Monday, August 20, 2007

Super Massive Super Hot

NASA's Spitzer Space Telescope shows that supermassive black holes at the centers of elliptical galaxies keep the galactic "thermostat" so high gas cannot cool, stunting the birth of new stars.

Astronomers have detected dust grains mingling with blazing hot gas at temperatures of 10 million degrees Kelvin (10 million Celsius) or 17 million Fahrenheit, in an area surrounding the elliptical-shaped galaxy called NGC 5044.

Similar to raindrops forming in Earth's clouds, stars form when dense cosmic clouds of gas and dust condense. Scientists suspect that if the gas surrounding a galaxy never cools enough to condense, then new stars cannot form.

Galaxies in the universe come in many shapes and sizes. Spiral galaxies, like the Milky Way, are usually active in star formation.

By contrast, elliptical galaxies are stellar retirement communities because they are made up of older stars, and don't form many new stars. Many elliptical galaxies, like NGC 5044, are found at the centers of galaxy clusters that are filled with enormous amounts of hot gas. Why the gas doesn't cool and form new stars is a subject of intense debate among astronomers.
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Feedback heating

Observations with NASA's Hubble Space Telescope have shown small, massive clouds of dusty gas near the cores of many elliptical galaxies. Astronomers think these clouds may play a crucial role in feedback heating. They suspect this material probably gravitated toward the galaxy's center after being ejected by nearby dying stars, as part of their normal life cycle.

When some of this dusty gas approaches the host galaxy's central supermassive black hole, a large amount of energy is released -- enough to heat nearby gas to extremely high temperatures, making it buoyant. Like smoke carrying ashes away from a fire, scientists believe that this buoyant gas floats away from the galaxy's center carrying some dust with it. As plumes of this dusty smoke fill the galaxy's surrounding area, gas around the galaxy is also heated. Temi's team was the first to see this cosmic smoke with Spitzer's super-sensitive infrared eyes.

Whenever the central back hole takes another gulp of the dusty gas hovering around the galaxy's center, enough energy will be fed back to heat up more of the surrounding gas, and feedback heating will happen all over again, maintaining the temperature of the surrounding gas. Both the heating and buoyant removal of gas from the galaxy's center reduces the likelihood of star formation.

Astronomers have long hypothesized about feedback heating in the hot cluster gas surrounding elliptical galaxies, but Spitzer has given us the first piece of observational evidence that this might actually be occurring in elliptical galaxies across the universe.

Original Source: Do Supermassive Black Holes Stunt Stellar Birth in Galaxies?
by Linda Vu, Spitzer Science Center.


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