Super massive black holes in the formation of galaxies

Galaxies begin as small clouds full of gas and dark matter. The gravity in the cloud gathers all the gas together to make a star which is ready to begin nuclear fusion. Nuclear Fusion is the process of combining four hydrogen nuclei to make helium nuclei in three stages.

Stage 1: 2 protons fuse to make deuterium:

1H + 1H –> 2H + β+

Stage 2: Hydrogen fuses with deuterium to make helium-3:

2H + 1H –> 3He

Stage 3: 2 helium-3 nuclei fuse to make helium-4 and 2 protons:

3He + 3He –> 4He + 2 1H

This process creates a lot of energy. Each gas cloud that has formed stars then come together to make a bigger cloud, eventually the cloud will contain millions and millions of stars which signifies the beginning of a baby galaxy[i].


Data has shown that the mass of a super massive black hole is directly proportional to the mass of the central area in the galaxy, known as the bulge[ii]. Furthermore, we know that the speed of the bulge in a galaxy is directly proportional to the mass of the super massive black hole. This vital information is that star formation inside the galaxy is closely linked to the mass of the super massive black hole, this means that we can suggest explanations on how the two are linked (See Figure 1).

fig 1 black holes

The graph on the left shows how the bulge speed is proportional to the mass of the super massive black hole. The graph on the right shows how the mass of the bulge is proportional to the mass of the super massive black hole. (Figure 1)[iii]


The first solution states that the process of a super massive black hole absorbing matter via gravity to grow, also known as accretion, and the birth of new stars in the galaxy happen at the same time[iv]. This is due to the fact that both procedures are being fuelled by the same gas. The super massive black hole eventually stops attracting matter because the new created stars have used all of the gas[v].


Another explanation for the link between the mass of a super massive black hole and the mass of the bulge is that the birth of new stars comes to a halt when the super massive black hole essentially blows out the gas from the galaxy, which is used for star formation. This occurs when the super massive black holes mass is absorbing something large like a star, this is known as Active Galactic Nucleation. For a given mass of the black hole there is a maximum amount of brightness that the Active galactic nuclei (AGN) can have, this is called the maximum Active Galactic Nuclei luminosity or the Eddington limit. If the luminosity of the AGN is larger than the Eddington limit then the Gravitational force acting inwards produced by the super massive black hole will be overpowered by the outwards radiation pressure produced by the AGN.


This is possible because the AGN can relocate its energy to the nearby gas. It does this by heating the gas which can either cause the gas to enlarge known as thermal energy driven winds, or the gas gets forced out of the galaxy known as pressure momentum driven winds. These winds are produced by a process called Photoionisation; this is where photons from the AGN will hit metals in the galaxy[vi]. This causes the surrounding gas to be heated due to the electromagnetic radiation given off by the electrons in the metals that absorb the photons from the AGN. The heat gets spread by a photon being absorbed by an electron and then it gets emitted to hit another nearby electron to cause a chain reaction, this is known as Compton Scattering[vii]. The Radiation can be absorbed by the gas very easily and it has a very large dispersion area9.

fig 2 black holesJets produced by Active Galactic Nuclei [viii]

However, even if the AGN is very powerful and has the potential to produce a vast amount of winds which can push all the gas out of the galaxy, the processes which create these winds are highly inefficient. This is because the only way of spreading the heat throughout the galaxy via photoionisation is for the photons produced by the AGN to be absorbed by the metals in the surrounding gas. However, the metals only make up a small amount of the total gas; the rest is made up of simple atoms such as Hydrogen and Helium. Additionally, despite the fact that photoionisation has a large dispersion area, it can only achieve this if the photons emitted from the AGN have a particular frequency. Furthermore, when the photon scatters across different electrons it only deposits a small amount of energy; this means that each electron gives off a tiny amount of electromagnetic radiation which is used to produce these winds that force all the gas out of the galaxy. When a super massive black hole is active it emits 2 jets opposite to each other9.


Using Optical and X-ray spectroscopy we can show that winds produced by quasars travel at speeds of 103 km/s. Models have replicated the thermal and radiation pressure driven winds, which have identified the similarities between the mass of the black hole and the mass of the bulge. These replicates have been tested and the results from both models appear to be similar, this shows that we have an inconclusive decision on which model will be the best to use. By using these techniques we can identify that the speed of winds in a galaxy where star formation is prolific are between 500 to 2000km/s. These are known as starburst galaxies, they have an incredibly fast rate of star formation[ix].


Further research of starburst galaxies shows that the speed of the winds in that particular galaxy are very high, proving that there is an AGN. Also, in usual galaxies, the rate of star formation predictably decreases at a very consistent rate, as long as there are no critical events which could affect this such as galaxy collisions. On the other hand, a galaxy with an AGN will have a drop in star formation that is a lot more significant. However, this is inconclusive due to the fact that galaxies that are further away from earth could have much more powerful AGN and are a lot harder to observe in detail9.



[i] Accessed on 24/09/2016 (YouTube)

[ii] Accessed on 08/08/2016 (HubbleSite)

[iii] Accessed on 14/02/2017

[iv] Accessed on 10/08/2016 (StrongGravity)

[v] Accessed on 08/08/2016

[vi] Accessed on 08/08/2016 (Britannica)

[vii] Accessed on 08/08/2016

[viii]  Accessed on 27/11/2016 (University Of Leicester)

[ix] Accessed on 09/08/2016 (Cosmos)

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