Seyfert galaxies are one of the two largest groups of active galaxies, along with quasar host galaxies. They have quasar-like nuclei (very luminous sources of electromagnetic radiation that are outside of our own galaxy) with very high surface brightnesses whose spectra reveal strong, high-ionisationemission lines, but unlike quasars, their host galaxies are clearly detectable.
Seyfert galaxies account for about 10% of all galaxies and are some of the most intensely studied objects in astronomy,
as they are thought to be powered by the same phenomena that occur in
quasars, although they are closer and less luminous than quasars. These
galaxies have supermassive black holes at their centers which are surrounded by accretion discs of in-falling material. The accretion discs are believed to be the source of the observed ultraviolet radiation. Ultraviolet emission and absorption lines provide the best diagnostics for the composition of the surrounding material.
Seen in visible light, most Seyfert galaxies look like normal spiral galaxies, but when studied under other wavelengths, it becomes clear that the luminosity of their cores is of comparable intensity to the luminosity of whole galaxies the size of the Milky Way.
Seyfert galaxies are named after Carl Seyfert, who first described this class in 1943.
Discovery
NGC 1068 (Messier 77), one of the first Seyfert galaxies classified
In 1926, Edwin Hubble looked at the emission lines of NGC 1068 and two other such "nebulae" and classified them as extragalactic objects. In 1943, Carl Keenan Seyfert
discovered more galaxies similar to NGC 1068 and reported that these
galaxies have very bright stellar-like nuclei that produce broad
emission lines. In 1944 Cygnus A was detected at 160 MHz, and detection was confirmed in 1948 when it was established that it was a discrete source. Its double radio structure became apparent with the use of interferometry. In the next few years, other radio sources such as supernova
remnants were discovered. By the end of the 1950s, more important
characteristics of Seyfert galaxies were discovered, including the fact
that their nuclei are extremely compact (< 100 pc, i.e.
"unresolved"), have high mass (≈109±1 solar masses), and the duration of peak nuclear emissions is relatively short (> 108 years).
NGC 5793 is a Seyfert galaxy located over 150 million light-years away in the constellation of Libra.
In the 1960s and 1970s, research to further understand the properties
of Seyfert galaxies was carried out. A few direct measurements of the
actual sizes of Seyfert nuclei were taken, and it was established that
the emission lines in NGC 1068 were produced in a region over a thousand
light years in diameter. Controversy existed over whether Seyfert redshifts were of cosmological origin.
Confirming estimates of the distance to Seyfert galaxies and their age
were limited since their nuclei vary in brightness over a time scale of a
few years; therefore arguments involving distance to such galaxies and
the constant speed of light cannot always be used to determine their
age.
In the same time period, research had been undertaken to survey,
identify and catalogue galaxies, including Seyferts. Beginning in 1967, Benjamin Markarian
published lists containing a few hundred galaxies distinguished by
their very strong ultraviolet emission, with measurements on the
position of some of them being improved in 1973 by other researchers. At the time, it was believed that 1% of spiral galaxies are Seyferts. By 1977, it was found that very few Seyfert galaxies are ellipticals, most of them being spiral or barred spiral galaxies. During the same time period, efforts have been made to gather spectrophotometric
data for Seyfert galaxies. It became obvious that not all spectra from
Seyfert galaxies look the same, so they have been subclassified
according to the characteristics of their emission spectra. A simple division into types I and II has been devised, with the classes depending on the relative width of their emission lines.
It has been later noticed that some Seyfert nuclei show intermediate
properties, resulting in their being further subclassified into types
1.2, 1.5, 1.8 and 1.9 (see Classification). Early surveys for Seyfert galaxies were biased in counting only the
brightest representatives of this group. More recent surveys that count
galaxies with low-luminosity and obscured Seyfert nuclei suggest that
the Seyfert phenomenon is actually quite common, occurring in 16% ± 5%
of galaxies; indeed, several dozen galaxies exhibiting the Seyfert
phenomenon exist in the close vicinity (≈27 Mpc) of our own galaxy. Seyfert galaxies form a substantial fraction of the galaxies appearing in the Markarian catalog, a list of galaxies displaying an ultraviolet excess in their nuclei.
Characteristics
Optical and ultraviolet images of the black hole in the center of NGC 4151, a Seyfert galaxy
An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a higher than normal luminosity over portions of the electromagnetic spectrum.
A galaxy having an active nucleus is called an active galaxy. Active
galactic nuclei are the most luminous sources of electromagnetic
radiation in the Universe, and their evolution puts constraints on
cosmological models. Depending on the type, their luminosity varies over
a timescale from a few hours to a few years. The two largest subclasses
of active galaxies are quasars and Seyfert galaxies, the main
difference between the two being the amount of radiation they emit. In a
typical Seyfert galaxy, the nuclear source emits at visible wavelengths
an amount of radiation comparable to that of the whole galaxy's
constituent stars, while in a quasar, the nuclear source is brighter
than the constituent stars by at least a factor of 100. Seyfert galaxies have extremely bright nuclei, with luminosities ranging between 108 and 1011 solar luminosities. Only about 5% of them are radio bright; their emissions are moderate in gamma rays and bright in X-rays. Their visible and infrared spectra show very bright emission lines of hydrogen, helium, nitrogen, and oxygen. These emission lines exhibit strong Doppler broadening, which implies velocities from 500 to 4,000 km/s (310 to 2,490 mi/s), and are believed to originate near an accretion disc surrounding the central black hole.
A lower limit to the mass of the central black hole can be calculated using the Eddington luminosity. This limit arises because light exhibits radiation pressure. Assume that a black hole is surrounded by a disc of luminous gas.
Both the attractive gravitational force acting on electron-ion pairs in
the disc and the repulsive force exerted by radiation pressure follow
an inverse-square law. If the gravitational force exerted by the black
hole is less than the repulsive force due to radiation pressure, the
disc will be blown away by radiation pressure.
