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How do broad emission lines form in a quasar spectrum?
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Quasar
Six quasar host galaxies, as observed by the Hubble Space Telescope.
Shown are apparently normal, solitary galaxies (left), colliding galaxies (centre), and merging galaxies (right).
Broad emission lines in a quasar spectrum are produced by clouds of gas located just beyond the accretion disk surrounding the central supermassive black hole.1
These gas clouds move at high velocities around the inner structure of the quasar. They absorb high-energy radiation emitted by the accretion disk and reprocess it into the characteristic broad emission lines of hydrogen and various ions that serve as the primary signatures of quasar spectra.1