Cepheid variable

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RS Puppis as imaged by Hubble (HST)

A Cepheid (/ˈsɛfiːɪd/ or /ˈsiːfiːɪd/) is a star that varies between a larger, brighter state and a smaller, denser one. They are very luminous variable stars, of a class that was especially massive and hot, using up their fuel early, leaving them in this pulsating condition. The strong direct relationship between a Cepheid variable's luminosity and pulsation period^[1][2] secures for Cepheids their status as important distance indicators for establishing the galactic and extragalactic distance scales.^[3][4][5][6]

Cepheid variables are divided into several subclasses which exhibit markedly different masses, ages, and evolutionary histories: classical Cepheids, type II Cepheids, anomalous Cepheids, and dwarf Cepheids.

The term cepheid originates from Delta Cephei in the constellation Cepheus, the first star of this type identified, by John Goodricke in 1784. Delta Cephei is also of particular importance as a calibrator of the Cepheid period-luminosity relation since its distance is among the most precisely established for a Cepheid, thanks in part to its membership in a star cluster^[7][8] and the availability of precise Hubble Space Telescope/Hipparcos parallaxes.^[9] The accuracy of the distance measurements to Cepheid variables and other bodies within 7,500 lightyears is vastly improved by combining images from Hubble taken six months apart when the Earth and Hubble are on opposite sides of the sun.^[10]

Classes

Classical Cepheids

Classical Cepheids (also known as Population I Cepheids, type I Cepheids, or Delta Cepheid variables) undergo pulsations with very regular periods on the order of days to months. Classical Cepheids are Population I variable stars which are 4–20 times more massive than the Sun,^[11] and up to 100,000 times more luminous.^[12] Cepheids are yellow supergiants of spectral class F6 – K2 and their radii change by (~25% for the longer-period I Carinae) millions of kilometers during a pulsation cycle.^[13][14]
Classical Cepheids are used to determine distances to galaxies within the Local Group and beyond, and are a means by which the Hubble constant can be established.^{[3][4][6][15][16]} Classical Cepheids have also been used to clarify many characteristics of our galaxy, such as the Sun's height above the galactic plane and the Galaxy's local spiral structure.^[5]

Type II Cepheids

Type II Cepheids (also termed Population II Cepheids) are population II variable stars which pulsate with periods typically between 1 and 50 days.^[17][18] Type II Cepheids are typically metal-poor, old (~10 Gyr), low mass objects (~half the mass of the Sun). Type II Cepheids are divided into several subgroups by period. Stars with periods between 1 and 4 days are of the BL Her subclass, 10–20 days belong to the W Virginis subclass, and stars with periods greater than 20 days belong to the RV Tauri subclass.^[17][18]
Type II Cepheids are used to establish the distance to the Galactic Center, globular clusters, and galaxies.^{[5][19][20][21][22][23][24]}

History

On September 10, 1784, Edward Pigott detected the variability of Eta Aquilae, the first known representative of the class of classical Cepheid variables. However, the eponymous star for classical Cepheids is Delta Cephei, discovered to be variable by John Goodricke a few months later.
A relationship between the period and luminosity for classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt in an investigation of thousands of variable stars in the Magellanic Clouds.^[25] She published it in 1912^[26] with further evidence.

In 1913, Ejnar Hertzsprung conducted research on Cepheids. His research would later require revision, however.

In 1915, Harlow Shapley used Cepheids to place initial constraints on the size and shape of the Milky Way, and of the placement of our Sun within it.

In 1924, Edwin Hubble established the distance to classical Cepheid variables in the Andromeda Galaxy, and showed that the variables were not members of the Milky Way. That settled the Island Universe debate which was concerned with whether the Milky Way and the Universe were synonymous, or was the Milky Way merely one in a plethora of galaxies that constitute the Universe.^[27]

In 1929, Hubble and Milton L. Humason formulated what is now known as Hubble's Law by combining Cepheid distances to several galaxies with Vesto Slipher's measurements of the speed at which those galaxies recede from us. They discovered that the Universe is expanding (see the expansion of the Universe). However, the expansion of the Universe was posited several years before by Georges Lemaître.^[28]

In the mid 20th century, significant problems with the astronomical distance scale were resolved by dividing the Cepheids into different classes with very different properties. In the 1940s, Walter Baade recognized two separate populations of Cepheids (classical and type II). Classical Cepheids are younger and more massive population I stars, whereas type II Cepheids are older fainter Population II stars.^[17] Classical Cepheids and type II Cepheids follow different period-luminosity relationships. The luminosity of type II Cepheids is, on average, less than classical Cepheids by about 1.5 magnitudes (but still brighter than RR Lyrae stars). Initial studies of Cepheid variable distances were complicated by the inadvertent admixture of classical Cepheids and type II Cepheids.^[29] Walter Baade's seminal discovery led to a fourfold increase in the distance to M31, and the extragalactic distance scale. RR Lyrae stars were recognized fairly early (by the 1930s) as being a separate class of variable, due in part to their short periods.

Uncertainties in Cepheid determined distances

Chief among the uncertainties tied to the classical and type II Cepheid distance scale are: the nature of the period-luminosity relation in various passbands, the impact of metallicity on both the zero-point and slope of those relations, and the effects of photometric contamination (blending) and a changing (typically unknown) extinction law on Cepheid distances. All these topics are actively debated in the literature.^{[4][12][15][22][30][31][32][33][34][35][36][37]}

These unresolved matters have resulted in cited values for the Hubble constant (established from Classical Cepheids) ranging between 60 km/s/Mpc and 80 km/s/Mpc.^{[3][4][6][15][16]} Resolving this discrepancy is one of the foremost problems in astronomy since the cosmological parameters of the Universe may be constrained by supplying a precise value of the Hubble constant.^[6][16]

Dynamics of the pulsation

The accepted explanation for the pulsation of Cepheids is called the Eddington valve,^[38] or κ-mechanism, where the Greek letter κ (kappa) denotes gas opacity. Helium is the gas thought to be most active in the process. Doubly ionized helium (helium whose atoms are missing two electrons) is more opaque than singly ionized helium. The more helium is heated, the more ionized it becomes. At the dimmest part of a Cepheid's cycle, the ionized gas in the outer layers of the star is opaque, and so is heated by the star's radiation, and due to the increased temperature, begins to expand. As it expands, it cools, and so becomes less ionized and therefore more transparent, allowing the radiation to escape. Then the expansion stops, and reverses due to the star's gravitational attraction. The process then repeats.