A
model of an active galactic nucleus. The central black hole is
surrounded by an accretion disc, which is surrounded by a torus. The
broad line region and narrow line emission region are shown, as well as
jets coming out of the nucleus.
Emissions
The
emission lines seen on the spectrum of a Seyfert galaxy may come from
the surface of the accretion disc itself, or may come from clouds of gas
illuminated by the central engine in an ionization cone. The exact
geometry of the emitting region is difficult to determine due to poor
resolution of the galactic center. However, each part of the accretion
disc has a different velocity relative to our line of sight, and the
faster the gas is rotating around the black hole, the broader the
emission line will be. Similarly, an illuminated disc wind also has a position-dependent velocity.
The narrow lines are believed to originate from the outer part of
the active galactic nucleus, where velocities are lower, while the
broad lines originate closer to the black hole. This is confirmed by the
fact that the narrow lines do not vary detectably, which implies that
the emitting region is large, contrary to the broad lines which can vary
on relatively short timescales. Reverberation mapping
is a technique which uses this variability to try to determine the
location and morphology of the emitting region. This technique measures
the structure and kinematics of the broad line emitting region by
observing the changes in the emitted lines as a response to changes in
the continuum. The use of reverberation mapping requires the assumption
that the continuum originates in a single central source.
For 35 AGN, reverberation mapping has been used to calculate the mass
of the central black holes and the size of the broad line regions.
In the few radio-loud Seyfert galaxies that have been observed, the radio emission is believed to represent synchrotron emission
from the jet. The infrared emission is due to radiation in other bands
being reprocessed by dust near the nucleus. The highest energy photons
are believed to be created by inverse Compton scattering by a high temperature corona near the black hole.
Classification
NGC 1097
is an example of a Seyfert galaxy. A supermassive black hole with a
mass of 100 million solar masses lies at the center of the galaxy. The
area around the black hole emits large amounts of radiation from the
matter falling into the black hole.
Seyferts were first classified as Type I or II, depending on the
emission lines shown by their spectra. The spectra of Type I Seyfert
galaxies show broad lines that include both allowed lines, like H I,
He I or He II and narrower forbidden lines, like O III. They show some
narrower allowed lines as well, but even these narrow lines are much
broader than the lines shown by normal galaxies. However, the spectra of
Type II Seyfert galaxies show only narrow lines, both permitted and
forbidden. Forbidden lines are spectral lines that occur due to electron transitions not normally allowed by the selection rules of quantum mechanics,
but that still have a small probability of spontaneously occurring. The
term "forbidden" is slightly misleading, as the electron transitions
causing them are not forbidden but highly improbable.
NGC 6300 is a Type II galaxy in the southern constellation of Ara.
In some cases, the spectra show both broad and narrow permitted
lines, which is why they are classified as an intermediate type between
Type I and Type II, such as Type 1.5 Seyfert. The spectra of some of
these galaxies have changed from Type 1.5 to Type II in a matter of a
few years. However, the characteristic broad Hα emission line has rarely, if ever, disappeared.
The origin of the differences between Type I and Type II Seyfert
galaxies is not known yet. There are a few cases where galaxies have
been identified as Type II only because the broad components of the
spectral lines have been very hard to detect. It is believed by some
that all Type II Seyferts are in fact Type I, where the broad components
of the lines are impossible to detect because of the angle we are at
with respect to the galaxy. Specifically, in Type I Seyfert galaxies, we
observe the central compact source more or less directly, therefore
sampling the high velocity clouds in the broad line emission region
moving around the supermassive black hole thought to be at the center of
the galaxy. By contrast, in Type II Seyfert galaxies, the active nuclei
are obscured and only the colder outer regions located further away
from the clouds' broad line emission region are seen. This theory is
known as the "Unification scheme" of Seyfert galaxies. However, it is not yet clear if this hypothesis can explain all the observed differences between the two types.
Type I Seyfert galaxies
NGC 6814 is a Seyfert galaxy with a highly variable source of X-ray radiation.
Type I Seyferts are very bright sources of ultraviolet light and X-rays
in addition to the visible light coming from their cores. They have two
sets of emission lines on their spectra: narrow lines with widths
(measured in velocity units) of several hundred km/s, and broad lines
with widths up to 104 km/s.
The broad lines originate above the accretion disc of the supermassive
black hole thought to power the galaxy, while the narrow lines occur
beyond the broad line region of the accretion disc. Both emissions are
caused by heavily ionised gas. The broad line emission arises in a
region 0.1–1 parsec across. The broad line emission region, RBLR,
can be estimated from the time delay corresponding to the time taken by
light to travel from the continuum source to the line-emitting gas.
Type II Seyfert galaxies
NGC 3081 is known as a Type II Seyfert galaxy, characterised by its dazzling nucleus.
Type II Seyfert galaxies have the characteristic bright core, as well as appearing bright when viewed at infrared wavelengths.
Their spectra contain narrow lines associated with forbidden
transitions, and broader lines associated with allowed strong dipole or
intercombination transitions. NGC 3147 is considered the best candidate to be a true Type II Seyfert galaxy. In some Type II Seyfert galaxies, analysis with a technique called spectro-polarimetry (spectroscopy of polarised light component) revealed obscured Type I regions. In the case of NGC 1068, nuclear light reflected off a dust cloud was measured, which led scientists to believe in the presence of an obscuring dust torus
around a bright continuum and broad emission line nucleus. When the
galaxy is viewed from the side, the nucleus is indirectly observed
through reflection by gas and dust above and below the torus. This reflection causes the polarisation.
