Richard Ellis, Astronomy Department, Caltech
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For many years astronomers have sought to locate the most distant objects in the Universe. As light travels at a finite speed, we see back in time and the most powerful facilities, such as the Keck Telescopes in Hawaii and Hubble Space Telescope, can readily glimpse sources viewed when the Universe was only a billion years old. However, only the most spectacularly luminous objects are normally detectable at these vast distances. For many reasons, astronomers seek to find more representative objects at early times. Finding the earliest sources, forming stars for the first time, is currently a major goal in cosmology. However, most astronomers believe these will be faint low mass clusters of, at most, a few million stars. As such they lie beyond reach of current facilities. Locating and characterizing this primeval population is a major motivation for constructing Hubble's successor, the Next Generation Space Telescope (NGST) , a 6.5 meter telescope to be launched in 2009.

Magnifying Faint Objects via Gravitational Lensing:

Gravitational lensing , a feature of Einstein's theory of General Relativity, offers a unique short cut to solving this problem while we wait for NGST to be launched. By viewing the deep universe through a foreground massive cluster, faint background sources can be magnified by factors of as much as 50 by foreground structures. The breakthrough in our article is that we believe we have secured a first glimpse of this predicted population of faint star-forming systems at early times. Continued work in this direction using the Keck telescopes will give astronomers valuable information on how best to exploit NGST and provide the first constraint on when the first stars in the Universe formed.

How it works:

Using the Keck I telescope we carefully select those regions in powerful lensing clusters where the magnification of background sources will be considerable (more than a factor of 30) . We search these areas for signals of the first forming stars, hydrogen gas heated by young, newly-formed stars. Using the Keck II telescope we follow up our candidates more carefully to be doubly-sure these are very distant, magnified sources. Gravitational lensing can, like a terrestrial mirage, produce two identical images of the same source. The location and relative brightnesses in such cases tells us precisely how much magnification occurred for each image.

First Light Viewed Through the Rich Cluster Abell 2218

So far we have found 3 promising examples in our survey but the most distant, at a redshift 5.6 (corresponding to 13.4 billion light years distance in current cosmological models), is the most intriguing. We found this source magnified by the foreground cluster Abell 2218. This is a stunning cluster with many lensed `arc-like' features of known redshift. Such data, gathered by our team, has enabled us to determine very precisely where the regions of maximum magnification occur. By scanning this area we encountered the source which forms the basis of our recent article.
We discovered a pair of images which represent the magnified result of a single source whose light is magnified 30-fold and thus would ordinarily (without the lensing boost) be undetectable except with the future Next Generation Space Telescope. The Keck spectra show both images arise from the same source, which is actively forming stars but, crucially, those stars have not yet aged sufficiently to form a mature underlying population. In short, the object is being viewed less than 1 billion years after the Big Bang but is only about a million years old. This makes it a promising candidate for a primeval star cluster, possibly one of many at that time, which subsequently merge and assemble to form the large bright galaxies we see today.

Further Information:

Scientific article (to appear in Astrophysical Journal) (Postscript format) (Acrobat Format)


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