Paula Cleggett-Haleim Headquarters, Washington, D.C. May 25, 1993 (Phone: 202/358-0883) Embargoed Until 1 p.m. EDT Jim Elliott Goddard Space Flight Center, Greenbelt, Md. (Phone: 301/286-6255) Ray Villard Space Telescope Science Institute, Baltimore, Md. (Phone: 410/338-4514) RELEASE: 93-97 HUBBLE LOOKS AT THE HEART OF A GALAXY COLLISION NASA's Hubble Space Telescope has looked into the heart of a galaxy created by the collision of two galaxies and peering deeply into its nucleus, discovered a remarkable pinwheel-shaped disk of gas surrounded by clusters of young stars born as a result of the merger. The star clusters apparently were born as a result of the collision of two disk-shaped galaxies. The galaxy merger, which occurred about 1 billion years ago, triggered an infall of the gas which fueled the birth of new stars around the center of the galaxy. "This may unlock the key for understanding how all globular clusters formed in ellipticals," said Dr. Brad Whitmore of the Space Telescope Science Institute (STScI), Baltimore, Md. "The Hubble observation also shows how tiny disk-like structures might have formed in many other galaxies." This discovery provides some of the best evidence to date for explaining the origin of giant elliptical galaxies. For more than a half century, astronomers have theorized about how such galaxies formed. Some theories propose that ellipticals formed from collisions between disk galaxies -- flattened stellar systems resembling the Milky Way galaxy. These results are being presented in a press conference today at NASA Headquarters, Washington, D.C., by Whitmore and Dr. Francois Schweizer of the Carnegie Institution of Washington. Co-investigators are Claus Leitherer, Kirk Borne and Carmelle Robert of STScI. - more - - 2 - Pinwheel of Stars and Gas The striking Hubble image shows a spiral pattern at the galaxy's core, surrounded by bright star clusters. "I knew I had a major result within 10 seconds of looking at the Hubble picture," said Whitmore. The pinwheel shaped disk has an uncanny resemblance to a face-on spiral galaxy, yet it is only 10 thousand light-years across -- about 1/20 the size of the total galaxy. The gas and stars in the disk swirl around the nucleus, making a spiral pattern like cream poured in a cup of coffee. The mini-spiral contains enough gas to make 8 billion stars like the sun. Though several of the clusters were first spotted from ground- based telescopes, their true nature was uncertain until the Hubble observations. Hubble's resolution is so good that the astronomers can measure the diameters (0.04 arc seconds, the apparent size of a dime at a distance of 80 miles) of the bright star clusters seen in the same image as the spiral disk. They turn out to be about 60 light years across, the same size as globular clusters that orbit the Milky Way galaxy. The globular clusters found in NGC 7252 are considered the progenitors of similar clusters that orbit the Milky Way galaxy. Since globular clusters normally contain ancient red giant stars, they provide a fossil record of the formation and evolution of galaxies. Globular clusters contain about 1 million stars each, arranged in a tight, spherical swarm and generally are found to be about 15 billion years old. However, the "ultra-luminous clusters" found in NGC 7252 contain hot bluish stars. Because these blue stars are short-lived, the clusters in NGC 7252 are estimated to be mostly between 50 and 500 million years old. The blue stars make the globular clusters up to several hundred times brighter than the clusters that orbit the Milky Way galaxy. If the Milky Way's globular clusters were as bright, they could be seen with the naked-eye and would be brighter than the stars in the Big Dipper. In the 1920's, American astronomer Edwin Hubble classified galaxies according to their spiral or elliptical shape. A key difference is that stars are concentrated in a disk in spirals, but are distributed in a diffuse, roughly spherical distribution in ellipticals. Since Edwin Hubble's time, astronomers have sought an explanation for why there are two different types of galaxies. During the past decade, the hypothesis that spiral galaxies can collide and merge to form elliptical galaxies has become increasingly popular. - more - - 3 - Located 300 million light-years away in the constellation Aquarius, NGC 7252 has been considered the prototypical example of a merger between two disk-shaped galaxies. The galaxy has a pair of long tails that are unambiguous evidence of the effects of gravitational tidal forces from a galaxy merger. The galaxy NGC 7252 is nicknamed the "Atoms-for-Peace" galaxy because its stars form a bizarre loop-like structure that resembles a schematic diagram of an electron orbiting and an atomic nucleus. (In December 1953. U.S. President Dwight D. Eisenhower made his "Atoms for Peace" speech to foster peaceful applications of nuclear energy. If globular clusters can be born during galaxy collisions, it reinforces the theory that disk galaxies merge to make giant elliptical galaxies. One argument against this theory is that elliptical galaxies have more globular clusters than expected if disk galaxies were simply combined, since disk galaxies have relatively few clusters. Hubble Picture Helps to Solve Mystery The new Hubble Space Telescope observation solves this dilemma by showing that when disk galaxies collide they can form new globular clusters. Rather than being a problem for the merger scenario, an increase in the number of globular clusters is a natural consequence of galaxy mergers. The existence of a "mini-disk" also fits with the merger scenario since similar disk-like features appear to exist in many elliptical galaxies. Another clear indication that the material originated from the collision of two galaxies is that the mini-spiral is rotating in a direction opposite to the rest of the galaxy. This discovery is the latest in a series of disk-like structures that Hubble has uncovered at the cores of galaxies. Previously, HST found a giant disk of cool dust and gas orbiting a suspected black hole in the active galaxy NGC 4261 and discovered an edge-on "donut" of dust in the spiral galaxy M51. The astronomers predict that in a few billion years the gas in NGC 7252 will be exhausted. The galaxy will look like a normal elliptical galaxy with a small inner disk. - end - NOTE TO EDITORS: Color and black and white images are available to media from NASA's Broadcast and Imaging Branch, 358-1741. The photo numbers are: B&W: 93-H-205, -206 Color: 93-HC-190