Lyman continuum

The Lyman continuum is a spectral feature due to absorption of photons by neutral atomic hydrogen: a reduced continuum in the spectrum for photon energies above the Lyman limit, reduced from the corresponding continuum of a black-body spectrum. Photons with energies above this limit (Lyman continuum photons, i.e., those with photon energy values above 13.6 eV) can be absorbed by any neutral hydrogen atoms (which ionizes them), thus fewer of such photons proceed. At the limit is a Lyman jump (aka Lyman break) spectral feature: a distinct drop in the spectral energy distribution (SED). The absorption reduces the strength of the most energetic ultraviolet as well as X-rays and gamma rays. Early-type stars display the Lyman jump but the feature cannot be observed from the ground because its not within an atmospheric window. However, ground observations of distant, redshifted galaxies sense it and extremely distant galaxies with the feature are detected through the dropouts that result (Lyman-break galaxies).

Galaxies from which Lyman continuum photons emerge, i.e., such absorption is avoided (Lyman continuum escape, LyC escape) are of research interest. Distant galaxies from which this occurred have been called Lyman continuum leakers (LyC leakers).