Ionization cones are cones of ionized material extending from active galactic nuclei, predominantly observed in type II Seyfert galaxies. They are detected through their emission of electromagnetic radiation in the visible and infrared parts of the spectrum. The main method of observation is through spectroscopy, using spectral line analysis to measure the shape of the ionized region and the condition of the material such as temperature, density, composition, and degree of ionization.
Ionization cones have a distinct conical shape, with the galactic center at the apex. Galaxies with ionization cones are thought to have a dense torus-like structure surrounding the central black hole, co-planar with the accretion disk. [1] The material in this torus, consisting of interstellar gas and dust, obstructs the photons coming from the inner area around the black hole and prevents ionization of the galactic matter outside the torus. [2] Along the symmetry axis, however, the density of interstellar matter is much lower, allowing for ionization. Radiation pressure then forces this matter away from the center, resulting in a cone of ionized material. [3]
There is no scientific consensus yet for the orientation of ionization cones with respect to the galactic plane (galactic disc), [4] however, ionization cones usually align with radio sources near the galactic nucleus. [5]
The two large ionization cones in the type II Seyfert galaxy NGC 5252 align with a radio source 10 kiloparsecs (approx. 32.6 kly) from the galactic nucleus instead of the nucleus itself. [5] In NGC 5728, its 1.8 kiloparsec (approx. 5.9 kly) long ionization cone aligns within 3 degrees of a radio source near its nucleus. [6] Conversely, in Messier 77, the ionization cone's alignment is thought to be influenced by factors such as central radio sources and the torus angle, yet it aligns closely with the radio emissions in the vicinity of the nucleus.[ citation needed ] The explanation for deviations from the galactic plane tend to focus on the complex interactions of the torus material with both the inflowing gas and dust in the accretion disk and the gas and radiation being pushed outward by the action of the central black hole. In the process of forming the shape of the ionization cone and containing its ionized material away from the interstellar medium, these interactions may lead to possible deviations from a co-planar alignment with the accretion disk and the galactic plane.[ citation needed ]
Using soft X-ray spectroscopy performed with the Chandra X-ray Observatory, plasma in Markarian 3 photoionized from collision with the interstellar medium was observed to emit almost no X-rays. Markarian 3 is known to have a bi-conical ionization cone, indicating high ionization activity, yet the spectroscopy results indicate that ionization cones usually do not emit significant quantities of X-rays. [7]
 | This section may contain information not important or relevant to the article's subject.(January 2023) |
The accretion disk of the galactic black hole's radiation is aligned with the obscuring torus. [8] Usually, ultraviolet or extreme ultraviolet radiation ionizes the nearby interstellar medium, enlarging the ionization cone. [9] [4] For example, Messier 77 has an ionization cone produced from the hypersonic collision of ejecta from its galactic nucleus with material in the interstellar medium of the galaxy. This collision produces extreme ultraviolet photons. These photons ionize the colliding material through photoionization. [9]
| This section may contain information not important or relevant to the article's subject.(January 2023) |
Ionization cones tend to emit infrared light. Infrared emissions can be used to explain the properties of ionization cones. For example, tori that obscure the nuclei of Seyfert galaxies may have an effect on the photoionization by ultraviolet photons of the material found in an ionization cone. This can be determined with infrared emissions as the infrared emissions of ionization cones are not affected by the ionization of the matter in the cone. For example, the ionization cone in Messier 77 has the same symmetry in visible light and in infrared light, showing there is no impact of the torus on the ionization cone. [4]
In line with the prevalence of Seyfert galaxies, ionization cones are thought to be present only in a small fraction of galaxies. [3]
The Circinus Galaxy, which is the closest Seyfert type II galaxy to the Milky Way, [10] has been observed to have a large, prominent ionization cone. [11] Messier 77's ionization cone was produced from the hypersonic collision of ejecta from its galactic nucleus with material in the interstellar medium of the galaxy. This collision produced extreme ultraviolet photons which then ionized the colliding material which is highly visible in an emission spectrum. The Seyfert galaxies Markarian 348 (type II) and Markarian 3 (disputed type II, possible type I) have very intense inverse bi-conical ionization cones. [7] [12]
Although ionization cones have to date been observed mostly in Seyfert galaxies, they are not exclusive to active galactic nuclei. The X-ray binary LMC-X1 contains an ionization cone similar to those found in galactic nuclei. [13] As X-ray binary systems have comparable X-ray power density to active galactic nuclei, they have the ability to generate similar ionization cones, although the scale of the regions differ by many orders of magnitude. [14]
The study of ionization cones has been used to support the existence of a Seyfert flare event that is thought to have occurred in the Milky Way several million years ago. Interstellar clouds at the Milky Way's galactic poles have been observed to be highly ionized, which could indicate a period of high activity in the Galactic Center that sent pockets of ionized gas outside of the Milky Way's central bulge. [15]
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that the luminosity is not produced by stars. Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.
Seyfert galaxies are one of the two largest groups of active galaxies, along with quasars. They have quasar-like nuclei with very high surface brightnesses whose spectra reveal strong, high-ionisation emission lines, but unlike quasars, their host galaxies are clearly detectable.
