Going Further 8.3: What Is Between the Coma Cluster’s Galaxies?

Is dark matter the only stuff between the galaxies in the Coma Cluster? In fact, it is not. With the advent of space-based observatories sensitive to the x-ray emissions from hot gas, an additional method of measuring the mass of galaxy clusters became available. x-ray observations showed that galaxy clusters contain a tremendous amount of gas that lies outside and between the galaxies within a cluster. In fact, the mass of the hot gas is comparable to, or even in excess of, the mass of all the galaxies in the cluster combined. The gas is often referred to as the intra-cluster medium. It has a low density, but a very high temperature. As a result, it emits x-rays.

Figure B.8.2: The Coma Cluster in x-ray Wavelengths. This x-ray image from the Chandra Observatory shows the central 1.5 million light-years of the Coma Cluster. The blue haze represents a vast 100-million-degree-Celsius gas cloud which envelops the galaxies in the cluster. Compare this to Figure 8.19, which shows the Hubble Space Telescope visible light image of the cluster. Credit: NASA/CXC/SAO/A.Vikhlinin et al.

The temperature of a gas is related to the average speed of the gas particles, and the higher the temperature, the faster the particles are moving. This is because temperature is a measure of the average kinetic energy of the gas particles. The expression for kinetic energy is ½mv² for a particle of mass m and velocity v. So, using the temperature of the gas, we can calculate a velocity measurement for the gas. The velocity of the gas particles is determined by the gravity of the cluster—remember that if the velocity of the gas particles was too high, they would fly out of the cluster. So the cluster must be massive enough and have enough gravity to hold onto the gas. We can use the velocity measurement of the particles in the gas to determine the galaxy cluster’s mass. This method is similar to using stellar motions or galaxy motions to determine mass.

Astronomers have found that the mass of the luminous, or light-emitting, matter from the galaxies in the cluster and from the gas in between the galaxies makes up only about 10% of the cluster’s total mass. The Coma Cluster contains about 90% dark matter!