Supermassive black hole

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The first direct image of a supermassive black hole, found in the galactic core of Messier 87. This view is somewhat from above, looking down on one of its galactic jets. Rather than an accretion disc, it shows synchrotron radiation in the microwave range (1.3 mm). This light was emitted by electrons captured in the plasma vortex at the base of a jet. Radiation of this wavelength does not reveal the thermal features thought to dominate the emissions of an accretion disc. The synchrotron radiation is shown after its interaction with the black hole's photon sphere, which generates the ring. The dark central feature indicates the region where no path exists between the event horizon and Earth. The edge of the photon sphere shows an asymmetry in brightness because of Doppler beaming. The image was released in 2019 by the Event Horizon Telescope Collaboration.

A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (M_☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, including light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way galaxy has a supermassive black hole at its center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei (AGNs) and quasars.

Two supermassive black holes have been directly imaged by the Event Horizon Telescope: the black hole in the giant elliptical galaxy Messier 87 and the black hole at the Milky Way’s center (Sagittarius A*).

Description

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 (${\displaystyle r_{\text{s}}}$) 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 planet Uranus, 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 10⁵–10⁹ M_☉. 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 0.75×10⁶ M_☉ 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 10⁸ M_☉ 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 10¹⁰ M_☉, or a large number of smaller black holes with masses below 10³ M_☉. Dynamical evidence for a massive dark object was found at the core of the active elliptical galaxy Messier 87 in 1978, initially estimated at 5×10⁹ M_☉. 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)×10⁹ M_☉ 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 3.6×10⁷ M_☉, 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.

Formation

An artist's conception of a supermassive black hole surrounded by an accretion disk and emitting a relativistic jet.

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.

First stars

Main articles: Population III star, Quasi-star, and Dark star (dark matter)

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 5×10¹⁰ M_☉ in all but rare cases.

Although modern UMBHs within quasars and galactic nuclei cannot grow beyond around (5–27)×10¹⁰ M_☉ 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 10¹⁴ M_☉ during the collapse of superclusters of galaxies in the extremely far future of the universe.

Activity and galactic evolution

Main articles: Active galactic nucleus and Galaxy formation and evolution

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 ${\displaystyle \sigma }$ 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

Main article: Hawking radiation

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 1×10¹¹ M_☉ will evaporate in around 2.1×10¹⁰⁰ years. Black holes formed during the predicted collapse of superclusters of galaxies in the far future with 1×10¹⁴ M_☉ 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

Evidence indicates that the Milky Way galaxy has a supermassive black hole at its center, 26,000 light-years from the Solar System, in a region called Sagittarius A* because:

• The star S2 follows an elliptical orbit with a period of 15.2 years and a pericenter (closest distance) of 17 light-hours (1.8×10¹³ m or 120 AU) from the center of the central object.
• 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

 

Sagittarius A* imaged by the Event Horizon Telescope

Outside the Milky Way

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 6.5×10⁹ M_☉ 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 galaxy NGC 1277, which lies 220 million light-years away in the constellation Perseus. The putative black hole has approximately 59 percent of the mass of the bulge of this lenticular galaxy (14 percent of the total stellar mass of the galaxy). 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 galaxy M60-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 10¹⁰ M_☉, 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
−4.1×10¹⁰ M_☉ or as low as (6–11)×10⁹ M_☉. 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 10¹⁰ M_☉.

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

The supermassive black hole of NeVe 1 is responsible for the Ophiuchus Supercluster eruption – the most energetic eruption ever detected.
From: Chandra X-ray Observatory

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.

In February 2021, astronomers released, for the first time, a very high-resolution image of 25,000 active supermassive black holes, covering four percent of the Northern celestial hemisphere, based on ultra-low radio wavelengths, as detected by the Low-Frequency Array (LOFAR) in Europe.

Dwarf galaxy

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Dwarf_galaxy

The Large Magellanic Cloud, a satellite galaxy of the Milky Way

A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

Formation

Dwarf galaxies like NGC 5264 typically possess around a billion stars.

One theory states that most galaxies, including dwarf galaxies, form in association with dark matter, or from gas that contains metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases with low metallicity. These galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo.

Because of their small size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies, resulting in stellar streams and eventually galaxy merger.