The mechanics of the pulsation as a heat-engine was proposed in 1917 by Arthur Stanley Eddington^[39] (who wrote at length on the dynamics of Cepheids), but it was not until 1953 that S. A. Zhevakin identified ionized helium^[40] as a likely valve for the engine.

Examples

• Classical Cepheids include: Eta Aquilae, Zeta Geminorum, Beta Doradus, RT Aurigae, Polaris, as well as Delta Cephei. The North Star (Polaris) is the closest classical Cepheid, although the star exhibits many peculiarities and its distance is a topic of active debate.^[41]
• Type II Cepheids include: W Virginis and BL Herculis.
• Dwarf Cepheids include: Delta Scuti, SX Phoenicis.

Supermassive black hole

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Play media

 

A gas cloud with several times the mass of the Earth is accelerating towards a supermassive black hole at the centre of the Milky Way.

Top: artist's conception of a supermassive black hole tearing apart a star. Bottom: images believed to show a supermassive black hole devouring a star in galaxy RX J1242-11. Left: X-ray image, Right: optical image.^[1]

A supermassive black hole (SMBH) is the largest type of black hole, on the order of hundreds of thousands to billions of solar masses. Most—and possibly all—galaxies are inferred to contain a supermassive black hole at their centers.^[2][3] In the case of the Milky Way, the SMBH is believed to correspond with the location of Sagittarius A*.^[4]

Supermassive black holes have properties which distinguish them from lower-mass classifications. First, the average density of a supermassive black hole (defined as the mass of the black hole divided by the volume within its Schwarzschild radius) can be less than the density of water in the case of some supermassive black holes.^[5] This is because the Schwarzschild radius is directly proportional to mass, while density is inversely proportional to the volume. Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, the density of a black hole is inversely proportional to the square of the mass, and thus higher mass black holes have lower average density. In addition, the tidal forces in the vicinity of the event horizon are significantly weaker for massive black holes. As with density, the tidal force on a body at the event horizon is inversely proportional to the square of the mass: a person on the surface of the Earth and one at the event horizon of a 10 million solar mass black hole experience about the same tidal force between their head and feet. Unlike with stellar mass black holes, one would not experience significant tidal force until very deep into the black hole.

History of research

Donald Lynden-Bell and Martin Rees hypothesized in 1971 that the center of the Milky Way galaxy would contain a supermassive black hole. Sagittarius A* was discovered and named on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the baseline interferometer of the National Radio Astronomy Observatory.^[6] They discovered a radio source that emits synchrotron radiation; it was found to be dense and immobile because of its gravitation. This was, therefore, the first indication that a supermassive black hole exists in the center of the Milky Way.

Formation

An artist's conception of a supermassive black hole and accretion disk.

The origin of supermassive black holes remains an open field of research. Astrophysicists agree that once a black hole is in place in the center of a galaxy, it can grow by accretion of matter and by merging with other black holes. There are, however, several hypotheses for the formation mechanisms and initial masses of the progenitors, or "seeds", of supermassive black holes. The most obvious hypothesis is that the seeds are black holes of tens or perhaps hundreds of solar masses that are left behind by the explosions of massive stars and grow by accretion of matter. Another model involves a large gas cloud in the period before the first stars formed collapsing into a “quasi-star” and then a black hole of initially only around ~20 solar masses, and then rapidly accreting to become relatively quickly an intermediate-mass black hole, and possibly a SMBH if the accretion-rate is not quenched at higher masses.^[7] The initial “quasi-star” would become unstable to radial perturbations because of electron-positron pair production in its core, and may collapse directly into a black hole without a supernova explosion, which would eject most of its mass and prevent it from leaving a black hole as a remnant. Yet another model^[8] involves a dense stellar cluster undergoing core-collapse as the negative heat capacity of the system drives the velocity dispersion in the core to relativistic speeds. Finally, primordial black holes may have been produced directly from external pressure in the first moments after the Big Bang. Formation of black holes from the deaths of the first stars has been extensively studied and corroborated by observations. The other models for black hole formation listed above are theoretical.

Artist’s impression of the huge outflow ejected from the quasar SDSS J1106+1939.^[9]

The difficulty in forming a supermassive black hole resides in the need for enough matter to be in a small enough volume. This matter needs to have very little angular momentum in order for this to happen. Normally, the process of accretion involves transporting a large initial endowment of angular momentum outwards, and this appears to be the limiting factor in black hole growth. This is a major component of the theory of accretion disks. Gas accretion is the most efficient, and also the most conspicuous, way in which black holes grow. The majority of the mass growth of supermassive black holes is thought to occur through episodes of rapid gas accretion, which are observable as active galactic nuclei or quasars. Observations reveal that quasars were much more frequent when the Universe was younger, indicating that supermassive black holes formed and grew early. A major constraining factor for theories of supermassive black hole formation is the observation of distant luminous quasars, which indicate that supermassive black holes of billions of solar masses had already formed when the Universe was less than one billion years old. This suggests that supermassive black holes arose very early in the Universe, inside the first massive galaxies.

Currently, there appears to be a gap in the observed mass distribution of black holes. There are stellar-mass black holes, generated from collapsing stars, which range up to perhaps 33 solar masses. The minimal supermassive black hole is in the range of a hundred thousand solar masses. Between these regimes there appears to be a dearth of intermediate-mass black holes. Such a gap would suggest qualitatively different formation processes. However, some models^[10] suggest that ultraluminous X-ray sources (ULXs) may be black holes from this missing group.

