A titanic collision between two galaxy clusters is creating the most massive collection of galaxies ever observed in the distance Universe. The observation of this new super-cluster, nicknamed ‘El Gordo’ after the Spanish for ‘fat one’, fits in neatly with models of dark matter and dark energy in the Universe.

Galaxies tend to arrange themselves in clusters at the nodes of the ‘cosmic web’ – the filaments of matter distributed across the Universe. Our Milky Way exists in a small cluster, the Local Group, which also incorporates the Triangulum and Andromeda spiral galaxies as well as a handful of diminutive dwarf galaxies. The Local Group, however, exists close to the edge of the Virgo Supercluster, a gargantuan agglomeration of at least 1,300 galaxies. Large clusters are common in the local Universe, but become more scarce the farther back in time we look. We see El Gordo (official designation: ACT-CL-J0102-4915) as it was seven billion years ago at a redshift of 0.87. It’s not the most distant galaxy cluster ever seen, given that some have been identified at almost ten billion light years (see our news story here) but it is the most massive, measuring in at 2 x 10¹⁵ (2,000 trillion) times the mass of the Sun. To put that into context, the mass of the Milky Way is on the order of a trillion solar masses. El Gordo is composed of two smaller clusters that are merging at velocities of several million kilometres per hour.

El Gordo was discovered by the six-metre Atacama Cosmology Telescope at the European Southern Observatory in Chile after a disturbance in the cosmic microwave background (CMB) radiation was detected. The CMB is the now faint radiation emitted in the first light after the big bang and its now long-cooled microwave photons permeate the Universe. When these photons interact with electrons in hot gas the CMB becomes distorted. The more hot the gas – like in the environment of a galaxy cluster – the greater the distortion, which is known as the Sunyaev-Zel’dovich effect. One of these distortions tipped off a team led by Felipe Menanteau of Rutgers University in New Jersey, USA and, using the Atacama telescope, they identified El Gordo.

Subsequent observations by NASA’s orbiting Chandra X-ray Observatory measured the position and abundance of hot X-ray emitting ‘intra-cluster’ gas and found the distribution to be almost identical to previous studies of other colliding galaxy clusters such as the famed Bullet Cluster, which has a similar mass (about 3 x 10¹⁵ solar masses) to El Gordo (see our previous news story here). In the Bullet Cluster the galaxies themselves had become separated from the intra-cluster gas, but there was something else. Gravitational lensing, whereby more distant objects in the background are magnified by the mass of the cluster warping space-time, revealed that there was unseen, dark matter present in the cluster, that had decoupled from both the gas – the hot gas had slowed during the collision, but the dark matter had drifted right through. This is strong evidence for the existence of dark matter.

“For El Gordo we have made the educated assumption that galaxies are a good tracer of dark matter,” Menanteau told Astronomy Now. “So we have used the galaxies in the cluster in lieu of dark matter maps from galaxy lensing and we see the gas and galaxies (and dark matter) are decoupled, just as in the Bullet Cluster.”

To be sure, Menanteau’s team plan on making Hubble Space Telescope observations of the cluster in order to identify any weak gravitational lensing that may hint at where the dark matter is present.

The detection of the cluster also tells us about the strength of dark energy at the time the cluster was forming. Dark energy is the mysterious force that is causing the expansion of the Universe to accelerate. As it does so, it makes it more difficult for galaxy clusters to grow as it is continually working against gravity, pulling small galaxy groups away from one another. By measuring the mass and growth rate of clusters through cosmic history, it is possible to constrain the strength of dark energy.

“The mass of El Gordo, although rare, is still consistent with the predictions of the current cosmological model and it validates it independently via the growth of structure,” says Menanteau. “However, a merger like El Gordo is not expected, so we hope that its discovery will shed light in the merging process of clusters in the early Universe.”