Supermassive black hole

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*).^[10][11]

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_☉.^[12] 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:^[13] 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.^[14]

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.^[15] This is because the Schwarzschild radius (${\displaystyle r_{\text{s}}}$) is directly

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.^[17][18]

Some astronomers refer to black holes of greater than 5 billion M_☉ as ultramassive black holes (UMBHs or UBHs),^[19] 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,^[20] 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

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_☉.^[31] 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_☉.^[32] Discovery of similar behavior in other galaxies soon followed, including the Andromeda Galaxy in 1984 and the Sombrero Galaxy in 1988.^[5]

Donald Lynden-Bell and

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.^[36] Accompanying this observation which provided the first confirmation of supermassive black holes was the discovery^[37] 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.^[2] 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.^[38][39]

Formation

The origin of supermassive black holes remains an active field of research. Astrophysicists agree that black holes can grow by

This section needs to be updated. Please help update this article to reflect recent events or newly available information. (November 2022)

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.^[50][51]

Before the first stars, large gas clouds could collapse into a "

Large, high-redshift clouds of metal-free gas,[55] when irradiated by a sufficiently intense flux of Lyman–Werner photons,^[56] can avoid cooling and fragmenting, thus collapsing as a single object due to self-gravitation.^[57][58] The core of the collapsing object reaches extremely large values of the matter density, of the order of about 10⁷ g/cm³, and triggers a general relativistic instability.^[59] 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.^[60]

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.^[61][62]

Artist's impression of the huge outflow ejected from the quasar SDSS J1106+1939^[63]

Artist's illustration of galaxy with jets from a supermassive black hole^[64]

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

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.^{[citation needed]}

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.^{[21][66][67][68]} 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.^[21]

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,^[24][21] although the maximum limit for a black hole's spin parameter is very slightly lower at a = 0.9982.^[69] 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.^[24]

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.^[21] 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.^[21]

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.^[21] This would lead the accretion statistically to spin-down, due to retrograde events having larger lever arms than prograde, and occurring almost as often.^[21] 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.^[21] 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.^[21]

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.^[70]

Activity and galactic 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.^[71][72] An empirical correlation between the size of supermassive black holes and the stellar velocity dispersion ${\displaystyle \sigma }$ of a galaxy bulge^[73] 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.^[74]

Mergers and recoiled SMBHs

The

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.^[76] 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.^[77] An ejected black hole is called a runaway black hole.^[78]

There are different ways to detect recoiling black holes. Often a displacement of a quasar/AGN from the center of a galaxy

Candidate recoiling black holes include NGC 3718,^[81] SDSS1133,^[82] 3C 186,^[83] E1821+643^[84] and SDSSJ0927+2943.^[80] Candidate runaway black holes are HE0450–2958,^[79] CID-42^[85] and objects around RCP 28.^[86] Runaway super massive black holes may trigger star formation in their wakes.^[78] A linear feature near the dwarf galaxy RCP 28 was interpreted as the star-forming wake of a candidate runaway black hole.^[86][87][88]

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.^[89][18] 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.^[70][18]

Evidence

Doppler measurements

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*^[92] because:

• The star
  light-hours (1.8×10¹³ m or 120 AU) from the center of the central object.^[93]
• From the motion of star S2, the object's mass can be estimated as 4.0 million M_☉,[94] 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^[95] 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.^[95]

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.^[96]

On January 5, 2015, NASA reported observing an

Detection of an unusually bright X-ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy^[97]

Sagittarius A* imaged by the Event Horizon Telescope

Outside the Milky Way

Unambiguous dynamical evidence for supermassive black holes exists only for a handful of galaxies;

Nevertheless, it is commonly accepted that the center of nearly every galaxy contains a supermassive black hole.[100] 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.^[101] 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.^[100]

On March 28, 2011, a supermassive black hole was seen tearing a mid-size star apart.^[102] That is the only likely explanation of the observations that day of sudden X-ray radiation and the follow-up broad-band observations.^[103][104] 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

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.^[105] 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.^[106] 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_☉.^[107]

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_☉.^[108] 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_☉,^[109] 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.^[110]

Some galaxies, such as the galaxy

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_☉.^[120]

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.^{[121][122][123]}

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

Black hole disk flares in galaxy OJ 287
(1:22; animation; 28 April 2020)

Notes

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Further reading

• Fulvio Melia (2003). The Edge of Infinity. Supermassive Black Holes in the Universe. Cambridge University Press.
  OL 22546388M
  .
• Carr, Bernard; Kühnel, Florian (2022). "Primordial black holes as dark matter candidates". SciPost Physics Lecture Notes.
  S2CID 238407875
  .
• Chakraborty, Amlan; Chanda, Prolay K.; Pandey, Kanhaiya Lal; Das, Subinoy (2022). "Formation and Abundance of Late-forming Primordial Black Holes as Dark Matter". The Astrophysical Journal. 932 (2): 119.
  S2CID 248266315
  .
• S2CID 5266031
  .
• Krolik, Julian (1999). Active Galactic Nuclei. Princeton University Press.
  OL 361705M
  .
• ISBN 978-0-691-12101-7
  .
• Dotan, Calanit; Rossi, Elena M.; Shaviv, Nir J. (2011). "A lower limit on the halo mass to form supermassive black holes". Monthly Notices of the Royal Astronomical Society. 417 (4): 3035–3046.
  S2CID 54854781
  .
• Argüelles, Carlos R.; Díaz, Manuel I.; Krut, Andreas; Yunis, Rafael (2021). "On the formation and stability of fermionic dark matter haloes in a cosmological framework". Monthly Notices of the Royal Astronomical Society. 502 (3): 4227–4246.
  doi:10.1093/mnras/staa3986
  .
• Fiacconi, Davide; Rossi, Elena M. (2017). "Light or heavy supermassive black hole seeds: The role of internal rotation in the fate of supermassive stars". Monthly Notices of the Royal Astronomical Society. 464 (2): 2259–2269.
  doi:10.1093/mnras/stw2505
  .
• Davelaar, Jordy; Bronzwaer, Thomas; Kok, Daniel; Younsi, Ziri; Mościbrodzka, Monika; Falcke, Heino (2018). "Observing supermassive black holes in virtual reality". Computational Astrophysics and Cosmology. 5 (1): 1.
  doi:10.1186/s40668-018-0023-7
  .

External links

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