ASCA sheds light on galaxy formation.

They have the spherical shape of ordinary galaxy clusters and radiate X rays and visible light at the expected intensity. In fact, the four groupings of galaxies recently observed with the Japanese X-ray satellite ASCA probably are ordinary clusters--which is precisely what intrigues Michael Loewenstein and Richard F. Mushotzky.

Shortly after its 1993 launch, ASCA recorded X rays from the hot gas swaddling four galaxy clusters that lie within a few hundred million light-years of the Milky Way. For the first time, astronomers could closely examine X-ray emissions from two elements--silicon and oxygen--that provide a fossil record of ancient starbirth. Analysis of these data now reveals that the four clusters--Abell 496, Abell 1060, Abell 2199, and AWM7--contain enormous amounts of silicon and oxygen.

Only massive stars can forge these elements. When such stars explode as type II supernovas, they hurl silicon and oxygen into space, enriching the intracluster medium. The abundance of the two elements indicates that the clusters' assortment of galaxies contained in the distant past an unusually high proportion of massive stars, creating a hotbed of violence, assert Loewenstein and Mushotzky of NASA's Goddard Space Flight Center in Greenbelt, Md.

These supernovas, energetic and numerous, ejected from their home galaxies as much visible mass as remains in the galaxies today, the astronomers estimate. And if these galactic groupings resemble others in the universe, then many apparently sedate clusters suffered similarly violent pasts, Loewenstein and Mushotzky reported last month at a meeting of the Astronomical Society of the Pacific in College Park, Md.

Since the 1970s, several craft have found that clusters contain substantial amounts of heavy elements. But nearly all have relied on X-ray emissions from iron, which can enter the intracluster medium in two ways--the explosion of many massive stars as type II supernovas or the detonation of far fewer, lower-mass stars as type I supernovas.

The ASCA data firmly point to the type II origin.

The researchers say their findings have several implications for understanding galaxy formation. Because the energy generated by the supernovas would have exceeded the gravitational tug from visible matter, fledgling galaxies must have harbored an enormous amount of invisible, or dark, matter.

"In this very violent environment, you need a very massive dark-matter halo to keep [the galaxy] from blowing apart," says Loewenstein.

In suggesting that infant galaxies blazed with light from a huge number of massive stars, the findings add to a continuing puzzle: Searches for primeval galaxies have come up empty-handed. The astronomers cite several explanations. Dust may veil the light; starbirth may have occurred over an extended time; or the first generation of stars formed so long ago that they are now are too distant, and thus too dim, to see.

Stanislav G. Djorgovski of the California Institute of Technology in Pasadena suggests another alternative: Some bright objects identified as distant quasars may in fact represent infant galaxies fiery with starbirth.