Very high energy gamma-ray emission from a radio galaxy
Giant elliptical galaxies, the oldest known large galactic structures in the universe, have no spiral arms and little or no current star formation activity, but their central supermassive black holes are often active galactic nuclei (AGN). While nearly all galaxies host a supermassive black hole in their nuclei, most nuclei are not AGN. Astronomers think that giant ellipticals formed in the early universe, less than a billion years after the big bang, after a phase of rapid star-formation, and then evolved to become even larger through galaxy mergers and accretion of gas from the intergalactic medium. The same accretion helps feed the AGN that drive the ejection of powerful jets of rapidly moving charged particles. The particles emit strongly at radio frequencies, making these objects bright targets for radio telescopes, and many of these galaxies were first discovered in radio surveys.
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