A low-ionization nuclear emission-line region (LINER) is a type of galactic nucleus 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.
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, lenticular galaxies, or S0/a-Sab galaxies (spiral galaxies with large bulges and tightly wound spiral arms). LINERs are found less frequently in Sb-Scd galaxies (spiral galaxies with small bulges and loosely wound spiral arms), and they are very rare in nearby irregular galaxies. LINERs also may be commonly found in luminous infrared galaxies (LIRGs), a class of galaxies defined by their infrared luminosities that are frequently formed when two galaxies collide with each other. Approximately one-quarter of LIRGs may contain LINERs.
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 (AGN) with supermassive black holes 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 waves propagating through the gas may ionize the gas, while others have suggested that photoionization (ionization by ultraviolet 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.
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.
Recent observations with the Spitzer Space Telescope show a clear connection between LINER emission in luminous infrared galaxies (LIRGs) and star formation activity. The mid-infrared spectra of LIRGs with LINERs have been shown to look similar to the mid-infrared spectra of starburst galaxies, 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.
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.
The index of physics articles is split into multiple pages due to its size.
To navigate by individual letter use the table of contents below.Messier 87
Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the local Universe, it is notable for its large population of globular clusters—about 12,000 compared to the 150–200 orbiting the Milky Way—and its jet of energetic plasma that originates at the core and extends at least 1,500 parsecs (4,900 light-years), traveling at relativistic speed. It is one of the brightest radio sources in the sky, and a popular target for both amateur and professional astronomers.
The French astronomer Charles Messier discovered M87 in 1781, and catalogued it as a nebulous feature while searching for objects that would otherwise confuse comet hunters. M87 is located about 16.4 million parsecs (53 million light-years) from Earth and is the second-brightest galaxy within the northern Virgo Cluster, having many satellite galaxies. Unlike a disk-shaped spiral galaxy, M87 has no distinctive dust lanes. Instead, it has an almost featureless, ellipsoidal shape typical of most giant elliptical galaxies, diminishing in luminosity with distance from the center. Forming around one sixth of its mass, M87's stars have a nearly spherically symmetric distribution. Their population density decreases with increasing distance from the core. It has an active supermassive black hole at its core, which forms the primary component of an active galactic nucleus.
The galaxy is a strong source of multiwavelength radiation, particularly radio waves. Its galactic envelope extends to a radius of about 150 kiloparsecs (490 thousand light-years), where it is truncated—possibly by an encounter with another galaxy. Its interstellar medium consists of diffuse gas enriched by elements emitted from evolved stars.NGC 1672
NGC 1672 is a barred spiral galaxy located in the constellation Dorado. It was originally thought to be a member of the Dorado Group, however, this membership was later rejected. NGC 1672 has a large bar which is estimated to measure around 20 kpc. It has very strong radio emissions emanating from its nucleus, bar, and the inner portion of the spiral arm region. The nucleus is Seyfert type 2 and is engulfed by a starburst region. The strongest polarized emissions come from the northeastern region which is upstream from its dust lanes. Magnetic field lines are at large angles with respect to the bar and turn smoothly to the center.NGC 2681
