A photon emitted with wavelength shorter than 912 Angstroms (Lyman continuum) will be completely absorbed by hydrogen gas both in a galaxy and along the line of sight to us. Essentially, there will be no light making it to us from a galaxy with wavelengths shorter than that. We see a "break" (the Lyman break) in the spectrum of the galaxy. From 912-1216 A (Lyman alpha), photons can also be absorbed by intervening gas.

For high redshift galaxies, this Lyman break redshifts into the optical. By looking at the colors of galaxies, we should see high redshift star forming galaxies as objects that "disappear" in the bluest filters, or more correctly are red in blue colors (ie U-B) and blue in red colors (B-V or V-I). We can identify high redshift candidates this way and do follow-up spectroscopy using big telescopes to confirm their redshifts.

	intrinsic	z=3.3	z=4.3
Ly alpha	1216 A	5228 A	6444 A
Ly cont	912 A	3921 A	4888 A

The Highest Redshift Galaxies: Lyman Break Technique

The Lyman Break