The past of the galaxies



How do the galaxies evolve? We have seen that the angular momentum and the density of the initial gas cloud can act upon the shape of the galaxies, but can we found other factors able to influence this shape? Especially, can the shape of a galaxy be modified, or in other words, is the Hubble sequence an evolutionnary one ?

When we look faraway, we also look far into the past. And this allows us to conduct a general survey of the evolution of galaxies. Of course, we cannot see the same galaxies at different stages, but we can have a good idea, by galaxy type.

Galaxies were formed from clouds of gas at the beginning of the universe. These clouds were agglomerated by gravity, and inside them, molecular clouds appeared, which allow the formation of stars.
So, we can assume that when we look far into the galaxies' past, we will find more star forming areas, and hence more young, hot stars, hence bluer ones.

A possible scenario for the evolution of galaxies begins with a great number of dwarf irregular galaxies, full of gas. These galaxies will condense, by gravitational interactions, into well shaped galaxies, spiral or elliptical ones, depending upon initial conditions, like the angular momentum.

faraway galaxies are more blue

Computer simulations show that galaxies should have been more numerous in the past, leading to more collisions between galaxies. By passing near each other, they are attracted by their mutual gravity, and so are able to encounter.

But, unlike two solid objects, like cars, when they collide, two galaxies simply give birth to a new galaxy.

Galactic antenna
LThe two galaxies skimmed each other, and the result is a bending of the arms, i.e. a change in the orbits of the stars. This change can result in such structures, like these antennae.
Cartwheel Galaxy
Here there was a head-on collision : not only are the galaxies out of shape, but the dust disks of the two galaxies have disappeared.

Cartwheel galaxy, 150 Mpc away in the constellation of Sculptor.

Source NASA/HST.

In such a collision, there is no risk for the stars, because they are widely separated from each other. We only see a distortion of the two galaxies.
Usually, this distortion déformation conduit en général les galaxies en contact à perdre leur forme initiale. That is how spiral galaxies can transform into elliptical ones, the inverse evolution is impossible.

On the other hand, if there is a head-on collision, the gas disks will strike. They will be compressed, hence heated : the result will be an outburst of star formation, and in extreme situations, the two gas and dust disks can diseappear.
In the above example, the Cartwheel galaxy, one of the galaxies on the right of the picture has crossed the disk of the left one, throwing it away with a speed greater than 300 000 km/h. The shock wave has favoured the formation of stars in the ring, hence its blue color.

If one of the two galaxies is much larger than the other one, it can 'swallow' the little one ( the expression is 'galactic cannibalism'). Because of this, it can loose its initial shape and become elliptical. It is in this way that we can explain the frequent presence of a giant elliptical galaxy at the center of the clusters.


So, we have two aspects to the evolution of galaxies :

0,5 billion years 1 billion years 2 billion years
0-0.5 billion years : regular distribution of matter, start of gathering by gravity of the biggest parts. 0.5-1 billion years : start of structuring by merging of small blocks. 1-2 billion years : the various mergings reach a fraction of the Milky Way : the Hubble Telescope is able to see them.
4 billion years 13 billion years
2-4 billion years : owing to mergings and collisions, appearance of massive and irregular objects. 4-13 billion years : galaxies have gained their final shape, elliptical galaxies have formed first.

Summary of the evolution of galaxies in the universe.
Source Sam Pascarelle (Arizona State University)


By looking faraway in the universe, therefore far in the past, we will see a greater density of blue galaxies than in our nearby neighborhood. And that is exactly the result of the deep sky observations of the Hubble Space Telescope.

Deep Field
This deep sky picture was taken in the constellation of Hercules.
One can see 18 galaxies, 11 billion light years away, in an area less than 2 million light years wide. (such an area corresponds to 1/100th of the diameter of the full moon).
Each galaxy is between 30 and 50 times smaller than the Milky Way.



References :
The Dynamical Evolution of Galaxies in Clusters (J. Dubinski)
Collision-induced galaxy formation (J.E. Devriendt)