Local dwarf galaxies

The Phoenix Dwarf Galaxy is a dwarf irregular galaxy, featuring younger stars in its inner regions and older ones at its outskirts.

There are many dwarf galaxies in the Local Group; these small galaxies frequently orbit larger galaxies, such as the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy. A 2007 paper has suggested that many dwarf galaxies were created by galactic tides during the early evolutions of the Milky Way and Andromeda. Tidal dwarf galaxies are produced when galaxies collide and their gravitational masses interact. Streams of galactic material are pulled away from the parent galaxies and the halos of dark matter that surround them. A 2018 study suggests that some local dwarf galaxies formed extremely early, during the Dark Ages within the first billion years after the Big Bang.

More than 20 known dwarf galaxies orbit the Milky Way, and recent observations have also led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with a black hole at its centre, which was at some time absorbed by the Milky Way.

Common types

UGC 11411 is a galaxy known as an irregular blue compact dwarf (BCD) galaxy.

• Elliptical galaxy: dwarf elliptical galaxy (dE)
• Dwarf spheroidal galaxy (dSph): Once a subtype of dwarf ellipticals, now regarded as a distinct type 
• Irregular galaxy: dwarf irregular galaxy (dIrr)
• Spiral galaxy: dwarf spiral galaxy (dS) 
• Magellanic type dwarfs
• Blue compact dwarf galaxies (see section below)
• Ultra-compact dwarf galaxies (see section below)

Blue compact dwarf galaxies

Blue compact dwarf PGC 51017.

In astronomy, a blue compact dwarf galaxy (BCD galaxy) is a small galaxy which contains large clusters of young, hot, massive stars. These stars, the brightest of which are blue, cause the galaxy itself to appear blue in colour. Most BCD galaxies are also classified as dwarf irregular galaxies or as dwarf lenticular galaxies. Because they are composed of star clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become very violent when forming.

BCD galaxies cool in the process of forming new stars. The galaxies' stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies.

Nearby examples include NGC 1705, NGC 2915, NGC 3353 and UGCA 281.

Ultra-faint dwarf galaxies

Ultra-faint dwarf galaxies (UFDs) are a class of galaxies that contain from a few hundred to one hundred thousand stars, making them the faintest galaxies in the Universe. UFDs resemble globular clusters (GCs) in appearance but have very different properties. Unlike GCs, UFDs contain a significant amount of dark matter and are more extended. UFDs were first discovered with the advent of digital sky surveys in 2005, in particular with the Sloan Digital Sky Survey (SDSS).

UFDs are the most dark matter-dominated systems known. Astronomers believe that UFDs encode valuable information about the early Universe, as all UFDs discovered so far are ancient systems that have likely formed very early on, only a few million years after the Big Bang and before the epoch of reionization. Recent theoretical work has hypothesised the existence of a population of young UFDs that form at a much later time than the ancient UFDs. These galaxies have not been observed in our Universe so far.

Ultra-compact dwarfs

Ultra-compact dwarf galaxies (UCD) are a class of very compact galaxies with very high stellar densities, discovered in the 2000s. They are thought to be on the order of 200 light years across, containing about 100 million stars. It is theorised that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars by tidal interactions, travelling through the hearts of rich clusters. UCDs have been found in the Virgo Cluster, Fornax Cluster, Abell 1689, and the Coma Cluster, amongst others. In particular, an unprecedentedly large sample of ~ 100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team. The first ever relatively robust studies of the global properties of Virgo UCDs suggest that UCDs have distinct dynamical and structural properties from normal globular clusters. An extreme example of UCD is M60-UCD1, about 54 million light years away, which contains approximately 200 million solar masses within a 160 light year radius; the stars in its central region are packed 25 times more densely than stars in Earth's region in the Milky Way. M59-UCD3 is approximately the same size as M60-UCD1 with a half-light radius, r_h, of approximately 20 parsecs but is 40% more luminous with an absolute visual magnitude of approximately −14.6. This makes M59-UCD3 the densest known galaxy. Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to have supermassive black holes weighing 13% and 18% of the galaxies' masses.

