Galaxy Formation and Evolution


From the observations and conclusions made from the Cosmic Microwave Background (CMB), the era that succeeded the big bang was characterized by homogeneity in the universe that registered variations not exceeding 1 in every 100,000. As it is generally agreed today, the current universe structure was generated as a result of development of primordial variations due to unstable gravitational forces. Gravitational changes decelerated the growth of the high density parts in a more rapid speed than that experienced in the low density parts. The increased variation over the low and high density parts eventually led to the formation of the structures of the universe as well as the galaxies.

The mutual contact and interaction between galaxies is a significant way in the typical scenario of the formation of galaxies as the gravity pulls the neighbouring galaxies towards each other. This is followed by nimbus of the dark matter which converges to constitute a bigger dark nimbus with the gas disks also merging to constitute a bigger disk. Any black holes at the centre of the merging nimbus will also merge in the process and generate an outstanding discharge of energy in the form of gravitational field forces or neutrinos. As the merging of the gas disks is realized a great portion of the gas is forced into the black hole at the centre while other outer gases catalyzes into stars (Ying 2005, p.620).

It has been demonstrated that merging of spiral galaxies can result to residual masses with the same characteristics as that of the elliptical galaxies. The active parts of the star formation is usually attributed to the spiral structure and the spiral arms of the galaxies are conspicuous due to the presence of very hot blue and white stars as well as numerous gas and dust. Recent observations of the galaxy have also revealed bimodal allocation of the colours with discrete luminosity during the time of merging with both the mean and the variance of the distribution being independent of environmental changes (Ying 2005, p.620).

Recent scientific researches have revealed that the whole mass of stars in the modern galaxy matches up with the size of the black hole as shown in figure 1. The scientists have however not been able to explain the idea behind this interesting relationship. The same results have been confirmed using the NASA space telescope that held a detailed scan for over thirty galaxies. The telescope which is equipped with a modern black-hole searching spectrograph established a link between the sizes of the black hole and the total mass of the stars raising debate on whether the black holes preceded the formation of the galaxies or it evolved together with the galaxy.

The scientists have embraced the latter theory of the black hole evolving together with the galaxy by entrapping corresponding masses of stars and gases within the given galaxies. This means that the resultant mass of the black hole is not primordial but it is dictated upon at the time when the galaxies are forming (Ying 2005, p.620).

The relationship between the star mass and the black hole.
Figure 1: The relationship between the star mass and the black hole.

Formation of supermassive black hole

Super massive black holes are ever-present elements of the nuclei of all galaxies and its a common assumption that when two big galaxies combine, a super massive black hole develops and ends up shrinking due to gas active processes. They are the largest form of black holes in a galaxy and almost all galaxies contain super massive black holes at their centres. Eventually, the super massive black holes coalesce by releasing gravitational waves (Mayer 2007, p.1874). The formation of super massive black holes and the manner through which they interact with host galaxies is a very essential aspect in the process of understanding the configuration of galaxies.

There exist diverse models that attempts to define formation and development of super massive black hole. Some of these include:

  • Accretion of matter- these are assumed to move slowly from a stellar size black hole.
  • Collapsing of a gas cloud, very large, into a relativistic star assumed to be of hundred thousand masses and above. Consequently, the star becomes unstable to radial movements hence disintegrates into a black hole.
  • Possible disintegration of a dense stellar cluster due to conversion of the system velocity to relativistic velocities as a result of changes in the system heat conditions.
  • Another model ascertains that super massive black holes could have come up as a result of external pressure after the Big Bang.

According to the standard theory, the formation of black holes is divided into three phases:

  1. Transfer of black holes kinetic energy to the surrounding field stars due to dynamical friction. This makes the black holes to spiral towards the centre where they form a binary.
  2. Reduction of the dynamical friction effect and the evolution of the super massive black hole becomes dominated by super elastic processes. In this case, there is an interaction of field stars hence increasing the binding energy.
  3. Coalescing of black holes all through the emission of gravitational radiation.

The problem that occurs during formation of super massive black holes dwells in the need for adequate matter to fit in a small volume. The matter has to be in a tiny angular momentum for the black hole to be formed. In addition, the process of accretion includes the transportation of a large endowment of angular momentum outwards. This is a limiting factor in the development of the black hole hence explains the formation of accretion disks (Penrose 2002, p.1151).

Evolution of Galaxy

This regards the processes the established processes assumed to be behind the relative transformation of cosmos from a homogenous to a heterogeneous state. It also involves aspects of how galaxies change through time and the various processes that have led to the generation of the diverse structures that are seen today in the galaxies. The big bang theory asserts that possibilities of small quantum’s fluctuations that occurred after the bang may have led to galaxies formation. The universe was uniform as seen in the cosmic microwave background and there was no significant structure hence no galaxies (Navarro 2002, p.155).

