Lymanbreak

Lymanbreak is the sharp discontinuity in a spectrum caused by the absorption of photons with energies above the hydrogen ionization threshold, known as the Lyman limit at 912 angstroms. In regions containing neutral hydrogen, such as a galaxy’s interstellar medium and the intervening intergalactic medium, photons with wavelengths shorter than 912 Å are absorbed, producing a pronounced drop in flux. The cumulative effect of this absorption, including contributions from the Lyman-alpha forest at wavelengths longer than 912 Å but shorter than 1216 Å, further suppresses blueward light.

In extragalactic astronomy, distant star-forming galaxies exhibit a redshifted Lyman break that shifts into optical or near-infrared bands. This enables the Lyman-break, or dropout, technique to identify high-redshift galaxy candidates by comparing their colors across filters and selecting objects that disappear (“drop out”) in the bluer band while remaining detected in redder bands. Galaxies selected this way are commonly called Lyman-break galaxies (LBGs). The method was developed in the mid-1990s, notably by Steidel, Hamilton, and collaborators, and has since become a standard approach for assembling samples of galaxies at redshifts roughly from z ~ 2 to z ~ 7 and beyond.

Lyman-break studies have informed our understanding of the cosmic star formation history, the buildup of stellar mass in the early universe, and the evolution of the ultraviolet luminosity function. The same absorption processes that create the Lyman break in galaxies also imprint features in quasar spectra, revealing the state of the intergalactic medium along the line of sight.