Supermassive black holes

Introduction

Black holes are monstruous remnants of massive stars. They weigh more than three times the mass of our sun (1 solar mass is approx. 2 x 10³⁰ kg). They are so small, and incredibly compact, that light itself, which travels at 300 000 km/s cannot escape from a certain point from the black hole. They warp space and time around them.
And then we have supermassive black holes. They weigh millions, or sometimes billion times more than our sun (lower limit seems to
be above 10⁵ M_Sun). They can be so large, they would reach the far end of our solarsystem, had they been in the sun's place. Given the sheer mass of the supemassive black hole, the density can be very low, in fact it can be lower than the density of water. See Schwarzschild radius.
When they are feeding, they can swallow entire stars, or even clusters of stars. During the feeding, the gas that isnt swallowed is heated to millions or billions degrees Kelvin, which makes the gas shine extremely brightly. Thus, making the galactic core visible at great distances from space. These galaxies that harbor a feeding supermassive black hole are called active galaxies. They were much more abundant early in the universe when matter still was flowing into them. Supermassive black holes may have been what powered quasars, when the universe was still young.

Many astronomers believe that supermassive black holes are lurking at the center of nearly every galaxy, including our own, the Milky Way. Our supermassive black hole is estimated to be 2-3 million solar masses heavy.

How supermassive black holes form

There are different theories as to how supermassive black holes are formed. Some state that the giant black hole was formed early in the galaxy's history, as a normal stellar corpse, and was able to gradually swallow more and more matter due to the high abundancy of it in the vicinity. Other theories state that a supermassive black hole is formed when several black holes merge and then continue feeding on the rest of the matter. However, astronomers do not yet know how they are actually formed.
When there is greater abundancy of matter falling in than the black hole is able to swallow, an accretion disc forms around it. And if this accretion disc becomes hot enough (millions or billion degrees Kelvin), it starts to repel other infalling matter through radiation pressure. Thus, the matter approaching the supermassive black hole is halted. Hence, supermassive black holes feed in cycles of varying length of time.
What is more intriguing is that the size of the supermassive black hole seems correlated to the size of the entire galaxy. Furthermore, the size of the supermassive black hole also correlates to the speed of the stars orbiting the galactic center throughout the galaxy. A supermassive black hole plays a role in the evolution of the galaxy which harbours it: During the early stages of the formation of the galaxy itself, the intense heat around an active supermassive black hole repels gas in a direction outward from center, triggering starbirth. Though astronomers do not know what is formed first, the giant black hole, or the galaxy itself?

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Evidence for supermassive black holes

A black hole cannot be directly seen as it doesn't let any light escape from it. So astronomers have to rely on other methods to detect a black hole, or even a supermassive black hole.
When astronomers set out to discover these giants of apocalyptic size, they looked at the center of many galaxies. In most cases they found that stars very close to the center were orbiting much more rapidly, than the total mass of the apparent stars allows. The astronomers found this by studying doppler shift variations of stars very close to the center. They aso discovered that the mass that made the stars orbit so fast had to be concentrated in a very small area. Only one thing fits into this category: A supermassive black hole.

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Known supermassive black holes

One of the most known supermassive black holes is the one located at the center of our own galaxy; Sagittarius A (Sgr A for short). Astronomers have calculated it's mass to be 2-3 million solar masses.
Another famous supermassive black hole is the one lurking at the center of the elliptical galaxy Messier 87. It is colossally massive: 3 billion M_Sun (again 1 M_Sun is approximately 2*10³⁰ Kg) and it is the size of our solarsystem! Astronomers have detected a massive jet stretching 6 500 light years, that is probably being radiated from one of the poles of the supermassive black hole.

Below is a list of some supermassive black holes found.

Galaxy	Constellation	Type	Distance, ly	Mass, MSun
Milky Way	-	Sbc	28,000	2 Million
NGC 224, M31	Andromeda	Sb	2.3 Million	30 Million
NGC 221, M32	Andromeda	E2	2.3 Million	3 Million
NGC 3115	Sextans	SO	27 Million	2 Million
NGC 4258	Canes Venacti	Sbc	24 Million	40 Million
NGC 4261	Virgo	E2	90 Million	400 Million
NGC 4486, M87	Virgo	E0	50 Million	3 Billion
NGC 4594, M104	Virgo	Sa	30 Million	1 Billion

Source: HubbleSite

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How supermassive black holes form
Evidence for supermassive black holes
Known supermassive black holes