In 1981, Donald Osterbrock
introduced the notations Type 1.5, 1.8 and 1.9, where the subclasses
are based on the optical appearance of the spectrum, with the
numerically larger subclasses having weaker broad-line components
relative to the narrow lines. For example, Type 1.9 only shows a broad component in the Hα line, and not in higher order Balmer lines. In Type 1.8, very weak broad lines can be detected in the Hβ
lines as well as Hα, even if they are very weak compared to the Hα. In
Type 1.5, the strength of the Hα and Hβ lines are comparable.
In addition to the Seyfert progression from Type I to Type II
(including Type 1.2 to Type 1.9), there are other types of galaxies that
are very similar to Seyferts or that can be considered as subclasses of
them. Very similar to Seyferts are the low-ionisation narrow-line
emission radio galaxies (LINER), discovered in 1980. These galaxies have
strong emission lines from weakly ionised or neutral atoms, while the
emission lines from strongly ionised atoms are relatively weak by
comparison. LINERs share a large amount of traits with low luminosity
Seyferts. In fact, when seen in visible light, the global
characteristics of their host galaxies are indistinguishable. Also, they
both show a broad line emission region, but the line emitting region in
LINERs has a lower density than in Seyferts. An example of such a galaxy is M104 in the Virgo constellation, also known as the Sombrero Galaxy. A galaxy that is both a LINER and a Type I Seyfert is NGC 7213, a galaxy that is relatively close compared to other AGNs.
Another very interesting subclass are the narrow line Type I galaxies
(NLSy1), which have been subject to extensive research in recent years.
They have much narrower lines than the broad lines from classic Type I
galaxies, steep hard and soft X-ray spectra and strong Fe[II] emission.
Their properties suggest that NLSy1 galaxies are young AGNs with high
accretion rates, suggesting a relatively small but growing central black
hole mass.
There are theories suggesting that NLSy1s are galaxies in an early
stage of evolution, and links between them and ultraluminous infrared
galaxies or Type II galaxies have been proposed.
Evolution
The majority of active galaxies are very distant and show large Doppler shifts. This suggests that active galaxies occurred in the early Universe and, due to cosmic expansion, are receding away from the Milky Way
at very high speeds. Quasars are the furthest active galaxies, some of
them being observed at distances 12 billion light years away. Seyfert
galaxies are much closer than quasars.
Because light has a finite speed, looking across large distances in the
Universe is equivalent to looking back in time. Therefore, the
observation of active galactic nuclei at large distances and their
scarcity in the nearby Universe suggests that they were much more common
in the early Universe, implying that active galactic nuclei could be early stages of galactic evolution.
This leads to the question about what would be the local (modern-day)
counterparts of AGNs found at large redshifts. It has been proposed that
NLSy1s could be the small redshift counterparts of quasars found at
large redshifts (z > 4). The two have many similar properties, for example: high metallicities or similar pattern of emission lines (strong Fe [II], weak O [III]).
Some observations suggest that AGN emission from the nucleus is not
spherically symmetric and that the nucleus often shows axial symmetry,
with radiation escaping in a conical region. Based on these
observations, models have been devised to explain the different classes
of AGNs as due to their different orientations with respect to the
observational line of sight. Such models are called unified models.
Unified models explain the difference between Type I and Type II
galaxies as being the result of Type II galaxies being surrounded by
obscuring toruses which prevent telescopes from seeing the broad line
region. Quasars and blazars can be fit quite easily in this model.
The main problem of such a unification scheme is trying to explain why
some AGN are radio loud while others are radio quiet. It has been
suggested that these differences may be due to differences in the spin
of the central black hole.
In general relativity, the sticky bead argument is a simple thought experiment designed to show that gravitational radiation is indeed predicted by general relativity,
and can have physical effects. These claims were not widely accepted
prior to about 1955, but after the introduction of the bead argument, any remaining doubts soon disappeared from the research literature.
The argument is often credited to Hermann Bondi, who popularized it, but it was originally proposed by Richard Feynman.
Description
The thought experiment was first described by Feynman in 1957 at a conference at Chapel Hill, North Carolina, and later addressed in his private letter to Victor Weisskopf:
Feynman’s gravitational wave detector: It is simply two beads
sliding freely (but with a small amount of friction) on a rigid rod. As
the wave passes over the rod, atomic forces hold the length of the rod
fixed, but the proper distance between the two beads oscillates. Thus,
the beads rub against the rod, dissipating heat.
As the gravitational waves are mainly transverse, the rod has to be
oriented perpendicular to the propagation direction of the wave.
History of arguments on the properties of gravitational waves
Einstein's double reversal
The creator of the theory of general relativity, Albert Einstein, argued in 1916
that gravitational radiation should be produced, according to his
theory, by any mass-energy configuration that has a time-varying quadrupole moment (or higher multipole moment). Using a linearized field equation (appropriate for the study of weak gravitational fields), he derived the famous quadrupole formula quantifying the rate at which such radiation should carry away energy.
Examples of systems with time varying quadrupole moments include
vibrating strings, bars rotating about an axis perpendicular to the
symmetry axis of the bar, and binary star systems, but not rotating
disks.
In 1922, Arthur Stanley Eddington
wrote a paper expressing (apparently for the first time) the view that
gravitational waves are in essence ripples in coordinates, and have no
physical meaning. He did not appreciate Einstein's arguments that the
waves are real.
In 1936, together with Nathan Rosen, Einstein rediscovered the Beck vacuums, a family of exact gravitational wave solutions with cylindrical symmetry (sometimes also called Einstein–Rosen waves).
While investigating the motion of test particles in these solutions,
Einstein and Rosen became convinced that gravitational waves were
unstable to collapse. Einstein reversed himself and declared that
gravitational radiation was not after all a prediction of his theory. Einstein wrote to his friend Max Born
Together with a young collaborator, I arrived at the interesting
result that gravitational waves do not exist, though they had been
assumed a certainty to the first approximation. This shows that the
nonlinear field equations can show us more, or rather limit us more,
than we have believed up till now.
