Gravitational Lens Helps Hubble and Keck Discover Galaxy Building Block

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October 5, 2001: A very small, faint galaxy -- possibly one of the long sought "building blocks" of present-day galaxies -- has been discovered by a collaboration between NASA's Hubble Space Telescope and the 10-meter Keck Telescopes at a tremendous distance of 13.4 billion light-years (based on the estimate of 14 billion years as the age of the universe). The discovery was made possible by examining small areas of the sky viewed through massive intervening clusters of galaxies. These act as a powerful gravitational lens, magnifying distant objects and allowing scientists to probe how galaxies assemble at very early times. This has profound implications for our understanding of how and when the first stars and galaxies formed in the universe.

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Q & A: Understanding the Discovery

1. 1. What are gravitational lenses and how do they work?




The gravitational field from any huge concentration of matter like a cluster of galaxies distorts and magnifies the light from distant galaxies that are behind it but in the same line of sight according to Einstein's General Theory of Relativity. Gravitational lenses, the equivalent of nature's magnifying glasses in space, give researchers a unique tool with which to learn more about the detailed physics of very early galaxies that are undetectable with current technology and telescopes.

2. 2. What is the nearer object that is acting as a lens?




Abell 2218 is a rich galaxy cluster composed of thousands of galaxies and a mass equivalent to ten thousand galaxies interspersed throughout the cluster. The cluster is located relatively nearby — at a distance of 2 billion light-years (a redshift of 0.18). Redshift is the stretching of light waves as they travel across expanding space. The longer they travel, the more they are stretched, and the higher the measured redshift.

3. 3. What is the further object that is being lensed by Abell 2218?




The recently discovered "baby galaxy" has a redshift of 5.58, corresponding to a distance of about 13.4 billion light-years. The galaxy's light has been magnified more than 30 times by Abell 2218 and split into two "images" by the uneven distribution of matter in the cluster. It has been determined that the amount of matter in the object is astoundingly low for a galaxy — only a few million times the mass of our Sun, or about one hundred thousand times less than the amount of matter in our own Milky Way galaxy. The object is only around 500 light-years across, as compared to the 100,000 light-year diameter of the Milky Way.