Learn · Other galaxies
Galaxy groupings — from group portrait to cosmic web
Next to the long disk of NGC 891 the image holds tiny oval patches of light, and around NGC 7331 four small galaxies stand like chicks around a hen. Both look like neighbourhood. In fact we are looking through several layers of depth — from a star of our Milky Way a few hundred light-years away to a galaxy cluster a quarter of a billion light-years out. To sort that out you need a feel for the orders of magnitude.
The ladder of scales
A galaxy like the Milky Way measures about a hundred thousand light-years. The next step, our Local Group, is a hundred times larger: some ten million light-years, with two large spirals and about a hundred known, mostly tiny members. The nearest galaxy cluster, the Virgo Cluster, lies a good fifty million light-years away and gathers more than a thousand galaxies. Clusters and groups line up into superclusters of a few hundred million light-years; our own, Laniakea, comprises about a hundred thousand galaxies. The largest known structures, walls and filaments over a billion light-years long, are already a noticeable fraction of the observable universe with its roughly 93 billion light-years of diameter. Each rung of the ladder is about a power of ten larger than the previous one — and each has its own physics. NASA: Large-scale structures
Groups: the rule, not the exception
Most galaxies live in groups — assemblies of a handful of bright and many faint members, bound together by their shared dark matter. The Local Group is a typical example: two large spirals, the Milky Way and M 31, each with its own retinue of dwarf galaxies, plus M 33 as the third large one and dozens of small systems. Groups are where galaxies most often merge: relative velocities of one to two hundred kilometres per second are low enough for an encounter to bind. Andromeda and the Milky Way are approaching each other for exactly that reason. Nor are groups static: the Local Group as a whole is falling towards the Virgo Cluster at a few hundred kilometres per second — too slowly ever to reach it, because the expansion of the universe is faster.
Clusters: hot gas and dark matter
A galaxy cluster is a different class of object. The Virgo Cluster or the Coma Cluster gather thousands of galaxies in a volume of ten to twenty million light-years, and their total mass reaches a thousand times that of the Milky Way. As early as 1933 Fritz Zwicky noticed that the galaxies in the Coma Cluster move far too fast for their visible mass to hold them together — the first hint of dark matter. Today we know: the galaxies are the smallest part of a cluster. Between them lies a tenuous gas heated to ten to a hundred million degrees that glows in X-rays and contains several times as much mass as all the stars of the cluster combined. And both together are again only a sixth of the dark matter, which can be mapped directly through the deflection of light from distant galaxies — gravitational lensing. Clusters change their members: a spiral plunging into the hot cluster gas has its own cold gas swept out — the headwind alone is enough. That is why elliptical and lenticular galaxies dominate cluster centres, and why a cluster is the fastest way to turn a spiral into a red system. Zwicky (1933): The redshift of extragalactic nebulae; Clowe et al. (2006): Direct detection of dark matter in the Bullet Cluster
Filaments, walls and voids
How are the clusters themselves distributed? The answer came in 1986 from a survey that mapped thousands of galaxy distances: galaxies do not sit evenly in space but on thin walls and filaments enclosing huge, almost empty voids — like the skins of a foam. The famous “Great Wall” stretches over more than five hundred million light-years; the Boötes Void is an emptiness over three hundred million light-years across with only a handful of galaxies. Later surveys with millions of galaxies have confirmed and refined this picture. The structure is the direct consequence of gravity: matter first falls out of the voids onto sheets, then along the sheets into filaments, and finally along the filaments into the nodes — where the rich clusters sit. Simulations such as the Millennium Simulation reproduce this cosmic web from a few initial conditions, provided dark matter is included. Voids fill most of the volume but contain only a small fraction of the galaxies; the filaments carry roughly half of all matter. And they probably also hold a good part of the “missing” ordinary matter of the universe: a gas too hot to shine in hydrogen lines and too thin for ordinary X-ray telescopes. de Lapparent, Geller & Huchra (1986): A slice of the universe; Springel et al. (2005): The Millennium Simulation
AI-generated illustration of the cosmic web: bright nodes and fine filaments stand for denser regions of matter, dark gaps for voids. Colours and brightness serve the visualisation; this is neither a sky photograph nor a computed simulation. This image is AI-generated — not a photograph but a visualisation.