In other words, Einstein believed that he and Rosen had established
that their new argument showed that the prediction of gravitational
radiation was a mathematical artifact of the linear approximation
he had employed in 1916. Einstein believed these plane waves would
gravitationally collapse into points; he had long hoped something like
this would explain quantum mechanical wave-particle duality.
Einstein and Rosen accordingly submitted a paper entitled Do gravitational waves exist? to a leading physics journal, Physical Review,
in which they described their wave solutions and concluded that the
"radiation" that seemed to appear in general relativity was not genuine
radiation capable of transporting energy or having (in principle)
measurable physical effects. The anonymous referee, who—as the current editor of Physical Review recently confirmed, all parties now being deceased—was the combative cosmologist, Howard Percy Robertson,
pointed out the error described below, and the manuscript was returned
to the authors with a note from the editor asking them to revise the
paper to address these concerns. Quite uncharacteristically, Einstein
took this criticism very badly, angrily replying "I see no reason to
address the, in any case erroneous, opinion expressed by your referee."
He vowed never again to submit a paper to Physical Review. Instead, Einstein and Rosen resubmitted the paper without change to another and much less well known journal, The Journal of the Franklin Institute. He kept his vow regarding Physical Review.
Leopold Infeld, who arrived at Princeton University
at this time, later remembered his utter astonishment on hearing of
this development, since radiation is such an essential element for any classical field theory
worthy of the name. Infeld expressed his doubts to a leading expert on
general relativity: H. P. Robertson, who had just returned from a visit
to Caltech.
Going over the argument as Infeld remembered it, Robertson was able to
show Infeld the mistake: locally, the Einstein–Rosen waves are gravitational plane waves. Einstein and Rosen had correctly shown that a cloud of test particles would, in sinusoidal plane waves, form caustics, but changing to another chart (essentially the Brinkmann coordinates) shows that the formation of the caustic is not a contradiction at all,
but in fact just what one would expect in this situation. Infeld then
approached Einstein, who concurred with Robertson's analysis (still not
knowing it was he who reviewed the Physical Review submission).
Since Rosen had recently departed for the Soviet Union, Einstein
acted alone in promptly and thoroughly revising their joint paper. This
third version was retitled On gravitational waves, and, following
Robertson's suggestion of a transformation to cylindrical coordinates,
presented what are now called Einstein–Rosen cylindrical waves (these
are locally isometric to plane waves). This is the version that
eventually appeared. However, Rosen was unhappy with this revision and
eventually published his own version, which retained the erroneous
"disproof" of the prediction of gravitational radiation.
In a letter to the editor of Physical Review, Robertson wryly reported that in the end, Einstein had fully accepted the objections that had initially so upset him.
Bern and Chapel Hill conferences
In 1955, an important conference honoring the semi-centennial of special relativity was held in Bern, the Swiss capital city where Einstein was working in the famous patent office during the Annus mirabilis. Rosen attended and gave a talk in which he computed the Einstein pseudotensor and Landau–Lifshitz pseudotensor (two alternative, non-covariant, descriptions of the energy carried by a gravitational
field, a notion that is notoriously difficult to pin down in general
relativity). These turn out to be zero for the Einstein–Rosen waves, and
Rosen argued that this reaffirmed the negative conclusion he had
reached with Einstein in 1936.
However, by this time a few physicists, such as Felix Pirani and Ivor Robinson,
had come to appreciate the role played by curvature in producing tidal
accelerations, and were able to convince many peers that gravitational
radiation would indeed be produced, at least in cases such as a
vibrating spring where different pieces of the system were clearly not
in inertial motion. Nonetheless, some physicists continued to doubt whether radiation would be produced by a binary star system, where the world lines of the centers of mass of the two stars should, according to the EIH approximation (dating from 1938 and due to Einstein, Infeld, and Banesh Hoffmann), follow timelike geodesics.
Inspired by conversations by Felix Pirani,
Hermann Bondi took up the study of gravitational radiation, in
particular the question of quantifying the energy and momentum carried
off 'to infinity' by a radiating system. During the next few years,
Bondi developed the Bondi radiating chart and the notion of Bondi energy to rigorously study this question in maximal generality.
In 1957, at a conference at Chapel Hill, North Carolina, appealing to various mathematical tools developed by John Lighton Synge, A. Z. Petrov and André Lichnerowicz, Pirani explained more clearly than had previously been possible the central role played by the Riemann tensor and in particular the tidal tensor in general relativity.
He gave the first correct description of the relative (tidal)
acceleration of initially mutually static test particles that encounter a
sinusoidal gravitational plane wave.
Feynman's argument
Later in the Chapel Hill conference, Richard Feynman
used Pirani's description to point out that a passing gravitational
wave should in principle cause a bead on a stick (oriented transversely
to the direction of propagation of the wave) to slide back and forth,
thus heating the bead and the stick by friction.
This heating, said Feynman, showed that the wave did indeed impart
energy to the bead and stick system, so it must indeed transport energy,
contrary to the view expressed in 1955 by Rosen.
In two 1957 papers, Bondi and (separately) Joseph Weber and John Archibald Wheeler used this bead argument to present detailed refutations of Rosen's argument.
Rosen's final views
Nathan Rosen continued to argue as late as the 1970s, on the basis of a supposed paradox involving the radiation reaction,
that gravitational radiation is not in fact predicted by general
relativity. His arguments were generally regarded as invalid, but in any
case the sticky bead argument had by then long since convinced other
physicists of the reality of the prediction of gravitational radiation.