NGC 2681 is a lenticular galaxy in the constellation Ursa Major. The galaxy lies 50 million light years away from Earth, which means, given its apparent dimensions, that NGC 2681 is approximately 55,000 light years across. NGC 2681 has an active galactic nucleus and it is a type 3 Seyfert galaxy. Its nucleus is also a low-ionization nuclear emission-line region.NGC 2681 has possibly three bars, with a relatively large bar at the outer side. Because the galaxy is seen nearly face-on, the bar like structures cannot be projection effects. From earth based observations, in B-I images the galaxy showed neither grand design spirals nor a ring, but only two symmetrical spiral arms starting from the end of the primary bar. Ia Hα images some HII regions were observed in the spiral arms. A dust spiral is seen in Hubble space telescope images extending to the centre. The lack of stellar gradient in the central regions and the data from Faint Object Camera, Faint Object Spectrograph and International Ultraviolet Explorer indicate that the galaxy had a starburst event approximately one billion years ago, possibly after the tidal interaction with another galaxy, which involved all the galaxy.Dynamical modeling of the velocity dispersions suggests that NGC 2681 hosts a supermassive black hole whose upper mass limit was set at 6×107 M⊙. As observed from Chandra X-ray Observatory, NGC 2681 displayed three stellar sources within the central kiloparsec of the galaxy. The active galactic nucleus had luminosity 1.8 × 1038 erg/s, which accounts for approximately the 20% of the total luminosity of the central kiloparsec.NGC 2787
NGC 2787 is a barred lenticular galaxy approximately 24 million light-years away in the constellation Ursa Major. In 1999, the Hubble Space Telescope took a look at NGC 2787. The supermassive black hole at the core has a mass of 4.1+0.4−0.5×107 M☉.NGC 2841
NGC 2841 is an inclined unbarred spiral galaxy exhibiting a prominent inner ring structure in the constellation Ursa Major, it was discovered on 9 March 1788 by William Herschel. Initially thought to be about 30 million light years distant, a 2001 Hubble Space Telescope survey of the galaxy's Cepheid variables determined that it was approximately 14.1 megaparsecs or 46 million light years distant.NGC 3226
NGC 3226 is a dwarf elliptical galaxy that is interacting with the spiral galaxy NGC 3227. The two galaxies are one of several examples of a spiral with a dwarf elliptical companion that are listed in the Atlas of Peculiar Galaxies. Both galaxies may be found in the constellation Leo.NGC 3521
NGC 3521 is a flocculent intermediate spiral galaxy located around 26 million light-years away from Earth in the constellation Leo. It has a morphological classification of SAB(rs)bc, which indicates that it is a spiral galaxy with a trace of a bar structure (SAB), a weak inner ring (rs), and moderate to loosely wound arm structure (bc). The bar structure is difficult to discern, both because it has a low ellipticity and the galaxy is at a high inclination of 72.7° to the line of sight. The relatively bright bulge is nearly 3/4 the size of the bar, which may indicate the former is quite massive. The nucleus of this galaxy is classified as an HII LINER, as there is an H II region at the core and the nucleus forms a low-ionization nuclear emission-line region.NGC 3642
NGC 3642 is a spiral galaxy in constellation Ursa Major. The galaxy has a low-ionization nuclear emission-line region. It is located at a distance of circa 30 million light years from Earth, which, given its apparent dimensions, means that NGC 3642 is about 50,000 light years across. The galaxy is characterised by an outer pseudoring, which was probably formed after the accretion of gas rich dwarf galaxy.NGC 3675
NGC 3675 is a spiral galaxy located in the constellation Ursa Major. It is located at a distance of circa 50 million light years from Earth, which, given its apparent dimensions, means that NGC 3675 is about 100,000 light years across. It was discovered by William Herschel in 1788.
It hosts a low-ionization nuclear emission-line region (LINER). In the nucleus there is a supermassive black hole with an estimated mass of 10-39 million M☉, based on the intrinsic velocity dispersion as measured by the Hubble Space Telescope. Although the galaxy was reported to have a strong bar visible in infrared images, it has not been any indication of bar in further observations. Its spiral disk in of type III and there is a dust structure which is more prominent to the east. The galaxy features two ring structures, with diametre 1.62 and 2.42 arcminutes. The spiral arms are tightly wound and form an inner pseudoring and they continue for one revolution outside the ring. The outer arms are very patchy and filamentary.One supernova has been observed in NGC 3675, SN 1984R. NGC 3675 belongs to the Ursa Major Cluster, part of the Virgo Supercluster.NGC 404
NGC 404 is a field galaxy located about 10 million light years away in the constellation Andromeda. It was discovered by William Herschel in 1784, and is visible through small telescopes. NGC 404 lies just beyond the Local Group and does not appear gravitationally bound to it. It is located within 7 arc-minutes of second magnitude star Mirach, making it a difficult target to observe or photograph and granting it the nickname "Mirach's Ghost".NGC 4111
