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92-517: Accretion disks are a ubiquitous phenomenon in astrophysics; active galactic nuclei , protoplanetary disks , and gamma ray bursts all involve accretion disks. These disks very often give rise to astrophysical jets coming from the vicinity of the central object. Jets are an efficient way for the star-disk system to shed angular momentum without losing too much mass . The most prominent accretion disks are those of active galactic nuclei and of quasars , which are thought to be massive black holes at

184-852: A s {\displaystyle \nu \propto \alpha p_{\mathrm {gas} }} . In the standard Shakura–Sunyaev model, viscosity is assumed to be proportional to the total pressure p t o t = p r a d + p g a s = ρ c s 2 {\displaystyle p_{\mathrm {tot} }=p_{\mathrm {rad} }+p_{\mathrm {gas} }=\rho c_{\rm {s}}^{2}} since ν = α c s H = α c s 2 / Ω = α p t o t / ( ρ Ω ) {\displaystyle \nu =\alpha c_{\rm {s}}H=\alpha c_{s}^{2}/\Omega =\alpha p_{\mathrm {tot} }/(\rho \Omega )} . The Shakura–Sunyaev model assumes that

276-538: A blazar of some variety. However, the population of radio galaxies is completely dominated by low-luminosity, low-excitation objects. These do not show strong nuclear emission lines—broad or narrow—they have optical continua which appear to be entirely jet-related, and their X-ray emission is also consistent with coming purely from a jet, with no heavily absorbed nuclear component in general. These objects cannot be unified with quasars, even though they include some high-luminosity objects when looking at radio emission, since

368-498: A connection between AGN type, host galaxy morphology and collision history. Moreover, angular clustering studies of the two AGN types confirm that they reside in different environments and show that they reside within dark matter halos of different masses. The AGN environment studies are in line with evolution-based unification models where Seyfert 2s transform into Seyfert 1s during merger, supporting earlier models of merger-driven activation of Seyfert 1 nuclei. While controversy about

460-427: A direct view of the optical continuum, broad-line region or (soft) X-ray emission. The key insight of orientation-dependent accretion models is that the two types of object can be the same if only certain angles to the line of sight are observed. The standard picture is of a torus of obscuring material surrounding the accretion disc. It must be large enough to obscure the broad-line region but not large enough to obscure

552-557: A hidden broad-line region and thus split Seyfert 2 galaxies into two populations. The two classes of populations appear to differ by their luminosity, where the Seyfert 2s without a hidden broad-line region are generally less luminous. This suggests absence of broad-line region is connected to low Eddington ratio, and not to obscuration. The covering factor of the torus might play an important role. Some torus models predict how Seyfert 1s and Seyfert 2s can obtain different covering factors from

644-404: A luminosity and accretion rate dependence of the torus covering factor, something supported by studies in the x-ray of AGN. The models also suggest an accretion-rate dependence of the broad-line region and provide a natural evolution from more active engines in Seyfert 1s to more "dead" Seyfert 2s and can explain the observed break-down of the unified model at low luminosities and the evolution of

736-474: A manner directly analogous to the Seyfert 1/2 unification (but without the complication of much in the way of a reflection component: narrow-line radio galaxies show no nuclear optical continuum or reflected X-ray component, although they do occasionally show polarized broad-line emission). The large-scale radio structures of these objects provide compelling evidence that the orientation-based unified models really are true. X-ray evidence, where available, supports

828-434: A mechanism which involves magnetic fields to generate the angular momentum transport. A simple system displaying this mechanism is a gas disk in the presence of a weak axial magnetic field. Two radially neighboring fluid elements will behave as two mass points connected by a massless spring, the spring tension playing the role of the magnetic tension. In a Keplerian disk the inner fluid element would be orbiting more rapidly than

920-505: A reduction in velocity; at a slower velocity, the particle must adopt a lower orbit. As the particle falls to this lower orbit, a portion of its gravitational potential energy is converted to increased velocity and the particle gains speed. Thus, the particle has lost energy even though it is now travelling faster than before; however, it has lost angular momentum. As a particle orbits closer and closer, its velocity increases; as velocity increases frictional heating increases as more and more of

