A Tiny Galaxy Is Born

Evidence of the galaxy's youthfulness can be seen in the burst of newborn stars and its disturbed shape. This evidence indicates that the galaxy, called POX 186, formed when two smaller clumps of gas and stars collided less than 100 million years ago (a relatively recent event in the universe's 13-billion-year history), triggering more star formation. Most large galaxies, such as our Milky Way, are thought to have formed the bulk of their stars billions of years ago.

The Hubble images of POX 186 support theories that all galaxies originally formed through the assembly of smaller "building blocks" of gas and stars. These galactic building blocks formed shortly after the Big Bang, the event that created the universe. Astronomers Michael Corbin of the Space Telescope Science Institute in Baltimore, Md., and William Vacca of the Max-Planck Institute for Extraterrestrial Physics in Garching, Germany, used the telescope's Wide Field and Planetary Camera 2 to study POX 186 in March and June 2000. Their results will appear in the Dec. 20 issue of the Astrophysical Journal.

"This is a surprising find," Corbin says. "We didn't expect to see any galaxies forming in the nearby universe. POX 186 lies only about 68 million light-years away, which means that it is relatively close to us in both space and time."

Adds Vacca: "POX 186 may be giving us a glimpse of the early stages of the formation process of all galaxies."

POX 186 is a member of a class of galaxies called blue compact dwarfs because of its small size and its collection of hot blue stars. [The term "POX" is derived from the French "prism objectif," or objective prism, a device that astronomers place in front of a telescope to photograph spectra of all objects in its field of view.] POX 186 was discovered 20 years ago, but ground-based telescopes resolved few details of the galaxy's structure because it is so tiny. To probe the galaxy's complex structure, astronomers used the sharp vision of the Hubble telescope. The Hubble pictures reveal that the system is puny by galaxy standards, measuring only about 900 light-years across, and containing just 10 million stars. By contrast, our Milky Way is about 100,000 light-years across and contains more than 100 billion stars.

So why did POX 186 lag behind its larger galactic cousins in forming? Corbin and Vacca find that the young system sits in a region of comparatively empty space known as a void. Its closest galactic neighbors are about 30 million light-years away. The two small clumps of gas and stars that are merging to form POX 186 would have taken longer to be drawn together by gravity than similar clumps in denser regions of space. The Hubble data don't reveal the ages of the stars in the clumps. Corbin, however, suspects that the oldest stars may be about 1 billion years old, which is young on the cosmic time scale.

The youthful galaxy's puny size may support a recent theory of galaxy formation known as "downsizing," which proposes that the least massive galaxies in the universe are the last to form. In clear contrast to POX 186, the most massive galaxies in the universe, known as giant ellipticals, have a generally spherical structure with few or no young stars, indicating that they formed many billions of years in the past. To actually see the formation process of stars in such large galaxies, astronomers are awaiting the deployment of Hubble's successor, the James Webb Space Telescope. This telescope is designed, in part, to study faint objects whose light left them early in the 13-billion-year history of the universe.

Although the POX 186 results are tantalizing, Corbin and Vacca realize that one galaxy is not enough evidence to support the idea that galaxy formation is an ongoing process. They are proposing to use Hubble to study nine other blue compact dwarfs for similar evidence of recent formation.