Doppler measurements

Direct Doppler measures of water masers surrounding the nuclei of nearby galaxies have revealed a very fast Keplerian motion, only possible with a high concentration of matter in the center. Currently, the only known objects that can pack enough matter in such a small space are black holes, or things that will evolve into black holes within astrophysically short timescales. For active galaxies farther away, the width of broad spectral lines can be used to probe the gas orbiting near the event horizon.
The technique of reverberation mapping uses variability of these lines to measure the mass and perhaps the spin of the black hole that powers active galaxies.

Gravitation from supermassive black holes in the center of many galaxies is thought to power active objects such as Seyfert galaxies and quasars.

An empirical correlation between the size of supermassive black holes and the stellar velocity dispersion of a galaxy bulge^[11] is called the M-sigma relation.

Milky Way galactic center black hole

Inferred orbits of 6 stars around supermassive black hole candidate Sagittarius A* at the Milky Way galactic centre.^[12]

Astronomers are confident that our own Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System, in a region called Sagittarius A*^[13] because:

• The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light-hours (1.8×10¹³ m or 120 AU) from the center of the central object.^[14]
• From the motion of star S2, the object's mass can be estimated as 4.1 million solar masses,^[15][16] or about 8.2×10³⁶ kg.
• The radius of the central object must be less than 17 light-hours, because otherwise, S2 would collide with it. In fact, recent observations^[17] indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit. However, applying the formula for the Schwarzschild radius yields just about 41 light-seconds, making it consistent with the escape velocity being the speed of light.
• No known astronomical object other than a black hole can contain 4.1 million solar masses in this volume of space.

The Max Planck Institute for Extraterrestrial Physics and UCLA Galactic Center Group^[18] have provided the strongest evidence to date that Sagittarius A* is the site of a supermassive black hole,^[13] based on data from ESO's Very Large Telescope^[19] and the Keck telescope.^[20]

Supermassive black holes outside the Milky Way

Artist's impression of the surroundings of the supermassive black hole in NGC 3783^[21]

Unambiguous dynamical evidence for supermassive black holes exists only in a handful of galaxies;^[22] these include the Milky Way, the Local Group galaxies M31 and M32, and a few galaxies beyond the Local Group, e.g. NGC 4395. In these galaxies, the mean square (or rms) velocities of the stars or gas rises as ~1/r near the center, indicating a central point mass. In all other galaxies observed to date, the rms velocities are flat, or even falling, toward the center, making it impossible to state with certainty that a supermassive black hole is present.^[22] Nevertheless it is commonly accepted that the center of nearly every galaxy contains a supermassive black hole.^[23] The reason for this assumption is the M-sigma relation, a tight (low scatter) relation between the mass of the hole in the ~10 galaxies with secure detections, and the velocity dispersion of the stars in the bulges of those galaxies.^[24] This correlation, although based on just a handful of galaxies, suggests to many astronomers a strong connection between the formation of the black hole and the galaxy itself.^[23]

The nearby Andromeda Galaxy, 2.5 million light-years away, contains a (1.1–2.3) × 10⁸ (110-230 million) solar mass central black hole, significantly larger than the Milky Way's.^[25] The largest supermassive black hole in the Milky Way's neighborhood appears to be that of M87, weighing in at (6.4 ± 0.5) × 10⁹ (~6.4 billion) solar masses at a distance of 53.5 million light-years.^[26][27] On 5 December 2011 astronomers discovered the largest supermassive black hole yet found, that of NGC 4889, weighing in at 21 billion solar masses at a distance of 336 million light-years away in the constellation Coma.^[28]

Some galaxies, such as Galaxy 0402+379, appear to have two supermassive black holes at their centers, forming a binary system. If they collided, the event would create strong gravitational waves.^[29] Binary supermassive black holes are believed to be a common consequence of galactic mergers.^[30] The binary pair in OJ 287, 3.5 billion light-years away, contains the previous most massive black hole known (until the discovery in NGC 4889 ^[31]), with a mass estimated at 18 billion solar masses.^[32] A supermassive black hole was recently discovered in the dwarf galaxy Henize 2-10, which has no bulge. The precise implications for this discovery on black hole formation are unknown, but may indicate that black holes formed before bulges.^[33]

On March 28, 2011, a supermassive black hole was seen tearing a mid-size star apart.^[34] That is, according to astronomers, the only likely explanation of the observations that day of sudden X-ray radiation and the follow-up broad-band observations.^[35][36] The source was previously an inactive galactic nucleus, and from study of the outburst the galactic nucleus is estimated to be a SMBH with mass of the order of a million solar masses. This rare event is assumed to be a relativistic outflow (material being emitted in a jet at a significant fraction of the speed of light) from a star tidally disrupted by the SMBH. A significant fraction of a solar mass of material is expected to have accreted onto the SMBH. Subsequent long-term observation will allow this assumption to be confirmed if the emission from the jet decays at the expected rate for mass accretion onto a SMBH.

In 2012, astronomers reported an unusually large mass of approximately 17 billion solar masses for the black hole in the compact, lenticular galaxy NGC 1277, which lies 220 million light-years away in the constellation Perseus. The putative black hole has approximately 59 percent of the mass of the bulge of this lenticular galaxy (14 percent of the total stellar mass of the galaxy).^[37] Another study reached a very different conclusion: this black hole is not particularly overmassive, estimated at between 2 and 5 billion solar masses with 5 billion being the most likely value.^[38] On 28 February 2013 astronomers reported on the use of the NuSTAR satellite to accurately measure the spin of a supermassive black hole for the first time, reporting that the event horizon was spinning at almost the speed of light.^[39]

Active galactic nucleus

From Wikipedia, the free encyclopedia

An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much higher than normal luminosity over at least some portion, and possibly all, of the electromagnetic spectrum. Such excess emission has been observed in the radio, microwaves, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The radiation from AGN is believed to be a result of accretion of mass by a supermassive black hole at the centre of its host galaxy. AGN are the most luminous and 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.

