Galaxy Origins

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A more recent variation of the "top-down" model says that there were extremely large gas clouds that fragmented into smaller clouds. Each of the smaller clouds then formed a galaxy. This explains why galaxies are grouped in clusters and even clusters of galaxy clusters (superclusters). However, the model predicts a very long time for the collapse of the super-large clouds and fragmentation into individual galaxy clouds. There should still be galaxies forming today. One galaxy called "I Zwicky 18" was recently discovered to be a very young galaxy that began forming stars only 500 million years ago. However, it is a dwarf galaxy whose formation might better be explained by the "bottom-up" model called "hierarchical clustering".

I Zwicky 18 started forming stars only 500 million years ago

I Zwicky 18 is a nearby dwarf galaxy that started forming stars only 500 million to one billion years ago. It may be an example of what the galaxies were like over 12 billion years ago. How this galaxy remained in an embryonic state for almost the entire history of the universe is unknown. Is it an example of a "dark galaxy"---a dark matter clump with cold primeval hydrogen and helium gas---in which the gas only now got compressed enough to form stars? One variation of the "botton-up" galaxy formation model says that such "dark galaxies" are very numerous but very hard to see.

The "bottom-up" model builds galaxies from the merging of smaller clumps about the size of a million solar masses (the sizes of the globular clusters). These clumps would have been able to start collapsing when the universe was still very young. Then galaxies would be drawn into clusters and clusters into superclusters by their mutual gravity. This model predicts that there should be many more small galaxies than large galaxies---that is observed to be true. The dwarf irregular galaxies may be from cloud fragments that did not get incorporated into larger galaxies. Also, the galaxy clusters and superclusters should still be in the process of forming---observations suggest this to be true, as well.

"Spiderweb Galaxy": a galaxy in the making

The radio galaxy MRC 1138-262, also called the "Spiderweb Galaxy" is a large galaxy in the making. At 10.6 billion light years away, we see it in the process of forming only 3 billion years after the Big Bang. Note the small, thin "tadpole" and "chain" galaxies that are merging together to create a giant galaxy.

Astronomers are now exploring formation models that combine the "top-down" and "bottom-up" models. These models also incorporate the dark matter that is now known to make up most of the mass of the universe. Huge dark matter clumps the size of superclusters gather together under the action of gravity into a network of filaments. Where the dark matter filaments intersect, regular matter concentrates into galaxies and galaxy clusters. The densest places would have had more rapid star formation to make the elliptical galaxies while the lower density concentrations would have made the spiral galaxies and dwarf galaxies. In such a model, the visible galaxies ablaze in starlight are like the tip of an iceberg---the visible matter is at the very densest part of much larger dark matter chunks. Some dark matter clumps may have cold hydrogen and helium gas making "dark galaxies" that have not become concentrated enough to start star formation---I Zwicky 18 may be an example of one that did just recently.

A dark matter map was published at the beginning of 2007 that probed the dark matter distribution over a large expanse of sky and depth (distance---see the "Distribution of Dark Matter" figure below). The map is large enough and has high enough resolution to show the dark matter becoming more concentrated with time. The map stretches halfway back to the beginning of the universe. It also shows the visible matter clumping at the densest areas of the dark matter filaments (see the "Distribution of Visible and Dark Matter" figure below). The dark matter distribution was measured by the weak gravitational lensing of the light from visible galaxies by the dark matter (see the relativity chapter).

dark matter map shows increasing clumping with time

Visible matter compared with dark matter

Galaxy Collisions and Mergers


Collisions take place over very long timescales compared to the length of our lifetime---several tens of millions of years. In order to study the collisions, astronomers use powerful computers to simulate the gravitational interactions between galaxies. The computer can run through a simulation in several hours to a few days depending on the computer hardware and the number of interacting points. The results are checked with observations of galaxies in different stages of interaction. Note that this is the same process used to study the evolution of stars. The physics of stellar interiors are input into the computer model and the entire star's life cycle is simulated in a short time. Then the results are checked with observations of stars in different stages of their life.




the gravity fields of colliding galaxies are greatly distorted

Select the "Antennae Galaxies formation movie" link below to show a movie of a computer simulation from Joshua Barnes showing the formation of the Antennae Galaxies. It is a Quicktime movie, so you will need a Quicktime viewer. The red particles are the dark matter particles and the white and green are stars and gas, respectively. Other collision movies are available on Barnes' Galaxy Transformations web site and on Chris Mihos' Galaxy Collisions and Mergers website.

Antennae Galaxies formation movie (will display in another window).


Antennae Galaxies

Cartwheel Galaxy: ring galaxy


Messier 82 as seen by the Hubble Space Telescope
Messier 82 (a starburst in the M81 group)


Some satellite galaxies of the Milky Way are in the process of merging with our galaxy. The dwarf elliptical galaxy SagDEG in the direction of the Milky Way's center is stretched and distorted from the tidal effects of the Milky Way's strong gravity. The Canis Major Dwarf galaxy about 25,000 light years from us is in a more advanced stage of "digestion" by the Milky Way---just the nucleus of a former galaxy is all that is left. A narrow band of neutral hydrogen from other satellite galaxies, the Magellanic Clouds, appears to be trailing behind those galaxies as they orbit the Milky Way. The band of hydrogen gas, called the "Magellanic Stream", extends almost 90 degrees across the sky away from the Magellanic Clouds and may be the result of an encounter they experienced with the Milky Way about 200 million years ago. At least eight other streams in the Milky Way from other dwarf galaxies have been found. The Andromeda Galaxy and the Milky Way will collide with each other 4 billion years from now and over the following two billion years after that initial encounter, they will merge to form an elliptical galaxy. The smaller spiral galaxy of the Local Group, the Triangulum Galaxy (M33) will also probably merge with us after that.


M 87

If collisions and mergers do happen, then more interactions should be seen when looking at regions of space at very great distances. When you look out to great distances, you see the universe as it was long ago because the light from those places takes such a long time to reach us over the billions of light years of intervening space. Edwin Hubble's discovery of the expansion of the universe means that the galaxies were once much closer together, so collisions should have been more common. Pictures from the Hubble Space Telescope of very distant galaxies show more distorted shapes, bent spiral arms, and irregular fragments than in nearby galaxies (seen in a more recent stage of their evolution).

The Hubble Ultra Deep Field -10000 galaxies!


The formation of galaxies is one major field of current research in astronomy. Astronomers are close to solving the engineering problem of computer hardware speeds and simulation techniques so that they can focus on the physical principles of galaxy formation. One major roadblock in their progress is the lack of understanding of the role that dark matter plays in the formation and interaction of galaxies. Since the dark matter's composition is unknown and how far out it extends in the galaxies and galaxy clusters is only beginning to be mapped (and see also link), it is not known how to best incorporate it into the computer simulations. Faced with such ignorance of the nature of dark matter, astronomers try inputting different models of the dark matter into the simulations and see if the results match the observations. As mentioned in the previous section, models that use "cold dark matter" of "WIMPs" provide the best fit to the observed structures. The recent discovery of "dark energy" is another major unknown in galaxy evolution models, though its effect may be more important to the future of the universe than to the origin and early history of the galaxies in which gravity and gas dynamics played the significant role.


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last updated: May 1, 2013

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel