June 19, 2003: Two of NASA's Great Observatories, bolstered by the largest ground-based telescopes around the world, are beginning to harvest new clues to the origin and evolution of the universe's largest building blocks, the galaxies. It's a bit like finding a family scrapbook containing snapshots that capture the lives of family members from infancy through adolescence to adulthood. The Hubble Space Telescope has joined forces with the Chandra X-ray Observatory to survey a relatively broad swath of sky encompassing tens of thousands of galaxies stretching far back in time. Called the Great Observatories Origins Deep Survey (GOODS), astronomers are studying galaxy formation and evolution over a wide range of distances and ages.See the rest:
In preliminary results soon to be published, Hubble astronomers report that the sizes of galaxies clearly increase continuously from when the universe was about 1 billion to 6 billion years old. (Many astronomers believe that the universe is about 13.7 billion years old.) GOODS astronomers also find that the star-birth rate rose slightly between the time the universe was about 1 and 1.5 billion years old, and remained high until about 7 billion years ago, when it quickly dropped. This is further evidence that major galaxy building trailed off when the universe was about half its current age.
The Chandra observations amounted to a "high-energy core sample" of the early universe. One of the fascinating findings in this deepest X-ray image ever taken is the discovery of seven mysterious black holes that do not correspond to the galaxies seen in the Hubble image. Astronomers suspect that these objects are the most distant black holes ever detected, or the galaxies in which they reside cannot be seen because they are heavily enshrouded in dust.
When comparing the Hubble and Chandra fields, astronomers also found that active black holes in distant, relatively small galaxies were more rare than expected. This may be due to the effects of early generations of massive stars that exploded as supernovas, evacuating galactic gas and thus reducing the supply of gas needed to feed a supermassive black hole.
Astronomers used several observatories for this study because they want to build a coherent picture of galaxy evolution. The two orbiting observatories, Chandra and Hubble, analyzed different wavelengths of light. Chandra studied X-rays from supermassive black holes; the Hubble telescope's Advanced Camera for Surveys examined visible light from stars in galaxies. Another space observatory, the Space Infrared Telescope Facility (SIRTF), will sample the same regions of sky in infrared light soon after it is launched in August 2003.
Light yields information about an object. X-rays trace the hottest gas surrounding a supermassive black hole at a galaxy's center. Visible light yields clues about a galaxy's structure. Infrared light penetrates the dust surrounding young stars, collecting information about a galaxy's total stellar population.