Hubble Space Telescope image of a 5000-light-year-long (1.5-kiloparsec-long) jet being ejected from the active nucleus of the active galaxy M87, a radio galaxy. The blue synchrotron radiation of the jet contrasts with the yellow starlight from the host galaxy.

 

Models of the active nucleus

For a long time it has been argued^[1] that an AGN must be powered by accretion of mass onto massive black holes (10⁶ to 10¹⁰ times the Solar mass).^[2] 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. Evidence for that is that the mass of the black hole correlates well with the velocity dispersion of the galactic bulge (the M-sigma relation) or with bulge luminosity (e.g.).^[3] Thus AGN-like characteristics are expected whenever a supply of material for accretion comes within the sphere of influence of the central black hole.

Accretion disc

In the standard model of AGN, cold material close to a black hole forms an accretion disc.
Dissipative processes in 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 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.

Relativistic jets

Some accretion discs produce jets of twin, highly collimated, and fast outflows that emerge in opposite directions from close to 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 low resolution of astronomical instruments, and as a result, observations cannot provide enough evidence to support one of the various theoretical models of jet production over the many that exist. The jets have their most obvious observational effects in the radio waveband, where Very Long Baseline Interferometry can be used to study 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.

Radiatively inefficient AGN

There exists a class of 'radiatively inefficient' solutions to the equations that govern accretion. The most widely known of these is the Advection Dominated Accretion Flow (ADAF),^[4] but other theories exist. 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 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.^[5] Radiatively inefficient AGN would be expected to lack many of the characteristic features of standard AGN with an accretion disc.

Observational characteristics

There is no single observational signature of an AGN. The list below covers some of the historically important features that have allowed systems to be identified as AGN.

• Nuclear optical continuum emission. This is visible whenever there is a direct view of the accretion disc. Jets can also contribute to this component of the AGN emission. The optical emission has a roughly power-law dependence on wavelength.
• Nuclear infra-red emission. This is visible whenever the accretion disc and its environment are obscured by gas and dust close to the nucleus and then re-emitted ('reprocessing'). As it is thermal emission, it can be distinguished from any jet or disc-related emission.
• Broad optical emission lines. These come from cold material close to the central black hole. The lines are broad because the emitting material is revolving around the black hole with high speeds causing a range of Doppler shifts of the emitted photons.
• Narrow optical emission lines. These come from more distant cold material, and so are narrower than the broad lines.
• Radio continuum emission. This is always due to a jet. It shows a spectrum characteristic of synchrotron radiation.
• X-ray continuum emission. This can arise both from a jet and from the hot corona of the accretion disc via a scattering process: in both cases it shows a power-law spectrum. In some radio-quiet AGN there is an excess of soft X-ray emission in addition to the power-law component. The origin of the soft X-rays is not clear at present.
• X-ray line emission. This is a result of illumination of cold heavy elements by the X-ray continuum that causes fluorescence of X-ray emission lines, the best-known of which is the iron feature around 6.4 keV. This line may be narrow or broad: relativistically broadened iron lines can be used to study the dynamics of the accretion disc very close to the nucleus and therefore the nature of the central black hole.

Types of active galaxy

It is convenient to divide AGN into two classes, conventionally called radio-quiet and radio-loud. In the radio-loud objects the emission contribution from the jet(s) and the lobes that they inflate dominates the luminosity of the AGN, at least at radio wavelengths but possibly at some or all others. Radio-quiet objects are simpler since jet and jet-related emission can be neglected.
AGN terminology 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.

Radio-quiet AGN

• Low-ionization nuclear emission-line regions (LINERs). As the name suggests, these systems show only weak nuclear emission-line regions, and no other signatures of AGN emission. It is debatable whether all such systems are true AGN (powered by accretion on to a supermassive black hole). If they are, they constitute the lowest-luminosity class of radio-quiet AGN. Some may be radio-quiet analogues of the low-excitation radio galaxies (see below).
• Seyfert galaxies. Seyferts were the earliest distinct class of AGN to be identified. They show optical range nuclear continuum emission, narrow and occasionally broad emission lines, occasionally strong nuclear X-ray emission and sometimes a weak small-scale radio jet. Originally they were divided into two types known as Seyfert 1 and 2: Seyfert 1s show strong broad emission lines while Seyfert 2s do not, and Seyfert 1s are more likely to show strong low-energy X-ray emission. Various forms of elaboration on this scheme exist: for example, Seyfert 1s with relatively narrow broad lines are sometimes referred to as narrow-line Seyfert 1s. The host galaxies of Seyferts are usually spiral or irregular galaxies.
• Radio-quiet quasars/QSOs. These are essentially more luminous versions of Seyfert 1s: the distinction is arbitrary and is usually expressed in terms of a limiting optical magnitude. Quasars were originally 'quasi-stellar' in optical images as they had optical luminosities that were greater than that of their host galaxy. They always show strong optical continuum emission, X-ray continuum emission, and broad and narrow optical emission lines. Some astronomers use the term QSO (Quasi-Stellar Object) for this class of AGN, reserving 'quasar' for radio-loud objects, while others talk about radio-quiet and radio-loud quasars. The host galaxies of quasars can be spirals, irregulars or ellipticals. There is a correlation between the quasar's luminosity and the mass of its host galaxy, in that the most luminous quasars inhabit the most massive galaxies (ellipticals).
• 'Quasar 2s'. By analogy with Seyfert 2s, these are objects with quasar-like luminosities but without strong optical nuclear continuum emission or broad line emission. They are scarce in surveys, though a number of possible candidate quasar 2s have been identified.

Radio-loud AGN

See main article Radio galaxy for a discussion of the large-scale behaviour of the jets. Here, only the active nuclei are discussed.

