and Cosmology

9.1 Galaxies at High Redshift
one expects that some fraction of galaxies feature a Lyα
emission line (as do all QSOs). By comparing two sky
images, one taken with a narrow-band filter centered
on a wavelength λ, the other with a broader filter also
centered roughly on λ, this line emission can be searched
for specifically. If a galaxy at z ≈ λ/(1216 Å) − 1has
a strong Lyα emission line, it should be particularly
bright in the narrow-band image in comparison to the
broad-band image, relative to other sources. This search
for Lyα emission line galaxies had been almost without
success until the mid-1990s. Among other reasons, one
did not know what to expect, e.g., how faint galaxies at
z ∼ 3 are and how strong their Lyα line would be.
357
The Lyman-Break Method. The breakthrough was
obtained with a method that became known as the
Lyman-break method. Since hydrogen is so abundant
and its ionization cross-section so large, one can expect
that photons with λ