Supermassive black hole

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 that nothing, not even light, can escape.

The first direct image of a supermassive black hole, found in the galactic core of Messier 87.

Observational evidence shows 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, which corresponds to the radio source Sagittarius A*. The process of accretion of interstellar gas onto these black holes powers active galactic nuclei (AGNs) and quasars, some of the brightest objects in the universe.

Two supermassive black holes have been directly imaged by the Event Horizon Telescope; these are Sagittarius A* at the center of the Milky Way, and the black hole at the center of Messier 87, a giant elliptical galaxy. Studying these fascinating objects helps scientists understand the universe and how galaxies form and grow.

Description

Supermassive black holes are the largest type of black hole, with masses ranging from hundreds of thousands to billions of times the mass of the Sun solar masses (M_☉). Unlike smaller black holes, supermassive black holes have weaker tidal forces near their event horizon. This means a person near the edge of such a black hole might feel forces similar to standing on Earth.

Interestingly, the density of a supermassive black hole can be less than that of water. This is because the space they occupy grows quickly with their mass, making them less dense overall. Some of the biggest known supermassive black holes are found in galaxies like TON 618, NGC 6166, ESO 444-46, and NGC 4889. Recent studies even suggest the existence of extremely large black holes, called stupendously large black holes, with masses over 100 billion times that of the Sun, possibly including the black hole in Phoenix A.

History of research

The search for supermassive black holes began in 1963 when Maarten Schmidt studied a bright radio source called 3C 273. At first, people thought it was a star, but they discovered it was moving very far away and giving off huge amounts of energy. This led scientists to think it might be a very special kind of object called a quasar.

As scientists learned more, they began to understand that these powerful objects might be explained by supermassive black holes. In the 1970s, observations of stars moving quickly around the centers of galaxies showed there must be something very heavy there. Later, telescopes like the Hubble Space Telescope helped scientists find strong evidence for these black holes. In 2019, the Event Horizon Telescope Collaboration even took the first picture of a black hole’s edge, confirming these giant objects really exist at the centers of many galaxies.

Formation

The origin of supermassive black holes is still being studied. Scientists know that black holes can grow by taking in matter and by joining with other black holes. There are several ideas about how the first black holes, called "seeds," formed. No matter how these seeds started, if they had enough material around them, they could grow by taking in more matter to become larger black holes.

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

Some very distant and ancient supermassive black holes are hard to explain because they appeared so soon after the Big Bang. One idea is that they might have formed from the direct collapse of dark matter. Another idea suggests they could be evidence that the Universe resulted from a "Big Bounce" instead of a Big Bang. These black holes might have formed before the Big Bang.

The first stars might have left behind black holes with masses of tens or hundreds of times the mass of the Sun. These could grow by taking in more matter. Another idea is that large clouds of gas could collapse directly into black holes without first becoming stars. These black holes could have masses of about 100,000 times the mass of the Sun.

An artist's impression of stars born in winds from supermassive black holes.

Supermassive black holes can also form from very dense clouds of gas that collapse directly into a black hole. These black holes would have masses of about 100,000 times the mass of the Sun.

There is a natural limit to how large supermassive black holes can grow. Theories suggest this limit is around 50 billion times the mass of the Sun. Beyond this limit, growth slows down a lot. Some theories suggest an absolute maximum mass of about 270 billion times the mass of the Sun, but this is very rare. Most supermassive black holes are thought to stay below this limit.

In the very far future, some of these black holes might continue to grow to incredibly large masses during the collapse of groups of galaxies.

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

Activity and galactic evolution

Main articles: Active galactic nucleus and Galaxy formation and evolution

Candidate SMBHs suspected to be recoiled or ejected black holes

The gravity of supermassive black holes at the centers of galaxies is thought to power bright objects like Seyfert galaxies and quasars. There is a special link between the size of these black holes and the mass of the galaxy they live in, depending on the kind of galaxy. Scientists call this link the M–sigma relation.

When galaxies collide and merge, their supermassive black holes can also come together. Over time, these black holes might move closer and eventually join into one. This process can send out powerful waves called gravitational waves, which might push the new black hole away from the center of the galaxy. Scientists are searching for these moving black holes by looking for special signs in space.

Evidence

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 ways we know black holes exist come from watching how stars and gas move around them. When material moves toward a black hole, its light changes color due to the Doppler effect — it gets redder when moving away and bluer when moving closer. This helps scientists figure out how fast things are moving near a black hole.

We have strong evidence that our own Milky Way galaxy has a supermassive black hole at its center, called Sagittarius A*. One star, called S2, orbits this black hole very quickly, which lets scientists calculate the black hole’s mass — about 4 million times the mass of our Sun. Other galaxies also show signs of having supermassive black holes in their centers, even though we can’t always see them directly.

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 Andromeda Galaxy, located 2.5 million light-years away, has a central black hole about 140 million times the mass of the Sun. One of the largest known supermassive black holes is in Messier 87, weighing about 6.5 billion times the Sun's mass and located 48.92 million light-years away.

Other huge black holes are found in distant objects called quasars. For example, TON 618 has a black hole estimated to be 40.7 billion times the mass of the Sun. Some galaxies even have two supermassive black holes close together, which might eventually merge and create strong gravitational waves.