A new interferometer combining infrared light collected by two of the Very Large Telescope’s four 8-meter telescopes in Paranal, Chile, has finally provided a view that’s sharp enough. A belt of dust about 11 light-years across and 7 light-years thick surrounds a supermassive black hole at the core of the galaxy NGC 1068, Walter Jaffe of Leiden University in the Netherlands and his colleagues report in the May 6 Nature. That galaxy lies 50 million light-years from Earth.

“This is the first time we’ve been able to see the [doughnut] directly,” comments Julian Krolik of Johns Hopkins University in Baltimore. The high-resolution images, which show NGC 1068 structures as small as 3 light-years across, are the first to be derived from infrared interferometry of any object outside our own galaxy, he notes.

The outer part of the newly imaged dust veil has a temperature of about 50°C, while the inner part is about 500°C, Jaffe’s team says. Spectra of the dust indicate that the veil is probably made of calcium aluminum silicate, a material also found in the outer atmosphere of some stars.

While the interferometric detection of the veil in NGC 1068 confirms the cornerstones of the dust model, it also poses a puzzle. The thickness of the dusty doughnut relative to its diameter suggests that it can survive only if it gets a continuous injection of energy. Although theorists have proposed a grab bag of ideas, no one has found a convincing source of such energy. Additional high-resolution images may point researchers in the right direction, says Krolik.

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