Quasar spectra and the Lyman alpha forest
I. Quasar spectra
Quasars were originally called QSOs, for quasi-stellar objects, because their spectra reminded us
of stars-- there was a continuum, peppered with absorption lines in the bright domains, and emission
lines in the dimmer domains. However, it was quickly apparent that the continua were much too broad,
spanning the entire IR, optical, and UV domains, to be from a single star, so we now successfully model these
spectra as being from an accretion disk over a wide range of temperature. Even more surprising, the lines
as significantly redshifted, with z of several. So quasars are very luminous and very far away. This makes
them excellent probes of the history of our universe, not only because they show us conditions at the time
of the origination of the quasar emission, but also because the light passes through all the material in
between. That material leaves its mark on the quasar light by scattering out some of the light at particular
wavelengths, and it does this according to the redshift of the intervening matter, not the redshift of the quasar.
Scattering away of quasar light happens most effectively in the strong Lyman alpha line of neutral hydrogen, so
quasars create excellent probes of structure formation in neutral hydrogen at all epochs following the quasar
emission.
II. The Lyman alpha forest
The spectra of quasars are thus peppered with absorption by the Lyman alpha line, but at different redshifts.
For a distant enough quasar, there are lots of different redshifts at which we see this Lyman alpha absorption,
creating a kind of "forest"
of absorption lines. Each line in this forest gives us information about the history
of formation of dense clumps of neutral hydrogen, which then go on to because reionized when the gas forms galaxies
and stars.