Tracking the evolution of galaxies.

Two teams of astronomers using the Hubble Space Telescope have found evidence that mergers have played a key role in creating today's collection of galaxies.

One of the research groups, which includes Steven B. Mutz and Rogier A. Windhorst of Arizona State University in Tempe, measured the size of several hundred spiral and elliptical galaxies with Hubble's wide-field and planetary camera. In their analysis, the astronomers examined 75 of these galaxies in greater detail, calculating their distance from Earth by taking spectra with two ground-based telescopes. The galaxies -- 47 spirals and 28 ellipticals -- ranged in distance from about 2.6 billion to 7.8 billion light-years.

Mutz and his coworkers estimated what sizes nearby spiral and elliptical gala:des would appear to be if these bodies were located billions of light-years from Earth. They then compared these calculated sizes with those measured by Hubble for galaxies that actually reside at such faraway distances. The elliptical galaxies seen by the telescope are about one-half to one-third the size of the ellipticals that lie closer to Earth, Mutz says. In contrast, the faraway spirals are about the same size as those nearby,

Galaxies that lie billions of light-years from Earth are observed as they appeared in their youth, billions of years ago. Thus, the finding indicates that elliptical galaxies in the distant past were smaller than ellipticals of today.

But how did these small ellipticals become the larger elliptical galaxies of the present-day universe? A second study now adds to the body of evidence suggesting that groups of the smaller galaxies merged under the influence of gravity to form the larger bodies.

In this study, Windhorst, Barbara Franklin of Arizona State University, and William C. Keel of the University of Alabama in Tuscaloosa and their colleagues compared the likelihood that nearby galaxies, observed with ground-based telescopes, and distant galaxies, detected by Hubble, occur in pairs. The team found that slightly more than one-third of the distant galaxies pair off, compared with just 7 percent of nearby galaxies. Studies suggest that pairing is a prelude to merging.

The new finding suggests that mergers were more common in the early universe and have played a significant role in the evolution of galaxies, the researchers conclude.

However, Mutz and his colleagues also note an alternative explanation for their fIndIng that ellipticals appear smaller than expected in the past. Because the team measured the dimensions of galaxies based on the location of starlight, it's possible that ellipticals in the early universe were the same size as those today but that stars had only ignited in a narrow, core region of the galaxies.

If this scenario proves correct, then stars at the center of elliptical galaxies should be older, and thus redder, than those in outlying areas. This April, Mutz and his colleagues will start putting this theory to the test, surveying in the near-infrared some of the elliptical galaxies previously imaged in their Hubble study. For this study, the team will use a high-resolution telescope at Steward Observatory near Tucson recently outfitted with a mirror that changes position to compensate for the distorting influence of atmospheric turbulence.