CU researcher heads international team in a galactic discovery

Research associate Vassilis Charmandaris, shown here at the Space Sciences Building, has been studying the giant elliptical galaxy Centaurus A. Matthew Fondeur/University Photography

By David Brand

Astronomers working with the 15-meter Swedish-European Southern Observatory (ESO) Submillimetre Telescope at the La Silla observatory in Chile, led by Cornell researcher Vassilis Charmandaris, have shown for the first time that the nearby giant elliptical galaxy Centaurus A is surrounded by shells in which carbon monoxide (CO) molecules are present.

The CO molecules were detected in two of the surrounding shells, located at a distance of about 50,000 light-years from the nucleus of the galaxy.

The discovery opens very interesting avenues for future research on the evolution of gas in interacting galaxies, said Charmandaris, who before arriving as a research associate in the Cornell astronomy department was a postdoctoral fellow at the Department of Millimetric Radioastronomy (DEMIRM) at the Paris Observatory, France. His team's findings will appear in the April issue of the European journal Astronomy and Astrophysics and were the subject of a recent ESO press release.

One of the major differences between elliptical and spiral galaxies is that the former do not possess extensive gaseous discs in which young stars can be formed. This is despite the fact that most elliptical galaxies are probably formed by the merger of two or more spiral galaxies. However, during such a process most of the gas in the spirals is either quickly turned into stars by massive bursts of star formation or is completely lost into the surrounding space.

In 1994, atomic hydrogen gas was discovered in the outer shells of some elliptical galaxies. Charmandaris and other astronomers believed that a possible explanation might be that this diffuse atomic gas is located, not in vast, very dilute clouds, but rather around smaller, much denser molecular clouds, such as those detected in our own galaxy, the Milky Way. But due to their relative compactness, said Charmandaris, these molecular clouds would be expected to be found at significant distances from the remnants of galactic collisions.

To test this idea, the astronomers decided to look for the possible presence in the shells around some nearby elliptical galaxies of specific molecules that are typical of dense gas clouds. Their search was successful, notably in the case of Centaurus A, a galaxy with strong radio emissions and an active nucleus.

What is the likely fate of these gaseous shells? Charmandaris said that most shells are gravitationally bound to Centaurus A and cannot escape into the surrounding intergalactic space. But while the thin and diffuse hydrogen gas probably will move towards the galaxy center fairly quickly, the more clumpy clouds and their molecular gas could remain in the outer shells over long periods, he said, becoming less prominent over time as the clouds -- and the gas -- slowly disperse.

Also on the team were Françoise Combes, DEMIRM, and Thijs van der Hulst, the Kapteyn Institute, University of Groningen, the Netherlands. The work was supported by the European Union under a Marie-Curie fellowship.