What are Galaxy Clusters or Supergalaxies

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 enormous mass of galaxies makes them naturally gregarious – their gravity extends across millions of light years of surrounding space, influencing the motion of other galaxies passing nearby and, if conditions are right, pulling them into orbit. Large galaxies like the Milky Way and the nearby Andromeda spiral move through space with a halo of smaller galaxies in orbit around them. Close encounters or collisions with these satellite galaxies are common on a cosmic timescale, and can result in the smaller galaxy being completely cannibalized and absorbed. Big galaxies also influence each other over larger distances – for instance, Andromeda and the Milky Way are being pulled towards each other and are doomed to collide in around 4 billion years’ time.

On the largest scales, gravitational attraction also plays a key role in the evolution of supergalaxies, but it is not enough to explain their concentration in filaments and sheets in the first place. In the 13.7 billion years since the universe was born from the Big Bang, there has simply not been enough time for galaxies to come together under the influence of gravity alone. Instead, astronomers think that supergalaxies were born out of denser knots of matter in a universe that had already separated into large regions of differing density in the immediate aftermath of the Big Bang.

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.

Measuring the orbits of individual galaxies, especially on the supergalaxy’s outer edges, can also provide useful data like the cluster’s overall mass. This led, in the Thirties, to the discovery that supergalaxies contain far more mass than their luminous contents suggest -one of the first hints that a large proportion of cosmic mass is mysterious dark matter.

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).

The crowded conditions inside supergalaxies mean that even their larger individual members may be separated by just a few times their own diameter, or even less. As a result, spectacular intergalactic close encounters and even head-on collisions are common. Close encounters typically unleash powerful tidal forces that may cause spiral arms to unwind into long tails stretching across space, or create shockwaves that trigger new waves of star birth, resulting in a variety of ‘peculiar’ galaxies, each unique in appearance.

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.

Experts think that such collisions are responsible for generating most of the intracluster gas that lies between galaxies. They also help change the structure of galaxies themselves, using up or driving out the raw materials of star formation, while at the same time mixing up the orbits of surviving stars into a chaotic melee. The end result is that a gas-rich system such as an irregular or spiral galaxy may ultimately be transformed into a spheroidal or elliptical galaxy – a system devoid of ISM or young stars, and dominated by older stars whose overlapping orbits form a more or less structureless ball or ellipse.

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

Between the filaments and sheets of supergalaxies lie enormous gulfs of space known as voids. Ranging in size between 40 million and 500 million light years, they are defined by a lack of clusters or superclusters, but are still home to a few scattered ‘field galaxies’. Along with superclusters, they were identified for the first time in the Harvard-Smithsonian Center for Astrophysics (CfA) Galaxy Redshift Survey, which attempted to map out the large-scale universe for the first time from 1977-82. Today, voids are thought to have arisen as a result of pressure waves called baryon acoustic oscillations (BAOs) that rippled through the fireball of the early universe and helped concentrate both dark and baryonic (luminous) matter in certain regions, while leaving others empty. The closest void to Earth – known as the Local Void – is around 200 million light years across, with the Local Sheet (including the Virgo Cluster and our own small Local Group of galaxies) defining one of its edges.

A sense of scale

Galaxies like our Milky Way are so big that light takes 100,000 years to cross it, travelling at 300,000 kilometres (186,000 miles) per second. The commonly used measure of distance – a light year – is equivalent to 9.5 trillion kilometres (5.9 trillion miles); with figures this large, it’s little wonder astronomers prefer not to use everyday units! In a supergalaxy, large galaxies are separated by a couple of million light years at most – just a few times their own diameter. In contrast, light takes little more than a second to travel between the Moon and Earth, and about 500 seconds to arrive at Earth from the Sun. To put it another way, if our world was a one-millimetre (0.04-inch) dot, the Moon would be just three centimetres (1.2 inches) away, but our galaxy would stretch as far as the Sun, and the Local Group supergalaxy would be about the size of our Solar System out to the orbit of Neptune.

Shining a light on dark matter

In 1932, Swiss-American astronomer Fritz Zwicky made the first attempt to measure the motion of individual galaxies within a supergalaxy, targeting the rich Coma Cluster some 320 million light years from Earth. When he discovered that the motion of the galaxies was much faster than the cluster’s visible matter could account for, he coined the phrase ‘dunkle materie’ (dark matter) to describe it. Zwicky believed that his dark matter outweighed luminous material in the Coma Cluster by around 400 times, but the discovery of intracluster gas, along with improved measurements, now suggests that dark matter accounts for approximately 85 per cent of the supergalaxy’s mass.

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.