Observations with NASA's Hubble Space Telescope (HST) have given a tantalizing glimpse of the time soon after galaxies formed, suggesting that these huge systems of stars formed over a wider span of time than once believed.
These data, part of a study of galaxy evolution being conducted by astronomers from Arizona State University (Dr. Rogier Windhorst and students) and the University of Alabama (Dr. William Keel), were presented at a HST symposium held in Bala Chia, Italy this week.
Images of the distant radio galaxy 53W002 were obtained in the visible and near-infrared ranges using the telescope's Wide Field and Planetary Camera (WFPC). After processing to remove the aberration caused by the spherical aberration in the primary mirror, using programs developed at UA, the pictures show that the galaxy already has a regular shape, surprisingly similar to present-day galaxies.
The discovery will allow for further study and hypothesis about the formation of the Milky Way and other galaxies. "Determining the early history of galaxies was one of the key goals of the HST . . . and we are now actually seeing the process of a galaxy being formed," said Keel.
The galaxy is seen as it was when the universe was less than one third of its present age. The galaxy's spectrum indicates that most of its stars formed about 500 million years before this point. The HST images show that the galaxy has had time to collapse to a compact form, which simulations show would also take about 500 million years. It had been widely assumed that star formation and galaxy collapse occur together, but the HST data make it possible to verify this for the first time.
"What we are seeing is how the galaxy looked billions of years ago," Keel said. "If galaxy 53W002 were developing concurrently with our galaxy, it would appear much older through the telescope. However, it appears younger, suggesting it is about two billion years younger than our galaxy."
This galaxy, first detected in a survey done with the Westerbork Synthesis Radio Telescope in the Netherlands and mapped in more detail using the Very Large Array radio telescope in New Mexico, is one of the few relatively normal galaxies known at such a great distance and early time. Most other distant galaxies have such a strong component from an active nucleus that the galaxies' populations of stars are difficult to separate from the role of the core. The much weaker radio source in 53W002 indicates that the nonstellar processes in the nucleus are much less important and can be accounted for. This galaxy is very faint, magnitude 23 in visible light. This is nearly 10 million times fainter than the dimmest stars visible to the unaided eye. These images required four hours HST exposure each, spread over 24 orbits in a 36 hour period in late 1991.