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Galaxy Formation and Evolution

Jeff Stanger


Since the earliest studies of our galaxy astronomers have attempted to understand galactic formation and evolution. The steady advancement of technology has allowed us to peer further into the universe to glimpse galaxies at progressively earlier stages of evolution. In the last decade the use of instruments such as the Hubble Space Telescope (HST) has allowed us to study galaxies that existed when the universe was only 10% its current age9. This has finally yielded evidence that is leading us towards a favoured model for the mechanisms of the formation and evolution of galaxies.

Competing Models

Two main theories of galaxy formation and evolution were proposed in the 1960’s11. As observational evidence was gathered over time both these theories were found to have shortcomings. Over the past twenty years the two competing models of galaxy formation and evolution have been refined but neither is consistent with the available evidence10.

The classical or monolithic model (Figure 1, left) proposes that galaxies form and evolve as relatively isolated bodies4,12. In this scenario galaxies evolve in a pre-determined fashion dependant on the initial conditions and with relatively little impact from the surrounding environment. The hierarchical model (Figure 1, right) proposes that galaxies form and evolve through successive mergers of smaller bodies and their fate is more dependent on the environment which they inhabit.

This two model scenario is very reminiscent of the nature vs. nurture debate in psychology. This debate is centred on the roles of heredity and environment in human development. Much like this debate the puzzle of galaxy formation and evolution does not have a simple solution and still remains hotly debated.

Despite the controversy a general trend is beginning to emerge from research of the last ten years. This is that galaxy formation and evolution is likely to involve a combination of the two competing models. This view has developed in light of new observations of the distant universe and these have only been made possible recently by new technology.

The Hybrid Hierarchical Model

Our current understanding of galaxy formation and evolution has been shaped by many observations over the last decade. In combination the results of these observations favour a hybrid hierarchical model. This model provides a good explanation of the observed galaxies although the numerical evidence is less certain12. This model is a combination of the models outlined in Figure 1 and a simplistic outline is shown in Figure 2.

In this hybrid model12, galaxy formation is thought to be seeded from the very earliest times in the universe. The Cosmic Microwave Background (CMB) radiation is the remnant radiation we can observe that was emitted around 300000 years after the birth of the universe (commonly called the big bang). The small variations we detect in this radiation across the sky are thought to show that the distribution of matter in the early universe was slightly ‘lumpy’ or clumped. These clumps of matter are thought to have formed the cores of the first protogalaxies (or gas clouds) that would eventually mature into the galaxies we see in the universe today1.

The first protogalaxies are thought to have merged to form larger structures1. Stars formed in these larger structures and the first population of galaxies as we know them followed. If these newly formed galaxies merged with others over a time when they were still condensing or collapsing they would have produced spheroidal shaped galaxies4,12. As they condensed further a disk would form leading to spiral galaxies. We see these spirals today with spheroidal bulges at their centres as remnants of their past form (Figure 3). If major mergers only occurred after the time period of collapse then elliptical galaxies form (Figure 4). These spheroidal structures in galaxies contain approximately half the stars in the observable universe and provide a valuable window into galactic evolution8.

Much of the theoretical work in this field has been concerned with modelling gaseous collapse or accretion. In the hybrid hierarchical model of galaxy formation, if gas collapses inwards (accretes) in a smooth fashion it builds disks and stars form slowly. The merger of these disks would lead to spherical systems and if there is sufficient gas a slow accretion would again form a disk4. One of the strengths of this model is that it explains the large variation in different types of galaxies that we observe throughout the history of the universe. This is because there is no one time when certain types of galaxies had to form and galaxies can evolve through many forms throughout their history.

There are two central factors in developing a theory of galaxy formation and evolution. One is an understanding of the star formation rate within galaxies. The other is understanding the dynamics of these systems that leads to the different forms we see today. By developing a model that can make predictions of these two factors astronomers can then see if observations are in agreement.

The Evidence

Dark Matter

Since the 1930’s there has been a growing suspicion that most of the mass in the universe was not visible2. By the 1980’s the evidence was overwhelming that this ‘dark matter’ may be up to 90% of the matter in the universe4. The current theory is that dark matter consists mainly of exotic particles or cold dark matter (CDM). The use of models based on the theories of CDM have been extremely successful in predicting many of the features of the universe. Galaxy formation and evolution are no exception and CDM models have given us our best tool so far to model galaxies from their earliest seeds to their present day forms12.

Before the CDM models most cosmologists believed that at least some galaxies formed through classical or monolithic collapse12. The subsequent predictions of CDM models and their agreement with recent observations are numerous and this is why the hybrid hierarchical model has gained credibility12. This model is indicated as the most successful so far and predicts hierarchical formation and clustering on many scales within the universe.

Small Blue Galaxies

Observations by the HST have been crucial in forming our current view. Most notably, a new population of galaxies was discovered by the HST in its deepest images called the Hubble Deep Fields5,6. These small blue galaxies (Figure 5) fill the early universe but are not seen in the universe today. Their absence today is possible evidence for a hierarchical model as these galaxies may have merged to form the large galaxies we see today7.

