Damped Lyman alpha systems or Damped Lyman alpha absorption systems is a term used by astronomers for concentrations of neutral hydrogen gas that are detected in the spectra of

quasar A quasar is an extremely Luminosity, luminous active galactic nucleus (AGN). It is pronounced , and sometimes known as a quasi-stellar object, abbreviated QSO. This emission from a galaxy nucleus is powered by a supermassive black hole with a m ...

s – a class of distant Active Galactic Nuclei. They are defined to be systems where the column density (density projected along the line of sight to the quasar) of hydrogen is larger than 2 x 10²⁰ atoms/cm². The observed spectra consist of neutral hydrogen Lyman alpha absorption lines which are broadened by radiation damping. These systems can be observed in quantity at relatively high

redshift In physics, a redshift is an increase in the wavelength, and corresponding decrease in the frequency and photon energy, of electromagnetic radiation (such as light). The opposite change, a decrease in wavelength and simultaneous increase in f ...

s of 2–4, when they contained most of the neutral hydrogen in the universe. They are believed to be associated with the early stages of

galaxy formation The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have gen ...

, as the high neutral hydrogen column densities of DLAs are also typical of sightlines in the Milky Way, and other nearby galaxies. Since they are observed in absorption rather than emissions by their stars, they offer the opportunity to study the dynamics of the gas in early galaxies directly.

References

Physical cosmology Astronomical spectroscopy Intergalactic media {{physical-cosmology-stub

See also

References