Luminous arcs discovered between galaxies.

Luminous Arcs Discovered Between Galaxies

While surveying clusters of galaxies forother purposes, Roger Lynds of Kitt Peak National Observatory at the National Optical Astronomy Observatories in Tucson, Ariz., found two, possibly three examples of bright luminous arcs stretching between galaxies in three of the 58 clusters of galaxies he surveyed. The discovery of such intergalactic arcs is unprecedented. They were unknown before now, and what they are made of and how they got where they are remain mysteries, Lynds and theorist Vahe Petrosian of Stanford University told last week's meeting of the American Astronomical Society in Pasadena, Calif.

The arcs are more than 100,000 parsecs(300,000 light-years) long, blue in color, sharply defined and shaped in almost perfect circular arcs. They are very luminous, with powers equal to 100 billion suns, but they are also very far from us.

The geometric perfection is especiallyintriguing, Lynds and Petrosian say, as nature rarely deals in such perfect shapes. "It's as if God took a piece of rope and bent it there," Petrosian says.

The blueness may indicate that thearcs are made of young stars. Young stars are blue; mature stars tend to be yellow or white. Spectra will tell whether there are stars in the arcs, or whether they are simply made of some luminous gas. So far the observers have not been able to obtain spectra, but that is one of their aims for the future.

Whether the arcs are made of stars or ofother matter taken out of the galaxies or from the intergalactic space of the cluster, how they got where they are is another puzzle. Petrosian says the three most obvious possibilities don't work. If they were stars blown out by a blast wave, they should form fragments of spherical shells, and they don't seem to. They seem like pieces of rope -- or, as one scientist put it, sausages. Furthermore, such an explosion would require the energy of 100 million supernova explosions -- the thermonuclear explosions of 100 million large stars -- to drive it. Such an explosion should leave other evidence behind, and there is none of that.

If the arcs were dragged out of the galaxiesin their clusters by some kind of tidal distortion by a mass in the center of each cluster, there would have to be a mass there equal to 100 trillion (10.sup.14.) suns, something many times larger than most galaxies. Moreover, the shape of the arcs is too smooth for them to have been made that way.

If the arcs were made of nonstellar matter -- thatis, if they were giant examples of the jets associated with quasars and some galaxies -- they would have to be strong radio emitters. One attempt to find radio waves from them came up negative, but that is not conclusive. However, such jets radiate by mechanisms that involve large amounts of highly energetic electrons. To energize jets of this size in that way, Petrosian estimates, would take the energy of a billion supernovas. The source of such energy should be obvious, and it isn't.

A fourth, less prominent possibility isthat they are something primordial, left over from the time the galaxies formed. But there is not yet much theory about that one way or the other.

Future plans include looking for moreexamples of these jets, and trying to get their spectra. Lynds also says he would like to use the Very Large Array of radiotelescopes near Socorro, N.M., to see whether they show up as radio emitters. Future activity will also undoubtedly include some more theorizing.