Supermassive black holes are classically defined as black holes with a mass above 100,000 (105) solar masses (M☉); some have masses of several billion M☉. Supermassive black holes have physical properties that clearly distinguish them from lower-mass classifications. First, the tidal forces in the vicinity of the event horizon
are significantly weaker for supermassive black holes. The tidal force
on a body at a black hole's event horizon is inversely proportional to
the square of the black hole's mass: a person at the event horizon of a 10 million M☉
black hole experiences about the same tidal force between their head
and feet as a person on the surface of the Earth. Unlike with stellar-mass black holes, one would not experience significant tidal force until very deep into the black hole's event horizon.
It is somewhat counterintuitive to note that the average density of a SMBH within its event horizon (defined as the mass of the black hole divided by the volume of space within its Schwarzschild radius) can be smaller than the density of water. This is because the Schwarzschild radius () is directly proportional
to its mass. 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 a lower average density.
The Schwarzschild radius of the event horizon of a nonrotating and uncharged supermassive black hole of around 1 billion M☉ is comparable to the semi-major axis of the orbit of planetUranus, which is about 19 AU.
Some astronomers refer to black holes of greater than 5 billion M☉ as ultramassive black holes (UMBHs or UBHs), but the term is not broadly used. Possible examples include the black holes at the cores of TON 618, NGC 6166, ESO 444-46 and NGC 4889, which are among the most massive black holes known.
Some studies have suggested that the maximum natural mass that a
black hole can reach, while being luminous accretors (featuring an accretion disk), is typically on the order of about 50 billion M☉.[However, a 2020 study suggested even larger black holes, dubbed stupendously large black holes (SLABs), with masses greater than 100 billion M☉, could exist based on used models; some estimates place the black hole at the core of Phoenix A in this category.
History of research
The story of how supermassive black holes were found began with the investigation by Maarten Schmidt of the radio source 3C 273 in 1963. Initially this was thought to be a star, but the spectrum proved puzzling. It was determined to be hydrogen emission lines that had been red shifted, indicating the object was moving away from the Earth. Hubble's law showed that the object was located several billion light-years
away, and thus must be emitting the energy equivalent of hundreds of
galaxies. The rate of light variations of the source dubbed a quasi-stellar object, or quasar, suggested the emitting region had a diameter of one parsec or less. Four such sources had been identified by 1964.
In 1963, Fred Hoyle and W. A. Fowler proposed the existence of hydrogen-burning supermassive stars (SMS) as an explanation for the compact dimensions and high energy output of quasars. These would have a mass of about 105–109M☉. However, Richard Feynman
noted stars above a certain critical mass are dynamically unstable and
would collapse into a black hole, at least if they were non-rotating.
Fowler then proposed that these supermassive stars would undergo a
series of collapse and explosion oscillations, thereby explaining the
energy output pattern. Appenzeller and Fricke (1972) built models of
this behavior, but found that the resulting star would still undergo
collapse, concluding that a non-rotating 0.75×106M☉ SMS "cannot escape collapse to a black hole by burning its hydrogen through the CNO cycle".
Edwin E. Salpeter and Yakov Zeldovich
made the proposal in 1964 that matter falling onto a massive compact
object would explain the properties of quasars. It would require a mass
of around 108M☉ to match the output of these objects. Donald Lynden-Bell noted in 1969 that the infalling gas would form a flat disk that spirals into the central "Schwarzschild throat". He noted that the relatively low output of nearby galactic cores implied these were old, inactive quasars. Meanwhile, in 1967, Martin Ryle and Malcolm Longair
suggested that nearly all sources of extra-galactic radio emission
could be explained by a model in which particles are ejected from
galaxies at relativistic velocities, meaning they are moving near the speed of light. Martin Ryle, Malcolm Longair, and Peter Scheuer then proposed in 1973 that the compact central nucleus could be the original energy source for these relativistic jets.
Arthur M. Wolfe and Geoffrey Burbidge noted in 1970 that the large velocity dispersion of the stars in the nuclear region of elliptical galaxies
could only be explained by a large mass concentration at the nucleus;
larger than could be explained by ordinary stars. They showed that the
behavior could be explained by a massive black hole with up to 1010M☉, or a large number of smaller black holes with masses below 103M☉. Dynamical evidence for a massive dark object was found at the core of the active elliptical galaxy Messier 87 in 1978, initially estimated at 5×109M☉. Discovery of similar behavior in other galaxies soon followed, including the Andromeda Galaxy in 1984 and the Sombrero Galaxy in 1988.
Donald Lynden-Bell and Martin Rees hypothesized in 1971 that the center of the Milky Way galaxy would contain a massive black hole. Sagittarius A* was discovered and named on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the Green Bank Interferometer of the National Radio Astronomy Observatory. 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.
The Hubble Space Telescope, launched in 1990, provided the resolution needed to perform more refined observations of galactic nuclei. In 1994 the Faint Object Spectrograph
on the Hubble was used to observe Messier 87, finding that ionized gas
was orbiting the central part of the nucleus at a velocity of ±500 km/s.
The data indicated a concentrated mass of (2.4±0.7)×109M☉ lay within a 0.25″ span, providing strong evidence of a supermassive black hole.
Using the Very Long Baseline Array to observe Messier 106, Miyoshi et al. (1995) were able to demonstrate that the emission from an H2O maser in this galaxy came from a gaseous disk in the nucleus that orbited a concentrated mass of 3.6×107M☉,
which was constrained to a radius of 0.13 parsecs. Their
ground-breaking research noted that a swarm of solar mass black holes
within a radius this small would not survive for long without undergoing
collisions, making a supermassive black hole the sole viable candidate. Accompanying this observation which provided the first confirmation of supermassive black holes was the discovery
of the highly broadened, ionised iron Kα emission line (6.4 keV) from
the galaxy MCG-6-30-15. The broadening was due to the gravitational
redshift of the light as it escaped from just 3 to 10 Schwarzschild
radii from the black hole.
