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.

Allan Sandage

From Wikipedia, the free encyclopedia

 

Allan Sandage
Born	June 18, 1926 Iowa City, Iowa
Died	November 13, 2010 (aged 84) San Gabriel, California
Nationality	United States
Fields	astronomy
Institutions	Carnegie Observatories
Alma mater	California Institute of Technology University of Illinois
Doctoral advisor	Walter Baade
Known for	cosmology
Influences	Walter Baade Edwin Hubble
Notable awards	Helen B. Warner Prize for Astronomy (1957) Eddington Medal (1963) National Medal of Science (1970) Elliott Cresson Medal (1973) Bruce Medal (1975) Crafoord Prize (1991) Fellow of the Royal Society[1]

Allan Rex Sandage (June 18, 1926 – November 13, 2010) was an American astronomer. He was Staff Member Emeritus with the Carnegie Observatories in Pasadena, California.^[2] He is best known for determining the first reasonably accurate value for the Hubble constant and the age of the universe. He is also the discoverer of the first quasar.^[3][4]

Career

Sandage was one of the most influential astronomers of the 20th century.^[5] He graduated from the University of Illinois in 1948. In 1953 he received a Ph.D. from the California Institute of Technology; the German-born Wilson Observatory-based astronomer Walter Baade was his advisor.
During this time Sandage was a graduate student assistant to cosmologist Edwin Hubble. He continued Hubble's research program after Hubble died in 1953. In 1952 Baade had shaken the astronomical world by announcing^[6] his determination of two separate populations of Cepheid variable stars in the Andromeda Galaxy, resulted in a doubling of the estimated age of the universe (from 1.8 to 3.6 billion years). Hubble had posited the earlier value; he had considered only the weaker Population II Cepheid variables as standard candles. Following Baade's pronouncements, Sandage showed that astronomers' previous assumption, that the brightest stars in galaxies were of approximately equal inherent intensity, was mistaken in the case of H II regions which he found not to be stars and inherently brighter than the brightest stars in distant galaxies. This resulted in another 1.5 factor increase in the calculated age of the universe, to approximately 5.5 billion years.^[7] Throughout the 1950s and well into the 1980s Sandage was regarded as the pre-eminent observational cosmologist, making contributions to all aspects of the cosmological distance scale, ranging from calibrators within our own Milky Way Galaxy, to cosmologically distant galaxies.
Sandage began working at the Palomar Observatory. In 1958 he published^[8] the first good estimate for the Hubble constant, revising Hubble's value of 250 down to 75 km/s/Mpc, which is close to today's accepted value. Later he became the chief advocate of an even lower value, around 50, corresponding to a Hubble age of around 20 billion years.

Sandage performed photometric studies of globular clusters, and calculated their age to be at least 25 billion years. This led him to speculate that the universe did not merely expand, but actually expanded and contracted with a period of 80 billion years. The current cosmological estimates of the age of the universe, in contrast, are typically of the order of 14 billion years. As part of his studies on the formation of galaxies in the early universe, he co-wrote the paper^[9] now called ELS after the authors Olin J. Eggen, Donald Lynden-Bell and Sandage, first describing the collapse of a proto-galactic gas cloud into our present Milky Way Galaxy. He later defended the paper in 1990.^[10]

In his 1961 paper "The Ability of the 200-inch Telescope to Discriminate Between Selected World Models,"^[11] he suggested that the future of observational cosmology would be the search for two parameters: the Hubble constant H₀ and the deceleration parameter q₀. This paper influenced observational cosmology for at least three decades as it carefully laid out the types of observational tests that could be performed with a large telescope. He also published two atlases of galaxies, in 1961^[12] and in 1981,^[13] based on the Hubble classification scheme.

In 1962 Sandage studied the possibility of directly measuring the temporal variation of the redshift of extra-galactic sources.^[14] This analysis became known as the "Sandage–Loeb test".^[15]

Sandage is noted for his discovery in the M82 galaxy of jets erupting from the core. These must have been caused by massive explosions in the core, and they have apparently been occurring for at least 1.5 million years.^[16]

Sandage was a prolific researcher; during his career he published over 500 papers. Until his death he continued to be an active researcher at the Carnegie Observatories, still publishing several papers a year.^[17]

Personal life

In 1959, Sandage married Mary Connelley, also an astronomer, with whom he had two sons, David and John.^[18] In his later life Sandage became a Christian^[19] and wrote essays on the subject of religion and science.^[20] On November 13, 2010, Sandage died of pancreatic cancer at his home in San Gabriel, California. He was 84 years old.^{[21][22][23][18]}

Honors

Awards

• Helen B. Warner Prize for Astronomy (1957)
• Eddington Medal (1963)
• Gold Medal of the Royal Astronomical Society (1967)
• National Medal of Science (1970)
• Henry Norris Russell Lectureship (1972)
• Elliott Cresson Medal (1973)
• Fellow of the Royal Society^[1]
• Bruce Medal (1975)
• Crafoord Prize (1991)
• Gruber Prize in Cosmology (2000)

Named after him

• Main-belt asteroid, 9963 Sandage (1992 AN)