A low-ionization nuclear emission-line region (LINER) is a type of

galactic Galactic is an American jam band from New Orleans, Louisiana. Origins and background Formed in 1994 as an octet (under the name Galactic Prophylactic) and including singer Chris Lane and guitarist Rob Gowen, the group was soon pared down to a ...

nucleus Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom * Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucl ...

that is defined by its spectral line emission. The spectra typically include line emission from weakly ionized or neutral atoms, such as O, O⁺, N⁺, and S⁺. Conversely, the spectral line emission from strongly ionized atoms, such as O⁺⁺, Ne⁺⁺, and He⁺, is relatively weak. The class of galactic nuclei was first identified by Timothy Heckman in the third of a series of papers on the spectra of galactic nuclei that were published in 1980.

Demographics of LINER galaxies

Galaxies that contain LINERs are often referred to as ''LINER galaxies''. LINER galaxies are very common; approximately one-third of all nearby galaxies (galaxies within approximately 20-40 Mpc) may be classified as LINER galaxies. Approximately 75% of LINER galaxies are either

elliptical galaxies An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The Real ...

lenticular galaxies A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. ...

, or S0/a-Sab galaxies (

spiral galaxies Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''irregular galaxies An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, ...

. LINERs also may be commonly found in

luminous infrared galaxies Luminous infrared galaxies or LIRGs are galaxies with luminosities, the measurement of brightness, above . They are also referred to as submillimeter galaxies (SMGs) through their normal method of detection. LIRGs are more abundant than starburst ...

(LIRGs), a class of galaxies defined by their

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...

luminosities that are frequently formed when two galaxies collide with each other. Approximately one-quarter of LIRGs may contain LINERs.

Scientific debates: energy sources and ionization mechanisms

LINERs have been at the center of two major debates. First, astronomers have debated the source of energy that excites the ionized gas in the centers of these galaxies. Some astronomers have proposed that

active galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not prod ...

(AGN) with

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions of times the mass of the Sun (). Black holes are a class of astronomical ob ...

s are responsible for the LINER spectral emission. Other astronomers have asserted that the emission is powered by star formation regions. The other major issue is related to how the ions are excited. Some astronomers have suggested that

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

s propagating through the gas may ionize the gas, while others have suggested that

photoionization Photoionization is the physical process in which an ion is formed from the interaction of a photon with an atom or molecule. Cross section Not every interaction between a photon and an atom, or molecule, will result in photoionization. The prob ...

(ionization by

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation ...

light) may be responsible. These debates are complicated by the fact that LINERs are found in a wide variety of objects with different brightnesses and morphologies. Moreover, the debate over the energy sources for LINERs is entangled with a similar debate over whether the light from star formation regions or the light from AGN produce the high infrared luminosities seen in LIRGs. Although both the energy sources and the excitation mechanisms for LINER emission are still being studied, many LINERs are frequently referred to as AGN.

Star formation in LINERs

A number of surveys have been performed to explore the connection between star formation and LINER activity. If a connection can be found between star formation activity and LINER activity, then this strengthens the possibility that LINERs are powered by the hot gas found in star formation regions. However, if star formation cannot be found in LINERs, then this definitively excludes star formation as powering LINER emission.

Star formation in LIRGs with LINERs

Recent observations with the Spitzer Space Telescope show a clear connection between LINER emission in

(LIRGs) and star formation activity. The mid-

spectra of LIRGs with LINERs have been shown to look similar to the mid-infrared spectra of

starburst galaxies A starburst galaxy is one undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star formatio ...

, which suggest that infrared-bright LINERs are powered by star formation activity. However, some mid-infrared spectral line emission from AGN have also been detected in these galaxies, indicating that star formation may not be the only energy sources in these galaxies.

Star formation in normal galaxies with LINERs

Normal nearby galaxies with LINERs, however, appear to be different. A few near-infrared spectroscopic surveys have identified some LINERs in normal galaxies that may be powered by star formation. However, most LINERs in nearby galaxies have low levels of star formation activity. Moreover, the stellar populations of many LINERs appear to be very old, and the mid-infrared spectra, as observed by the Spitzer Space Telescope, do not appear similar to the spectra expected from star formation. These results demonstrate that most LINER in nearby normal galaxies may not be powered by star formation, although a few exceptions clearly exist.

Notable LINER galaxies

Messier 94 Messier 94 (also known as NGC 4736) is a spiral galaxy in the mid-northern constellation Canes Venatici. It was discovered by Pierre Méchain in 1781, and catalogued by Charles Messier two days later. Although some references describe M94 as ...

NGC 5005 NGC 5005, also known as Caldwell 29, is an inclined spiral galaxy in the constellation Canes Venatici. The galaxy has a relatively bright nucleus and a bright disk that contains multiple dust lanes. The galaxy's high surface brightness makes it ...

NGC 5195 NGC 5195 (also known as Messier 51b or M51b) is a dwarf galaxy that is interacting with the Whirlpool Galaxy (also known as M51a or NGC 5194). Both galaxies are located approximately 25 million light-years away in the constellation Canes Venat ...

Sombrero Galaxy The Sombrero Galaxy (also known as Messier Object 104, M104 or NGC 4594) is a peculiar galaxy of unclear classification in the constellation borders of Virgo and Corvus, being about from the Milky Way galaxy. It is a member of the Virgo II Grou ...

References

{{Galaxy Active galaxy types -LINER 1980 in science