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Astronomers have long puzzled over why some elliptical galaxies stop forming many new stars, despite having the materials to do so. Now, an observation using NASA’s Chandra X-ray Observatory by a team of astronomers including assistant professor Mateusz Ruszkowski has an answer. X-ray observations of galaxies show that many galaxies are surrounded by halos of hot gas. “For decades astronomers were puzzled by the presence of the warm gas around these objects. The gas was expected to cool down and form a lot of stars” said Prof. Ruszkowski in an interview with the UofM News Service. The X-ray observation of M84, a giant elliptical galaxy in the Virgo cluster around 55 million light years away, show how the supermassive black hole at the center of the galaxy may be heating the gas. The image shows that the black hole has regular, repeated outbursts, which heats the gas in the halo. “Now, we see clear and direct evidence that the heating mechanism of black holes is persistent, producing enough heat to significantly suppress star formation. These plasma bubbles are caused by bursts of energy that happen one after another rather than occasionally, and the direct evidence for such periodic behavior is difficult to find.” The image to the right is an x-ray image of M84, with red lines showing the location of the bubbles. Some of the bubbles are inside others, like a Russian matryoshka doll. In the X-ray image, the topmost bubble appears to be in the process of popping, releasing new superheated gas into the halo and the space between galaxies. The team also produced a numerical simulation of the waves produced as the bubbles expand. The simulation shows that multiple outbursts can lead to the nested bubbles in the observation. Click the image at right to see the animation of the simulation. The repeated outbursts pump energy into the gas and dust on the galaxy and between it and other galaxies. This prevents the gas from cooling enough to form new stars. The lead author of the paper in the astrophysical journal, Alexis Finoguenov, of UMBC and the Max-Planck Institute for Extraterrestrial Physics in Germany, compares the actions of the black hole to a human heart. “Just like our hearts periodically pump our circulatory systems to keep us alive, black holes give galaxies a vital warm component. They are a careful creation of nature, allowing a galaxy to maintain a fragile equilibrium 这篇呢？ 还有LZ CR33卡车和直升机 您选的是啥？ | 
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发表于 2009-5-13 08:47:00
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