1012-402: A report arguing that "explosions in galactic nuclei cause large amounts of mass to be expelled. For these explosions to occur, galactic nuclei must contain bodies of huge mass and unknown nature. From this point forward Active Galactic Nuclei (AGN) became a key component in theories of galactic evolution." His idea was initially accepted skeptically. A major breakthrough was the measurement of

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1104-459: A sphere (or a "corona") rather than a disk, and very hot (close to the virial temperature). Because of their low efficiency, ADAFs are much less luminous than the Shakura–Sunyaev thin disks. ADAFs emit a power-law, non-thermal radiation, often with a strong Compton component. Credit: NASA/JPL-Caltech The theory of highly super-Eddington black hole accretion, M ≫ M Edd , was developed in

1196-512: A straight flow from one star to the other and an accretion disk forms instead. Accretion disks surrounding T Tauri stars or Herbig stars are called protoplanetary disks because they are thought to be the progenitors of planetary systems . The accreted gas in this case comes from the molecular cloud out of which the star has formed rather than a companion star. In the 1940s, models were first derived from basic physical principles. In order to agree with observations, those models had to invoke

1288-423: A sum of black bodies. Radiative cooling is very efficient in thin disks. The classic 1974 work by Shakura and Sunyaev on thin accretion disks is one of the most often quoted papers in modern astrophysics. Thin disks were independently worked out by Lynden-Bell, Pringle, and Rees. Pringle contributed in the past thirty years many key results to accretion disk theory, and wrote the classic 1981 review that for many years

1380-597: A weakly magnetized disk accreting around a heavy, compact central object would be highly unstable, providing a direct mechanism for angular-momentum redistribution. Shakura and Sunyaev (1973) proposed turbulence in the gas as the source of an increased viscosity. Assuming subsonic turbulence and the disk height as an upper limit for the size of the eddies, the disk viscosity can be estimated as ν = α c s H {\displaystyle \nu =\alpha c_{\rm {s}}H} where c s {\displaystyle c_{\rm {s}}}

1472-470: A yet unknown mechanism for angular momentum redistribution. If matter is to fall inward it must lose not only gravitational energy but also lose angular momentum . Since the total angular momentum of the disk is conserved, the angular momentum loss of the mass falling into the center has to be compensated by an angular momentum gain of the mass far from the center. In other words, angular momentum should be transported outward for matter to accrete. According to

1564-413: Is a very thin sheet of air lying over the surface of the wing (and all other surfaces of the aircraft). Because air has viscosity , this layer of air tends to adhere to the wing. As the wing moves forward through the air, the boundary layer at first flows smoothly over the streamlined shape of the airfoil . Here, the flow is laminar and the boundary layer is a laminar layer. Prandtl applied the concept of

1656-452: Is an excretion disk where instead of material accreting from a disk on to a central object, material is excreted from the center outward onto the disk. Excretion disks are formed when stars merge. Active galactic nucleus 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 this luminosity

1748-408: Is assuming a viscosity much larger than the magnetic diffusivity in the disk. However, numerical simulations and theoretical models show that the viscosity and magnetic diffusivity have almost the same order of magnitude in magneto-rotationally turbulent disks. Some other factors may possibly affect the advection/diffusion rate: reduced turbulent magnetic diffusion on the surface layers; reduction of

1840-412: Is characterized by eddies or small packets of fluid particles, which result in lateral mixing. In non-scientific terms, laminar flow is smooth , while turbulent flow is rough . The type of flow occurring in a fluid in a channel is important in fluid-dynamics problems and subsequently affects heat and mass transfer in fluid systems. The dimensionless Reynolds number is an important parameter in

1932-427: Is in the smooth flow of a viscous liquid through a tube or pipe. In that case, the velocity of flow varies from zero at the walls to a maximum along the cross-sectional centre of the vessel. The flow profile of laminar flow in a tube can be calculated by dividing the flow into thin cylindrical elements and applying the viscous force to them. Another example is the flow of air over an aircraft wing . The boundary layer

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2024-428: Is not produced by the 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. Active galactic nuclei are

2116-555: Is often confusing, since the distinctions between different types of AGN sometimes reflect historical differences in how the objects were discovered or initially classified, rather than real physical differences. There are several subtypes of radio-loud active galactic nuclei. nuclei radio loud Unified models propose that different observational classes of AGN are a single type of physical object observed under different conditions. The currently favoured unified models are 'orientation-based unified models' meaning that they propose that