On April 10, 2019, the Event Horizon Telescope collaboration released the first horizon-scale image of a black hole, in the center of the galaxy Messier 87. In March 2020, astronomers suggested that additional subrings should form the photon ring, proposing a way of better detecting these signatures in the first black hole image.
The origin of supermassive black holes remains an active field of research. Astrophysicists agree that black holes can grow by accretion of matter and by merging with other black holes.
There are several hypotheses for the formation mechanisms and initial
masses of the progenitors, or "seeds", of supermassive black holes.
Independently of the specific formation channel for the black hole seed,
given sufficient mass nearby, it could accrete to become an intermediate-mass black hole and possibly a SMBH if the accretion rate persists.
Distant and early supermassive black holes, such as J0313–1806, and ULAS J1342+0928,
are hard to explain so soon after the Big Bang. Some postulate they
might come from direct collapse of dark matter with self-interaction. A small minority of sources argue that they may be evidence that the Universe is the result of a Big Bounce, instead of a Big Bang, with these supermassive black holes being formed before the Big Bounce.
The early progenitor seeds may be black holes of tens or perhaps hundreds of M☉
that are left behind by the explosions of massive stars and grow by
accretion of matter. Another model 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.
Before the first stars, large gas clouds could collapse into a "quasi-star", which would in turn collapse into a black hole of around 20 M☉. These stars may have also been formed by dark matter halos drawing in enormous amounts of gas by gravity, which would then produce supermassive stars with tens of thousands of M☉.
The "quasi-star" becomes unstable to radial perturbations because of
electron-positron pair production in its core and could collapse
directly into a black hole without a supernova explosion (which would eject most of its mass, preventing the black hole from growing as fast).
A more recent theory proposes that SMBH seeds were formed in the very early universe each from the collapse of a supermassive star with mass of around 100,000 M☉.
Direct-collapse and primordial black holes
Large, high-redshift clouds of metal-free gas, when irradiated by a sufficiently intense flux of Lyman–Werner photons, can avoid cooling and fragmenting, thus collapsing as a single object due to self-gravitation. The core of the collapsing object reaches extremely large values of matter density, of the order of about 107 g/cm3, and triggers a general relativistic instability.
Thus, the object collapses directly into a black hole, without passing
from the intermediate phase of a star, or of a quasi-star. These objects
have a typical mass of about 100,000 M☉ and are named direct collapse black holes.
A 2022 computer simulation showed that the first supermassive
black holes can arise in rare turbulent clumps of gas, called primordial
halos, that were fed by unusually strong streams of cold gas. The key
simulation result was that cold flows suppressed star formation in the
turbulent halo until the halo’s gravity was finally able to overcome the
turbulence and formed two direct-collapse black holes of 31,000 M☉ and 40,000 M☉.
The birth of the first SMBHs can therefore be a result of standard
cosmological structure formation — contrary to what had been thought for
almost two decades.
Artist's impression of the huge outflow ejected from the quasar SDSS J1106+1939
Artist's illustration of galaxy with jets from a supermassive black hole
Primordial black holes
(PBHs) could have been produced directly from external pressure in the
first moments after the Big Bang. These black holes would then have more
time than any of the above models to accrete, allowing them sufficient
time to reach supermassive sizes. 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.
The formation of a supermassive black hole requires a relatively small volume of highly dense matter having small angular momentum.
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 both the most efficient and 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 M☉
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.
An artist's impression of stars born in winds from supermassive black holes.
Maximum mass limit
There is a natural upper limit to how large supermassive black holes can grow. Supermassive black holes in any quasar or active galactic nucleus (AGN) appear to have a theoretical upper limit of physically around 50 billion M☉ for typical parameters, as anything above this slows growth down to a crawl (the slowdown tends to start around 10 billion M☉) and causes the unstable accretion disk surrounding the black hole to coalesce into stars that orbit it. A study concluded that the radius of the innermost stable circular orbit (ISCO) for SMBH masses above this limit exceeds the self-gravity radius, making disc formation no longer possible.
A larger upper limit of around 270 billion M☉
was represented as the absolute maximum mass limit for an accreting
SMBH in extreme cases, for example its maximal prograde spin with a
dimensionless spin parameter of a = 1, although the maximum limit for a black hole's spin parameter is very slightly lower at a = 0.9982. At masses just below the limit, the disc luminosity of a field galaxy is likely to be below the Eddington limit and not strong enough to trigger the feedback underlying the M–sigma relation, so SMBHs close to the limit can evolve above this.
It was noted that, black holes close to this limit are likely to
be rather even rarer, as it would require the accretion disc to be
almost permanently prograde because the black hole grows and the
spin-down effect of retrograde accretion is larger than the spin-up by
prograde accretion, due to its ISCO and therefore its lever arm.
This would require the hole spin to be permanently correlated with a
fixed direction of the potential controlling gas flow, within the black
hole's host galaxy, and thus would tend to produce a spin axis and hence
AGN jet direction, which is similarly aligned with the galaxy. Current
observations do not support this correlation.
The so-called 'chaotic accretion' presumably has to involve
multiple small-scale events, essentially random in time and orientation
if it is not controlled by a large-scale potential in this way.
This would lead the accretion statistically to spin-down, due to
retrograde events having larger lever arms than prograde, and occurring
almost as often.
There is also other interactions with large SMBHs that trend to reduce
their spin, including particularly mergers with other black holes, which
can statistically decrease the spin.
All of these considerations suggested that SMBHs usually cross the
critical theoretical mass limit at modest values of their spin
parameters, so that 5×1010M☉ in all but rare cases.
