They are the smallest, faintest, and most numerous galaxies ever seen in the remote universe, captured by Hubble deep exposures taken in ultraviolet light.
The galaxies spied by Hubble were photographed as they appeared more than 10 billion years ago, during the heyday of star birth. The newly discovered galaxies are 100 times more numerous than their more massive cousins. But they are 100 times fainter than galaxies typically detected in previous deep-field surveys of the early universe. These galaxies would normally be too faint for Hubble to see. To detect them, astronomers teamed Hubble with a natural zoom lens in space, produced by the gravity of a giant foreground galaxy cluster, Abell 1689.
The cluster is so massive that it magnifies the light from faraway galaxies behind it due to a phenomenon called gravitational lensing, where the curvature of space acts like
a giant funhouse mirror to stretch and brighten distant objects.
"There's always been a concern that we've only found the brightest of the distant galaxies," said study leader Brian Siana of the University of California, Riverside.
"The bright galaxies, however, represent the tip of the iceberg. We believe most of the stars forming in the early universe are occurring in galaxies we normally can't see at all. Now we have found those 'unseen' galaxies, and we're really confident that we're seeing the rest of the iceberg," Siana said.
Siana's team believes it has completed the census of galaxies at an epoch when the universe was roughly 3.4 billion years old.
If this sample of galaxies is representative of the entire population at this early time, then 80 per cent of new stars formed in these small galaxies, researchers said.
"Though these galaxies are very faint, their increased numbers mean that they account for the majority of star formation during this epoch," said team member Anahita Alavi,
also of the University of California, Riverside. The results of the study are being presented at a meeting of the American Astronomical Society in Washington, DC.