Our universe is a big place – looking out from Earth, we can see a huge sphere of space stretching for billions of light years in every direction, its darkness illuminated by the glow of distant galaxies like our own.
But the distribution of galaxies is not random – while roughly half are lone wanderers called field galaxies, the rest (including our own Milky Way) are gathered together in galaxy clusters, or ‘supergalaxies’ – conglomerations that may contain anything from a few dozen to a few thousand separate galaxies.
Supergalaxies essentially form the large-scale structure of the universe. Merging together at their edges to form even larger superclusters, they fill the cosmos with a network of thread-like filaments and thin sheets, surrounding enormous and apparently empty dark areas called voids.
Their distribution gives us clues to the way in which the cosmos developed, while the close encounters that occur within them are thought to play a vital role in the evolution of galaxies. They can even create an entirely new class of galaxy – monstrous giant ellipticals that are the largest star systems known, with up to 200 times the mass of the Milky Way.
The term ‘supergalaxy’ is quite loosely defined – some astronomers use it to refer to galaxy clusters on all scales, while others reserve it for only the richest and densest, classifying less impressive gatherings as mere galaxy groups. Our own Local Group, for instance, contains just the Milky Way and Andromeda galaxies, along with the smaller Triangulum spiral and several dozen much smaller dwarf systems, scattered across about 10 million light years of space. The far more impressive Virgo Cluster – some 60 million light years from Earth – incorporates around 1,300 galaxies including dozens of large spirals and ellipticals, yet it occupies a surprisingly similar diameter of just 15 million light years across. Curiously, even the most impressive and distant supergalaxies, which can contain as many as 3,000 galaxies, have similar diameters of 10-30 million light years.
Inside a supergalaxy, each individual member follows its own unique path through space, however this path betrays the influence of its neighbours. The members of a cluster are bound together in orbit around a common centre of gravity, and astronomers can measure both their speed and direction of travel by analysing the rainbow-like spectra of their light. This provides a good way of testing whether a galaxy is actually a true member of a cluster, or just a field galaxy that happens to be passing through.
Individual orbits also help to distinguish between supergalaxies whose members have been locked in their gravitational waltz for some considerable time, and those whose tracks through space are more mixed – perhaps as a result of collisions and mergers. The powerful gravity of supergalaxies draws them toward one another, leading to epic cosmic impacts or the formation of extended superclusters (the Virgo Cluster, for example, forms the core of a ‘Local Supercluster’ that stretches as far as our own Local Group).
Since the Fifties, detectors and telescopes sent into space have revealed supergalaxies are among the most powerful X-ray sources in the cosmos. These high-energy rays are produced by huge quantities of gas heated to millions of degrees, lying in the heart of supergalaxies. The distribution of this intracluster gas is patchy in smaller clusters, but smoother in the larger ones, and often centred on one or more giant elliptical galaxies at the cluster’s heart. Intracluster gas is thought to outweigh all the other luminous material in a supergalaxy by a factor of 2:1 (though this is still not enough to resolve the dark matter problem).
In a direct collision between galaxies, individual stars are usually spread out so widely that they survive unharmed. However, most galaxies are also filled with huge clouds of raw stellar material – a mix of light hydrogen and helium gas and dust, known as the interstellar medium (ISM). As these ISM clouds plough into each other, the shock can trigger spectacular ‘starburst’ events in which the rate of star birth in a galaxy can be boosted by up to a millionfold. Heated to great temperatures, some of the ISM gains enough energy to escape the galaxies’ gravity entirely, becoming intracluster gas. Here, it is soon supplemented as short-lived heavyweight stars born in the starburst die in spectacular supernova explosions and scatter their heavier elements across interstellar and intergalactic space.
This, it seems, is the reason why large elliptical galaxies are only found in the heart of rich supergalaxies. A result of repeated mergers, these galactic monsters may weigh as much as several hundred Milky Ways and contain trillions of stars. They give their origins away through the presence of huge numbers of ‘globular’ star clusters cannibalised from the galaxies that they have subsumed in the past. Sitting at the centre of a cosmic web, they exert their influence over tens of millions of light years, ruling over an entire supercluster.
Enter the void
A sense of scale
Shining a light on dark matter
What’s more, dark matter seems to be widespread throughout the universe, concentrated in and around individual galaxies and clusters. This mysterious substance is not only dark but entirely transparent in all radiations, and astronomers can only measure its presence through the gravity it exerts. Perhaps the cleverest of these techniques uses gravitational lensing – the way in which large concentrations of mass distort the path of light from more distant objects beyond them. By measuring such distortions, scientists can estimate both the mass and distribution of dark matter within them, confirming that it tends to concentrate around individual galaxies.