Spaceflight Now: Breaking News

Raining deuterium gives new clues to Milky Way formation
Posted: June 29, 2000

Twelve-meter telescope. Photo: NRAO
A team of researchers led by Hofstra University Professor Donald Lubowich and Williams College Professor Jay M. Pasachoff recently discovered a significant amount of deuterium (a rare isotope of hydrogen) raining into the center of our Milky Way Galaxy. They reported their findings, and its implications for understanding the origin of the light elements in the Universe, in the June 29 issue of Nature.

The abundance of deuterium provides important insights into the formation of the Milky Way and demonstrates that deuterium is not made by stars or stellar activity. (Stars burn deuterium into helium, leaving less deuterium.) Thus, the deuterium they have detected in the Milky Way was created in the first few minutes after the Big Bang.

Lubowich and Pasachoff were joined at the telescope by Prof. Thomas J. Balonek of Colgate U, a collaborator through the Keck Northeast Astronomy Consortium, and Hofstra student Ann Mancuso, Williams student Robert P. Galloway, and Colgate student Christy Tremonti. For their publication, they enlisted Prof. Tom Millar of Manchester University and his student Helen Roberts to calculate some theoretical models of how deuterium is incorporated in molecules.

The researchers used the 12-meter (40-foot) radiotelescope (similar to a large satellite dish) at the National Radio Astronomy Observatory's site on Kitt Peak outside Tucson, AZ, to determine the abundance of deuterium from microwave observations of a molecule containing one atom of deuterium, carbon and nitrogen, a form of hydrocyanic acid. They observed the molecule, known as DCN, in the Sagittarius A cloud, only about 30 light years from the center of the Milky Way Galaxy, which is about 25,000 light years from Earth.

The existence of deuterium in the galactic center had been suggested in observations by Nobel Prize winner Arno Penzias. But the Lubowich and Pasachoff observations pin down the ratio of deuterium (heavy hydrogen) to ordinary hydrogen, allowing quantitative conclusions to be drawn. They find only one part per million of deuterium compared with hydrogen, a million times more deuterium than had been expected in that location. Comparing the abundance of deuterium found with the abundances known for other light elements like lithium and for heavier elements like oxygen rules out local formation of the deuterium. The most likely source of the added deuterium is clouds of primordial matter from the halo of our galaxy or from intergalactic space, with that primordial material raining down into our galaxy's center. The definitive detection of deuterium in the amounts reported also is important observational confirmation of the theoretical idea that all the deuterium in the Universe is primordial, since the center of our galaxy would be the place that more deuterium could be formed by cosmic rays, if that were a viable process.

The abundance of deuterium found, together with analysis based on earlier work by Nobel Prize winning physicist William A. Fowler and colleagues, indicates that only 4% of the mass of the Universe is in the form of ordinary matter, a value also indicated by other recent results. Pasachoff and Lubowich had earlier participated in setting a limit for the amount of deuterium in the galactic center through observations at the radio wavelength of atomic deuterium, as reported in the Astrophysical Journal in 1989, in collaboration with K. Anantharamiah. Another third of the mass is thought to be "dark matter" in some unknown form. The rest of the Universe is thought to be filled with some type of energy that leads, on a large scale, to an antigravity.

The observations were made with a telescope of the National Radio Astronomy Observatory that is to be closed on July 1, for financial reasons. The national radio astronomy observatory is putting its effort into erecting an array of radio telescopes in Chile, jointly with the European Southern Observatory. The observations were made in May and June 1993 and in February 2000.

The article will appear in the June 29 issue of the British journal Nature, where it will be a featured article and will be accompanied by a "News and Views" description and analysis in Nature's "News and Views" section by Francesca Matteuci, of the Universita degli Studi di Trieste, Italy, who puts the scientific results in context.