University of California, San Diego
Center for Astrophysics & Space Sciences

H. E. Smith   Winter 2007

Physics 7 - Lecture Summary #16

Clusters of Galaxies; Dark Matter

Clusters of Galaxies
Most galaxies are found in gravitationally-bound groups called "clusters". Clusters can be rich, with several thousand galaxies, or poor, with only 20 or 30 members. The Local Group, the cluster to which our own Milky Way galaxy belongs, is made up of about 30 galaxies.

Clusters of galaxies are classified by their properties;richness (number of members),shape (spherical,flattened, or irregular),and galactic content (spiral-rich,spiral-poor, or elliptical-rich),for example. Some are strong radio sources, while others emit x-rays. The richest nearby cluster is Virgo, 60 million light years from the Milky Way. It contains about 2500 galaxies, mostly ellipticals.

The Local Group of Galaxies

We reside in a small group called the Local Group which is dominated by two giant spiral galaxies, Andromeda and our own Milky Way. In addition to Messier 33, an intermediate mass Sc galaxy, there are 15 ellipticals and 13 irregular galaxies in the cluster, including the Magellanic Clouds, our Galaxy's satellites, Messier 32 and NGC 205, satellites of Andromeda. The group has a size of about 3 million l.y., and has a total mass of 5 x 1012M

The Virgo Cluster

The Virgo Cluster, about 50 million l.y. away, is the nearest regular cluster of galaxies with several hundred members. Our Local Group is an outlying member of a "supercluster" of galaxies of which the Virgo Cluster is the dominant member.

The Hercules Cluster of Galaxies

The Hubble Space Telescope has provided the first opportunity to look back into the early universe at clusters. Billions of years ago, clusters contained many more spiral galaxies than they do today. They were probably disrupted over time by collisions and mergers within the clusters.

HST Image of CL0024+1654

CL 0024+1654 is a large cluster of galaxies located 5 billion light-years from Earth. It is distinctive because of its richness (large number of member galaxies), and its magnificent gravitational lens. The blue loops in the foreground are lensed images of a spiral galaxy located behind the cluster.

Cluster Links

Gravitational Lenses

Einstein's General Theory of Relativity demonstrates that a large mass can deform spacetime and bend the path of light. So, a very massive object, such as a cluster of galaxies can act as a gravitational lens. When light passes through the cluster from an object lying behind it, the light is bent and focused to produce an image or images of the source. The image may be magnified, distorted, or multiplied by the lens, depending upon the position of the source with respect to the lesnsing mass.

Schematic Diagram of a Galaxy acting as a Gravitational Lens

The characteristics of the gravitationally lensed image depend upon the alignment of the observer, the lens and the background object. If the alignment is perfect, the resulting image is an Einstein Ring, shown to the left. This object, discovered by Radio Astronomers at England's Jodrell Bank Radio Observatory, then imaged in the near infrared with HST is probably a distant background galaxy lensed by an intermediate distance Elliptical galaxy, shown as the bright image at the center of the ring. The Ring is estimated to be about 10-20 times brighter than the background galaxy would appear without lensing.

If the alignment is not perfect, then multiple images are formed rather than a ring. The object to the right, called the Einstein Cross, shows four images of a distant quasar at a redshift, z=1.7, imaged by an intervening spiral galaxy with z=0.04. HST images have been processed to separate the galaxy and quasar images.

A gravitational lens is most effective if it is near midway between the observer and the distant object which is being lensed. The amplification, ratio of the lensed brightness of an object to its unlensed value, is larger if the line of sight passes very close to the lens. Lenses can amplify brightnesses by a few times, up to over a factor of 100 - this means that lenses offer the potential to "see" objects over 10 times further away.

Microlensing of a star in the LMC by an object in the Milky Way, from the MACHO Project

We see three types of gravitational lenses:

  1. Stars/Remnants/Brown Dwarfs/Planets - As an object in the Milky Way passes in between us and a distant star, it will focus and amplify the light of the background star as shown in the light curve above. Several events of this type have been observed in the Large Magellanic Cloud, a small galaxy near our Milky Way. MACHO Project
  2. Galaxies- Massive galaxies can also act as gravitational lenses. Light from a source lying behind the galaxy is bent and focused to create an image or images of the source. Because the mass in a galaxy is not evenly distributed, the images are often deformed or magnified.
  3. Clusters of Galaxies- As shown above for CL 0024+1654, a massive cluster can create multiple images of a distant object lying behind it. Cluster gravitational lenses allow us to observe objects that are much too distant or too faint to be seen directly. And, since looking out to greater distances means looking farther back in time, we gain access to information about the early universe.

Gravitational Lens Links

Dark Matter

Observations of clusters and their galaxies, have uncovered one of the major mysteries in astronomy today. Clusters appear to be very stable entities- they contain mature galaxies with old stars, and seem to have been formed billions of years ago. But, when we calculate the amount of mass in a cluster using the orbital motions of its member galaxies, the result is too low for the cluster to be gravitationally bound. If the cluster contains only the mass we can observe, the gravitational force is insufficient to prevent the galaxies from "escaping".There must be more mass in the cluster than what we see.

Rotation Curves for 3 Spiral Galaxies - Galaxy Image(left), Spectrum (center - photographic negative), & Plot (right).
The flatness of the rotation curve with no downward turn indicates that the mass distribution extends far beyond
the measured values, probably in the form of a massive halo of dark matter.

The same problem arises when we look at the galaxies themselves. The rotation curve of a galaxy shows how the orbital velocities of the stars change with distance from the center. If the galaxy rotated as a solid disk, the velocity would increase linearly with distance. If most of the mass were concentrated at the the center, as in our solar system, the velocities of the stars would decrease with the square root of the distance. But, that is not what is observed. Far past the point where no mass is visible, the rotation curves are flat! This means that the mass is still increasing as we move outward, even though we can't see anything! One again we have to call upon "dark matter". The galaxy must extend much farther out than the luminous matter indicates. In fact, the calculations require that there be at least 10 times more mass than we can see! Calculations suggest that this dark matter is likely to be in an extensive halo of dark matter.

The nature of this dark matter or "missing mass" is unknown. There are theories ranging from the bizzare to the mundane, none of which successfully answer all of the questions.

Dark Matter Links

Gamma-Ray Bursts

Gamma-Ray Bursts are turning out to be one of the most explosive new areas of astronomical research. Gamma-ray bursts were discovered in 1967 by the Vela Satellites launched to monitor compliance with the 1963 Nuclear Test-Ban Treaty by detecting gamma-rays from atmospheric nuclear tests. Fortunately, it was quickly established that the bursts are of extraterrestrial origin (no, not ET!) rather than some rogue state initiating nuclear war. Initially the number of bursts was too small and the ability to pinpoint the bursts too poor to determine their origin.

Gamma-ray burst observed by BATSE
In 1991 the Compton Gamma-Ray Observatory carried its Burst and Transient Source Experiment (BATSE), built with participation by UCSD's high energy astrophysicists. into orbit. The gamma-ray bursts last from a few seconds to a couple of minutes and, until very recently, were undetected at other wavelengths. BATSE has observed a couple thousand bursts and determined that they are distributed uniformly around the sky.

Optical "Afterglow" of a Gamma Ray Burst in a Galaxy estimated to be !2 Billion light-years away.

Gamma Ray Burst Links

Quasars & AGN   Galaxies   Physics 7 Lectures   Physics 7 Home  

Conducted by Gene Smith, CASS/UCSD.
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Prof. H. E. (Gene) Smith
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Last updated: 29 Jan 2000