Superclusters: Laniakea and the limits of binding
A supercluster gathers groups and clusters on still larger scales — but unlike a group or cluster it is not a gravitationally bound object. On these scales the expansion of the universe wins, and the accelerating expansion will pull the superclusters apart in the distant future, while the Local Group stays together as a bound system. How to delimit a supercluster at all was answered anew by a 2014 study: not by the density of galaxies but by their motion. One maps where the galaxies are falling and gathers everything that streams towards the same centre of attraction — like a river's catchment basin on a map. Defined this way, the Milky Way belongs to Laniakea, a region of a good five hundred million light-years whose flows converge on the “Great Attractor” behind the plane of the Milky Way. The older term “Local Supercluster” or “Virgo Supercluster” denotes only the inner part of it. “Galaxy grouping” is therefore a good umbrella term; group, cluster, filament and supercluster say more precisely which kind of structure is meant — and whether it is bound. Tully et al. (2014): The Laniakea supercluster; Chon et al. (2015): How to define superclusters
Measuring depth: redshift as a tape measure
How do we know that the small patches next to NGC 891 are eight times further away than the galaxy itself? Not from their size — a small nearby galaxy and a large distant one look alike. From their light. The universe is expanding, so distant galaxies recede from us, the faster the further away they are: about 21 kilometres per second for every million light-years. The motion stretches the wavelength of the light, and in the spectrum all known lines shift towards the red by the same factor. From this redshift follows the recession velocity and from that the distance. NGC 891 recedes at about 500 kilometres per second, the galaxies of the cluster Abell 347 behind it at some 5000 — so they are about ten times as far away. The method has a limit at close range: galaxies also have peculiar motions of a few hundred kilometres per second, in rich clusters over a thousand; M 31 is even coming towards us. Within the Local Group redshift is therefore useless as a distance measure; beyond a hundred million light-years it is the most important one we have.
The examples: NGC 891 and NGC 7331
NGC 891, at about 30 million light-years, lies well in front of the cluster Abell 347 at some 240 million light-years, which belongs to the Perseus–Pisces Supercluster — one of the most conspicuous filaments of the nearby universe, running as a chain of clusters across the autumn sky. Our markers identify individual catalogued galaxies in this field; whether each one is a cluster member is decided by its redshift, not by its position in the image. With NGC 7331 the situation is similar: the galaxy itself is about 45 million light-years away, the four small “companions” of the Deer Lick Group stand at about 300 million — the group name describes an arrangement on the sky, not a physical system. Half a degree further south, outside our field, lies Stephan's Quintet, the textbook example of the same problem: four of its five galaxies really do interact at about 290 million light-years, the fifth happens to stand in front at 40 million. Smaller apparent size alone is never a reliable distance gauge. NASA/APOD: NGC 891 and Abell 347; Hubble: NGC 7331 in Pegasus
A few things worth knowing:
- We are moving. Relative to the cosmic microwave background, the most restful frame of reference there is, the Local Group flies through space at about 600 kilometres per second — the sum of all the attractions of the cosmic web around us.
- How many galaxies there are is open. Estimates for the observable universe range from a few hundred billion to two trillion, depending on how many faint dwarfs are counted. The number mostly tells how incomplete our census is at the small end.
- A cluster does not see its galaxies as points. In cluster centres encounters are too fast for mergers but frequent enough to warp disks and strip gas. The “jellyfish galaxies” with their trailing gas tails are the result.
- The web is dark. Stars and gas merely trace it. The structure itself is carried by dark matter, which weighs five times as much as all ordinary matter combined — which is why the illustration above shows structures of matter, not threads of light.
- It will not last. Because the expansion is accelerating, superclusters and filaments will have been pulled apart in a few hundred billion years. Only what is bound today stays bound: groups and clusters — and with them the Local Group, merged by then.
In my own sky
Four fields in which several layers of depth overlap — from the dust of the Milky Way to the galaxy cluster:
