Low-ionization nuclear emission-line region

A galactic nucleus whose optical emission-line spectrum is dominated by low-ionization species. Its emission can arise from a weak active galactic nucleus, hot evolved stars, or other sources.

Photoionization: The production of ions when atoms or molecules absorb photons energetic enough to remove electrons. Ultraviolet photons from an active nucleus or stars can ionize the gas emitting LINER lines.
Emission-line spectrum: A pattern of bright spectral lines emitted by atoms or ions in excited gas. LINER classification begins with the relative strengths of optical emission lines.
Active galactic nucleus: A compact, energetic galactic center powered by matter accreting onto a supermassive black hole. A LINER is a spectral classification, while an active nucleus is one possible physical engine.
Integral field spectroscopy: A method that records a spectrum at each position across a two-dimensional astronomical field. Spatially resolved spectra reveal whether LINER emission is nuclear or distributed.
Shock ionization: Ionization and heating of gas by shock waves that convert bulk motion into thermal energy. Shocks can produce strong low-ionization lines without requiring a luminous central source.
Forbidden line: A spectral transition unlikely under ordinary laboratory conditions but observable in low-density astronomical gas. Several characteristic LINER diagnostics are forbidden transitions from low-ionization ions.
Low-ionization emission region: A region of ionized gas with strong low-ionization emission lines, not necessarily located in a galactic nucleus. The broader term distinguishes LINER-like gas outside a galaxy’s central region.
Aperture bias: A distortion in measured properties caused by observing only a limited portion of an extended object. A small or large spectroscopic aperture can change whether a galaxy appears LINER-like.
Ionization parameter: A dimensionless measure of the ratio of ionizing-photon density to gas density. A low ionization parameter helps explain LINERs’ prominent low-ionization species.
Seyfert galaxy: A galaxy with a bright active nucleus whose spectrum shows strong emission lines. Seyfert nuclei typically show stronger high-ionization emission than LINERs.

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Messier 81Broader topic: M81’s nucleus is a well-studied example of this low-luminosity active-galaxy class.