The theory of Einstein-de-sitter and Friedmann on galaxy formation ascertains that these structures developed as an effect of augmentation of primordial oscillations. These are the tiny changes in the cosmos mass in a restricted area. The process of galaxy formation began with formation of dark matter haloes which are ascertained to be more plentiful than any other matter hence dominating the development of the total density oscillations as the baryons fell into the prospective wells of dark matter (Thompson 2010, p.354). Hence, as the universe chilled, dark matter began condensing together with the gas in them. The result of these, supposedly led to accumulation of dark matters and gas into denser areas where they gravitated to form structures making the first galaxies.

During this era, the universe was mainly composed of hydrogen, helium and the dark matter but after the formation of the first proto galaxies, these gases condensed to form stars. With time most massive stars ceased to be because they blazed away more quickly than the tiny ones leading to the formation of black holes. The black holes further collected in the middle of the proto galaxy and integrated to form super massive black holes that were seen as quasars once they accreted gas (Charlton 2010, p.10).

There are two types of galaxies that arose after the formation and evolution process. They include the blue star galaxies which are more of the spiral form and the red non-star forming ones which are the elliptical galaxies. Formation of the elliptical galaxy mostly occurred in huge dark matter haloes that corresponded to the available group of galaxies. The elliptical galaxies contained very huge black holes whose function was to prevent any other hot gas from cooling onto the galaxy. Therefore, no more stars could form but the galaxy kept on growing by coalescing with other big galaxies which happened to fall in the cluster (Thompson 2003, p.353).

Image of an elliptical galaxy
Figure 2: image of an elliptical galaxy.
Image of a spiral galaxy.
Figure 3: image of a spiral galaxy.

The role of supermassive black hole in the formation of galaxy

Most galaxies host central black holes that vary in mass from millions to billions of solar masses and the growth of these black holes normally emit huge amounts of energy that power quasars and other weaker active galactic nuclei. A small amount of this energy could easily stop the formation of stars by heating and casting out the ambient gas if it’s absorbed by the host galaxy. There are two types of galaxies; the football shaped elliptical and the pancake shaped spirals.

It’s the spirals that structurally contain central bulges and each bulge consists of a central black hole. The mass of the bulges and the black holes are directly proportional because the two are formed at the same epoch (Stuart 2003, p.600). This clearly indicates that the formation of black holes and that of bulges occur at the same time or are associated. Hence black holes are essential in the galaxy formation process.

It’s believed that the universe was formed through the Big Bang with small haloes in homogeneities that developed into lumps called haloes with time. Galaxies grew through accretion of gas that fell to the centre in cold flows in low-mass haloes. On the other hand, high-mass haloes did not form galaxies. This is because they were dominated by heat hence the gas did not accrete onto galaxies. Combination of tiny haloes formed big ones which contained a huge number of galaxies that are referred to as clusters or groups. As a result, the merging of galaxies within haloes led to the transformation of discs into bulges which provided a chance for the development of galaxies when they ceased to accrete gas.

In cases where there was merging of galaxies that were still accreting, the gas fell into the centre, triggered stars to burst out and fed the rapid growth of black holes. In response, the black holes released energy into the surrounding gas and produced winds which compressed the gas that accelerated the rate at which stars were formed. In galaxies which fail to accrete gas, there is lack of star formation and black hole accretion (Munster 2008, p.44). Research has also revealed that massive black holes dwell in most local galaxies and other previous studies have also established that a number of relations exist between the super massive black holes masses and the properties of host galaxies (Volonteri 2010, p.1). Therefore all these findings imply that black holes play a very fundamental role in the formation of galaxies.

On the other hand, black holes have been known to be closely associated with the stellar mass and its speed distribution within the host bulges. This shows an informal relationship between the formation of black holes and that of bulges though they can be interpreted in two ways. First, the formation of stars and black holes occurs simultaneously because both of them feed from a similar gas and they get to the centre by disc instabilities. In addition, the accretion of black hole ceases when all the gases have been used up by star formation hence the two are interdependent on each other. Secondly, the formation of stars normally ceases when the black holes blow away all the gas outside the host galaxies (Volonteri 2010, p.1).


In conclusion, galaxies were formed and evolved from a uniform universe and different theories have been developed to explain the processes that led to the formation of these structures. The big bang theory relates galaxy formation to small quantum fluctuation while the Einstein –de-sitter and Friedmann theory associates it with the augmentation of primordial oscillations. It is evident that the formation of super massive black holes occurs as a result of the merging of various galaxies and in the process there is emission of gravitational waves. There are several models that explain the formation of the super massive black holes and the standard theory divides the process of formation in to three phases as discussed in the essay.

There is a close association in the formation of super massive black holes and galaxies because bulges and black holes are formed at the same positions. In addition, research has also found out that the two have densities that are directly proportional. This gives a clear indication that galaxies cannot be formed in the absence of super massive black holes.


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