Anthony P. Fairall
Superclusters are large conglomerations of clusters and groups of
galaxies on a scale exceeding 100 million light years [1021 km or 30
megaparsecs (Mpc)]. Individual superclusters and ``complexes'' of
superclusters interconnect to form the largest known structures in the
universe: a sponge-like network of high-density regions , spaced apart
by a labyrinth of voids (Fig. 1). The tendency
in recent years is for
ever-larger structures to be recognized, so that we may not have yet
established the top of the hierarchy of clustering, that is, the
largest inhomogeneities in the universe. The greatest strides have
been made since the mid-1970s, as large numbers of galaxy redshifts
have been obtained.
The first examinations of the large-scale distribution of galaxies
were carried out in the 1930s independently by Edwin Hubble and Harlow
Shapley. Hubble worked in numerous narrow selected fields. Shapley
covered most of the sky using wide-angle photographs. He noted regions
where the galaxy count was much higher than average and labeled these
as ``clouds'' of galaxies. Many of his clouds are recognized today as
superclusters.
Improved wide-angle photographic surveys, such as the classic
National Geographic Society-Palomar Observatory Sky Survey, do reveal
hundreds of thousands of galaxies, but give only a two-dimensional
view of their distribution. To some extent, the third dimension
(distance) can be gauged by the angular diameters of the galaxies. A
nearby galaxy appears larger than a more distant one, although one may
sometimes be misled by a small nearby galaxy mimicking the appearance
of a larger distant galaxy. However, there would be little problem in
deciding between nearby and distant clusters of galaxies. In this way,
in the 1950s and 1960s, George Abell and Fritz Zwicky independently
gauged the relative distances of clusters on the Palomar Sky
Survey. Abell concentrated on rich clusters, whereas Zwicky's cluster
boundaries lay far from the central condensations. Abell also noted
that certain regions of the sky had a greater number of clusters than
others. Thus, both suspected much larger entities. In a similar way,
C.D. Shane, working from Lick photographs, described ``superclusters''
or (as he preferred to describe them, using Shapley's term) ``clouds of
galaxies.''
The true recognition of superclusters required a three-dimensional
view. When distances to relatively nearby galaxies were calibrated,
Gerard de Vaucouleurs advocated the existence of a ``supergalaxy,'' now
recognized as our local supercluster.
At larger distances, the most effective way of obtaining galaxy
distances is to measure redshifts. Due to the overall expansion of the
universe, an observer in any galaxy would see its neighbors moving
away from it. The velocity of recession increases with distance from
the observer's galaxy according to Hubble's well known relation
where V is the velocity of recession, d is the distance,
and H0 is the
Hubble constant (in the range 50-100 km s-1 Mpc-1
or 15-30 km s-1 per
million light-years). The velocity can be measured by the Doppler
shift of spectral features (redshift): either absorption or emission
lines in the optical spectra, or 21-cm neutral hydrogen emission in
the radio region. knowing the velocity, the distance can be inferred.
Strictly speaking the observed velocity is not entirely cosmological
but should be expressed as
where the velocity V0 is now corrected for our Sun's
motion within the
Galaxy and for the streaming motion of our galaxy, Vs
is the streaming
motion associated with the observed galaxy, and
Vp is the observed
galaxy's own individual motion over and above systematic streaming;
Vp
is significant (several hundred kilometers per second) in rich
clusters. Because it is difficult to disentangle these velocities,
and because of the uncertainty in the value of the Hubble constant, it
is customary to plot data in ``redshift space'' (with dimensions shown
as kilometers per second) rather than in conventional
three-dimensional space. Gross structures are much the same in either
space, but the peculiar velocities in rich clusters make the clusters
appear stretched radially (the ``Finger of God'' effect) in redshift
space (examples of this can be seen in Fig. 2).
In the past, obtaining the spectrum of a single galaxy called for a
photographic exposure of some hours duration. The advent of electronic
image intensifiers and the replacement of the photographic plate by
charge-coupled devices (CCDs) and Reticon arrays has greatly
accelerated the acquisition of redshifts. Around 1950 little more than
100 redshifts were known, by 1960, 1000 were known and twice that
number had been collected by 10 years later. Yet, by 1980 the figure
exceeded 10,000 and by 1990 it was around 40,000.
With greater numbers of redshifts available in the mid-1970s, Guido
Chincarini pointed out that, even when clusters are avoided in a
study, redshifts seem to favor certain values and to avoid others (for
the region of sky involved). Thus superclustering was revealed in the
third dimension, and three-dimensional mapping became possible. Much
pioneering work was done in the region of the Coma cluster where a
bridge to a neighboring cluster, Abell 1367, was discerned, with a
void immediately in front of the structure.
In order to map completely the neighboring superclusters to our own
``Virgo supercluster,'' one would need to work out to
a redshift
corresponding to several thousand kilometers per second. Within such a
volume of space, there are hundreds of thousands of giant
galaxies. Even with present technology, it is quite impossible to
obtain all their redshifts (the number to be observed would be much
greater because a vast number of background galaxies would have to be
candidates). Thus, it is necessary to restrict observations to a
``representative'' sample. The most common approach is to observe all
galaxies brighter than a selected limiting apparent luminosity
(apparent magnitude). Such a choice is satisfactory for the
capabilities of the telescopes involved, but nearer low-luminosity
galaxies are included in the sample, whereas distant high-luminosity
galaxies may be excluded. The data thin with distance, and low
galactic latitudes (where light from distant objects is subject to
extinction by matter in the MilKy Way) have to be
avoided. Nevertheless, knowledge of the galaxy luminosity function
(the relative numbers of galaxies versus luminosity) allows one to
derive true number densities and other statistics. Difficulties could
arise if the luminosity function varies with environment; such
tendencies have been claimed in the literature. The alternative
approach is to disregard a strict magnitude limit and to observe what
appears to constitute a representative sample on the sky. This allows
for initial mapping of superclusters (mainly because the intervening
voids are almost completely empty), but cannot produce quantitative
parameters. Whatever system of sampling is used, the outcome is plots
revealing filamentary or sponge-like structures, reminiscent of
aqueous media even though the data are in the form of discrete points.
