Hubble observations back merger theory.

Galaxies come in various shapes, including fuzzy footballs, elongated smears, hazy pinwheels, and glowing whirlpools. In the 1920s, astronomer Edwin Hubble resolved some of the confusion by classifying galaxies as either spiral (disk-like and compact) or elliptical (egg-shaped and diffuse). But the fundamental question remains: Why do galaxies look so different?

In recent decades, some astronomers have argued that spirals can merge to form larger, elliptical galaxies. Now, images from the Hubble space telescope provide some of the strongest evidence to date for the merger theory.

Astronomer Bradley C. Whitmore of the Space Telescope Science Institute in Baltimore and colleagues peered into the core of the elliptical galaxy NGC 7252, already suspected to be the product of a merger between two spiral galaxies, and saw something strange and unexpected. "Just for one terrible moment I thought, 'Oh my God, I gave them the wrong coordinates!'" Whitmore recalls.

Fortunately, Whitmore had indeed pointed Hubble in the right direction. And to his surprise, the telescope images revealed a pinwheel-shaped whorl of gas and stars in the galaxy's center. This "mini-spiral," as Whitmore calls it, measures 1/20 of the diameter of NGC 7252 and, in an unprecedented twist, rotates counter to the rest of the galaxy.

The astronomers also found at least 40 tightly packed, spherical knots of stars, called globular clusters, speckling the galaxy's central pinwheel. These young, blue clusters, previously detected with ground-based instruments but not seen clearly until now, provide a key piece of evidence for the merger theory. The clusters appear to be 50 to 500 million years old -- too young to have originated in the parental spiral galaxies. Thus, the clusters must have formed in the merger itself, says Whitmore.

About a billion years ago, Whitmore explains, two spiral galaxies had a fateful encounter where NGC 7252 now lies, some 300 million light-years from Earth in the constellation Aquarius. As the spinning galaxies merged, a powerful gravitational tug-of-war ensued among their billions of stars, tearing the spirals apart. Currents of gas streamed into the center of the merging mass, creating the swirling mini-spiral in the core of NGC 7252. This inrush of gas also created millions of stars, which gravitated together in dense clusters.

The new observations, says Jon A. Holtzman of Lowell Observatory in Flagstaff, Ariz., will help resolve the main objection to the merger theory: that ellipticals contain more clusters than expected from the simple addition of spiral galaxies. The Hubble images suggest that cluster birth may be a consequence of the merger process, says Holtzman, who has conducted similar research on the elliptical galaxy NGC 1275 (SN: 1/25/92, p.52).

Holtzman cautions, however, that "one case does not an entire theory prove." Both he and Whitmore emphasize that more work is needed to explain the physics of how new clusters form in mergers. Astronomers must also find more examples of the phenomenon. Only then might they agree that spiral mergers account for some elliptical galaxies.