• Radio-loud quasars behave exactly like radio-quiet quasars with the addition of emission from a jet. Thus they show strong optical continuum emission, broad and narrow emission lines, and strong X-ray emission, together with nuclear and often extended radio emission.
• “Blazars” (BL Lac objects and OVV quasars) classes are distinguished by rapidly variable, polarized optical, radio and X-ray emission. BL Lac objects show no optical emission lines, broad or narrow, so that their redshifts can only be determined from features in the spectra of their host galaxies. The emission-line features may be intrinsically absent or simply swamped by the additional variable component. In the latter case, emission lines may become visible when the variable component is at a low level.^[6] OVV quasars behave more like standard radio-loud quasars with the addition of a rapidly variable component. In both classes of source, the variable emission is believed to originate in a relativistic jet oriented close to the line of sight. Relativistic effects amplify both the luminosity of the jet and the amplitude of variability.
• Radio galaxies. These objects show nuclear and extended radio emission. Their other AGN properties are heterogeneous. They can broadly be divided into low-excitation and high-excitation classes.^[7][8] Low-excitation objects show no strong narrow or broad emission lines, and the emission lines they do have may be excited by a different mechanism.^[9] Their optical and X-ray nuclear emission is consistent with originating purely in a jet.^[10][11] They may be the best current candidates for AGN with radiatively inefficient accretion. By contrast, high-excitation objects (narrow-line radio galaxies) have emission-line spectra similar to those of Seyfert 2s. The small class of broad-line radio galaxies, which show relatively strong nuclear optical continuum emission^[12] probably includes some objects that are simply low-luminosity radio-loud quasars. The host galaxies of radio galaxies, whatever their emission-line type, are essentially always ellipticals.

Summary

These galaxies can be broadly summarised by the following table:

Differences between active galaxy types and normal galaxies.

Galaxy Type	Active Nuclei	Emission Lines		X-rays	Excess of		Strong Radio	Jets	Variable	Radio loud
		Narrow	Broad		UV	Far-IR
Normal	no	weak	no	weak	no	no	no	no	no	no
Starburst	no	yes	no	some	no	yes	some	no	no	no
Seyfert I	yes	yes	yes	some	some	yes	few	no	yes	no
Seyfert II	yes	yes	no	some	some	yes	few	yes	yes	no
Quasar	yes	yes	yes	some	yes	yes	some	some	yes	10%
Blazar	yes	no	some	yes	yes	no	yes	yes	yes	yes
BL Lac	yes	no	no/faint	yes	yes	no	yes	yes	yes	yes
OVV	yes	no	stronger than BL Lac	yes	yes	no	yes	yes	yes	yes
Radio galaxy	yes	some	some	some	some	yes	yes	yes	yes	yes

Unification of AGN species

Unification by viewing angle. From bottom to top: down the jet - Blazar, at an angle to the jet - Quasar/Seyfert 1 Galaxy, at 90 degrees from the jet - Radio galaxy / Seyfert 2 Galaxy^[13]

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 the apparent differences between different types of objects arise simply because of their different orientations to the observer.^[14][15] However, they are debated (see below).

 Radio-quiet unification

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 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 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 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.

Radio-loud unification

Historically, work on radio-loud unification has concentrated on high-luminosity radio-loud quasars. These can be unified with narrow-line radio galaxies in 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.^[16][17][18] X-ray evidence, where available, supports 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.^[19] At very small angles to the line of sight, relativistic beaming dominates, and we see 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,^[10] and their X-ray emission is also consistent with coming purely from a jet, with no heavily absorbed nuclear component in general.^[11] These objects cannot be unified with quasars, even though they include some high-luminosity objects when looking at radio emission, since 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:^[20] 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.^[21]

Criticism of the radio-quiet unification

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 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 ^[22] reveal that only 50% of Seyfert 2s show 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.^[23] 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 ^[24][25] predict how Seyfert 1s and Seyfert 2s can obtain different covering factors from a luminosity- and accretion rate- dependence of the torus covering factor, something supported by studies in the x-ray of AGN.^[26] 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 ^[27] and can explain the observed break-down of the unified model at low luminosities ^[28] and the evolution of the broad-line region.^[29]

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.^[30][31]

Studying neighbour galaxies rather than the AGN themselves ^[32][33] 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 ^[34] have shown that the neighbours of Seyfert 2s are intrinsically dustier and more star-forming than Seyfert 1s and a connection between AGN type, host galaxy morphology and collision history. Moreover, angular clustering studies ^[35] 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 ^[36] where Seyfert 2s transform into Seyfert 1s during merger, supporting earlier models of merger-driven activation of Seyfert 1 nuclei.

While controversy about the soundness of each individual study still prevails, they all agree on that the simplest viewing-angle based models of AGN Unification are incomplete. 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, 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.

Cosmological uses and evolution

For a long time, active galaxies held all the records for the highest-redshift objects known either in 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.

More interesting is the study of the evolution of the AGN population. Most luminous classes of AGN (radio-loud and radio-quiet) seem to have been much more numerous in the early universe. This suggests (1) that massive black holes formed early on and (2) that 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.

Quasar

From Wikipedia, the free encyclopedia

Artist's rendering of ULAS J1120+0641, a very distant quasar powered by a black hole with a mass two billion times that of the Sun.^[1] Credit: ESO/M. Kornmesser

Quasars (/ˈkweɪzɑr/) or quasi-stellar radio sources are the most energetic and distant members of a class of objects called active galactic nuclei (AGN). Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that appeared to be similar to stars, rather than extended sources similar to galaxies. Their spectra contain very broad emission lines, unlike any known from stars, hence the name "quasi-stellar". Their luminosity can be 100 times greater than that of the Milky Way.^[2]

While the nature of these objects was controversial until the early 1980s, there is now a scientific consensus that a quasar is a compact region in the center of a massive galaxy, that surrounds its central supermassive black hole.^[3] Its size is 10–10,000 times the Schwarzschild radius of the black hole. The energy emitted by a quasar derives from mass falling onto the accretion disc around the black hole.

