Other meanings of Supermassive black hole
Astronomy
A supermassive black hole (SMBH) is an astronomical object with a mass ranging from hundreds of thousands to billions of times that of the Sun, typically residing at the centers of most large galaxies, including the Milky Way. Its existence is inferred from gravitational effects on surrounding stars and gas, and from the emission of active galactic nuclei.
A supermassive black hole is a type of black hole whose mass is on the order of millions to billions of solar masses. Unlike stellar-mass black holes (3–100 solar masses), SMBHs are thought to reside at the centers of most massive galaxies. Their event horizons are correspondingly large: the Schwarzschild radius of a 4 million solar mass black hole is about 12 million kilometers, roughly 0.08 astronomical units.
The defining characteristic is the extreme gravitational pull, which prevents even light from escaping. The existence of SMBHs is inferred from the motion of stars and gas in their vicinity, as well as from the emission of active galactic nuclei (AGN) when matter accretes onto them.1
The strongest evidence for an SMBH comes from the Milky Way's center, Sagittarius A*. Monitoring of stars orbiting it, particularly the star S2, has allowed precise measurement of its mass and confirmation that it is a black hole. The 2020 Nobel Prize in Physics was awarded to Reinhard Genzel and Andrea Ghez for this work.2
In 2019, the Event Horizon Telescope (EHT) released the first direct image of a black hole's shadow, that of M87*, the central SMBH of the galaxy M87. The image showed a bright ring of emission surrounding a dark central region, consistent with predictions from general relativity.3 In 2022, the EHT imaged Sagittarius A* itself, confirming its nature.
The formation of SMBHs remains an open question. They likely grow through a combination of accretion of gas and mergers with other black holes. The existence of quasars at high redshift (e.g., at z > 6) implies that some SMBHs reached masses of a billion solar masses within the first billion years after the Big Bang, posing challenges for formation models.
Possible seeds include the collapse of massive early stars (producing ~100 solar mass black holes) or direct collapse of gas clouds in the early universe, forming 'seed' black holes of 10^4–10^5 solar masses. Subsequent accretion and mergers can then build them up.
SMBHs are not passive; they influence their host galaxies through feedback processes. When accreting matter, they can launch relativistic jets and outflows that heat and expel gas, regulating star formation. This is evidenced by the tight correlation between the mass of the central black hole and the velocity dispersion of the galaxy's bulge (the M–sigma relation).
Active galactic nuclei, powered by SMBH accretion, are among the most luminous objects in the universe. The energy output can affect the intergalactic medium and the evolution of the galaxy as a whole.4
Beyond the well-known examples, there are extreme SMBHs: some are 'ultramassive' with masses exceeding 10 billion solar masses, such as the one in NGC 1277. Others are 'dormant' or quiescent, like Sagittarius A*, which is currently accreting very little.
Some SMBHs exhibit 'recoil' after galaxy mergers, when gravitational waves carry away momentum, potentially ejecting the black hole from its host galaxy. There are also 'binary SMBHs' in merging galaxies, which are expected to be strong sources of gravitational waves for future detectors like LISA.
Additionally, the 'final parsec problem' refers to the difficulty in explaining how two SMBHs in a merging galaxy get close enough to merge, a topic of active research.5
This article focuses on the astronomical object known as a supermassive black hole, distinct from other types of black holes.
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