Galaxy Clusters and Large-Scale Structure of the Universe
Groups and clusters and superclusters of galaxies
Galaxies are preferentially found in groups or larger agglomerations called
clusters. The Local Group (seen below) consists of our own galaxy, the larger
spiral galaxy Andromeda (M31) and several smaller satellites, including the
Large and Small Magellenic Clouds.
Fornax (seen below) is a small cluster of spiral and elliptical galaxies near
our Local group.
Regular clusters have a concentrated central core and a well-defined spherical
structure. These are subdivided according to their richness, that is, the number
of galaxies within 1.5 Mpc of the centre (known as the Abell radius). Typically,
they have a size in the range 1-10Mpc and a mass M ~ 10^15 solar masses (one
followed by 15 zeros, that is, a million billion suns). The Coma cluster shown
here is a very rich cluster with thousands of ellipticals inside the Abell radius.
The central region of the Coma cluster (seen abiove) is populated with large
elliptical galaxies. This is one of the densest known regions on this scale
in the universe.
Irregular clusters have no well-defined centre, a similar range of sizes, but
they are generally poorer with a mass 10^12 - 10^14 solar masses (i.e. a thousand
to a hundred thousand million suns). An example is the nearby Virgo cluster.
Virgo (seen above), an irregular cluster, is the nearest large cluster of
Superclusters: These usually consist of chains of about a dozen clusters which
have a mass of about 10^16 solar masses (ten million billion suns). Our own
Local Supercluster is centered on Virgo and is relatively poor having a size
of 15Mpc. The largest superclusters, like that associated with Coma, are up
to 100Mpc in extent. The system of superclusters forms a network permeating
throughout space, on which about 90% of galaxies are located.
The Great Attractor: Measurements of peculiar velocities---deviations away
from the Hubble flow - are achieved by comparing redshifts and galactic distance
indicators. These have revealed enormous coherent motions on scales in excess
of 60Mpc. Consistent with these flows, our own galaxy is moving at about 600km/s
towards a distant object dubbed the `Great Attractor'. This lies at a distance
of 45Mpc and has a mass approaching 5x10^16 solar masses.
Voids, sheets & filaments: Deep redshift surveys reveal a very bubbly
structure to the universe with galaxies primarily confined to sheets and filaments.
Voids are the dominant feature and have a typical diameter of about 25Mpc. They
fill about 90\% of space and the largest observed, Bootes void, has a diameter
of about 124Mpc. Other features that have been observed are the `Great Wall',
an apparent sheet of galaxies 100Mpc long at a distance of about 100Mpc.
The CfA survey (below and left)showing large scale structures out to a distance
of 150 Mpc, that is, about 2% of the distance to the edge of the observed universe.
Galaxy positions are plotted as white points and large filamentary and sheet-like
structures are evident, as well as bubble-like voids (CfA).The APM survey (below
and right) show the same kind of filamentous structure.
Deep field surveys
A particularly exciting recent development in the study of large-scale structure
has been the advent of very deep galaxy surveys, notably those currently being
made by the Hubble Space Telescope. These images (below) show galaxies just
a couple of billions years after the Big Bang. One of the remarkable puzzles
presented by this work is that galaxies appear to form earlier than predicted
in most theoretical models.
The Universe is a pretty big place. To get an idea of just how much is out there
for our eyes to see, go to the Hubble Deep Field Lab and do just such an estimate.
Otherwise, you can go back to your learning about Galaxies,
Cosmological Principle, or simply drop into the Syllabus.
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