Figure 1 gives an indication of neighboring
superclusters. The
nearest of these is the Hydra-Centaurus-Pavo supercluster. The names
reflect the main constellations in which the structure is seen on the
sky (although constellations are based on nearby stars in our galaxy,
they conveniently represent general directions when looking far beyond
those stars). Although the Hydra-Centaurus portion is seen on the sky
on one side of the Milky Way and the Pavo portion on the other side,
the agreement in redshift and general continuity of structure point to
its being a single entity, with the main bulk lying in Centaurus or
probably behind the foreground obscuration of the Milky Way. The Hydra
condensation centers around the Hydra I cluster (redshift 3500 km
s-1) and there is only a relatively weak bridge to the Centaurus
concentration. The latter is dominated by the Centaurus cluster, which
shows a composite structure in redshift space, with concentrations at
both 3000 and 4500 km s-1. However, the weaker 4500 km
s-1
concentration may be, to some extent, background galaxies because 4500
km s-1 is the dominant redshift for the bulk of the extended
Centaurus
superclustering. The same redshift (4500 km s-1) is picked up on the
Pavo side, which contains a number of weaker clusters. All these
clusters contain both elliptical and spiral galaxies, but the cores of
both Hydra I and Centaurus have greater proportions of elliptical and
S0 galaxies. There seem to be some filamentary links between our own
Virgo supercluster and the Centaurus concentration,
almost as if our
supercluster were something of an appendage. We are separated from the
The Coma supercluster, mentioned earlier, is separated
from our own
supercluster and Hydra-Centaurus by a number of voids. Its structure
is centered on the Coma cluster, the nearest rich cluster of galaxies
(composed almost entirely of elliptical and S0 galaxies). From this
central concentration, extensions run out more or less perpendicular
to our line of sight. Although that running (west-wards) to the Abell 1367 cluster was the first discovered,
present-day surveys
(particularly the Harvard-Smithsonian ``slices'') show it extending both
east, north, and southward, so much so that it has been referred to as
the Great Wall, and its full extent is still being assessed.
Although the Virgo, Centaurus, and Coma superclusters dominate the
northern galactic hemisphere, the other side of the obscuring band of
the Milky Way is dominated by the Perseus-Pisces supercluster (which
has been mapped extensively by the radio redshifts of Martha P. Haynes
and Riccardo Giovanelli). Particularly interesting in this
supercluster is a filamentary central condensation that is well marked
by elliptical galaxies. It is some 4000 km s-1 long in
redshift space
and runs perpendicular to the line of sight. Toward one end lies the
Perseus cluster. Voids again intervene between this supercluster and
ours their peripheral galaxies provide tenuous
interconnections. Toward the south the supercluster continues and
connects to another heavy wall-like structure, in the Sculptor region,
that runs at an angle to our line of sight.
It has been remarked that these surrounding structures give a sort
of ``tree ring,'' appearance to the distribution within distances from
our galaxy that correspond to redshifts of several thousand kilometers
per second. More redshifts still are needed to define the nature and
extent of such larger patterns. What is relevant is that, whenever a
volume of space is sampled, there always seems to be structure with a
dimension comparable to that of the volume surveyed. This has led to
considerations of fractal structures (identical forms repeated on
ever-increasing scales) occurring in the universe. If this is correct,
although one would gain a geometrical interpretation to the nature of
the structures, it would make a physical explanation extremely
difficult.
Beyond redshifts of several thousand kilometers per second, a number
of further superclusters have been mapped tentatively. The volume is
incompletely sampled, although it is unlikely that very conspicuous
superclusters would have been overlooked. For example, in the north
there is a pair of superclusters in Hercules (at redshifts around
10,000 km s-1), whereas in the south Shapley's cloud of galaxies in
Horologium is resolved into two superclusters (at redshifts of 12,000
and 18,000 km s-1) seen along a common line of sight (see
Fig. 2).
An alternative approach for reaching out to larger distances is to
assume that Abell's clusters, which mark peak number densities, flag
the high points of superclusters. Thus, distant superclusters can be
recognized as groupings (in redshift space) of Abell clusters, and
examinations of even larger volumes of space can be carried out. On
this basis, ever larger conglomerations (to scales of 30,000 km
s-1) have been claimed. The work recently has been extended
to the southern skies.
The future holds exciting prospects. Just as human eyes scanned the
galaxies on the wide-angle photographs, the finest-quality photographs
of the U.K. Schmidt telescope and the new Palomar sky surveys are now
being scrutinized by machine and millions of galaxies already have
been detected and cataloged. From these sky densities comes evidence
of possible larger superclusters. In parallel, the development of
fiber-optic spectrographs that can be used to observe many galaxies
simultaneously may lead to the mass determination of redshifts. We can
look forward to finding even more remarkable structures in the
superclustering of galaxies.
SUPERCLUSTERS, OBSERVED PROPERTIES
NEARBY SUPERCLUSTERS
MORE DISTANT SUPERCLUSTERS
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See also Clusters of Galaxies; Galaxies, Local Supercluster;
Superclusters, Dynamics and Models; Voids, Extragalactic.