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Since the early 1960s astronomers have thought that the Milky Way and other galaxies were born early in cosmic history and then evolved slowly. Today, however, evidence indicates that galaxies are continuing to grow. They cannibalize their smaller brethren and gulp down fresh gas from intergalactic space.
In our Milky Way we have a close-up view of the ongoing construction work. The incoming gas takes the form of high-velocity clouds discovered decades ago. Only recently were some of these clouds proved to be fresh material; observationally, they get entangled with circulating gas.
These clouds come in several guises: clumps of neutral hydrogen reminiscent of intergalactic gas; a stream of gas torn out of nearby small galaxies; and highly ionized hot gas that may be dispersed throughout the intergalactic vicinity.
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