The Distribution of Galaxies

Long before the nature of galaxies was fully understood, astronomers were carefully mapping out their positions on the sky. It soon became clear that they are not entirely uniformly spread through space: galaxies tend to appear in small groups, and in some extreme cases many thousands of these objects are packed into “clusters” that occupy small regions of the sky.

With the advent of computers, it became possible to automate the mapping process by scanning in photographic plates of large areas of the sky and analysing the images automatically. These surveys revealed that the distribution of galaxies contained even larger scale structure, with clusters tending to group together into “superclusters.” There are even indications that the galaxies outside clusters tend to be arranged into enormous filament-like structures many millions of light years in length. However, just measuring the locations of galaxies on the sky, we obtain only a two-dimensional view of the Universe, and much of the detail is lost by projecting everything into such a flat image. Some measure of the distances to galaxies is needed if we are to discover the full three-dimensional picture.

The APM Galaxy Survey

A map showing the distribution of some three million galaxies in the southern sky. The picture’s intensity is scaled so that brighter pixels contain more galaxies. The colouring shows the average brightness of the galaxies in each pixel, with redder colours showing fainter objects. The irregular shape of the map is dictated by the photographic plates that the computer analyzed, and the “holes” are regions around bright stars where no galaxies could be detected. (Image by courtesy of S. Maddox, W. Sutherland, G. Efstathiou and J. Loveday).

Red-Shifts and the Third Dimension

Fortunately, a relatively simple measurement can be made to estimate the distance to a galaxy. If the light from a galaxy is passed through a prism, the resulting rainbow-like spectrum is found to contain dark bands at particular colours. These bands occur because the chemicals in the galaxy absorb light of very specific colours, so these colours are missing from the light that we detect. Early in the 20th century, it was discovered that these bands do not occur at exactly the same colours in all galaxies, and that they tend to be shifted toward the red end of the spectrum – a phenomenon known as “redshift.”

The simplest explanation for this redshift is that the galaxies are all receding from us, and the Doppler shift moves the features to redder wavelengths (analogous to the drop in frequency of sound from a receding police car’s siren).

The reason for this recession was understood in the 1920s when Edwin Hubble demonstrated that there is a simple relationship between the velocity of each galaxy, v, and its distance from us, D. He found that

v = H0 x D,

where H0 is now known as “Hubble’s constant.”

This simple formula, “The Hubble Law,” says that the velocity at which a galaxy is receding is directly proportional to its distance from us. Thus, for example, an object ten times as far away as a nearby galaxy will be receding from us at ten times the speed of the closer system.

A simple physical explanation for this relation comes from asking how long it must have taken the galaxies to travel from nearby to their current distances. The answer turns out to be the same for all galaxies: those at larger distances have had further to travel, but their greater speed gets them there in the same time. It seems that all the material in the Universe started flying apart simultaneously in a single huge explosive event, now generally known as “The Big Bang.”

The Hubble Law also provides a straightforward tool for mapping the positions of galaxies: by obtaining a spectrum of a galaxy, one can determine its redshift and hence its velocity. Using the Hubble Law, this velocity can be translated directly into the galaxy’s distance, allowing its location to be determined in all three dimensions. A “redshift survey” of many such measurements then provides a three-dimensional map of the Universe.

The Two-Degree Field

One of the shortcomings of the first redshift surveys undertaken in the 1980s was the amount of telescope time that was needed to acquire the data. Spectra might take an hour each to obtain, so that measuring the redshifts of even a few thousand galaxies could require months or even years of observations.

The 2dF mounted on the Anglo-Australian Telescope. This view is looking down the body of the telescope to the 3.9-metre diameter primary mirror. Light collected by this mirror is reflected back up to the 2dF instrument, which is the large cylindrical apparatus in the foreground. The robot positions fibres at one end of the cylinder while light from the telescope is collected in the fibres at the other, and the whole apparatus then rotates around to swap over the fibres in use. (Image courtesy of the 2dFGRS Team.)  

The solution was to obtain many spectra simultaneously by collecting the light from lots of galaxies within the telescope’s field of view, splitting each one’s light into a separate spectrum, and recording it. The Two Degree Field (“2dF”) system on the 3.9-metre Anglo-Australian Telescope in Siding Springs, Australia, has been designed for exactly this process. A robot positions 400 fibre-optic cables in the focal plane of the telescope to collect light from hundreds of galaxies simultaneously, and these fibres then channel the light into spectrographs, which record a spectrum for each observed galaxy. It can take the robot an hour to carefully position all the fibres, but no time is wasted waiting for it: the system has two complete sets of fibres, so while one set is being used to collect spectra of galaxies, the other set is being positioned ready for the next observation. Using this equipment, astronomers have been able to construct a three-dimensional map of the positions of more than 200,000 galaxies.

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