2208-527: Is the sound speed , H {\displaystyle H} is the scale height of the disk, and α {\displaystyle \alpha } is a free parameter between zero (no accretion) and approximately one. In a turbulent medium ν ≈ v t u r b l t u r b {\displaystyle \nu \approx v_{\rm {turb}}l_{\rm {turb}}} , where v t u r b {\displaystyle v_{\rm {turb}}}

2300-520: Is the Keplerian orbital angular velocity, r {\displaystyle r} is the radial distance from the central object of mass M {\displaystyle M} . By using the equation of hydrostatic equilibrium , combined with conservation of angular momentum and assuming that the disk is thin, the equations of disk structure may be solved in terms of the α {\displaystyle \alpha } parameter. Many of

2392-443: Is the accretion rate, in units of 10 16 g   s − 1 {\displaystyle 10^{16}{\rm {g\ s}}^{-1}} , m 1 {\displaystyle m_{1}} is the mass of the central accreting object in units of a solar mass, M ⨀ {\displaystyle M_{\bigodot }} , R 10 {\displaystyle R_{10}}

2484-412: Is the property of fluid particles in fluid dynamics to follow smooth paths in layers, with each layer moving smoothly past the adjacent layers with little or no mixing. At low velocities, the fluid tends to flow without lateral mixing, and adjacent layers slide past one another smoothly. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of fluids. In laminar flow,

2576-482: Is the radius of a point in the disk, in units of 10 10 c m {\displaystyle 10^{10}{\rm {cm}}} , and f = [ 1 − ( R ⋆ R ) 1 / 2 ] 1 / 4 {\displaystyle f=\left[1-\left({\frac {R_{\star }}{R}}\right)^{1/2}\right]^{1/4}} , where R ⋆ {\displaystyle R_{\star }}

2668-402: Is the radius where angular momentum stops being transported inward. The Shakura–Sunyaev α-disk model is both thermally and viscously unstable. An alternative model, known as the β {\displaystyle \beta } -disk, which is stable in both senses assumes that the viscosity is proportional to the gas pressure ν ∝ α p g

2760-739: Is the velocity of turbulent cells relative to the mean gas motion, and l t u r b {\displaystyle l_{\rm {turb}}} is the size of the largest turbulent cells, which is estimated as l t u r b ≈ H = c s / Ω {\displaystyle l_{\rm {turb}}\approx H=c_{\rm {s}}/\Omega } and v t u r b ≈ c s {\displaystyle v_{\rm {turb}}\approx c_{\rm {s}}} , where Ω = ( G M ) 1 / 2 r − 3 / 2 {\displaystyle \Omega =(GM)^{1/2}r^{-3/2}}

2852-455: The Rayleigh stability criterion , where Ω {\displaystyle \Omega } represents the angular velocity of a fluid element and R {\displaystyle R} its distance to the rotation center, an accretion disk is expected to be a laminar flow . This prevents the existence of a hydrodynamic mechanism for angular momentum transport. On one hand, it

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2944-552: The Shakura – Sunyaev viscosity by magnetic fields; and the generation of large scale fields by small scale MHD turbulence –a large scale dynamo. In fact, a combination of different mechanisms might be responsible for efficiently carrying the external field inward toward the central parts of the disk where the jet is launched. Magnetic buoyancy, turbulent pumping and turbulent diamagnetism exemplify such physical phenomena invoked to explain such efficient concentration of external fields. When

3036-467: The Solar mass ). AGN are both compact and persistently extremely luminous. Accretion can potentially give very efficient conversion of potential and kinetic energy to radiation, and a massive black hole has a high Eddington luminosity , and as a result, it can provide the observed high persistent luminosity. Supermassive black holes are now believed to exist in the centres of most if not all massive galaxies since

3128-460: The redshift of the quasar 3C 273 by Maarten Schmidt , published in 1963. Schmidt noted that if this object was extragalactic (outside the Milky Way , at a cosmological distance) then its large redshift of 0.158 implied that it was the nuclear region of a galaxy about 100 times more powerful than other radio galaxies that had been identified. Shortly afterward, optical spectra were used to measure

