The largest contiguous color image taken with the Hubble Space Telescope has been unveiled by an international team of astronomers at the American Astronomical Society meeting in Atlanta, Georgia. Dr. Eric F. Bell of the Max-Planck-Institute for Astronomy in Heidelberg, Germany, and Dr. Shardha Jogee of the Space Telescope Science Institute in Baltimore, Maryland presented an image in the constellation Fornax equal to the apparent size of the full moon containing more than 40,000 galaxies. The astronomers, members of a large team called the GEMS consortium and led by Dr. Hans-Walter Rix, Director of the Max-Planck-Institute for Astronomy, said the image will help them understand how large galaxies, like the Milky Way, evolved over the last nine billion years, about 2/3 the age of the universe. GEMS is an acronym for Galaxy Evolution from Morphology and Spectral energy distributions.
To construct the image, the team stitched together 78 separate exposures from Hubble's Advanced Camera for Surveys. "It's like making a big picture of a mountain range by pasting together individual pictures of each mountain — we have done exactly the same thing, only with the Hubble Space Telescope", said Dr. Daniel H. McIntosh, a researcher at the University of Massachusetts in Amherst, Massachusetts.
The main advantage of GEMS is its large area. "This is the largest color image ever taken by HST", continues McIntosh. "Because galaxies clump together in space, smaller images may accidentally land on unusual patches. For example, a picture of the USA at night shows bright areas near the cities and dark areas in the mountains, forest, and farmlands. To understand the population of the United States, we would need an image covering both the rare, bright cities and the dark but sparsely-populated farmland and wilderness. Furthermore, galaxies, like people are incredibly diverse. Only by having images of a large sample of galaxies can one explore the huge diversity of galaxy types, sizes and shapes, as well as discovering important but very brief episodes in their lives."
The team chose an area in which they already knew the distances to nearly ten thousand galaxies. Because of the expansion of the Universe, more distant galaxies are moving away from us faster than nearby galaxies. Astronomers use the Doppler shift of the galaxies’ light to measure this movement and compute the distance. And because the light from distant galaxies takes longer to reach Earth than that from the nearby objects, we see distant galaxies as they were in the past, giving astronomers a kind of cosmic ‘time machine.’ GEMS can see back accurately about 9 billion years, 4.5 billion years before the Sun and the Earth formed.
One of the many goals of GEMS is to investigate how galaxy interactions impact galaxy evolution. Interacting galaxies exert strong gravitational forces and shocks on each other, induce dramatic morphological transformations, and may even merge into a final product which is vastly different from the progenitors. "With GEMS, we identify interacting and merging galaxies over the last 9 billion years by capturing their tell-tale strongly disturbed morphologies: double bright nuclei, stellar tidal tails flung out over thousands of light years, and highly asymmetric star formation", says Jogee. Interactions may also play an important role in feeding gas to huge black holes in the centers of massive galaxies, ‘activating’ the nucleus into releasing prodigious amounts of energy. GEMS can bring valuable insights into the nature of galaxies hosting such active nuclei. "Furthermore, our current understanding of the physics of galaxy interactions is still pretty uncertain, and good observations of how frequently galaxies merge at different times in the Universe's history would be a really important part of this puzzle," said Dr. Rachel S. Somerville, a staff astronomer at the Space Telescope Science Institute in Baltimore, Maryland. First indications are that galaxy interactions are much more common in the past than they are today. "Most massive galaxies are simply aging now, fading slowly into obscurity", said Rix.
Another focus of GEMS is to explore the leftovers of particularly violent galaxy mergers — watermelon-shaped galaxies called elliptical galaxies. "With GEMS, we have proven for the first time that there are more elliptical galaxies today than there were in the past", explains Bell. "This is exciting first evidence for a merger origin of at least some elliptical galaxies."
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