When clusters of galaxies collide, something fascinating happens.
The “El Gordo” colliding galaxy cluster, the largest known in the observable universe, shows the same evidence of separation of dark matter and normal matter when galactic clusters collide as in other collision clusters. If gravity is to be explained by normal matter alone, its effects must be non-local: where gravity is found where matter/mass is not.
Without collisions, individual galaxies and dark matter simply pass through each other unscathed.
This Hubble Space Telescope image of the galaxy cluster Abell 1689 has been reconstructed using gravitational lensing, and this map is overlaid in blue over the optical image. If a large interaction can separate the gas in the intercluster medium from the position of the galaxies, the existence of dark matter can be put to the test. The differences between pre-collision and post-collision clusters are key evidence in concluding that dark matter is the main explanation for what we observe in our universe.
But the gas in each cluster collapses, heats up, and slows down.
By combining data from Pandora’s cluster, Abell 2744, from the infrared JWST and from the X-ray-sensitive Chandra space observatories, scientists were able to identify a number of lenticular galaxies, including one that emits large amounts of X-ray light from a very early age. in the history of the universe, despite having extremely little UV/optical/infrared light. This “supermassive” black hole holds key information about the formation and growth of black holes.
This creates the observed decoupling between the light-emitting gas and the gravitational effects of the total mass.
X-ray (pink) and total mass (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, one of the strongest evidences for dark matter. X-rays come in two varieties, soft (lower energy) and hard (higher energy), where colliding galaxies can produce temperatures ranging from a few hundred thousand degrees to ~100 million K. Meanwhile, the fact that gravitational effects (in blue) are displaced from the mass location away from normal matter (pink) shows that dark matter must be present. Without dark matter, these observations (along with many others) cannot be adequately explained.
In some collision clusters, the inferred velocities are very high: probably too fast for modern cosmology.
A full-scale image of the collision clusters of galaxies Abell 399 and Abell 401 showing X-ray data (red), Planck microwave data (yellow), and LOFAR radio data (blue) combined. Individual clusters of galaxies are clearly identifiable, but the radio bridge of relativistic electrons bound by a magnetic field 10 million light-years long is incredibly illuminating. One important lesson is that the predominant gas population in a galaxy cluster is in the intracluster environment rather than in the galaxies themselves: as is the total mass in the cluster.
But do we have the right speeds? Maybe not.
This four-panel animation shows the individual galaxies present in Abell 2744, the Pandora cluster, along with X-ray data from Chandra (red) and a lensing map created from gravitational lensing data (blue). The discrepancy between the X-rays and the lensing map, as shown in a wide range of X-ray emitting galaxy clusters, is one of the strongest indicators favoring the presence of dark matter. The Bullet cluster, like other galaxy clusters, shows similar features.
Most collisions in clusters are seen head-on: perpendicular to our line of sight.
The Bullet Cluster, the result of a galaxy cluster collision that occurred 3.8 billion years ago in a region of space ~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 viewed from the front: like looking at a collision from behind.
A full-field image of MACS J0717.5+3745 shows many thousands of galaxies in four separate subclusters within the large cluster. The blue contours show the inferred mass distribution from the effects of gravitational lensing on the background objects. Not shown in this diagram is the X-ray data that shows the shift between the X-ray emitting gas, which follows the normal mass distribution, and these blue contours, which map the total mass, including dark matter. This cluster collision took place mostly along the line of sight, which explains its apparent disorder.
One interesting test case is MACS J0018.5+1626.
This image shows the colliding galaxy cluster MACS J0018.5, although rotated to look like what we would see when viewed from the front, rather than from the front. Dark matter is shown in blue, floating ahead of the gas, while hot gas is slowing down and showing shocks, in orange.
Its line-of-sight collisions produce extensive X-ray and radio emissions.
X-ray data of the colliding galaxy cluster MACS J0018.5+1626, shown in colour, also emit radio signals, as shown in contours. This is an example of a head-on collision between two galaxy clusters with a total volume of more than a quadrillion solar masses.
We can measure these motions by heating the CMB through the kinetic Sunyaev-Zel’dovick phenomenon.
Planck’s CMB temperature measurements on small angular scales can reveal temperature increases or decreases of tens of microkelvins induced by the motions of objects: the kinetic Sunyaev-Zel’dovich effect. We can measure it for individual clusters of galaxies as well as clusters and determine the movement of matter in them.
Despite the presence of shocks, the collision is only at ~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 projected total mass (top) and optical depth of the intracluster medium (bottom). The frontal nature of this collision makes it uniquely informative.
New simulations show that normal matter separates much earlier than previously recognized.
Ordinary matter, which experiences shocks, turbulence, and frictional effects, lags behind dark matter, even at the beginning.
Two simulations of colliding galaxy clusters, showing normal matter and dark matter in different colors. The left simulation from 2007 implies enormous collision rates. The more modern one from 2024 (right) shows about half the speed while reproducing the same observed shocks.
The direct nature of MACS J0018.5+1626 reveals both normal matter and dark matter velocities.
Although MACS J0018.5 is exceptionally fast for a pair of colliding clusters, it is still much slower than earlier estimates for the speed of clusters like this one, such as the Bullet Cluster. With the lower relative velocities required to produce these observed X-ray (and radio) elements, the puzzle for our consensus cosmology is now solved.
Lower collision rates and complex gas effects are consistent with ΛCDM cosmology.
These animations show the simulated evolution of the environment inside the cluster and the separation of dark matter velocities along with the total density, gas densities and gas temperature. The changing angle shows how the DM dipoles vs. the gas velocities do not converge during the collision process.
Mostly, Silent Monday tells an astronomical story in images, visuals and no more than 200 words.