Overview

A Hubble picture showing a quasar core

Quasars show a very high redshift, which is an effect of the expansion of the universe between the quasar and the Earth.^[4] When combined with Hubble's law, the implication of the redshift is that the quasars are very distant in space and time, and it follows, very ancient objects. They tend to inhabit the very center of active, young galaxies, and are among the most luminous, powerful, and energetic objects known in the universe, emitting up to a thousand times the energy output of the Milky Way, which contains 200–400 billion stars. This radiation is emitted across the electromagnetic spectrum, almost uniformly, from X-rays to the far-infrared with a peak in the ultraviolet-optical bands, with some quasars also being strong sources of radio emission and of gamma-rays.

Quasar QSO-160913+653228 is so distant its light has taken nine billion years to reach us, two thirds of the time that has elapsed since the Big Bang.^[5]

In early optical images, quasars appeared as single points of light (i.e., point sources), indistinguishable from stars, except for their peculiar spectra. With infrared telescopes and the Hubble Space Telescope, the "host galaxies" surrounding the quasars have been detected in some cases.^[6] These galaxies are normally too dim to be seen against the glare of the quasar, except with special techniques. Most quasars, with the exception of 3C 273 whose average apparent magnitude is 12.9, cannot be seen with small telescopes.

Some quasars display changes in luminosity which are rapid in the optical range and even more rapid in the X-rays. Because these changes occur very rapidly they define an upper limit on the volume of a quasar; quasars are not much larger than the Solar System.^[7] This implies an astonishingly high energy density.^[8] The mechanism of brightness changes probably involves relativistic beaming of jets pointed nearly directly toward us. The highest redshift quasar known (as of June 2011) is ULAS J1120+0641, with a redshift of 7.085, which corresponds to a comoving distance of approximately 29 billion light-years from Earth (see more discussion of how cosmological distances can be greater than the light-travel time at Metric Expansion of Space).

Quasars are believed to be powered by accretion of material into supermassive black holes in the nuclei of distant galaxies, making these luminous versions of the general class of objects known as active galaxies. Since light cannot escape the black holes, the escaping energy is actually generated outside the event horizon by gravitational stresses and immense friction on the incoming material.^[9] Large central masses (10⁶ to 10⁹ Solar masses) have been measured in quasars using reverberation mapping. Several dozen nearby large galaxies, with no sign of a quasar nucleus, have been shown to contain a similar central black hole in their nuclei, so it is thought that all large galaxies have one, but only a small fraction are active (with enough accretion to power radiation) and so are seen as quasars. The matter accreting onto the black hole is unlikely to fall directly in, but will have some angular momentum around the black hole that will cause the matter to collect into an accretion disc. Quasars may also be ignited or re-ignited from normal galaxies when they merge and the black hole is infused with a fresh source of matter. In fact, it has been suggested that a quasar could form as the Andromeda Galaxy collides with our own Milky Way galaxy in approximately 3–5 billion years.^[9][10][11]

Properties

The Chandra X-ray image is of the quasar PKS 1127-145, a highly luminous source of X-rays and visible light about 10 billion light years from Earth. An enormous X-ray jet extends at least a million light years from the quasar. Image is 60 arcsec on a side. RA 11h 30m 7.10s Dec -14° 49' 27" in Crater. Observation date: May 28, 2000. Instrument: ACIS.

More than 200,000 quasars are known, most from the Sloan Digital Sky Survey. All observed quasar spectra have redshifts between 0.056 and 7.085. Applying Hubble's law to these redshifts, it can be shown that they are between 600 million^[12] and 28.85 billion light-years away (in terms of comoving distance). Because of the great distances to the farthest quasars and the finite velocity of light, we see them and their surrounding space as they existed in the very early universe.

The power of quasars originates from supermassive black holes that are believed to exist at the core of all galaxies. A survey of the 40 nearest galaxies with the Hubble Space Telescope in the 90's revealed Doppler shifts of the stars near the core of those galaxies were rotating about tremendous masses with very steep gravity gradients, suggesting black holes.

Although quasars appear faint when viewed from Earth, the fact that they are visible at all from so far is due to quasars being the most luminous objects in the known universe. The quasar that appears brightest in the sky is 3C 273 in the constellation of Virgo. It has an average apparent magnitude of 12.8 (bright enough to be seen through a medium-size amateur telescope), but it has an absolute magnitude of −26.7.^[2] From a distance of about 33 light-years, this object would shine in the sky about as brightly as our sun. This quasar's luminosity is, therefore, about 4 trillion (4 × 10¹²) times that of our sun, or about 100^[2] times that of the total light of giant galaxies like our Milky Way. However, this assumes the quasar is radiating energy in all directions, but the active galactic nucleus is believed to be radiating preferentially in the direction of its jet. In a universe containing hundreds of billions of galaxies, most of which had active nuclei billions of years ago but only seen today, it is statistically certain that thousands of energy jets should be pointed toward us, some more directly than others. In many cases it is likely that the brighter the quasar, the more directly its jet is aimed at us.

The hyperluminous quasar APM 08279+5255 was, when discovered in 1998, given an absolute magnitude of −32.2. High resolution imaging with the Hubble Space Telescope and the 10 m Keck Telescope revealed that this system is gravitationally lensed. A study of the gravitational lensing of this system suggests that the light emitted has been magnified by a factor of ~10. It is still substantially more luminous than nearby quasars such as 3C 273.

Quasars were much more common in the early universe. This discovery by Maarten Schmidt in 1967 was early strong evidence against the Steady State cosmology of Fred Hoyle, and in favor of the Big Bang cosmology. Quasars show the locations where massive black holes are growing rapidly (via accretion). These black holes grow in step with the mass of stars in their host galaxy in a way not understood at present. One idea is that jets, radiation and winds created by the quasars shut down the formation of new stars in the host galaxy, a process called 'feedback'. The jets that produce strong radio emission in some quasars at the centers of clusters of galaxies are known to have enough power to prevent the hot gas in those clusters from cooling and falling onto the central galaxy.

Quasars' luminosities are variable, with time scales that range from months to hours. This means that quasars generate and emit their energy from a very small region, since each part of the quasar would have to be in contact with other parts on such a time scale to allow the coordination of the luminosity variations. This would mean that a quasar varying on a time scale of a few weeks cannot be larger than a few light-weeks across. The emission of large amounts of power from a small region requires a power source far more efficient than the nuclear fusion that powers stars. The release of gravitational energy^[13] by matter falling towards a massive black hole is the only process known that can produce such high power continuously. Stellar explosions – supernovas and gamma-ray bursts – can do likewise, but only for a few weeks. Black holes were considered too exotic by some astronomers in the 1960s. They also suggested that the redshifts arose from some other (unknown) process, so that the quasars were not really so distant as the Hubble law implied. This 'redshift controversy' lasted for many years. Many lines of evidence (optical viewing of host galaxies, finding 'intervening' absorption lines, gravitational lensing) now demonstrate that the quasar redshifts are due to the Hubble expansion, and quasars are in fact as powerful as first thought.^[14]

Gravitationally lensed quasar HE 1104-1805.^[15]

Quasars have all the properties of other active galaxies such as Seyfert galaxies, but are more powerful: their radiation is partially 'nonthermal' (i.e., not due to black body radiation), and approximately 10 percent are observed to also have jets and lobes like those of radio galaxies that also carry significant (but poorly understood) amounts of energy in the form of particles moving at relativistic speeds. Quasars can be detected over the entire observable electromagnetic spectrum including radio, infrared, visible light, ultraviolet, X-ray and even gamma rays. Most quasars are brightest in their rest-frame near-ultraviolet wavelength of 121.6 nm Lyman-alpha emission line of hydrogen, but due to the tremendous redshifts of these sources, that peak luminosity has been observed as far to the red as 900.0 nm, in the near infrared. A minority of quasars show strong radio emission, which originates from jets of matter moving close to the speed of light. When looked at down the jet, these appear as blazars and often have regions that appear to move away from the center faster than the speed of light (superluminal expansion). This is an optical illusion due to the properties of special relativity.

Quasar redshifts are measured from the strong spectral lines that dominate their visible and ultraviolet spectra. These lines are brighter than the continuous spectrum, so they are called 'emission' lines. They have widths of several percent of the speed of light. These widths are due to Doppler shifts caused by the high speeds of the gas emitting the lines. Fast motions strongly indicate a large mass. Emission lines of hydrogen (mainly of the Lyman series and Balmer series), helium, carbon, magnesium, iron and oxygen are the brightest lines. The atoms emitting these lines range from neutral to highly ionized, i.e., many of the electrons are stripped off the atom, leaving it highly charged. This wide range of ionization shows that the gas is highly irradiated by the quasar, not merely hot, and not by stars, which cannot produce such a wide range of ionization.

Iron quasars show strong emission lines resulting from low ionization iron (FeII), such as IRAS 18508-7815.

Emission generation

This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination.

Since quasars exhibit properties common to all active galaxies, the emission from quasars can be readily compared to those of smaller active galaxies powered by smaller supermassive black holes. To create a luminosity of 10⁴⁰ watts (the typical brightness of a quasar), a super-massive black hole would have to consume the material equivalent of 10 stars per year. The brightest known quasars devour 1000 solar masses of material every year. The largest known is estimated to consume matter equivalent to 600 Earths per minute. Quasar luminosities can vary considerably over time, depending on their surroundings. Since it is difficult to fuel quasars for many billions of years, after a quasar finishes accreting the surrounding gas and dust, it becomes an ordinary galaxy.

Quasars also provide some clues as to the end of the Big Bang's reionization. The oldest known quasars (redshift ≥ 6) display a Gunn-Peterson trough and have absorption regions in front of them indicating that the intergalactic medium at that time was neutral gas. More recent quasars show no absorption region but rather their spectra contain a spiky area known as the Lyman-alpha forest; this indicates that the intergalactic medium has undergone reionization into plasma, and that neutral gas exists only in small clouds.

Quasars show evidence of elements heavier than helium, indicating that galaxies underwent a massive phase of star formation, creating population III stars between the time of the Big Bang and the first observed quasars. Light from these stars may have been observed in 2005 using NASA's Spitzer Space Telescope,^[16] although this observation remains to be confirmed.

Like all (unobscured) active galaxies, quasars can be strong X-ray sources. Radio-loud quasars can also produce X-rays and gamma rays by inverse Compton scattering of lower-energy photons by the radio-emitting electrons in the jet.^[17]

History of observation

Picture shows a famous cosmic mirage known as the Einstein Cross, and is a direct visual confirmation of the theory of general relativity.

The first quasars (3C 48 and 3C 273) were discovered in the early 1960s by Allan Sandage and others.^[18][19] Many were recorded as radio sources with no corresponding visible object. Using small telescopes and the Lovell Telescope as an interferometer, they were shown to have a very small angular size.^[20] Hundreds of these objects were recorded by 1960 and published in the Third Cambridge Catalogue as astronomers scanned the skies for their optical counterparts. In 1960, the radio source 3C 48 was finally tied to an optical object. Astronomers detected what appeared to be a faint blue star at the location of the radio source and obtained its spectrum. Containing many unknown broad emission lines, the anomalous spectrum defied interpretation—a claim by John Bolton of a large redshift was not generally accepted.

