Lyman break

A Lyman break (Lyman jump or Lyman edge) is a spectral feature consisting of a steep downward slope within an object's spectral energy distribution (SED) (sloping downward as you look at shorter and shorter wavelengths) at the Lyman limit, the "infinity end" of the Lyman series spectral lines, which is 91.18 nm. The flux is reduced because any neutral atomic hydrogen (HI) can absorb any photon with that energy or more, ionizing the atom.

The feature can occur in stars, but a common astronomical use is identifying high-redshift galaxies and quasars. Galaxies identified by this feature are termed Lyman-break galaxies (LBGs): the technique is useful because the feature is redshifted from ultraviolet (which wouldn't be visible from the ground) into visible light and infrared, making some viewable through the atmospheric windows. Photometry can reveal the likelihood of a high redshift, such as using dropouts. This has allowed such galaxies to be discovered in photometric survey images, such as the original Sloan Digital Sky Survey (SDSS). Details of the feature are also used in the analysis of accretion disks of active galactic nuclei (AGNs).

The term Lyman break is typically used regarding high-redshift galaxies and quasars, the term Lyman edge for accretion disks, and Lyman jump for stellar spectra.

Some spectra show an analogous Balmer break at the Balmer limit.

A Lyman-alpha break is a somewhat similar spectral feature, a lesser SED reduction for wavelengths shorter than Lyman alpha (121.6 nm). This reduction feature is due to the Lyman series lines and is not as prominent as the Lyman break.