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16.11 Clusters of Galaxies

The concentration of galaxies in the direction of Virgo is the nearest example of the many galaxy clusters that can be found in the universe. Galaxy clusters contain hundreds to thousands of galaxies. There is no fixed demarcation between a galaxy group and a galaxy cluster. However, in general, galaxy clusters not only have many more members than galaxy groups, but they also have a much higher concentration of galaxies in space. This means that galaxy clusters can be recognized out to extremely large distances from the Milky Way. In the 1960s, American astronomer George Abell cataloged over 2700 of the richest clusters of galaxies in the northern sky. He did not need red shifts to place galaxies in the third dimension; the concentration of galaxies on the plane of the sky was strong enough to spot the clusters. With large new surveys like the Sloan Digital Sky Survey, over 100,000 clusters have now been detected.

The nearby Virgo cluster lays at a distance of 18 Mpc, or about 60 million light years. Light from these galaxies began its journey to us long before humans evolved, not long after the catastrophic event that led to the death of the dinosaurs. Our modest Local Group is like a small suburb on the outskirts of Virgo's vast "city" of galaxies. The Virgo cluster is a sprawling mass of hundreds of galaxies, with three giant elliptical near its center — M 84, M 86, and M 87. It is about 2.2 Mpc across (50% larger than the Local Group), and the dense part of the cluster covers 6º on the sky, or 12 times the diameter of the Moon. Many Virgo cluster galaxies were discovered in the late 18th Century by Charles Messier, before the extra galactic nature of those objects was understood.

Despite its impressive size, Virgo is considered a poor cluster because it is only 2 or 3 times denser than the Local Group. Poor clusters usually have irregular shapes and a number of subgroups and concentrations within them. The bright galaxies in Virgo are more or less equally divided into spirals and ellipticals. The cluster also has a swarm of hundreds of dwarf galaxies, mostly dwarf elliptical like those in the Local Group. The Virgo cluster shows up prominently in the map of the brightest galaxies in the sky, along with the Centaurs cluster, which is a cluster of galaxies at a similar distance in the southern sky.

The nearest rich cluster is in the constellation Coma Berenices (Berenice's Hair). The Coma cluster, about 100 Mpc away and about 8 Mpc across, contains thousands of galaxies. At its center are two giant elliptical galaxies. Like most rich clusters, the Coma cluster is spherically symmetric, with a strong central concentration. Most of its galaxies are ellipticals, but it has a moderate number of S0 (or lenticular) galaxies, which are like spirals but without the spiral arms and interstellar matter. Only the most luminous galaxies in a cluster appear in CCD images. We are seeing the "tip of the iceberg" of a much larger population of (mostly dwarf) galaxies concentrated in space. The high velocities of Coma cluster galaxies were used by Fritz Zwicky in 1933 to infer the existence of dark matter.

Astronomers have discovered that the relative number of galaxies of different Hubble types in a region depends on the overall density of the environment. This morphology-density relation is a key to understanding how galaxies interact and evolve. Astronomers have measured the relative fraction of spiral, S0, and elliptical galaxies in regions spanning a range of a million in terms of galaxy space density. Below a density of about five galaxies per cubic mega parsec, the fraction of spirals is 60 to 70% and independent of density. