When astronomers look at the light of progressivly more distant quasars, they see in their spectrum the characteristic lines of hydrogen gas produced near the fundamental quantum transition of Lyman-alpha at a wavelength of 1215 angstroms. For nearby quasars, only a few of these lines appear, and have redshifts that are less than that of the quasar itself. For more and more distant quasars, so many of these lines appear in the spectrum of the quasar near its 'blue' end, that the spectrum is a 'forest' of emission spikes one on top of the other. In some cases, these can be separated into 'redshift' systems suggesting that whatever is producing the Lyman-alpha emission comes in distinct families at specific redshifts. The above computer model was presented at the Astronomy POD and was computed from spectral data for the structure at a redshift of about 3.
Astronomers believe that what they are seeing is the hydrogen emission from distant galaxies through which the quasar light is passing enroute to us. The galaxies are at their own particular distances along the line of sight, and that is why there are redshift systems. By examining other emission features in these spectra due to calcium and other elements, astronomers can probe the chemical abundances of these intervening galaxies without ever actually seeing them directly. Their own light is too feeble to use to get this information, but the enormously powerful light from the quasar makes this kind of study possible.
Copyright 1997 Dr. Sten Odenwald
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