Although modern UMBHs within quasars and galactic nuclei cannot grow beyond around (5–27)×1010M☉ through the accretion disk and as well given the current age of the universe,
some of these monster black holes in the universe are predicted to
still continue to grow up to stupendously large masses of perhaps 1014M☉ during the collapse of superclusters of galaxies in the extremely far future of the universe.
Gravitation from supermassive black holes in the center of many galaxies is thought to power active objects such as Seyfert galaxies and quasars, and the relationship between the mass of the central black hole and the mass of the host galaxy depends upon the galaxy type. An empirical correlation between the size of supermassive black holes and the stellar velocity dispersion of a galaxy bulge is called the M–sigma relation.
An AGN is now considered to be a galactic core hosting a massive
black hole that is accreting matter and displays a sufficiently strong
luminosity. The nuclear region of the Milky Way, for example, lacks
sufficient luminosity to satisfy this condition. The unified model of
AGN is the concept that the large range of observed properties of the
AGN taxonomy can be explained using just a small number of physical
parameters. For the initial model, these values consisted of the angle
of the accretion disk's torus to the line of sight and the luminosity of
the source. AGN can be divided into two main groups: a radiative mode
AGN in which most of the output is in the form of electromagnetic
radiation through an optically thick accretion disk, and a jet mode in
which relativistic jets emerge perpendicular to the disk.
Mergers and recoiled SMBHs
The interaction of a pair of SMBH-hosting galaxies can lead to merger events. Dynamic friction
on the hosted SMBH objects causes them to sink toward the center of the
merged mass, eventually forming a pair with a separation of under a
kiloparsec. The interaction of this pair with surrounding stars and gas
will then gradually bring the SMBH together as a gravitationally bound binary system with a separation of ten parsecs or less. Once the pair draw as close as 0.001 parsecs, gravitational radiation will cause them to merge. By the time this happens, the resulting galaxy will have long since relaxed from the merger event, with the initial starburst activity and AGN having faded away.
Candidate SMBHs suspected to be recoiled or ejected black holes
The gravitational waves
from this coalescence can give the resulting SMBH a velocity boost of
up to several thousand km/s, propelling it away from the galactic center
and possibly even ejecting it from the galaxy. This phenomenon is
called a gravitational recoil.
The other possible way to eject a black hole is the classical slingshot
scenario, also called slingshot recoil. In this scenario first a
long-lived binary black hole forms through a merger of two galaxies. A
third SMBH is introduced in a second merger and sinks into the center of
the galaxy. Due to the three-body interaction
one of the SMBHs, usually the lightest, is ejected. Due to conservation
of linear momentum the other two SMBHs are propelled in the opposite
direction as a binary. All SMBHs can be ejected in this scenario. An ejected black hole is called a runaway black hole.
There are different ways to detect recoiling black holes. Often a displacement of a quasar/AGN from the center of a galaxy or a spectroscopic binary nature of a quasar/AGN is seen as evidence for a recoiled black hole.
Candidate recoiling black holes include NGC 3718, SDSS1133, 3C 186, E1821+643 and SDSSJ0927+2943. Candidate runaway black holes are HE0450–2958, CID-42 and objects around RCP 28. Runaway super massive black holes may trigger star formation in their wakes. A linear feature near the dwarf galaxy RCP 28 was interpreted as the star-forming wake of a candidate runaway black hole.
Hawking radiation is black-body radiation that is predicted to be released by black holes,
due to quantum effects near the event horizon. This radiation reduces
the mass and energy of black holes, causing them to shrink and
ultimately vanish. If black holes evaporate via Hawking radiation, a non-rotating and uncharged stupendously large black hole with a mass of 1×1011M☉ will evaporate in around 2.1×10100years. Black holes formed during the predicted collapse of superclusters of galaxies in the far future with 1×1014M☉ would evaporate over a timescale of up to 2.1×10109 years.
Evidence
Doppler measurements
Simulation of a side view of a black hole with transparent toroidal ring of ionized matter according to a proposed model for Sgr A*. This image shows the result of bending of light from behind the black hole, and it also shows the asymmetry arising by the Doppler effect from the extremely high orbital speed of the matter in the ring.
Some of the best evidence for the presence of black holes is provided by the Doppler effect
whereby light from nearby orbiting matter is red-shifted when receding
and blue-shifted when advancing. For matter very close to a black hole
the orbital speed must be comparable with the speed of light, so
receding matter will appear very faint compared with advancing matter,
which means that systems with intrinsically symmetric discs and rings
will acquire a highly asymmetric visual appearance. This effect has been
allowed for in modern computer-generated images such as the example
presented here, based on a plausible model for the supermassive black hole in Sgr A*
at the center of the Milky Way. However, the resolution provided by
presently available telescope technology is still insufficient to
confirm such predictions directly.
What already has been observed directly in many systems are the
lower non-relativistic velocities of matter orbiting further out from
what are presumed to be black holes. 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.
In the Milky Way
Inferred orbits of six stars around supermassive black hole candidate Sagittarius A* at the Milky Way Galactic Center
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.
From the motion of star S2, the object's mass can be estimated as 4.0 million M☉, or about 7.96×1036 kg.
The radius of the central object must be less than 17 light-hours,
because otherwise S2 would collide with it. Observations of the star S14 indicate that the radius is no more than 6.25 light-hours, about the diameter of Uranus' orbit.
No known astronomical object other than a black hole can contain 4.0 million M☉ in this volume of space.
Infrared observations of bright flare activity near Sagittarius A* show orbital motion of plasma with a period of 45±15 min
at a separation of six to ten times the gravitational radius of the
candidate SMBH. This emission is consistent with a circularized orbit of
a polarized "hot spot" on an accretion disk in a strong magnetic field.
The radiating matter is orbiting at 30% of the speed of light just
outside the innermost stable circular orbit.
