With a steady hand and dark matter
The mysterious Dark Matter is only noticeable through its gravity. But just this pull of gravity is said to be missing in some galaxies. A contradiction that only a simulation of galaxy crashes helps to clarify.
Sun, moon and stars! – Anyone who describes the universe in this way has forgotten the vast majority. If the crowd has its way, the children's verse should start with "Dark Matter". He doesn't, because, firstly, no child knows what "dark matter" is actually supposed to be, and secondly, the scientists have no idea either. You can't see it because it's invisible across the spectrum, it just doesn't interact with electromagnetic radiation.
We only know that they exist because of their gravity. Because if the galaxies consisted only of stars, dust, gases and the rest of the usual matter, they would have flown apart long ago. The little bit of gravity of conventional particles couldn't stand up to the centrifugal forces - there must be a mysterious unknown pulling in on the rope, namely dark matter.
So it is that galaxies are more like a kind of dust that has collected in potential wells that are created by concentrations of dark matter. The galaxies lie embedded in a gigantic halo that is far larger than the visible stars. When two get too close and their gravity wells overlap, they merge into one another in a violent collision. An elliptical galaxy is forming - and astronomers have a problem with it.
By measuring the emission lines of a particular oxygen ion in planetary nebulae ejected from dying Sunlike stars, one can infer their motion. A procedure that astronomers have now performed on numerous galaxies. To their amazement, they found that the stars in the fringes of elliptical galaxies migrated very slowly. Too slow for the gravitational pull. Apparently, so some scientists concluded, there is probably no dark matter there after all. But it has to, others countered, because otherwise all the other observations would no longer fit.
A working group led by Avishai Dekel from the Hebrew University in Jerusalem is now offering a way out of the dilemma. On their computer, the scientists simulated what would happen when two disk-shaped galaxies collided, which merged into an elliptical one. They attached particular importance to the fate of the stars in the peripheral areas and the development of their orbits. As a starting point, they took data typical of disk galaxies, including the suspected amount and distribution of dark matter.
What the computer worked out gave the astronomers a sigh of relief: it is quite normal for some stars to move slowly. In the course of the collision, these have been forced into very elongated orbits and are currently pulling in a radial direction away from the center of the galaxy. In doing so, they slow down until they reach the turning point of their path and accelerate again.
So the dark problem with dark matter in elliptical galaxies has been resolved. We still don't know what this stuff is, though. And that is the really interesting question. Because who doesn't want to know what the universe and everything else is made of?