This Galaxy Has Good Teeth

A satellite instrument has detected fluoride molecules at the center of the Milky Way. The finding, to be published in the October issue of Astrophysical Journal Letters, provides the first evidence that the fluorine on Earth, such as the fluoride in toothpaste, originally came from interstellar space dust, rather than massive stellar explosions known as supernovae.

Astronomers can study the chemistry of interstellar gas and dust clouds by measuring how much radiation passes through, since each atom or molecule absorbs radiation at characteristic wavelengths. By analyzing these spectra, astronomers have found more than a hundred different molecules in interstellar gas clouds. The element fluorine, however, had proved difficult to detect because Earth's atmosphere blocks the wavelength absorbed by hydrogen fluoride, the fluorine compound thought to be most common in space.

David Neufeld and his colleagues at Johns Hopkins University took advantage of the Infrared Space Observatory, a European Space Agency satellite, to rise above the clouds. The researchers pointed a satellite-based instrument called the Long Wavelength Spectrometer at the Sagittarius B2 cloud, some 20,000 light-years from Earth. The amount of infrared radiation passing through the cloud dipped at the wavelength absorbed by hydrogen fluoride. By measuring the size of the dip, the team calculated the concentration at 0.3 parts per billion--just a tenth of the concentration of fluorine in our solar system. Neufeld thinks that the remainder is tied up in tiny frozen clumps of solid fluorine, which the infrared methods cannot detect.

Now that hydrogen fluoride has been detected in an interstellar cloud, experts say the chances are good that dust carrying the compound enriched rocky planets when they formed. "It would be reasonable to infer that most of the fluorine on Earth is stuff locked in this rocky material," says Leo Blitz, director of the radioastronomy laboratory at the University of California, Berkeley.