In 1962 a breakthrough was achieved. Another radio source, 3C 273, was predicted to undergo five occultations by the moon. Measurements taken by Cyril Hazard and John Bolton during one of the occultations using the Parkes Radio Telescope allowed Maarten Schmidt to optically identify the object and obtain an optical spectrum using the 200-inch Hale Telescope on Mount Palomar. This spectrum revealed the same strange emission lines. Schmidt realized that these were actually spectral lines of hydrogen redshifted at the rate of 15.8 percent. This discovery showed that 3C 273 was receding at a rate of 47,000 km/s.^[21] This discovery revolutionized quasar observation and allowed other astronomers to find redshifts from the emission lines from other radio sources. As predicted earlier by Bolton, 3C 48 was found to have a redshift of 37% of the speed of light.

The term quasar was coined by Chinese-born U.S. astrophysicist Hong-Yee Chiu in May 1964, in Physics Today, to describe these puzzling objects:

So far, the clumsily long name 'quasi-stellar radio sources' is used to describe these objects. Because the nature of these objects is entirely unknown, it is hard to prepare a short, appropriate nomenclature for them so that their essential properties are obvious from their name. For convenience, the abbreviated form 'quasar' will be used throughout this paper.

Later it was found that not all quasars have strong radio emission; in fact only about 10% are 'radio-loud'. Hence the name 'QSO' (quasi-stellar object) is used (in addition to 'quasar') to refer to these objects, including the 'radio-loud' and the 'radio-quiet' classes.

One great topic of debate during the 1960s was whether quasars were nearby objects or distant objects as implied by their redshift. It was suggested, for example, that the redshift of quasars was not due to the expansion of space but rather to light escaping a deep gravitational well. However a star of sufficient mass to form such a well would be unstable and in excess of the Hayashi limit.^[22] Quasars also show 'forbidden' spectral emission lines which were previously only seen in hot gaseous nebulae of low density, which would be too diffuse to both generate the observed power and fit within a deep gravitational well.^[23] There were also serious concerns regarding the idea of cosmologically distant quasars. One strong argument against them was that they implied energies that were far in excess of known energy conversion processes, including nuclear fusion. At this time, there were some suggestions that quasars were made of some hitherto unknown form of stable antimatter and that this might account for their brightness.^{[citation needed]} Others speculated that quasars were a white hole end of a wormhole.^{[citation needed]} However, when accretion disc energy-production mechanisms were successfully modeled in the 1970s, the argument that quasars were too luminous became moot and today the cosmological distance of quasars is accepted by almost all researchers.

In 1979 the gravitational lens effect predicted by Einstein's General Theory of Relativity was confirmed observationally for the first time with images of the double quasar 0957+561.^[24]
In the 1980s, unified models were developed in which quasars were classified as a particular kind of active galaxy, and a consensus emerged that in many cases it is simply the viewing angle that distinguishes them from other classes, such as blazars and radio galaxies.^[25] The huge luminosity of quasars results from the accretion discs of central supermassive black holes, which can convert on the order of 10% of the mass of an object into energy as compared to 0.7% for the p-p chain nuclear fusion process that dominates the energy production in sun-like stars.

This mechanism also explains why quasars were more common in the early universe, as this energy production ends when the supermassive black hole consumes all of the gas and dust near it. This means that it is possible that most galaxies, including our own Milky Way, have gone through an active stage (appearing as a quasar or some other class of active galaxy that depended on the black hole mass and the accretion rate) and are now quiescent because they lack a supply of matter to feed into their central black holes to generate radiation.

Role in celestial reference systems

The energetic radiation of the quasar makes dark galaxies glow, helping astronomers to understand the obscure early stages of galaxy formation.^[26]

Because quasars are extremely distant, bright, and small in apparent size, they are useful reference points in establishing a measurement grid on the sky.^[27] The International Celestial Reference System (ICRS) is based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured with the utmost accuracy by Very Long Baseline Interferometry (VLBI). The positions of most are known to 0.001 arcsecond or better, which is orders of magnitude more precise than the best optical measurements.

Multiple quasars

A multiple imaged quasar is a quasar whose light undergoes gravitational lensing, resulting in double, triple or quadruple images of the same quasar. The first such gravitational lens to be discovered was the double-imaged quasar Q0957+561 (or Twin Quasar) in 1979.^[28] A grouping of two or more quasars can result from a chance alignment, physical proximity, actual close physical interaction, or effects of gravity bending the light of a single quasar into two or more images.

As quasars are rare objects, the probability of three or more separate quasars being found near the same location is very low. The first true triple quasar was found in 2007 by observations at the W. M. Keck Observatory Mauna Kea, Hawaii.^[29] LBQS 1429-008 (or QQQ J1432−0106) was first observed in 1989 and was found to be a double quasar; itself a rare occurrence. When astronomers discovered the third member, they confirmed that the sources were separate and not the result of gravitational lensing. This triple quasar has a red shift of z = 2.076, which is equivalent to 10.5 billion light years.^[30] The components are separated by an estimated 30–50 kpc, which is typical of interacting galaxies.^[31] An example of a triple quasar that is formed by lensing is PG1115 +08.^[32]

In 2013, the second true triplet quasars QQQ J1519+0627 was found with redshift z = 1.51 (approx 9 billion light years) by an international team of astronomers led by Farina of the University of Insubria, the whole system is well accommodated within 25′′ (i.e., 200 kpc in projected distance). The team accessed data from observations collected at the La Silla Observatory with the New Technology Telescope (NTT) of the European Southern Observatory (ESO) and at the Calar Alto Observatory with the 3.5m telescope of the Centro Astronómico Hispano Alemán (CAHA).^[33][34]

When two quasars are so nearly in the same direction as seen from Earth that they appear to be a single quasar but may be separated by the use of telescopes, they are referred to as a "double quasar", such as the Twin Quasar.^[35] These are two different quasars, and not the same quasar that is gravitationally lensed. This configuration is similar to the optical double star. Two quasars, a "quasar pair", may be closely related in time and space, and be gravitationally bound to one another. These may take the form of two quasars in the same galaxy cluster. This configuration is similar to two prominent stars in a star cluster. A "binary quasar", may be closely linked gravitationally and form a pair of interacting galaxies. This configuration is similar to that of a binary star system.