3220-489: The visible-light sources associated with the radio emission. In photographic images, some of these objects were nearly point-like or quasi-stellar in appearance, and were classified as quasi-stellar radio sources (later abbreviated as "quasars"). Soviet Armenian astrophysicist Viktor Ambartsumian introduced Active Galactic Nuclei in the early 1950s. At the Solvay Conference on Physics in 1958, Ambartsumian presented

3312-422: The 1980s by Abramowicz, Jaroszynski, Paczyński , Sikora, and others in terms of "Polish doughnuts" (the name was coined by Rees). Polish doughnuts are low viscosity, optically thick, radiation pressure supported accretion disks cooled by advection . They are radiatively very inefficient. Polish doughnuts resemble in shape a fat torus (a doughnut) with two narrow funnels along the rotation axis. The funnels collimate

3404-519: The AGN themselves first suggested the numbers of neighbours were larger for Seyfert 2s than for Seyfert 1s, in contradiction with the Unified Model. Today, having overcome the previous limitations of small sample sizes and anisotropic selection, studies of neighbours of hundreds to thousands of AGN have shown that the neighbours of Seyfert 2s are intrinsically dustier and more star-forming than Seyfert 1s and

3496-476: The Reynolds number increases, such as by increasing the flow rate of the fluid, the flow will transition from laminar to turbulent flow at a specific range of Reynolds numbers, the laminar–turbulent transition range depending on small disturbance levels in the fluid or imperfections in the flow system. If the Reynolds number is very small, much less than 1, then the fluid will exhibit Stokes , or creeping, flow, where

3588-418: The accretion disc transport matter inwards and angular momentum outwards, while causing the accretion disc to heat up. The expected spectrum of an accretion disc peaks in the optical-ultraviolet waveband; in addition, a corona of hot material forms above the accretion disc and can inverse-Compton scatter photons up to X-ray energies. The radiation from the accretion disc excites cold atomic material close to

3680-480: The accretion rate is sub-Eddington and the opacity very high, the standard thin accretion disk is formed. It is geometrically thin in the vertical direction (has a disk-like shape), and is made of a relatively cold gas, with a negligible radiation pressure. The gas goes down on very tight spirals, resembling almost circular, almost free (Keplerian) orbits. Thin disks are relatively luminous and they have thermal electromagnetic spectra, i.e. not much different from that of

3772-452: The accretion rate is smaller than a few percent of the Eddington limit . Another extreme is the case of Saturn's rings , where the disk is so gas-poor that its angular momentum transport is dominated by solid body collisions and disk-moon gravitational interactions. The model is in agreement with recent astrophysical measurements using gravitational lensing . Balbus and Hawley (1991) proposed

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3864-429: The apparent differences between different types of objects arise simply because of their different orientations to the observer. However, they are debated (see below). At low luminosities, the objects to be unified are Seyfert galaxies. The unification models propose that in Seyfert 1s the observer has a direct view of the active nucleus. In Seyfert 2s the nucleus is observed through an obscuring structure which prevents

3956-432: The black hole and this in turn radiates at particular emission lines . A large fraction of the AGN's radiation may be obscured by interstellar gas and dust close to the accretion disc, but (in a steady-state situation) this will be re-radiated at some other waveband, most likely the infrared. Some accretion discs produce jets of twin, highly collimated , and fast outflows that emerge in opposite directions from close to

4048-429: The broad-line region. While studies of single AGN show important deviations from the expectations of the unified model, results from statistical tests have been contradictory. The most important short-coming of statistical tests by direct comparisons of statistical samples of Seyfert 1s and Seyfert 2s is the introduction of selection biases due to anisotropic selection criteria. Studying neighbour galaxies rather than

4140-428: The center of galaxies. As matter enters the accretion disc, it follows a trajectory called a tendex line , which describes an inward spiral. This is because particles rub and bounce against each other in a turbulent flow, causing frictional heating which radiates energy away, reducing the particles' angular momentum, allowing the particle to drift inward, driving the inward spiral. The loss of angular momentum manifests as

4232-421: The central star of the disk. High electric conductivity dictates that the magnetic field is frozen into the matter which is being accreted onto the central object with a slow velocity. However, the plasma is not a perfect electric conductor, so there is always some degree of dissipation. The magnetic field diffuses away faster than the rate at which it is being carried inward by accretion of matter. A simple solution

