Wind and reflections from supermassive black hole
Posted: July 28, 2003

NGC 1068. Credit: X-ray: NASA/CXC/MIT/UCSB/P.Ogle et al.; Optical: NASA/STScI/A.Capetti et al.
This composite X-ray (blue and green) and optical (red) image of the active galaxy, NGC 1068, shows gas blowing away in a high-speed wind from the vicinity of a central supermassive black hole. Regions of intense star formation in the inner spiral arms of the galaxy are highlighted by both optical and X-ray emission.

The elongated shape of the gas cloud is thought to be due to the funneling effect of a torus, or doughnut-shaped cloud, of cool gas and dust that surrounds the black hole. The torus, which appears as the elongated white spot in the accompanying 3-color X-ray images, has a mass of about 5 million Suns. Radio observations indicate that the torus extends from within a few light years of the black hole out to about 300 light years.

This illustration depicts a massive black hole at the center of a galaxy. Around it is a swirling disk of gas, which gradually pours down into the black hole. As the gas falls inward, it heats up and glows brightly, getting hotter and hotter the closer it is to the event horizon. Some of the gas is blown away from the disk like steam from a kettle. As this gas streams off the disk, the intense radiation generated by the very hot gas near the event horizon forces the escaping gas into a cone and accelerates it to speeds as high as a tenth the speed of light. Illustration: CXC/M.Weiss
The X-rays observed from the torus are scattered and reflected X-rays that are probably coming from a hidden disk of hot gas formed as matter swirls very near the black hole. The torus is one source of the gas in the high-speed wind, but the hidden disk may also be involved. X-ray heating of gas further out in the galaxy contributes to the slower, outer parts of the wind.

Observations with the spectrometers aboard Chandra enable scientists to estimate the composition, temperature and flow velocity of the gas. They show that the composition of the material in the wind is roughly similar to that of the Sun's atmosphere, except for a deficit of oxygen atoms, and that it has a temperature of about 100,000 degrees Celsius (180,000 degrees Fahrenheit). The average gas speed is about 1 million miles per hour.

These Chandra data on NGC 1068 are consistent with a picture where the observer is looking along the edge of a torus of cool gas and dust around a supermassive black hole. In this case we see the indirect effects of the black hole, but do not get a direct view. In contrast, an observer looking down into the hole of the torus would see a brilliant black hole source.

An artist's conception shows a black hole surrounded by a disk of hot gas, and a large doughnut or torus of cooler gas and dust. The light blue ring on the back of the torus is due to the fluorescence of iron atoms excited by X-rays from the hot gas disk. Illustration: CXC/M.Weiss

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