The Milky Way is not alone: it is accompanied by several smaller satellite galaxies forming a belt around it – and this has puzzled astronomers for decades. Because actually there shouldn’t be such a galaxy ring. The satellite galaxies should be randomly distributed around the Milky Way, according to the standard cosmological model. So what unknown force is forcing the galaxies into this plane? However, as an international research team has now shown on the basis of more precise measurements, this supposed cosmic mystery is not a mystery at all. The galaxies therefore move on completely different levels; that their arrangement looks like a ring is just coincidence. The scientists write that the supposed belt will dissolve again within a few hundred million years in the journal Nature Astronomy.
“The companions of the Milky Way are almost aligned in the sky – and this has puzzled astronomers for decades,” explains co-author Carlos Frenk of Durham University in the UK. Because if the satellite galaxies are aligned in the sky, they must all be within a thin disk in space. “And explaining that is extraordinarily difficult for cosmology.” So difficult that it has already cast doubt on the standard cosmological model and, in particular, on the researchers’ idea of the mysterious dark matter.
Because according to current knowledge, the visible matter – i.e. stars, planets or gas clouds – only contributes a fifth to the total mass of the cosmos. Eighty percent of the mass consists of dark matter. This is completely invisible and reveals itself only through its power of attraction. Dark matter plays a crucial role in the formation of galaxies in the cosmos.
The Gaia space telescope records the position and movement of about a billion stars
Computer simulations show how normal matter accumulates in large concentrations of dark matter. These concentrations are spherical, and the galaxies that form within them are randomly distributed. A concentration in a disk cannot be explained in this model and is also not shown in the computer simulations.
Carlos Frenk and his colleagues have now used the best data to date from the European Space Agency’s Gaia space telescope to solve this mystery. Gaia has been in space since 2013 and provides precise position and motion data for around a billion stars not only in our Milky Way, but also in its satellite galaxies.
The evaluation of the data led to a surprise. Until now, astronomers had assumed that the small galaxies always moved on orbits within the disk. But that is not the case. The conclusion of Frenk and his colleagues: The currently observed concentration of satellite galaxies in a disk is pure coincidence. Within a few hundred million years – cosmologically that is a short period of time – the disk will be gone.
The team then took another look at the best computer simulations of galaxy formation to date. And in fact, such random discs could also be detected there. Until now, such random phenomena had been overlooked because researchers had only been looking for evenly rotating disks from satellite galaxies. “We have thus solved one of the currently greatest challenges of the cosmological model,” says Frenk happily. “The dark matter model gives us a remarkably accurate description of how the Universe evolved.”