Magazine article Science News

Hint of a Burst of Supernova Activity in a Superluminous Galaxy

Magazine article Science News

Hint of a Burst of Supernova Activity in a Superluminous Galaxy

Article excerpt

Hint of a burst of supernova activity in a superluminous galaxy

The cosmic zoo holds many strange creatures, but NGC 6240 stands out as a particularly intriguing example. Catalogued as a galaxy, it displays a contorted structure and disturbed dust clouds -- features commonly seen when two spiral galaxies are in the process of merging. At the same time, it is about 1,000 times as luminous as the Milky Way. Even more remarkable, much of its light shines in the form of infrared radiation, especially those wavelengths emitted by excited molecules of hydrogen.

Two astrophysicists have now constructed a scenario to account for a key feature of that infrared spectrum. The scenario suggests NGC 6240 may be going through a period of unusually high supernova activity, with as many as three massive stars exploding every year. The supernova rate for the Milky Way is only a couple per century.

The argument, put forward by Bruce T. Draine of Princeton (N.J.) University and D. Tod Woods of the Lawrence Livermore (Calif.) National Laboratory, hinges on the strength of a single spectral line. In 1988, Dan F. Lester and his colleagues at the University of Texas at Austin reported the results of a detailed study of the spectrum of molecular-hydrogen lines emitted by NGC 6240. With the exception of one line that was much weaker than expected, they found that the intensity of the molecular-hydrogen lines fitted a model in which the emissions are caused by heating due to shock waves propagating through the galaxy's dense dust clouds.

"That [weak line] was a notable discrepancy that we didn't seem to be able to account for on the basis of our data," Lester says. "It was very perplexing to find that nine or so molecular-hydrogen emission lines all fit the model perfectly, and then to find one that was a factor of 10 fainter than what the model predicted it should be. …

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