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October 20, 2006

Hubble's greatest image... well, one of them...

Hubble's Deepest View of the Universe Unveils Bewildering Galaxies across Billions of Years

What did the first galaxies look like? To help answer this question, the Hubble Space Telescope has just finished taking the Hubble Ultra Deep Field (HUDF), the deepest image of the universe ever taken in visible light. Pictured above, the HUDF shows a sampling of the oldest galaxies ever seen, galaxies that formed just after the dark ages, 13 billion years ago, when the universe was only 5 percent of its present age. The Hubble Space Telescope's NICMOS and new ACS cameras took the image. Staring nearly 3 months at the same spot, the HUDF is four times more sensitive, in some colors, than the original Hubble Deep Field (HDF). Astronomers the world over will likely study the HUDF for years to come to better understand how stars and galaxies formed in the early universe.

Representing a narrow "keyhole" view stretching to the visible horizon of the universe, the Hubble Deep Field image covers a speck of the sky only about the width of a dime 75 feet away. Though the field is a very small sample of the heavens, it is considered representative of the typical distribution of galaxies in space, because the universe, statistically, looks largely the same in all directions. Gazing into this small field, Hubble uncovered a bewildering assortment of at least 3000 galaxies at various stages of evolution.

The field is so small that only a few foreground stars in the Milky Way lie within it; thus, almost all of the 3,000 objects in the image are galaxies, some of which are among the youngest and most distant known. By revealing such large numbers of very young galaxies, the HDF has become a landmark image in the study of the early universe, and it has been the source of almost 400 scientific papers since it was created.

As the universe expands, more distant objects recede from the Earth faster, in what is called the Hubble Flow. The light from very distant galaxies is significantly affected by doppler shifting, which reddens the radiation that we receive from them. While quasars with high redshifts were known, very few galaxies with redshifts greater than 1 were known before the HDF images were produced. The HDF, however, contained many galaxies with redshifts as high as 6, corresponding to distances of about 12 billion light years [2]. (Due to redshift the most distant objects in the HDF are not actually visible in the Hubble images; they can only be detected in images of the HDF taken at longer wavelengths by ground-based telescopes.)

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