Hint of a burst of supernova activity in a superluminous galaxy.

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. It was as if one line had been erased from the spectrum."

With that observation as a starting point, Draine and Woods looked for a mechanism that would change just this one line in the molecular-hydrogen spectrum. The line corresponds to a transition from one excited rotational and vibrational energy level of the molecule to a lower energy level. It happens that a molecule can stay in the higher excited state for a long time, which allows the molecule time to absorb, say, ultraviolet radiation to reach an even more excited state. Such a molecule has many paths by which it can release energy, so the intensity of the expected molecular-hydrogen line would be much reduced.

Draine and Woods say transient X-ray sources could provide the radiation necessary to heat and ionize gas, thereby exciting hydrogen atoms, which would then emit the ultraviolet light. One possible X-ray source could be the interaction of ambient dust clouds with material ejected by supernova explosions. A burst of star formation millions of years ago could have produced a large number of massive stars, which are now dying off.

"In order for our mechanism to work, we have to invoke a high supernova rate, even considering the large luminosity of this galaxy," Draine says. "We just calculate the X-ray flux that would emerge from a cooling blast wave, and we look at what that X-ray flux would do to the surrounding material."

"I think it's a really neat idea," Lester says. "Any explanation for what's happening has to be a clever one."

One way to check the scenario is to measure the intensity of additional molecular-hydrogen emission lines, especially those difficult to detect through the Earth's atmosphere. The model suggested by Draine and Woods predicts that other, not-yet-observed emission lines should also be weaker than expected.