Lyman-alpha blob

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A Lyman-alpha blob (LAB) is a large concentration of a gas emitting the Lyman-alpha emission line. The Lyman-alpha emission line is produced by recombination of electrons with ionized hydrogen atoms. LABs are one of the largest known objects in the Universe. Some gas bubbles are up to 400,000 light years across. So far they have only been found in the high-redshift universe. They have been discovered with the so-called narrow-band technique. The most famous ones were discovered in 2000 by Steidel et al. [1]. The Subaru Telescope of the National Astronomical Observatory of Japan extended the search for LABs [2] and found over 30 new LABs in the original field of Steidel et al. [1]. The new found LABs form a structure which is more than 200 million light-years large. LABs seems to trace overdensities of galaxies in the high-redshift universe. Currently it is not known which mechanism produces the Lyman-alpha emission line, and how the LABs are connected to the surrounding galaxies.