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Superclusters
of Galaxies
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Structure is observed on larger scales than that of clusters. The next
larger scale corresponds to
superclusters,
which can have extents of
order 100 Mpc. All known rich clusters are parts of larger superclusters,
suggesting that perhaps all galaxies belong to one supercluster or another, but
this is not yet certain.
An Example: the Local Supercluster
Our own galaxy
and its Local Group
belong to a supercluster called the
Local Supercluster. It
similar in shape to a flattened ellipse (pancake), with the
Virgo Cluster near its center and the Local Group near one end; its extent in
the longest direction is about 40-50 Mpc. The large concentration of galaxies
on the left side of the following diagram represents part of the Local
Supercluster (see this
explanation).
Other nearby superclusters
include Perseus-Pisces at a distance of about 70 Mpc and
Hydra-Centaurus, which is approximately 45 Mpc distant
(with the constellations in the names giving the approximate location on the
celestial sphere; thus, Perseus-Pisces lies partially in the constellation
Perseus and partially in Pisces, as viewed from the Earth).
Motion within the Local Supercluster
Shortly we shall discuss the
expansion of the Universe. This
general expansion, which increases the distances between galaxies steadily
with time, is called the Hubble flow. Deviations of the velocity
of a galaxy from the overall Hubble flow is termed the peculiar
velocity. By examining the peculiar velocities of clusters and
superclusters we can obtain estimates of local mass concentrations that may
be responsible for causing the deviation from the Hubble flow.
Mass of the Local Supercluster
As we shall see, the Virgo Cluster at a
distance of approximately
16 Mpc from
us should, by the
Hubble Law,
be receding from us at a velocity of
about 1100 km/s.
However, the measured recessional velocity of the Virgo Cluster is
approximately 170 km/s less than this. This difference, which is termed
the Virgocentric peculiar velocity
of the Local Group, presumably is due
to the higher than average
gravitational attraction felt between the Local Group
and the rest of the Local Supercluster.
This may be used to estimate a mass of 1015 solar masses
for the Local Supercluster.
The corresponding mass to light ratio for the Local Supercluster is
about 570, expressed in units of the ratio of the solar mass to the solar
luminosity,
indicating the presence of large amounts of dark matter in the Local
Supercluster.
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