4324-517: The conditions for the formation of luminous AGN were more common in the early universe, such as a much higher availability of cold gas near the centre of galaxies than at present. It also implies that many objects that were once luminous quasars are now much less luminous, or entirely quiescent. The evolution of the low-luminosity AGN population is much less well understood due to the difficulty of observing these objects at high redshifts. Laminar flow Laminar flow ( / ˈ l æ m ɪ n ər / )

4416-491: The connection to radio-loud AGN, the mechanisms of the variability of some AGN that vary between the two types at very short time scales, and the connection of the AGN type to small and large-scale environment remain important issues to incorporate into any unified model of active galactic nuclei. A study of Swift/BAT AGN published in July 2022 adds support to the "radiation-regulated unification model" outlined in 2017. In this model,

4508-442: The cosmic evolution and growth of black holes, studies of the physics of black hole accretion and the emission of electromagnetic radiation from AGN, examination of the properties of jets and outflows of matter from AGN, and the impact of black hole accretion and quasar activity on galaxy evolution . Since the late 1960s it has been argued that an AGN must be powered by accretion of mass onto massive black holes (10 to 10 times

4600-468: The disc. The direction of the jet ejection is determined either by the angular momentum axis of the accretion disc or the spin axis of the black hole. The jet production mechanism and indeed the jet composition on very small scales are not understood at present due to the resolution of astronomical instruments being too low. The jets have their most obvious observational effects in the radio waveband, where very-long-baseline interferometry can be used to study

4692-726: The discovery of the jet in Messier 87 by Heber Curtis (published in 1918). Further spectroscopic studies by astronomers including Vesto Slipher , Milton Humason , and Nicholas Mayall noted the presence of unusual emission lines in some galaxy nuclei. In 1943, Carl Seyfert published a paper in which he described observations of nearby galaxies having bright nuclei that were sources of unusually broad emission lines. Galaxies observed as part of this study included NGC 1068 , NGC 4151 , NGC 3516 , and NGC 7469 . Active galaxies such as these are known as Seyfert galaxies in honor of Seyfert's pioneering work. The development of radio astronomy

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4784-421: The disk is in local thermal equilibrium, and can radiate its heat efficiently. In this case, the disk radiates away the viscous heat, cools, and becomes geometrically thin. However, this assumption may break down. In the radiatively inefficient case, the disk may "puff up" into a torus or some other three-dimensional solution like an Advection Dominated Accretion Flow (ADAF). The ADAF solutions usually require that

4876-552: The early 1990s by Popham and Narayan in numerical models of accretion disk boundary layers. Self-similar solutions for advection-dominated accretion were found by Narayan and Yi, and independently by Abramowicz, Chen, Kato, Lasota (who coined the name ADAF), and Regev. Most important contributions to astrophysical applications of ADAFs have been made by Narayan and his collaborators. ADAFs are cooled by advection (heat captured in matter) rather than by radiation. They are very radiatively inefficient, geometrically extended, similar in shape to

4968-401: The equations that describe whether fully developed flow conditions lead to laminar or turbulent flow. The Reynolds number is the ratio of the inertial force to the shearing force of the fluid: how fast the fluid is moving relative to how viscous it is, irrespective of the scale of the fluid system. Laminar flow generally occurs when the fluid is moving slowly or the fluid is very viscous. As

5060-441: The external magnetic fields present in the interstellar medium . These fields are typically weak (about few micro-Gauss), but they can get anchored to the matter in the disk, because of its high electrical conductivity , and carried inward toward the central star . This process can concentrate the magnetic flux around the centre of the disk giving rise to very strong magnetic fields. Formation of powerful astrophysical jets along

5152-401: The first half of the 20th century, photographic observations of nearby galaxies detected some characteristic signatures of AGN emission, although there was not yet a physical understanding of the nature of the AGN phenomenon. Some early observations included the first spectroscopic detection of emission lines from the nuclei of NGC 1068 and Messier 81 by Edward Fath (published in 1909), and

5244-454: The fluid: laminar flow or turbulent flow . Laminar flow occurs at lower velocities, below a threshold at which the flow becomes turbulent. The threshold velocity is determined by a dimensionless parameter characterizing the flow called the Reynolds number , which also depends on the viscosity and density of the fluid and dimensions of the channel. Turbulent flow is a less orderly flow regime that