Omega Centauri

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Omega_Centauri

Omega Centauri
The globular cluster Omega Centauri
Observation data (J2000 epoch)
Class	VIII
Constellation	Centaurus
Right ascension	13h 26m 47.28s
Declination	−47° 28′ 46.1″
Distance	15.8 ± 1.1 kly (4.84 ± 0.34 kpc)
Apparent magnitude (V)	3.9
Apparent dimensions (V)	36′.3
Physical characteristics
Mass	(4.05±0.1)×106 M☉
Radius	86 ± 6 ly
Metallicity	${\displaystyle \begin{array}{c}\left[\text{Fe}/\text{H}\right]\end{array}}$ = –1.35 dex
Estimated age	11.52 Gyr
Other designations	NGC 5139, GCl 24, ω Centauri, Caldwell 80, Mel 118
See also: Globular cluster, List of globular clusters

Omega Centauri (ω Cen, NGC 5139, or Caldwell 80) is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of 17,090 light-years (5,240 parsecs), it is the largest-known globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, with a total mass of 4 million solar masses, making it the most massive known globular cluster in the Milky Way.

Omega Centauri is very different from most other galactic globular clusters to the extent that it is thought to have originated as the core remnant of a disrupted dwarf galaxy.

Observation history

Around 150 AD, Greco-Roman writer and astronomer Ptolemy catalogued this object in his Almagest as a star on the centaur's back, "Quae est in principio scapulae". German cartographer Johann Bayer used Ptolemy's data to designate this object "Omega Centauri" with his 1603 publication of Uranometria. Using a telescope from the South Atlantic island of Saint Helena, English astronomer Edmond Halley rediscovered this object in 1677, listing it as a non-stellar object. In 1716, it was published by Halley among his list of six "luminous spots or patches" in the Philosophical Transactions of the Royal Society.

Swiss astronomer Jean-Philippe de Cheseaux included Omega Centauri in his 1746 list of 21 nebulae, as did French astronomer Lacaille in 1755, whence the catalogue number is designated L I.5. It was first recognized as a globular cluster by Scottish astronomer James Dunlop in 1826, who described it as a "beautiful globe of stars very gradually and moderately compressed to the centre".

Properties

At a distance of about 15,800 light-years (4,800 parsecs) from Earth, Omega Centauri is one of the few globular clusters visible to the naked eye—and appears almost as large as the full Moon when seen from a dark, rural area. It is the brightest, largest and, at 4 million solar masses, the most massive-known globular cluster associated with the Milky Way. Of all the globular clusters in the Local Group of galaxies, only Mayall II in the Andromeda Galaxy is brighter and more massive. Orbiting through the Milky Way, Omega Centauri contains several million Population II stars and is about 12 billion years old.

The stars in the core of Omega Centauri are so crowded that they are estimated to average only 0.1 light-year away from each other. The internal dynamics have been analyzed using measurements of the radial velocities of 469 stars. The members of this cluster are orbiting the center of mass with a peak velocity dispersion of 7.9 km s⁻¹. The mass distribution inferred from the kinematics is slightly more extended than, though not strongly inconsistent with, the luminosity distribution.

Evidence of a central black hole

The central region of Omega Centauri. The lower illustration charts the future positions of the stars highlighted by the white box in the top image. Each streak represents the star's predicted motion over the next 600 years. The period between dots corresponds to 30 years. October 2010

A 2008 study presented evidence for an intermediate-mass black hole at the center of Omega Centauri, based on observations made by the Hubble Space Telescope and Gemini Observatory on Cerro Pachón in Chile. Hubble's Advanced Camera for Surveys showed that stars are bunching up near the center of Omega Centauri, as evidenced by the gradual increase in starlight near the center. Using instruments at the Gemini Observatory to measure the speed of stars swirling in the cluster's core, E. Noyola and colleagues found that stars closer to the core are moving faster than stars farther away. This measurement was interpreted to mean that unseen matter at the core is interacting gravitationally with nearby stars. By comparing these results with standard models, the astronomers concluded that the most likely cause was the gravitational pull of a dense, massive object such as a black hole. They calculated the object's mass at 40,000 solar masses.

More recent work has challenged conclusions that there is a black hole in the cluster's core, in particular disputing the proposed location of the cluster center. Calculations using a revised location for the center found that the velocity of core stars does not vary with distance, as would be expected if an intermediate-mass black hole were present. The same studies also found that starlight does not increase toward the center but instead remains relatively constant. The authors noted that their results do not entirely rule out the black hole proposed by Noyola and colleagues, but they do not confirm it, and they limit its maximum mass to 12,000 solar masses.