Another piece of evidence that supports this hierarchical view is that these blue galaxies have been found to be clustered together in clumps that contain enough mass to construct typical galaxies in our universe today10. The theories do not all support the hierarchical model however. Some studies have indicated that these faint blue galaxies are actually parts of larger galaxies that are made visible by high rates of star formation. If this is true then these larger galaxies may support a monolithic collapse theory.

Galaxy Diversity Over Time

Studies of galaxy types in galaxy clusters have produced results favouring both the hierarchical and classical models2. At a time when the universe was 30% younger clusters of galaxies were dominated by many more spiral galaxies10. Today we see these clusters are dominated by ellipticals. We are not able to simply say that spirals merge to give ellipticals because young clusters contain some mature ellipticals2,10. This situation is successfully explained by the hybrid hierarchical model because it incorporates early formation of mature galaxies by some monolithic collapse process and the evolution of spirals into ellipticals via hierarchical merger9,7.

Star Formation Rate (SFR)

Young hot stars emit much of their energy as ultraviolet (UV) light and therefore detection of the relative brightness of UV can be used to trace the SFR. Many surveys have been recently conducted to study the SFR over time and there is strong evidence for evolution (Figure 6)5.

One of the main questions that studies of the SFR hope to answer is when was the era of the peak SFR. This is uncertain because depending on the severity of the complicating factors involved, like extinction by dust, this peak could have occurred quite recently or so far in the past that we have not observed this era yet8.

Because the hierarchical theory predicts that galaxies form from smaller protogalaxies these may have escaped detection in these studies. This may correspond with the decline in the solid line in Figure 6 but this is by no means conclusive as there are many effects such as dust obscuration that can account for the reduced detection.

Further Work

Evidence for both the classical and hierarchical models of galaxy formation and evolution have been found in many studies. The success of CDM model predictions, the proliferation of smaller galaxies in the early universe, the range of galaxy diversity over time and the variation of the SFR over time all in some way favour hierarchical galaxy formation and evolution. There is also several lines of evidence indicating classical or monolithic collapse. A proposed solution to this inconsistent picture is the hybrid hierarchical model although there is much more work to be done.

The uncertainties in our understanding of the history of galaxies originate in the many methods used to gather the evidence. These methods involve complications that need to be understood or require technological improvements to enable more sensitive observations. As we improve the quality of our evidence we will be able to further test the current models and rule out the competing components.

The most significant factor in our ability to test these theories has been our ability to observe galaxies across much of the history of the universe using tools such as the HST. It has only been through the gradual improvement of technology that we have been able to discover the history of galaxies from progressively earlier views of the universe. New observatories like the Spitzer Space Telescope and the proposed Atacama Large Millimetre Array (ALMA) will gather further information that will clarify much of the evidence and lead us to a better understanding of galaxy formation and evolution.


Astronomers have made much progress with their attempts to understand galactic formation and evolution. A hybrid hierarchical model appears an extremely viable candidate in its ability to qualitatively explain the galaxies we have observed so far. Our current observations stretch back to when the universe was 10% its current age. Undoubtedly as technology allows us to increase the sensitivity and resolution of our observations we will be able to further test this theory.



1.      Djorgovski, S.G., 2001, Encyclopaedia of Astronomy and Astrophysics. Institute of Physics Publishing, USA. Accessed at: (27 March 2005).

 2.      Dressler, A., 1996, The Origin and evolution of the universe. Ed. Zuckerman, B. and Malkan, M.A. Jones and Bartlett publishers, Sudbury Massachusetts.

 3.      Ellis, R.S., 1997, Annu. Rev. Astron. Astrophys., vol. 35, p. 389.

 4.      Ellis, R.S., Abraham, R.G. and Menanteau, F., 2000, Astronomy and Geophysics, vol. 41, p. 2.10.

 5.      Ferguson, H.C., 2001, Encyclopaedia of Astronomy and Astrophysics. Institute of Physics Publishing, USA. Accessed at: (1 April 2005).

 6.      Ferguson, H.C., Dickinson, M. and Williams, R., 2000, Rev. Astron. Astrophys., vol. 38, p. 667.

 7.      Giavalisco, M., Steidel, C.C. and Macchetto, D., 1996, Astrophys. J. vol. 470, p. 189.

 8.      Giavalisco, M., 2001, Encyclopaedia of Astronomy and Astrophysics. Institute of Physics Publishing, USA. Accessed at: (27 March 2005).

 9.      Giavalisco, M., 2002, Annu. Rev. Astron. Astrophys., vol. 40, p. 579.

 10. Guiderdoni, B., (2002). Galaxy Evolution: An Overview. Astrophysics and Space Science, 281, 13-23.

 11. Seeds, M.A., 1997, Foundations of Astronomy, 4th Ed., Wadsworth Publishing Co., USA.

 12. Steinmetz, M., 2003, Astrophysics and Space Science, vol. 284, p. 325.

 Image Credits

 Figure 1. Ellis, R.S., Abraham, R.G. and Menanteau, F. 2000, Astronomy and Geophysics, vol. 41, p. 2.10.

 Figure 2. Original.

 Figure 3. Accessed at and adapted from on 8/4/05.

 Figure 4. Accessed at and adapted from on 8/4/05.

 Figure 5. Accessed at on 8/4/05.

 Figure 6. Guiderdoni, B., (2002). Galaxy Evolution: An Overview. Astrophysics and Space Science, 281, 13-23.


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