Why do galaxies have elliptical shapes

Formation and evolution of galaxies

It is estimated that there are around two hundred billion galaxies in the universe. The shape and color of these star clusters reveal a lot about their age and the history of their development. Franziska Konitzer spoke to Thorsten Naab from the Max Planck Institute for Astrophysics in Garching about what astronomers now know about galaxies.

On a clear night there are not only countless stars to be seen in the sky. If you take a closer look in the constellation Andromeda, you can see a blurry spot of light: the Andromeda Nebula. However, this is not a diffuse gas or dust cloud, but a galaxy - similar to the Milky Way system. The astronomer Edwin Hubble first came to this conclusion in 1923. It is now known that the Milky Way and the Andromeda Nebula are only two of billions of galaxies and that there are different types of galaxies.

Thorsten Naab from the Max Planck Institute for Astrophysics

Thorsten Naab: “So there are large galaxies that we call 'Giant Galaxies'. These elliptical galaxies are the largest in the universe. There are also spiral galaxies as well as smaller galaxies. These are dwarf galaxies, and they have much less mass, so much fewer stars. They are also called irregular galaxies. An example of this are the Large and Small Magellanic Clouds. These are satellite galaxies of the Milky Way and they have much less mass than the Milky Way itself. "

As a theoretical astrophysicist, Thorsten Naab researches the formation and evolution of galaxies using computer models. A galaxy is basically defined as a collection of matter held together by gravity, i.e. gas, dust, planets and stars. The shape and color of these systems give researchers clues about the age of the galaxy.

“The elliptical galaxies are very old galaxies. They typically consist of low-mass stars because the massive stars have all exploded. The light from low-mass stars is red, which is why these elliptical galaxies are also rather reddish. Spiral galaxies, on the other hand, are very blue because there are still a lot of young stars there. Active star formation also takes place there, for example in the spiral arms of the Milky Way. "

The Milky Way System is a spiral galaxy, just like the neighboring Andromeda Galaxy. It is estimated that there are a total of around two hundred billion galaxies in the universe. Most of them are younger spiral galaxies. The next largest share is accounted for by the older elliptical galaxies. But how old is old? The Big Bang, i.e. the beginning of the universe, probably occurred around 13.7 billion years ago. Shortly after the Big Bang, the entire universe was filled with hot gas, mainly hydrogen and helium. At this point there were no stars, let alone galaxies.

Milky Way and Andromeda Galaxy on a collision course

“The first galaxies formed very early after the Big Bang. If you look at the cosmic star formation rate, i.e. when most stars were formed in the development of the universe, then there was a peak about ten billion years ago. It is assumed that - in relative terms - at this point in time most of the stars and also mainly the massive galaxies were formed. "

With the Hubble space telescope, astronomers tracked down the oldest galaxies to date: These were probably only formed around five hundred million years after the Big Bang. This suggests that the first galaxies formed very quickly.

“We know that ten billion years ago there were galaxies with 1011 Solar masses existed. That's about ten times heavier than the Milky Way. These galaxies already existed at this point in time. We therefore suspect that these galaxies are located in very over-dense areas, where the processes of star formation and collapse proceed very, very quickly and which therefore completed their development relatively early. "

The early systems were flat and spiral-like in shape. Astrophysicists explain this structure with the help of dark matter - that invisible matter that interacts with normal matter only through its gravity.

The giant elliptical galaxy NGC 1132

“We imagine that every galaxy is surrounded by a dark matter halo. And within this dark matter halo, gas can enter, which in turn emits energy in the form of radiation. So the gas falls into the center, but its angular momentum cannot simply be given off. Therefore the gas falls into the center, cools down and forms a thin disc. If the gas density is high enough in this thin disk, then stars can form there. "

While in spiral galaxies like the Milky Way the star formation rate is around three solar masses per year, in elliptical galaxies almost no stars are formed any more. Until a few years ago, researchers assumed that elliptical galaxies were created solely by the merging of two spiral galaxies. The Milky Way System could also become part of a new galaxy in the distant future: The Andromeda Galaxy and our galaxy are on a collision course. However, it will be at least five billion years before the two systems may collide. Such collisions mainly result in lower mass elliptical galaxies.

“We don't believe that all elliptical galaxies are formed this way. The reason are relatively spectacular observations in recent years. About ten billion years ago, populations of elliptical galaxies were found that are very massive but already very compact. These will probably get much larger until they match our current population. And this increase in size cannot be explained by a merging of spiral galaxies. "

The massive elliptical galaxies instead grew due to their enormous gravity. They have absorbed smaller galaxies, so to speak, and merged with them. This scenario also explains their shape.

Deep view into space

“The elliptical shape comes from the fact that these galaxies are mainly formed by star systems merging with one another that have relatively little gas. Disk galaxies can only form if there is a lot of gas in the system. But if these merging systems arise from a large number of stars, then the merging product is round. "

Researchers have now understood the rough processes involved in galaxy formation. However, through observations, they only receive snapshots that need to be linked. Computer simulations, with which a galaxy can be tracked over billions of years, help here - from gas collapse in an accumulation of dark matter to the peak of the star formation rate to merging with other galaxies. Using these models, researchers can also look a little into the cosmic future: According to this, more and more galaxies are merging with one another and forming elliptical galaxies. The lack of free gas ultimately means that no new stars can form. The universe of the distant future will be completely different from today's one, full of elliptical galaxies and filled with the reddish light of old stars.