Question

2. Quickly explain how the Gunn-Peterson effect could be use to estimate the redshift of the end of re-ionisation.

Answer 1

Gunn and Peterson propose using Lyα absorption in the distant quasar spectrum to study the neutral hydrogen in the intergalactic medium (IGM). The quasar acts as the light source and provides a characteristic, relatively smooth spectrum. During the transmission of the light from the quasar to us, intervening neutral hydrogen clouds, along the line of sight to the quasar, cause absorptions on the quasar continuum at the neutral hydrogen HI transition 1215.67Å in the rest frame of the absorbing clouds. Because the characteristic quasar spectrum is well-understood, it’s relatively easy to isolate and study this absorption. The spectrum is continuously redshifted. Therefore the intervening neutral hydrogen at different redshifts will produce absorption at many different wavelengths blueward of the quasars’ Lyα emission. By studying that absorption spectra we can easily calculate the value of the redshift.