When galaxy clusters collide, something fascinating happens.
The colliding galaxy cluster “El Gordo”, the largest known in the observable Universe, shows the same evidence for separation of dark matter and normal matter during the collision of galaxy clusters as observed in other colliding clusters. If normal matter alone can explain gravity, its effects must be nonlocal: gravity is found where mass/matter is not present.
Individual galaxies and collisionless dark matter simply pass through each other, undamaged.
This Hubble Space Telescope image of the galaxy cluster Abell 1689 has had its mass distribution reconstructed using gravitational lensing effects, and this map is overlaid on the optical image in blue. If a major interaction can separate the gas in the intra-cluster medium from the position of galaxies, the existence of dark matter can be put to the test. The differences between pre-collisional and post-collisional clusters are key evidence to conclude that dark matter is the dominant explanation for what we observe in our Universe.
But the gas inside each cluster collides, heats up and slows down.
By combining data from the Pandora cluster, Abell 2744, obtained by the infrared JWST space telescope and the sensitive Chandra X-ray space observatories, scientists were able to identify a number of lensed galaxies, including one that has been emitting large amounts of X-rays since the early universe, although it receives very little ultraviolet/optical/infrared light. This “supermassive” black hole holds key information about the formation and growth of black holes.
This creates an observed separation between the light-emitting gas and the gravitational effects of the bulk mass.
X-ray (pink) and bulk matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, which is one of the strongest evidences for the existence of dark matter. X-rays come in two types, soft (low energy) and hard (high energy), where galaxy collisions can create temperatures ranging from several hundred thousand degrees to about 100 million K. At the same time, the fact that gravitational effects (blue) are displaced from the location of the mass of normal matter (pink) shows that dark matter must be present. Without dark matter, these observations (and many others) cannot be adequately explained.
In some colliding clusters, the deduced velocities are very fast: probably too fast for modern cosmology.
The large-scale image of the colliding galaxy clusters Abell 399 and Abell 401 shows X-ray data (red), Planck microwave data (yellow), and LOFAR radio data (blue) all together. The individual galaxy clusters are clearly identifiable, but the radio bridge of relativistic electrons connected by a 10 million light-year long magnetic field is incredibly illuminating. An important lesson is that the predominant population of gas within a galaxy cluster is in the intra-cluster medium, rather than in the galaxies themselves: as is the overall mass within the cluster.
But do we have the right speeds? Maybe not.
This four-panel animation shows individual galaxies in Abell 2744, the Pandora Cluster, along with Chandra X-ray data (in red) and the lensing map constructed from gravitational lensing data (in blue). The mismatch between the X-rays and the lensing map, as seen in a wide variety of X-ray emitting galaxy clusters, is one of the strongest indicators for the presence of dark matter. The Ball Cluster, as well as other galaxy clusters, exhibit similar features.
Most cluster collisions are viewed head-on: perpendicular to our line of sight.
The Ball Cluster, formed by a collision of galaxy clusters 3.8 billion years ago in a region of space about 3.7 billion light-years away, is very strong evidence for the existence of dark matter. The separation of gravitational effects (blue, reconstructed by gravitational lensing) from the location of most normal matter (pink, revealed by Chandra’s X-ray capabilities) is very difficult to explain without the presence of dark matter.
But others are observed from the front: as if watching a collision from behind.
The full-field image of MACS J0717.5+3745 shows several thousand galaxies in four distinct subclusters within the larger cluster. The blue contours show the mass distribution inferred from gravitational lensing of background objects. The X-ray data are not shown in this diagram because they show a mismatch between the X-ray-emitting gas, which traces the normal distribution of matter, and these blue contours, which map the total mass, including dark matter. This cluster collision occurred largely along the line of sight, which explains its apparent disorder.
An interesting test case is MACS J0018.5+1626.
This illustration shows the galaxy cluster MACS J0018.5 colliding, but rotated to look like what we would see if we were looking at it head-on, rather than straight on. Dark matter is shown in blue, sailing ahead of the gas, while the hot gas slows down and shows shocks, in orange.
Its collision, along the line of sight, creates significant X-ray and radio emissions.
X-ray data from the colliding galaxy cluster MACS J0018.5+1626, shown in color, also emit radio signals, as shown by the contours. This is an example of a head-on collision between two galaxy clusters, totaling more than a quadrillion solar masses in total.
We can measure these movements by heating the CMB via the Sunyaev-Zel’dovick kinetic effect.
Measurements of the CMB temperature at small angular scales by the Planck satellite can reveal temperature increases or decreases of several tens of microkelvins induced by the motions of the objects: the kinetic Sunyaev-Zel’dovich effect. We can measure this effect for individual galaxy clusters as well as for colliding clusters, and determine the motion of matter inside them.
Despite the presence of shocks, the collision only takes place at about 3000 km/s, or 1% of the speed of light.
The left column shows the relative motions of individual galaxies (top) and the intracluster medium (bottom) within MACS J0018.5, while the right column shows the total projected mass (top) and the optical depth of the intracluster medium (bottom). The head-on nature of this collision makes it particularly informative.
New simulations indicate that normal matter breaks apart much earlier than previously thought.
Experiencing shocks, turbulence and friction effects, normal matter lags behind dark matter even at the beginning.
Two simulations of colliding galaxy clusters, showing normal matter and dark matter in different colors. The simulation on the left, from 2007, implies enormous collision speeds. A more modern simulation, from 2024 (right), shows about half the speed, while reproducing the same observed shock signatures.
The frontal nature of MACS J0018.5+1626 reveals the velocities of both normal matter and dark matter.
Although the colliding cluster MACS J0018.5 is unusually fast for a pair of colliding clusters, it is still much slower than previous estimates for the speed of clusters such as this one, such as the Bullet cluster. With lower relative velocities required to produce these features observed in X-rays (and radio), a conundrum for our consensus cosmology has now been solved.
Slower collision speeds, as well as full gas effects, align with ΛCDM cosmology.
These animations show the simulated evolution of the intra-cluster medium and the decoupling of dark matter velocities, as well as the total density, gas densities, and gas temperature. The changing angle shows how the DM dipoles vs. gas velocity misalign throughout the collision process.
Mostly Mute Monday tells an astronomical story in pictures, visuals and 200 words or less.