Clusters of Galaxies

Cluster of Galaxies Formation

One of the cluster formation scenarios suggests that a group of newly formed galaxies (protogalaxy) are expanding away from each other some one billion years or so after the Big Bang. This grouping represented a fluctuation above the average density of its surroundings. Its density would grow and would exert a gravitational pull on itself that is strong enough to counteract the expansion of the universe, and the expansion of the region slowed down. It reached a maximum radius when its density is about five times greater than the background density as shown in Figure 04-06a.

Figure 04-06a Cluster of Galaxies Formation
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At its radius of maximum expansion, the cloud of galaxies had only gravitational potential energy and no kinetic energy. Then it begins to collapse, it loses potential energy and gains kinetic energy in the form of random motions of the galaxies. Once the kinetic energy equals one-half of the (negative) potential energy, the cloud reached a stage of energy balance and there is a dynamic equilibrium (where the Virial Theorem is applicable). The gravitational field of the cloud no longer changes with time. The kinetic energy of the randomly moving galaxies acts like a pressure that balances the force of gravitational attraction, the result is to produce a stable cluster of galaxies.

Cosmological simulations show that the progenitors of present-day galactic clusters were the largest structures at redshift of about 6. These proto-clusters are characterized by local over-densities of massive galaxies, and extend over tens of mega-parsecs. Owing to the high mass densities and correspondingly high merger rates, extreme phenomena such as star-bursts and quasars occurred frequently in these regions. One such proto-clusters at z = 5.3 have been detected (in 2010) by combined observations in X-ray, optical, infrared, microwave, and radio of the electromagnetic spectrum.

Figure 04-06b Proto-cluster
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Diagram (a) in Figure 04-06b shows the proto-cluster LBG among sundry nearby objects such as galaxies and stars. Diagram (b) in the same image is the blow-up of the core region. Analyse of these images and other data reveals some properties of this proto-cluster in remarkable agreement with computer simulations:

The green circle in Diagram (a) spans an area of about 2 Mpc radius. Members of the proto-cluster are marked in white circles. There are 8 of them around the core representing an over-density of 11-fold.
The galaxy with extreme star-burst is shown in Diagram (b) covering an area of about 0.865 Mpc. It is the optical counterpart of the sub-millimetre source.
A quasar (not shown) is detected (in X-ray and visible light) at a distance of 13 Mpc from the core. The associated black hole has a mass of about 10⁸ M_sun implying a stellar mass of about 10¹¹ M_sun for the entire system - among the most luminous and massive objects at this reshift.
The star formation rate of 1500 M_sun/year is estimated from the far-infrared luminosity of this system. It is more than 100 times the rate of an average galaxy at this redshift.
The 3 objects around the star-burst probably represent the progenitor of a massive central cluster galaxy (type cD) in later evolution. These objects are already within the radius of a typical local cD galaxy. However, the observed stellar mass in these galaxies is significantly less than the ~ 10¹¹ M_sun - 10¹² M_sun in a typical local cD galaxy, indicating that the majority of the stars have yet to form.
Actually, the galaxy spectra show absorption features of interstellar gas + young massive stars indicative of a stellar population less than 30 million years old.

Thus the observations show that this proto-cluster of massive galaxies extends over more than 13 Mpc and contains a luminous quasar, star-brust as well as a system rich in molecular gas. These massive galaxies place a lower limit of more than 4x10¹¹M_sun of dark and luminous matter in this region, in consistence with the cosmological simulations for the earliest galaxy clusters.