NGC 4111 is a lenticular galaxy in the constellation Canes Venatici. It is located at a distance of circa 50 million light years from Earth, which, given its apparent dimensions, means that NGC 4111 is about 55,000 light years across. It was discovered by William Herschel in 1788.NGC 4203
NGC 4203 is the New General Catalogue identifier for a lenticular galaxy in the northern constellation of Coma Berenices. It was discovered on March 20, 1787 by English astronomer William Herschel, and is situated 5.5° to the northwest of the 4th magnitude star Gamma Comae Berenices and can be viewed with a small telescope. The morphological classification of NGC 4203 is SAB0−, indicating that it has a lenticular form with tightly wound spiral arms and a weak bar structure at the nucleus.This galaxy has a fairly large reservoir of neutral hydrogen containing on the order of a billion solar masses (M☉), but it is only undergoing a low rate of new star formation. Hence, the inner star formation of the galaxy is fairly old; roughly ten billion years on average. The neutral hydrogen is arranged in two ring-like structures, with the outer ring having nine times the mass of the inner. In the central region there is around 2.5×107 M☉ of molecular hydrogen, plus dust structures within 980 ly (300 pc) of the nucleus. The gas in the outer disk may have been accreted from the inter-galactic medium, or captured during a close encounter with a dwarf galaxy.The nucleus of the galaxy contains a low-ionization nuclear emission-line region of type 1.9. This is being generated by a supermassive black hole of an estimated (6±1)×107 M☉. An influx of gas of around 2×10−2 M☉/yr is sufficient to explain the measured X-ray luminosity. The time-varying emissions from the region are perhaps best explained by an infalling asymptotic giant branch star that is losing mass to the black hole along a contrail.NGC 4203 is a member of the Coma I Group which is part of the Virgo Supercluster.NGC 536
NGC 536 is a barred spiral galaxy located in the constellation Andromeda. It is located at a distance of circa 200 million light years from Earth, which, given its apparent dimensions, means that NGC 536 is about 180,000 light years across. It was discovered by William Herschel on September 13, 1784. It is a member of Hickson Compact Group 10, which also includes the galaxies NGC 529, NGC 531, and NGC 542. It belongs to the Perseus-Pisces Supercluster.
The nucleus of NGC 536 is characterised as a low-ionization nuclear emission-line region (LINER), a type of active galactic nucleus. The galaxy features a bright inner region, surrounded by a ring from which emanate two faint arms with HII regions. These extened spiral arms has been suggested that they may be tidal tails. The galaxy has very weak Hα emission. The star formation rate in NGC 536 is estimated to be 1.16 – 1.25 M☉ per year. The galaxy is seen with inclination of 78 degrees.One supernova has been observed in NGC 536, SN 1963N. It was discovered by the Palomar Supernova Search on June 27, 1963, with mag 17.7.NGC 7013
NGC 7013 is a relatively nearby spiral or lenticular galaxy estimated to be around 37 to 41.4 million light-years away from Earth in the constellation of Cygnus. NGC 7013 was discovered by English astronomer William Herschel on July 17, 1784 and was also observed by his son, astronomer John Herschel on September 15, 1828.Seyfert galaxy
Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei (very luminous, distant and bright sources of electromagnetic radiation) with very high surface brightnesses whose spectra reveal strong, high-ionisation emission lines, but unlike quasars, their host galaxies are clearly detectable.Seyfert galaxies account for about 10% of all galaxies and are some of the most intensely studied objects in astronomy, as they are thought to be powered by the same phenomena that occur in quasars, although they are closer and less luminous than quasars. These galaxies have supermassive black holes at their centers which are surrounded by accretion discs of in-falling material. The accretion discs are believed to be the source of the observed ultraviolet radiation. Ultraviolet emission and absorption lines provide the best diagnostics for the composition of the surrounding material.Seen in visible light, most Seyfert galaxies look like normal spiral galaxies, but when studied under other wavelengths, it becomes clear that the luminosity of their cores is of comparable intensity to the luminosity of whole galaxies the size of the Milky Way.Seyfert galaxies are named after Carl Seyfert, who first described this class in 1943.