5336-428: The key predictions of the Unified Model, e.g. that each Seyfert 2 has an obscured Seyfert 1 nucleus (a hidden broad-line region). Therefore, one cannot know whether the gas in all Seyfert 2 galaxies is ionized due to photoionization from a single, non-stellar continuum source in the center or due to shock-ionization from e.g. intense, nuclear starbursts. Spectropolarimetric studies reveal that only 50% of Seyfert 2s show

5428-471: The lack of strong AGN-type radiation from massive black holes at the centres of elliptical galaxies in clusters, where otherwise we might expect high accretion rates and correspondingly high luminosities. Radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc. AGN are a candidate source of high and ultra-high energy cosmic rays (see also Centrifugal mechanism of acceleration ) . Among

5520-404: The laminar boundary layer to airfoils in 1904. An everyday example is the slow, smooth and optically transparent flow of shallow water over a smooth barrier. When water leaves a tap without an aerator with little force, it first exhibits laminar flow, but as acceleration by the force of gravity immediately sets in, the Reynolds number of the flow increases with speed, and the laminar flow of

5612-510: The many interesting characteristics of AGNs: It is convenient to divide AGN into two classes, conventionally called radio-quiet and radio-loud. Radio-loud objects have emission contributions from both the jet(s) and the lobes that the jets inflate. These emission contributions dominate the luminosity of the AGN at radio wavelengths and possibly at some or all other wavelengths. Radio-quiet objects are simpler since jet and any jet-related emission can be neglected at all wavelengths. AGN terminology

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5704-438: The mass of an object into energy as compared to around 0.7 percent for nuclear fusion processes. In close binary systems the more massive primary component evolves faster and has already become a white dwarf , a neutron star, or a black hole, when the less massive companion reaches the giant state and exceeds its Roche lobe . A gas flow then develops from the companion star to the primary. Angular momentum conservation prevents

5796-470: The mass of the black hole correlates well with the velocity dispersion of the galactic bulge (the M–sigma relation ) or with bulge luminosity. Thus, AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole. In the standard model of AGN, cold material close to a black hole forms an accretion disc . Dissipative processes in

5888-407: The most luminous persistent sources of electromagnetic radiation in the universe and, as such, can be used as a means of discovering distant objects; their evolution as a function of cosmic time also puts constraints on models of the cosmos . The observed characteristics of an AGN depend on several properties such as the mass of the central black hole, the rate of gas accretion onto the black hole,

5980-481: The most widely known of these is the Advection Dominated Accretion Flow (ADAF). In this type of accretion, which is important for accretion rates well below the Eddington limit , the accreting matter does not form a thin disc and consequently does not efficiently radiate away the energy that it acquired as it moved close to the black hole. Radiatively inefficient accretion has been used to explain

6072-429: The motion of the particles of the fluid is very orderly with particles close to a solid surface moving in straight lines parallel to that surface. Laminar flow is a flow regime characterized by high momentum diffusion and low momentum convection . When a fluid is flowing through a closed channel such as a pipe or between two flat plates, either of two types of flow may occur depending on the velocity and viscosity of

6164-465: The narrow-line region, which is seen in both classes of object. Seyfert 2s are seen through the torus. Outside the torus there is material that can scatter some of the nuclear emission into our line of sight, allowing us to see some optical and X-ray continuum and, in some cases, broad emission lines—which are strongly polarized, showing that they have been scattered and proving that some Seyfert 2s really do contain hidden Seyfert 1s. Infrared observations of

6256-510: The nuclei of Seyfert 2s also support this picture. At higher luminosities, quasars take the place of Seyfert 1s, but, as already mentioned, the corresponding 'quasar 2s' are elusive at present. If they do not have the scattering component of Seyfert 2s they would be hard to detect except through their luminous narrow-line and hard X-ray emission. Historically, work on radio-loud unification has concentrated on high-luminosity radio-loud quasars. These can be unified with narrow-line radio galaxies in

6348-492: The observables depend only weakly on α {\displaystyle \alpha } , so this theory is predictive even though it has a free parameter. Using Kramers' opacity law it is found that where T c {\displaystyle T_{c}} and ρ {\displaystyle \rho } are the mid-plane temperature and density respectively. M ˙ 16 {\displaystyle {\dot {M}}_{16}}

