Don Savage
NASA Headquarters, Washington, DC
(Phone: 202-358-1547)

Tammy Jones
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301-286-5566)

Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone: 410-338-4514)

PRESS RELEASE NO.: STScI-PR97-34

Hubble Reveals Stellar Fireworks Accompanying Galaxy Collisions

NASA's Hubble Space Telescope has uncovered over 1,000 bright, young star clusters bursting to life in a brief, intense, brilliant "fireworks show" at the heart of a pair of colliding galaxies.

"The sheer number of these young star clusters is amazing," says Brad Whitmore of the Space Telescope Science Institute (STScI), Baltimore, Maryland. "The discovery will help us put together a chronological sequence of how colliding galaxies evolve. This will help us address one of the fundamental questions in astronomy: why some galaxies are spirals while others are elliptical in shape."

"These spectacular images are helping us understand how globular star clusters formed from giant hydrogen clouds in space," adds Francois Schweizer of the Carnegie Institution of Washington, Washington, D.C. "This galaxy is an excellent laboratory for studying the formation of stars and star clusters since it is the nearest and youngest example of a pair of colliding galaxies."

By probing the Antennae galaxies (called the Antennae because a pair of long tails of luminous matter formed by the encounter resembles an insect's antennae) and some of the other nearby galactic-scale collisions, Hubble is coming up with a variety of surprises:

• Globular star clusters are not necessarily relics of the earliest generations of stars formed in a galaxy, as once commonly thought, but may also provide fossil records of more recent collisions.
• The "seeds" for star clusters appear to be huge clouds (tens to hundreds of light-years across) of cold hydrogen gas, called giant molecular clouds, which are squeezed by surrounding hot gas heated during the collision and then collapse under their own gravity. Like a string of firecrackers being ignited by the collision, these reservoirs of gas light up in a great burst of star formation.
• The ages of the resulting clusters provide a clock for estimating the age of a collision. This offers an unprecedented opportunity for understanding, step-by-step, the complex sequence of events which take place during a collision, and possibly even the evolution of spiral galaxies into elliptical galaxies.

Earlier Hubble pictures show that nearly a third of very distant galaxies, which existed early in the history of the universe, appear to be interacting galaxies, like the Antennae. In particular, the Hubble Deep Field (a "long-exposure" image from Hubble looking at galaxies far back into time), uncovered a plethora of odd-shaped, disrupted-looking galaxies. They offer direct visual evidence that galaxy collisions were more the rule than the exception in the early days of the universe.

These distant galaxy collisions are too faint and too small to study in much detail. Astronomers say we are fortunate to have such a nearby example as the Antennae to study, since collisions between galaxies are relatively rare today. "The degree of detail in these images is astounding, and represents both a dream come true and a nightmare when it comes to the analysis of such a large amount of data," Whitmore says.

In addition to providing a window into how stars and galaxies formed in the dim past, the Hubble views might also offer a glimpse of the future fate of Earth's home galaxy, the Milky Way, when it either sideswipes or plows head-on into the neighboring Andromeda galaxy billions of years from now.

The Hubble observations of the Antennae galaxies, as well as several other nearby colliding galaxies, were conducted by Whitmore (STScI) and co-investigators Francois Schweizer and Bryan Miller (Department of Terrestrial Magnetism, Carnegie Institution of Washington), and Michael Fall and Claus Leitherer (STScI) over the past several years.

Hubble's resolution and sensitivity allowed the team to uncover over 1,000 exceptionally bright young star clusters, sometimes called super star clusters, within the Antennae -- the prototypical galaxy smashup. Ground-based telescopes were only able to see the brightest of these clusters, and even in these cases were not able to show that the clusters were very compact with the sizes of normal globular clusters.

Observing other galaxy collisions, the Hubble team discovered the presence of young star clusters which were very bright and blue in the case of ongoing collisions, but had faded to become fainter and redder for the older merger remnants. This allowed them to place the snapshots of galaxy collisions into a chronological sequence.