Using the NASA / ESA Hubble Space Telescope, a team of astronomers has discovered that the brightest galaxies within galaxy clusters “wobble” in relation to the center of mass of the cluster. This unexpected result is inconsistent with the predictions made by the current standard dark matter model. Does it indicate the presence of a new physics?
Dark matter constitutes about 27% of all matter and energy in the universe, but it cannot be observed directly, so it is one of the greatest mysteries of modern astronomy. The invisible halos of the elusive dark matter enclose galaxies and clusters of galaxies alike. The clusters are massive groupings of up to 1,000 galaxies immersed in hot intergalactic gas, whose clusters have very dense nuclei, each with a massive galaxy called “the brightest galaxy“.
Current models of dark matter predict that galaxy clusters have very dense cores, and those cores contain a very massive galaxy that never moves from the center of the cluster, but instead is held in place by the enormous gravitational influence of matter dark
However, the international team formed by Swiss, French and English astronomers has analyzed 10 galaxy clusters observed with Hubble, discovering that these galaxies were not in a fixed position in the center as expected and that density is much lower than foreseen: the galaxy of the center really moves.
The researchers compared their observations with the predictions of the BAHAMAS series of cosmological hydrodynamic simulations and found that they did not coincide. According to the Standard Model of dark matter (called “cold dark matter”), this wobble does not exist because the enormous density of dark matter keeps it firmly attached to the center of the cluster of galaxies. Therefore, this clear mismatch suggests the existence of physics still unknown.
These galaxy clusters also act as immense gravitational lenses that deform space-time enough to distort the light that passes through them. Hence, we can use them to map dark matter, determine where the center is and then observe how the brightest galaxy wobbles or oscillates around this center.
“We discovered that they ‘overflow’ at the bottom of the haloes, indicating that, instead of a dense region at the center of the cluster of galaxies, there is a much shallower central density: a striking sign of exotic forms of dark matter right in the heart of galaxy clusters, “explains David Harvey, co-author of the work published by the Monthly Notices of the Royal Astronomical Society.
The wobble also shows that these galaxies can not exactly match the halo of the cluster, which means that certain models of galaxy clusters must also be modified.
Scientists will expand their research with broader studies of galaxy clusters and hope this will allow them to confirm their findings:
“We look forward to more extensive surveys, such as the Euclid survey, which will expand our data set, and then we can determine if the wobble of these galaxies is the result of a new astrophysical phenomenon or a new fundamental physics , both would be exciting!” , says Frederic Courbin, co – author of the study.