In the 1930's, the Swiss astronomer, Fritz
Zwicky was measuring the motions of galaxies
in a galaxy cluster called the Coma Cluster.
He found that, though they were gravitationally
bound to each other, their speeds around their
common center-of-mass indicated that they
are moving too fast to be bound.
That is, all the visible matter in those galaxies
would not provide enough gravity to hold that
cluster together.
There must be more matter there, matter not
visible through our telescopes, that is helping
to provide enough gravitational attraction
to accomplish this.
He called it 'dark matter'.
Dark matter doesn't seem to be black holes,
neutrinos or anything else we are already
familiar with.
Dark matter is not made of atoms or nuclei.
It exerts gravitational pull like ordinary
matter, but it doesn't seem to reflect, absorb
or emit electromagnetic radiation (or photons).
It is cold, in that it allows normal matter
to clump up around and inside it.
It moves slowly enough to be bound into clumps
around galaxies and galaxy clusters.
Whatever it is, there is a lot of it.
In order to learn more about what it is and
what it does, it is important for astronomers
to measure the amount of dark matter and how
it is distributed.
To quantify the amount of dark matter, astronomers
often use what is called the mass-to-light
ratio.
For example, one can use the measured flux
and distance to calculate the luminosity of
a distant galaxy (in solar luminosity units).
Once the mass of the galaxy is measured (in
solar mass units), this ratio can then be
calculated.
The greater this mass-to-light ratio, the
greater the amount of dark matter.
We first consider the measurement of dark
matter associated with galaxies.
To measure the mass of a distant spiral galaxy,
the speeds of stars and gas around the galaxy
center is measured.
This is done by observing the Doppler shift
of the 21-centimeter line of the atomic hydrogen.
It will be redshifted on one side of a spiral
galaxy and blueshifted on the other.
The Doppler shift is used to calculate the
speed of gas a certain distance from the galaxy
center.
This speed is then used to calculate the amount
of mass enclosed by that orbit.
This measurement can be done for distances
farther and farther from the galactic center.
Most of the mass of a spiral galaxy is concentrated
in its central bulge, at its center.
As one moves away from the center, one would
expect the speeds of the stars and gas to
be slower, according to Kepler's Laws.
However, as one measures speeds farther and
farther out, one finds that the speeds are
staying fairly constant rather than getting
slower (except for some gravitational effects
from the mass and gravity associated with
the spiral arms).
Even at great distances from the galactic
center, where there is no light-emitting matter,
clouds of hydrogen gas are moving at the same
speed.
Because of these speeds, if the galaxy consisted
only of normal matter, the galaxy should have
flown apart long ago.
It is the dark matter that is helping to provide
enough gravity to hold the galaxy together.
This dark matter extends to distances greater
than the visible matter in the galaxy.
It is like the galaxy is embedded in a large
clump of this dark matter.
Elliptical galaxies show little or no net
rotation.
The orbits of stars around the galactic center
are randomly oriented with a wide range of
speeds and eccentricities.
Some stars will be moving toward our line-of-sight
and will be blueshifted by various amounts
and other stars will be moving away from our
line-of-site and will be redshifted by various
amounts.
An effect of this chaotic motion is the broadened
absorption lines in the galaxy's spectrum.
The width of this absorption line can be used
to calculate what is called the velocity dispersion,
which can then be used to calculate the galaxy's
mass.
Similar to spiral galaxies, ellipticals also
seem to be embedded in larger clumps of dark
matter.
Interestingly, for both spiral and elliptical
galaxies, their mass-to-light ratios indicate
that 90-percent of these galaxies are composed
of dark matter.
For spirals, 99-percent of their halo mass
is dark matter.
This shouldn't be too surprising since there
is so little normal matter in those halos.
Three independent ways to measure the masses
of galaxy clusters will now be discussed.
The first way is the method used by Zwicky.
The distances of galaxies from their mutual
center-of-mass, and their speeds around that
center-of-mass, are measured and then used
to calculate the total cluster mass using
Kepler's Third Law.
A second method involves thermal emission
from hot gas at the center of the cluster.
Loose gas, not bound to a galaxy, is drawn
toward the cluster's center of mass.
Gas comprises about two-thirds of the normal
matter of a galaxy cluster.
The greater the cluster mass, the greater
the gravitational potential energy gas particles
begin with.
As gas particles fall toward the center, they
gain speed and thus gain kinetic energy.
Recall that the average kinetic energy of
these gas particles is the temperature of
the gas.
The greater the mass of the cluster, the higher
the temperature.
Gas is heated to tens of millions of Kelvin.
The higher the temperature the shorter the
average wavelength of radiation they emit.
Shown is the x-ray emission from a galaxy
cluster.
The last way involves gravitational lensing
as a result of the galaxy cluster curving
the spacetime around it.
When light from a more-distant background
galaxy passes the cluster, that light gets
curved, like a giant lens and the light gets
smeared out into arcs around the galaxy cluster.
Shown are the arcs of a smeared background
galaxy.
The distribution of the arcs is used to calculate
how much curvature there is in the spacetime
around the cluster and thus how much mass
the cluster must have in order to cause that
much curvature.
What is striking is that all three methods
yield the same result.
There is about 10 times more dark matter than
normal baryonic matter associated with galaxy
clusters.
Peaks in the cosmic microwave background also
confirm a 10-to-1 ratio of dark matter over
normal baryonic matter on an even grander
scale.
We now think that dark matter may have played
a major role in the formation of galaxies
and larger-scale structure in the universe.
Dark matter may have gravitationally formed
clumps that drew normal matter into it, forming
galaxies and galaxy clusters.
Without dark matter, there may have been no
structure at all.