Disrupted dwarf galaxy

It has been speculated that Omega Centauri is the core of a dwarf galaxy that was disrupted and absorbed by the Milky Way. Indeed, Kapteyn's Star, which is currently only 13 light-years away from Earth, is thought to originate from Omega Centauri. Omega Centauri's chemistry and motion in the Milky Way are also consistent with this picture. Like Mayall II, Omega Centauri has a range of metallicities and stellar ages that suggests that it did not all form at once (as globular clusters are thought to form) and may in fact be the remainder of the core of a smaller galaxy long since incorporated into the Milky Way.

In fiction

The novel Singularity (2012), by Ian Douglas, presents as fact that Omega Centauri and Kapteyn's Star originate from a disrupted dwarf galaxy, and this origin is central to the novel's plot. A number of scientific aspects of Omega Centauri are discussed as the story progresses, including the likely radiation environment inside the cluster and what the sky might look like from inside the cluster.

Active region

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Active_region 

In solar physics and observation, an active region is a temporary feature in the Sun's atmosphere characterized by a strong and complex magnetic field. They are often associated with sunspots and are commonly the source of violent eruptions such as coronal mass ejections and solar flares. The number and location of active regions on the solar disk at any given time is dependent on the solar cycle.

Region numbers

Newly observed active regions on the solar disk are assigned 4-digit region numbers by the Space Weather Prediction Center (SWPC) on the day following the initial observation. The region number assigned to a particular active region is one added to the previously assigned number. For example, the first observation of active region 8090, or AR8090, was followed by AR8091.

According to the SWPC, a number is assigned to a region if it meets at least one of the following criteria:

1. It contains a sunspot group of class C or larger based on the Modified Zurich Class sunspot classification system.
2. It contains a sunspot group of class A or B confirmed by at least two observers, preferably with observations more than one hour apart.
3. It has produced a solar flare with an X-ray burst.
4. It contains plage with a white-light brightness of at least 2.5 (on a linear scale 1-5, 5=flare) and has an extent of at least five heliographic degrees.
5. It contains plage that is bright near the west limb and is suspected of growing.

The region numbers reached 10,000 in July 2002. However, the SWPC continued using 4-digits, with the inclusion of leading zeros.

Magnetic field

A highly simplified diagram of the magnetic field of an active region illustrating its bipolar nature.

Mount Wilson magnetic classification

The Mount Wilson magnetic classification system, also known as the Hale magnetic classification system, is a method of classifying the magnetic field of active regions. It was first introduced in 1919 by George Ellery Hale and coworkers working at the Mount Wilson Observatory. It originally included only the α, β, and γ magnetic classifications, but it was later modified by H. Künzel in 1965 to include the δ qualifier.

Classification	Description
α	An active region containing a single sunspot or group of sunspots all having the same magnetic polarity. An opposite polarity counterpart is still present, but is weak or not concentrated enough to form sunspots.
β	An active region with at least two sunspots or sunspot groups that have opposite magnetic polarity. A simple neutral line between the two polarities is also present.
γ	An active region with sunspots having completely intermixed magnetic polarity.
β-γ	An active region with at least two sunspots or sunspot groups that have opposite magnetic polarity (hence β) but no well-defined neutral line dividing the opposite polarities (hence γ).
δ	A qualifier to the other classes indicating the presence of opposite polarity umbrae within a single penumbra separated by at most 2° in heliographic distance.
β-δ	An active region with a β magnetic field and at least one pair of opposite polarity umbrae within a single penumbra (hence δ).
β-γ-δ	An active region with a β-γ magnetic field and at least one pair of opposite polarity umbrae within a single penumbra (hence δ).
γ-δ	An active region with a γ magnetic field and at least one pair of opposite polarity umbrae within a single penumbra (hence δ).

Sunspots

Main article: Sunspot

An active region seen in visible light showing a group of sunspots.

The strong magnetic flux found in active regions is often strong enough to inhibit convection. Without convection transporting energy from the Sun's interior to the photosphere, surface temperature decreases along with the intensity of emitted black body radiation. These areas of cooler plasma are known as sunspots, and often appear in groups. However, not all active regions possess sunspots.

