Astronomers solve longstanding galaxy cluster collision puzzle

When galaxy clusters collide, something fascinating happens.

The colliding galaxy cluster “El Gordo,” the largest one known in the observable Universe, shows the same evidence of dark matter and normal matter separating when galaxy clusters collide, as seen in other colliding clusters. If normal matter alone is to explain gravity, its effects must be non-local: where gravity is found where the mass/matter isn’t.

The individual galaxies and collisionless dark matter simply pass through one another, unscathed.

This Hubble Space Telescope image of galaxy cluster Abell 1689 has had its mass distribution reconstructed via the effects of gravitational lensing, and that map is overlaid atop the optical image in blue. If a major interaction can separate the gas in the intracluster medium from the position of the galaxies, the existence of dark matter can be put to the test. Differences between pre-collisional and post-collisional clusters is key evidence in the conclusion that dark matter is the leading explanation for what we observe in our Universe.

But the gas within each cluster collides, heats up, and slows down.

By combining data of 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 lensed galaxies, including one that emits copious amounts of X-ray light from very early on in the Universe’s history, despite having extremely little ultraviolet/optical/infrared light. This “overmassive” 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 overall mass.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures ranging from several hundreds of thousands of degrees up to ~100 million K. Meanwhile, the fact that the gravitational effects (in blue) are displaced from the location of the mass from the normal matter (pink) shows that dark matter must be present. Without dark matter, these observations (along with many others) cannot be sufficiently explained.

In some colliding clusters, the inferred speeds are very fast: arguably too fast for modern cosmology.

The full-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 magnetic field 10 million light-years long is incredibly illuminating. One important lesson is that the predominant population of gas within a galaxy cluster is in the intracluster medium, rather than the galaxies themselves: just like the overall mass within the cluster.

But do we have the correct speeds? Perhaps not.

This four-panel animation shows the individual galaxies present within Abell 2744, Pandora’s Cluster, alongside the X-ray data from Chandra (red) and the lensing map constructed from gravitational lensing data (blue). The mismatch between the X-rays and the lensing map, as shown across a wide variety of X-ray emitting galaxy clusters, is one of the strongest indicators favoring the presence of dark matter. The Bullet Cluster, as well as other galaxy clusters, exhibit similar features.

Most cluster collisions are seen face on: perpendicular to our line-of-sight.

The Bullet Cluster, the aftermath of a galaxy cluster collision that occurred 3.8 billion years ago in a region of space located ~3.7 billion light-years away, represents very strong evidence for the existence of dark matter. The separation of the gravitational effects (blue, reconstructed through gravitational lensing) from the location of the majority of the normal matter (pink, revealed by Chandra’s X-ray capabilities) is very difficult to explain without dark matter’s presence.

But others are observed head-on: like watching a collision from behind.

The full-field image of MACS J0717.5+3745 shows many thousands of galaxies in four separate sub-clusters within the large cluster. The blue contours show the inferred mass distribution from the gravitational lensing effects on background objects. Not shown in this diagram is the X-ray data, which shows an offset between the X-ray emitting gas, which traces the normal matter distribution, and these blue contours, which map out the total mass, including dark matter. This cluster collision occurred largely along the line-of-sight, explaining its apparent messiness.

One interesting test case is MACS J0018.5+1626.

This illustration shows colliding galaxy cluster MACS J0018.5, although rotated to look like what we’d see if viewing it face-on, rather than head-on. The dark matter is shown in blue, sailing ahead of the gas, while the hot gas slows down and exhibits shocks, in orange.

Its collision, along the line-of-sight, creates expansive X-ray and radio emissions.

The X-ray data of colliding galaxy cluster MACS J0018.5+1626, as shown in color, also emits radio signals, as shown in the contours. This is an example of a head-on collision between two galaxy clusters, totaling more than a quadrillion solar masses all told.

We can measure these motions through CMB heating via the kinetic Sunyaev-Zel’dovick effect.

The Planck satellite’s measurements of the CMB temperature on small angular scales can reveal enhancements or suppressions of temperature by tens of microkelvin induced by the motions of objects: the kinetic Sunyaev-Zel’dovich effect. We can measure this for individual galaxy clusters as well as colliding clusters, and determine the motion of matter within them.

Despite the presence of shocks, the collision is only at ~3000 km/s, or 1% the speed of light.

The left hand 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 the optical depth of the intracluster medium (bottom). The head-on nature of this collision makes it uniquely informative.

Novel simulations indicate that normal matter separates much earlier than previously recognized.

Experiencing shocks, turbulence, and frictional effects, normal matter lags behind the dark matter, even early on.

Two simulations of colliding galaxy clusters, showcasing normal matter and dark matter in different colors. The left simulation, from 2007, implies enormous collisional speeds. A more modern one, from 2024 (at right), shows about half the speed, while reproducing the same shock signatures observed.

The head-on nature of MACS J0018.5+1626 reveals both normal matter’s and dark matter’s speeds.

Even though the colliding cluster 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 such as this, like the Bullet Cluster. With lower relative velocities needed to produce these observed X-ray (and radio) features, a puzzle for our consensus cosmology has now been solved.

Slower collisional speeds, plus comprehensive gas effects, aligns with ΛCDM cosmology.

These animations show the simulated evolution of the intracluster medium and the dark matter speeds decoupling from one another, along with the total density, gas densities, and gas temperature. The changing angle shows how the DM vs. gas velocity dipoles misalign throughout the collision process.

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