6440-447: The opacity very low, an ADAF (advection dominated accretion flow) is formed. This type of accretion disk was predicted in 1977 by Ichimaru. Although Ichimaru's paper was largely ignored, some elements of the ADAF model were present in the influential 1982 ion-tori paper by Rees, Phinney, Begelman, and Blandford. ADAFs started to be intensely studied by many authors only after their rediscovery in

6532-423: The optical or the radio spectrum, because of their high luminosity. They still have a role to play in studies of the early universe, but it is now recognised that an AGN gives a highly biased picture of the "typical" high-redshift galaxy. Most luminous classes of AGN (radio-loud and radio-quiet) seem to have been much more numerous in the early universe. This suggests that massive black holes formed early on and that

6624-461: The orientation of the accretion disk , the degree of obscuration of the nucleus by dust , and presence or absence of jets . Numerous subclasses of AGN have been defined on the basis of their observed characteristics; the most powerful AGN are classified as quasars . A blazar is an AGN with a jet pointed toward the Earth, in which radiation from the jet is enhanced by relativistic beaming . During

6716-470: The origin of the turbulence itself was not well understood. The conventional α {\displaystyle \alpha } -model (discussed below) introduces an adjustable parameter α {\displaystyle \alpha } describing the effective increase of viscosity due to turbulent eddies within the disk. In 1991, with the rediscovery of the magnetorotational instability (MRI), S. A. Balbus, and J. F. Hawley established that

6808-447: The outer, causing the spring to stretch. The inner fluid element is then forced by the spring to slow down, reduce correspondingly its angular momentum causing it to move to a lower orbit. The outer fluid element being pulled forward will speed up, increasing its angular momentum and move to a larger radius orbit. The spring tension will increase as the two fluid elements move further apart and the process runs away. It can be shown that in

6900-473: The particle's potential energy (relative to the black hole) is radiated away; the accretion disk of a black hole is hot enough to emit X-rays just outside the event horizon . The large luminosity of quasars is believed to be a result of gas being accreted by supermassive black holes. Elliptical accretion disks formed at tidal disruption of stars can be typical in galactic nuclei and quasars. The accretion process can convert about 10 percent to over 40 percent of

6992-457: The presence of such a spring-like tension the Rayleigh stability criterion is replaced by Most astrophysical disks do not meet this criterion and are therefore prone to this magnetorotational instability. The magnetic fields present in astrophysical objects (required for the instability to occur) are believed to be generated via dynamo action. Accretion disks are usually assumed to be threaded by

7084-407: The radiation into beams with highly super-Eddington luminosities. Slim disks (name coined by Kolakowska) have only moderately super-Eddington accretion rates, M ≥ M Edd , rather disk-like shapes, and almost thermal spectra. They are cooled by advection, and are radiatively ineffective. They were introduced by Abramowicz, Lasota, Czerny, and Szuszkiewicz in 1988. The opposite of an accretion disk

7176-630: The redshifts of a growing number of quasars including 3C 48 , even more distant at redshift 0.37. The enormous luminosities of these quasars as well as their unusual spectral properties indicated that their power source could not be ordinary stars. Accretion of gas onto a supermassive black hole was suggested as the source of quasars' power in papers by Edwin Salpeter and Yakov Zeldovich in 1964. In 1969 Donald Lynden-Bell proposed that nearby galaxies contain supermassive black holes at their centers as relics of "dead" quasars, and that black hole accretion

7268-403: The relative accretion rate (termed the "Eddington ratio") of the black hole has a significant impact on the observed features of the AGN. Black Holes with higher Eddington ratios appear to be more likely to be unobscured, having cleared away locally obscuring material in a very short timescale. For a long time, active galaxies held all the records for the highest- redshift objects known either in

7360-416: The relativistic rotation speed needed for centrifugal equilibrium in the very strong gravitational field near the black hole produces a strong Doppler redshift on the receding side (taken here to be on the right) whereas there will be a strong blueshift on the approaching side. Due to light bending, the disk appears distorted but is nowhere hidden by the black hole. When the accretion rate is sub-Eddington and

7452-455: The rotation axis of accretion disks requires a large scale poloidal magnetic field in the inner regions of the disk. Such magnetic fields may be advected inward from the interstellar medium or generated by a magnetic dynamo within the disk. Magnetic fields strengths at least of order 100 Gauss seem necessary for the magneto-centrifugal mechanism to launch powerful jets. There are problems, however, in carrying external magnetic flux inward toward

