Small galaxies have significantly less visible matter than big ones. A new model enables ETH Zurich researcher Lucio Mayer to explain why this is so. According to the model, the phase during which small and large galaxies come close together is decisive.

The majority of the universe is invisible to us. 90 percent of the cosmos consists of dark matter and dark energy, the exact composition of which has puzzled physicists for a long time. However, precisely which particles make up dark matter is not the only mystery, since it is also unclear why the proportion of visible matter, also known as baryonic matter and which makes up the world we can physically experience, is so different in the various galaxies. Large galaxies like the Milky Way or the Andromeda Nebula for example have about 10 percent visible matter whereas the smaller galaxies that orbit around the big ones have significantly less baryonic matter. Thus the smaller ones are the darkest galaxies in space.

Pressure in the halo

One widespread theory that is supposed to explain this unequal distribution postulates that in the early phase of the universe there was a tendency for visible matter to be preferentially attracted by large galaxies. The problem here is that this theory leads to contradictions with the current cosmological model. In the latest issue of the scientific journal “Nature” (1), Lucio Mayer, holder of a Swiss National Foundation (SNF) Professorship at the Institute of Astronomy of ETH Zurich, together with researchers from the University of Zurich and a Canadian colleague now present a model that can explain the non-uniform distribution of visible matter without contradicting the standard model. Mayer explains that every galaxy has a halo of bright gaseous matter. When a small galaxy comes near a larger one, enormous attraction forces from the larger galaxy at once come into play. This causes the small galaxy to become distorted. The phase during which the small galaxy passes through the larger one’s halo is now decisive. The gaseous baryonic matter of the small galaxy is exposed to a hydrodynamic pressure in the larger galaxy’s halo and as a result it is literally “blown away”. Mayer’s computer simulation illustrates how one must imagine it in concrete terms: a real tail of bright gaseous matter develops downstream of the small galaxy.

The timing is decisive

Under the prevailing conditions of today’s universe,, this phenomenon scarcely causes a small galaxy to lose any significant amount of visible matter because the majority of it is bound up in the stars. However, the situation looked rather different more than 10 billion years ago. At that time the majority of baryonic matter in small galaxies had not yet been compressed to form stars. That is why a small galaxy passing through the halo of a large galaxy under the conditions of that time lost a large fraction of its visible matter.

This approach not only allows Mayer to explain why small galaxies have much less bright matter than large ones, but also why they often have a distorted shape. The researcher can also solve another enigma that has preoccupied astronomers for a long time. Ten to twenty small galaxies have been discovered up to now around the Milky Way or the Andromeda Nebula. However, five to ten times as many would actually have been expected. Mayer theorises that in reality the number of such small galaxies is many more than we can observe. If a small galaxy passed through the halo of a large galaxy at a very early time when the small one still had no stars at all and the visible matter was present only in gaseous form, then the small galaxy lost practically the whole of its bright matter. Since then, a galaxy of this kind has consisted only of dark matter and therefore we can no longer observe it directly.

Mayer says that astrophysicists have argued about dark matter intensively in recent years. “If one really wants to understand how the universe developed in the course of time, then one really must also understand what happens to the bright matter even if it forms only a small fraction of the universe.”