"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."
The 58 young, diminutive galaxies spied by Hubble were photographed as they appeared more than 10 billion years ago. 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.
The research team used Hubble's Wide Field Camera 3 to search for faint, star-forming galaxies in ultraviolet light, a reliable tracer of star birth. The galaxies existed when the universe was undergoing a "baby boom" of star formation, estimated to have peaked between 9 billion and 12 billion years ago.
The results are being presented at the 223rd meeting of the American Astronomical Society in Washington, DC.
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.
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 percent of new stars formed in these small galaxies. "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, and first author on the science paper describing the results.
The Hubble analysis shows they are small, irregularly shaped objects measuring just a few thousand light-years across. Even when fully mature, these galaxies will be about one-tenth to one one-hundredth the mass the Milky Way. Because they are undergoing a firestorm of star birth, their light is dominated by the ultraviolet glow of fledgling stars.
This strategy of surveying large numbers of background galaxies with deep observations of lensing clusters is being used in a new three-year Hubble survey, called the Frontier Fields. Astronomers are using Hubble to exploit the magnification powers of six massive galaxy clusters in a hunt for small galaxies that existed more than 12 billion to 13 billion years ago.
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