Birth of galaxies

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At the very beginning of the universe, there was only gas clouds. So, we must assume that galaxies have been formed from small "germs", microscopic areas denser than their neighbourhood, which can attract the surrounding matter in the same way as stars which are born in molecular clouds, of course on a much larger scale.

To be able to grow with the gravitational attraction, a density fluctuation must include a mass of matter greater than a critical value called the Jeans' mass : below this value, it can only oscillate like a sound wave, but its amplitude is unable to increase.
If we consider only baryonic matter, ie matter as we can see it today, there appears a problem of time : the growth of fluctuations cann't be very rapid, because it is slowed down by the expansion of the universe. And it is quite impossible that galaxies may have been formed in such a short time in relation with the age of the universe, considering the very low amplitudes of initial fluctuations as we can see them in the cosmic microwave background.

Therefore, the two scenarios that we are exploring will have to make use of dark matter. This matter is so called "dark" because it does not interact, or only a little bit, with radiation. So we can't see it directly.

Top-down scenario

The first scenario was studied by Zel'dovich at the end of the 70's, and considers that dark matter was made up of light particles. These light particles were moving at relativistic speeds to form what is called the hot dark matter. From this fact, the Jeans' mass takes high values and the condensation of gas clouds can only happen at the scale of superclusters.

top-down scenario
These giant structures would have breaked up in smaller parts which would later become galaxies. Here comes the name of top-down scenario : from the larger to the smaller.

But there is a problem with this scenario : the necessary time for splitting of superclusters is much too long to be compatible with the presence of high redshift galaxies, as we can see them with the Hubble Space Telescope in its deep sky pictures. Following this scenario, galaxies can only form at a redshift z=2, corresponding with a 3 billion years old universe.

 

Bottom-up scenario

In this opposite scenario, dark matter is said to be cold, because it is made up of heavy and slow particles. In such a case, the Jeans' mass roughly corresponds to structures with the size of globular clusters or dwarf galaxies.

bottom-up scenario
These small proto-galaxies are first formed and then merge together to give birth to the great galactic structures as we know them today.
With such a theory, galaxies are formed at z=3 or 4. This is already a late value (corresponding age of the universe : about 1.5 to 2 billion years old), as we can observe still further galaxies.

Another problem of this scenario : how can the great structures, clusters and superclusters arise ?

 


The ΛCDM model

The bottom-up scenario is based on the hypothesis of a "Cold Dark Matter". By coupling it with the cosmological constant Λ we get a model called ΛCDM, which is the most commonly accepted nowadays.
In this model, as the black matter does not interact, it can begin to accumulate before the electromagnetic decoupling. After that, the matter can collapse in the potential wells already created by the black matter.
This mechanism will speed up the formation of the galaxies and so avoid the problem of late formation of the bottom-up model.
 
From this time, galaxies store matter :

It is demonstrated that the fusions of galaxies are much more efficient as long as they do not belong to more massive structures, like a cluster. In a hierarchical model, this corresponds to the beginning of the evolution.

For the large spiral galaxies, we get a growing by fusion of proto-galaxies, followed by an accretion of gas which will create the younger disk, where the stars are able to born.
In such a scenario, large elliptical galaxies are created after, by the fusion of spiral galaxies.
For the small elliptical galaxies, this scenario is at fault. They must then be considered as the remains of spiral galaxies which have lost their disk, or as aggregate of stellar groups.

The ΛCDM model is in fact much more than a simple model for the formation of galactic structures. It is a complete cosmological model which allows the description of the universe.
As such, it can claim for two important successes :

 

Unfortunately, beside these two successes, there are some problems with the model.
As usual, could we say...

 

To conclude, the ΛCDM model fits quite well with the observations of the universe for large structures and matter filaments, but lacks of precision at galaxies level.
It forecasts a too big quantity of black matter at the center of galaxies, with a too much concentrated density distribution (cupsy problem), which leads to too small galactic disks, with too many satellite dwarf galaxies.

We must remark that the smaller is the structure, the more non-linear are the physical phenomenons, which is always a problem for simulation.

Improving the model

To improve this model, which gives good results at the large scales, many hypothesis are tested : So, a lot of work is always necessary to fully understand the mystery of the formation of galaxies.

 


References :
Age dating of starbust galaxies (C. Leitherer)
Astrophysics and cosmology: the golden âge (M. Rowan-Robinson)
Sky surveys and deep fields of ground-bases and space telescopes (V.P. Reshetnikov)