7544-438: The soundness of each individual study still prevails, they all agree on that the simplest viewing-angle based models of AGN Unification are incomplete. Seyfert-1 and Seyfert-2 seem to differ in star formation and AGN engine power. While it still might be valid that an obscured Seyfert 1 can appear as a Seyfert 2, not all Seyfert 2s must host an obscured Seyfert 1. Understanding whether it is the same engine driving all Seyfert 2s,

7636-440: The synchrotron radiation they emit at resolutions of sub- parsec scales. However, they radiate in all wavebands from the radio through to the gamma-ray range via the synchrotron and the inverse-Compton scattering process, and so AGN jets are a second potential source of any observed continuum radiation. There exists a class of "radiatively inefficient" solutions to the equations that govern accretion. Several theories exist, but

7728-521: The torus can never hide the narrow-line region to the required extent, and since infrared studies show that they have no hidden nuclear component: in fact there is no evidence for a torus in these objects at all. Most likely, they form a separate class in which only jet-related emission is important. At small angles to the line of sight, they will appear as BL Lac objects. In the recent literature on AGN, being subject to an intense debate, an increasing set of observations appear to be in conflict with some of

7820-593: The transition range is typically between 1,800 and 2,100. For fluid systems occurring on external surfaces, such as flow past objects suspended in the fluid, other definitions for Reynolds numbers can be used to predict the type of flow around the object. The particle Reynolds number Re p would be used for particle suspended in flowing fluids, for example. As with flow in pipes, laminar flow typically occurs with lower Reynolds numbers, while turbulent flow and related phenomena, such as vortex shedding , occur with higher Reynolds numbers. A common application of laminar flow

7912-408: The unified picture: radio galaxies show evidence of obscuration from a torus, while quasars do not, although care must be taken since radio-loud objects also have a soft unabsorbed jet-related component, and high resolution is necessary to separate out thermal emission from the sources' large-scale hot-gas environment. At very small angles to the line of sight, relativistic beaming dominates, and we see

8004-435: The viscous forces of the fluid dominate the inertial forces. The specific calculation of the Reynolds number, and the values where laminar flow occurs, will depend on the geometry of the flow system and flow pattern. The common example is flow through a pipe , where the Reynolds number is defined as where: For such systems, laminar flow occurs when the Reynolds number is below a critical value of approximately 2,040, though

8096-504: The water downstream from the tap can transition to turbulent flow. Optical transparency is then reduced or lost entirely. Laminar airflow is used to separate volumes of air, or prevent airborne contaminants from entering an area. Laminar flow hoods are used to exclude contaminants from sensitive processes in science, electronics and medicine. Air curtains are frequently used in commercial settings to keep heated or refrigerated air from passing through doorways. A laminar flow reactor (LFR)

8188-499: Was a major catalyst to understanding AGN. Some of the earliest detected radio sources are nearby active elliptical galaxies such as Messier 87 and Centaurus A . Another radio source, Cygnus A , was identified by Walter Baade and Rudolph Minkowski as a tidally distorted galaxy with an unusual emission-line spectrum, having a recessional velocity of 16,700 kilometers per second. The 3C radio survey led to further progress in discovery of new radio sources as well as identifying

8280-405: Was clear that viscous stresses would eventually cause the matter toward the center to heat up and radiate away some of its gravitational energy. On the other hand, viscosity itself was not enough to explain the transport of angular momentum to the exterior parts of the disk. Turbulence -enhanced viscosity was the mechanism thought to be responsible for such angular-momentum redistribution, although

8372-409: Was the main source of information about accretion disks, and is still very useful today. A fully general relativistic treatment, as needed for the inner part of the disk when the central object is a black hole , has been provided by Page and Thorne, and used for producing simulated optical images by Luminet and Marck, in which, although such a system is intrinsically symmetric its image is not, because

8464-515: Was the power source for the non-stellar emission in nearby Seyfert galaxies. In the 1960s and 1970s, early X-ray astronomy observations demonstrated that Seyfert galaxies and quasars are powerful sources of X-ray emission, which originates from the inner regions of black hole accretion disks. Today, AGN are a major topic of astrophysical research, both observational and theoretical . AGN research encompasses observational surveys to find AGN over broad ranges of luminosity and redshift, examination of

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