Magnetic flux emergence

Active regions form through the process of magnetic flux emergence, during which magnetic fields generated by the solar dynamo emerge from the solar interior.

Galactic halo

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Galactic_halo

A galactic halo is an extended, roughly spherical component of a galaxy which extends beyond the main, visible component. Several distinct components of a galaxy comprise its halo:

• the stellar halo
• the galactic corona (hot gas, i.e. a plasma)
• the dark matter halo

The distinction between the halo and the main body of the galaxy is clearest in spiral galaxies, where the spherical shape of the halo contrasts with the flat disc. In an elliptical galaxy, there is no sharp transition between the other components of the galaxy and the halo.

A halo can be studied by observing its effect on the passage of light from distant bright objects like quasars that are in line of sight beyond the galaxy in question.

Components of the galactic halo

Stellar halo

The stellar halo is a nearly spherical population of field stars and globular clusters. It surrounds most disk galaxies as well as some elliptical galaxies of type cD. A low amount (about one percent) of a galaxy's stellar mass resides in the stellar halo, meaning its luminosity is much lower than other components of the galaxy.

The Milky Way's stellar halo contains globular clusters, RR Lyrae stars with low metal content, and subdwarfs. In our stellar halo, stars tend to be old (most are greater than 12 billion years old) and metal-poor, but there are also halo star clusters with observed metal content similar to disk stars. The halo stars of the Milky Way have an observed radial velocity dispersion of about 200 km/s and a low average velocity of rotation of about 50 km/s. Star formation in the stellar halo of the Milky Way ceased long ago.

Galactic corona

A galactic corona is a distribution of gas extending far away from the center of the galaxy. It can be detected by the distinct emission spectrum it gives off, showing the presence of HI gas (H one, 21 cm microwave line) and other features detectable by X-ray spectroscopy.

Dark matter halo

The dark matter halo is a theorized distribution of dark matter which extends throughout the galaxy extending far beyond its visible components. The mass of the dark matter halo is far greater than the mass of the other components of the galaxy. Its existence is hypothesized in order to account for the gravitational potential that determines the dynamics of bodies within galaxies. The nature of dark matter halos is an important area in current research in cosmology, in particular its relation to galactic formation and evolution.

The Navarro–Frenk–White profile is a widely accepted density profile of the dark matter halo determined through numerical simulations. It represents the mass density of the dark matter halo as a function of ${\displaystyle r}$, the distance from the galactic center:

$${\displaystyle \rho (r)=\frac{{\rho }_{\text{crit}}{\delta }_c}{(r/r_s)(1+r/r_s{)}^2}}$$

where ${\displaystyle r_s}$ is a characteristic radius for the model, ${\displaystyle {\rho }_{\text{crit}}=3H^2/8\pi G}$ is the critical density (with ${\displaystyle H}$ being the Hubble constant), and ${\displaystyle {\delta }_c}$ is a dimensionless constant. The invisible halo component cannot extend with this density profile indefinitely, however; this would lead to a diverging integral when calculating mass. It does, however, provide a finite gravitational potential for all ${\displaystyle r}$. Most measurements that can be made are relatively insensitive to the outer halo's mass distribution. This is a consequence of Newton's laws, which state that if the shape of the halo is spheroidal or elliptical there will be no net gravitational effect from halo mass a distance ${\displaystyle r}$ from the galactic center on an object that is closer to the galactic center than ${\displaystyle r}$. The only dynamical variable related to the extent of the halo that can be constrained is the escape velocity: the fastest-moving stellar objects still gravitationally bound to the Galaxy can give a lower bound on the mass profile of the outer edges of the dark halo.

Formation of galactic halos

The formation of stellar halos occurs naturally in a cold dark matter model of the universe in which the evolution of systems such as halos occurs from the bottom-up, meaning the large scale structure of galaxies is formed starting with small objects. Halos, which are composed of both baryonic and dark matter, form by merging with each other. Evidence suggests that the formation of galactic halos may also be due to the effects of increased gravity and the presence of primordial black holes. The gas from halo mergers goes toward the formation of the central galactic components, while stars and dark matter remain in the galactic halo.

On the other hand, the halo of the Milky Way Galaxy is thought to derive from the Gaia Sausage.