ATLANTA, GEORGIA--The Hubble Space Telescope's biggest image yet has revealed 10,000 galaxies scattered through 9 billion light-years of space. The contiguous mosaic of color photos, covering an area of sky the size of the full moon, contains so many galaxies of all shapes and sizes--at known distances from Earth--that astronomers can trace how each variety grew up, according to reports here 8 January at a meeting of the American Astronomical Society.
One of Hubble's best-known photos, the "Hubble Deep Field," earned acclaim in 1996 by exposing about 1500 galaxies in a tiny patch of sky. Many galaxies hadn't formed fully, providing the first visual clue that galaxies build up from star-filled scraps. But astronomers didn't know the distances to those objects, so they couldn't construct a timeline of galactic evolution.
Now, that information is in hand for the last two-thirds of cosmic history. The new mosaic, about 150 times larger than the Hubble Deep Field, combines photos from two ambitious surveys in the southern constellation Fornax. Astronomers already knew the distances to 10,000 galaxies in that area, thanks to a German-led study with a ground-based telescope. Hubble's photos now display their dramatic range of shapes and sizes, captured by sharp images from the telescope's sensitive Advanced Camera for Surveys.
Analysis is ongoing, but team member Eric Bell of the Max Planck Institute for Astronomy in Heidelberg, Germany, reported that some evolutionary trends are clear. For example, the universe has twice as many elliptical galaxies--giant star systems shaped like smooth blobs--today as it did 8 billion years ago. That's the first convincing evidence that ellipticals have continued to develop, rather than forming all at once when the universe was young. Indeed, Bell's colleagues are finding that collisions have transformed the shapes of many galaxies. Further, the survey reveals that barlike structures that develop within young shapeless galaxies are the key factor for turning some of them into grand spirals like our Milky Way.
The surveys are far more statistically powerful than previous attempts to watch galaxy evolution in action, says astronomer Rychard Bouwens of the University of California, Santa Cruz: "One can finally start to understand the evolution of [all galaxy types] and trace them from the present day back in time, almost to the epoch when they assembled."
Related sites
The main survey of
galaxy shapes, called GEMS
A Web tool for
navigating through the entire GEMS image
Advanced Camera for
Surveys
Information about the original Hubble Deep Field
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