On January 5, 2015, NASA reported observing an X-ray
flare 400 times brighter than usual, a record-breaker, from Sagittarius
A*. The unusual event may have been caused by the breaking apart of an asteroid falling into the black hole or by the entanglement of magnetic field lines within gas flowing into Sagittarius A*, according to astronomers.
Detection of an unusually bright X-ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy
Artist's
impression of a supermassive black hole tearing apart a star. Below:
supermassive black hole devouring a star in galaxy RX J1242−11 – X-ray (left) and optical (right).
Unambiguous dynamical evidence for supermassive black holes exists only for a handful of galaxies; these include the Milky Way, the Local Group galaxies M31 and M32, and a few galaxies beyond the Local Group, such as NGC 4395. In these galaxies, the root mean square (or rms) velocities of the stars or gas rises proportionally to 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.
Nevertheless, it is commonly accepted that the center of nearly every galaxy contains a supermassive black hole. The reason for this assumption is the M–sigma relation,
a tight (low scatter) relation between the mass of the hole in the 10
or so galaxies with secure detections, and the velocity dispersion of
the stars in the bulges of those galaxies.
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.
On March 28, 2011, a supermassive black hole was seen tearing a mid-size star apart.
That is the only likely explanation of the observations that day of
sudden X-ray radiation and the follow-up broad-band observations.
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 M☉. 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.
Individual studies
Hubble Space Telescope photograph of the 4,400 light-year-long relativistic jet of Messier 87, which is matter being ejected by the 6.5×109M☉ supermassive black hole at the center of the galaxy
The nearby Andromeda Galaxy, 2.5 million light-years away, contains a 1.4+0.65 −0.45×108 (140 million) M☉ central black hole, significantly larger than the Milky Way's. The largest supermassive black hole in the Milky Way's vicinity appears to be that of Messier 87 (i.e., M87*), at a mass of (6.5±0.7)×109 (c. 6.5 billion) M☉ at a distance of 48.92 million light-years. The supergiant elliptical galaxy NGC 4889, at a distance of 336 million light-years away in the Coma Berenices constellation, contains a black hole measured to be 2.1+3.5 −1.3×1010 (21 billion) M☉.
Masses of black holes in quasars can be estimated via indirect methods that are subject to substantial uncertainty. The quasar TON 618 is an example of an object with an extremely large black hole, estimated at 4.07×1010 (40.7 billion) M☉. Its redshift is 2.219. Other examples of quasars with large estimated black hole masses are the hyperluminous quasar APM 08279+5255, with an estimated mass of 1×1010 (10 billion) M☉, and the quasar SMSS J215728.21-360215.1, with a mass of (3.4±0.6)×1010 (34 billion) M☉, or nearly 10,000 times the mass of the black hole at the Milky Way's Galactic Center.
Some galaxies, such as the galaxy 4C +37.11, appear to have two supermassive black holes at their centers, forming a binary system. If they collided, the event would create strong gravitational waves. Binary supermassive black holes are believed to be a common consequence of galactic mergers. The binary pair in OJ 287, 3.5 billion light-years away, contains the most massive black hole in a pair, with a mass estimated at 18.348 billion M☉. In 2011, a super-massive black hole was 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.
In 2012, astronomers reported an unusually large mass of approximately 17 billion M☉ for the black hole in the compact, lenticular galaxyNGC 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).
Another study reached a very different conclusion: this black hole is
not particularly overmassive, estimated at between 2 and 5 billion M☉ with 5 billion M☉ being the most likely value. On February 28, 2013, astronomers reported on the use of the NuSTAR satellite to accurately measure the spin of a supermassive black hole for the first time, in NGC 1365, reporting that the event horizon was spinning at almost the speed of light.
In September 2014, data from different X-ray telescopes have shown that the extremely small, dense, ultracompact dwarf galaxyM60-UCD1
hosts a 20 million solar mass black hole at its center, accounting for
more than 10% of the total mass of the galaxy. The discovery is quite
surprising, since the black hole is five times more massive than the
Milky Way's black hole despite the galaxy being less than
five-thousandths the mass of the Milky Way.
Some galaxies lack any supermassive black holes in their centers.
Although most galaxies with no supermassive black holes are very small,
dwarf galaxies, one discovery remains mysterious: The supergiant
elliptical cD galaxy A2261-BCG has not been found to contain an active supermassive black hole of at least 1010M☉,
despite the galaxy being one of the largest galaxies known; over six
times the size and one thousand times the mass of the Milky Way. Despite
that, several studies gave very large mass values for a possible
central black hole inside A2261-BGC, such as about as large as 6.5+10.9 −4.1×1010M☉ or as low as (6–11)×109M☉.
Since a supermassive black hole will only be visible while it is
accreting, a supermassive black hole can be nearly invisible, except in
its effects on stellar orbits. This implies that either A2261-BGC has a
central black hole that is accreting at a low level or has a mass rather
below 1010M☉.
In December 2017, astronomers reported the detection of the most distant quasar known by this time, ULAS J1342+0928,
containing the most distant supermassive black hole, at a reported
redshift of z = 7.54, surpassing the redshift of 7 for the previously
known most distant quasar ULAS J1120+0641.
Supermassive black hole and smaller black hole in galaxy OJ 287
Comparisons of large and small black holes in galaxy OJ 287 to the Solar System
In February 2020, astronomers reported the discovery of the Ophiuchus Supercluster eruption, the most energetic event in the Universe ever detected since the Big Bang. It occurred in the Ophiuchus Cluster in the galaxy NeVe 1, caused by the accretion of nearly 270 million M☉
of material by its central supermassive black hole. The eruption lasted
for about 100 million years and released 5.7 million times more energy
than the most powerful gamma-ray burst known. The eruption released shock waves and jets of high-energy particles that punched the intracluster medium, creating a cavity about 1.5 million light-years wide – ten times the Milky Way's diameter.
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