NGC 3227 is an intermediate spiral galaxy that is interacting with the dwarf elliptical galaxy NGC 3226. 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. It is a member of the NGC 3227 Group of galaxies, which is a member of the Leo II Groups, a series of galaxies and galaxy clusters strung out from the right edge of the Virgo Supercluster.
NGC 5548 is a Type I Seyfert galaxy with a bright, active nucleus. This activity is caused by matter flowing onto a 65 million solar mass (M☉) supermassive black hole at the core. Morphologically, this is an unbarred lenticular galaxy with tightly-wound spiral arms, while shell and tidal tail features suggest that it has undergone a cosmologically-recent merger or interaction event. NGC 5548 is approximately 245 million light years away and appears in the constellation Boötes. The apparent visual magnitude of NGC 5548 is approximately 13.3 in the V band.
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 an object that is visible to amateur astronomers using large amateur telescopes.
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.
NGC 3783 is a barred spiral galaxy located about 135 million light years away in the constellation Centaurus. It is inclined by an angle of 23° to the line of sight from the Earth along a position angle of about 163°. The morphological classification of SBa indicates a bar structure across the center (B) and tightly-wound spiral arms (a). Although not shown by this classification, observers note the galaxy has a luminous inner ring surrounding the bar structure. The bright compact nucleus is active and categorized as a Seyfert 1 type. This nucleus is a strong source of X-ray emission and undergoes variations in emission across the electromagnetic spectrum.
NGC 7469 is an intermediate spiral galaxy in the constellation of Pegasus. NGC 7469 is located about 200 million light-years away from Earth, which means, given its apparent dimensions, that NGC 7469 is approximately 90,000 light-years across. It was discovered by William Herschel on November 12, 1784.
NGC 7213 is a lenticular galaxy located in the constellation Grus. It is located at a distance of circa 70 million light-years from Earth, which, given its apparent dimensions, means that NGC 7213 is about 75,000 light-years across. It was discovered by John Herschel on September 30, 1834. It is an active galaxy with characteristics between a type I Seyfert galaxy and LINER.
NGC 7130 is a spiral galaxy located in the constellation Piscis Austrinus. It is located at a distance of about 220 million light years from Earth, which, given its apparent dimensions, means that NGC 7130 is about 100,000 light years across. It was discovered by John Herschel on September 25, 1834, and discovered independently by Lewis Swift on September 17, 1897. The location of the galaxy given in the New General Catalogue was off by 30 arcminutes in declination from the location of the galaxy.
NGC 7674 is a spiral galaxy located in the constellation Pegasus. It is located at a distance of circa 350 million light years from Earth, which, given its apparent dimensions, means that NGC 7674 is about 125,000 light years across. It was discovered by John Herschel on August 16, 1830.
NGC 985 is a ring galaxy in the constellation of Cetus. It is located about 550 million light years away from Earth, which means, given its apparent dimensions, that NGC 985 is approximately 160,000 light years across. It was discovered by Francis Leavenworth in 1886. It is a type 1 Seyfert galaxy.
NGC 1386 is a spiral galaxy located in the constellation Eridanus. It is located at a distance of circa 53 million light years from Earth, which, given its apparent dimensions, means that NGC 1386 is about 50,000 light years across. It is a Seyfert galaxy, the only one in Fornax Cluster.
NGC 931 is a spiral galaxy located in the constellation Triangulum. It is located at a distance of circa 200 million light-years from Earth, which, given its apparent dimensions, means that NGC 931 is about 200,000 light years across. It was discovered by Heinrich d'Arrest on September 26, 1865. It is classified as a Seyfert galaxy.
NGC 1142 is a distorted spiral galaxy in the constellation of Cetus. It is located about 370 million light years away from Earth, which means, given its apparent dimensions, that NGC 1142 is approximately 170,000 light years across. It is a type 2 Seyfert galaxy. It interacts with the elliptical galaxy NGC 1141.
NGC 7592 is an interacting galaxy system located 300 million light years away in the constellation Aquarius. It was discovered by William Herschel on September 20, 1784. The total infrared luminosity is 1011.33 L☉, and thus it is categorised as a luminous infrared galaxy. One of the galaxies hosts a type 2 Seyfert nucleus.
Markarian 590, also known as NGC 863, NGC 866, and NGC 885, is a spiral galaxy located in the constellation Cetus. It is located at a distance of about 300 million light years from Earth, which, given its apparent dimensions, means that NGC 863 is about 110,000 light years across. It is a change looking Seyfert galaxy.
UGC 5101 is a galaxy merger located in the constellation Ursa Major. It is located at a distance of about 530 million light years from Earth. It is an ultraluminous infrared galaxy. The total infrared luminosity of the galaxy is estimated to be 1011.95 L☉ and the galaxy has a total star formation rate of 105 M☉ per year.
NGC 2110 is a lenticular galaxy located in the constellation Orion. It is located at a distance of about 120 million light years from Earth, which, given its apparent dimensions, means that NGC 2110 is about 90,000 light years across. It was discovered by William Herschel on October 5, 1785. It is a Seyfert galaxy.
NGC 5135 is a barred spiral galaxy located in the constellation Hydra. It is located at a distance of about 200 million light years from Earth. It was discovered by John Herschel on May 8, 1834. It is a Seyfert galaxy.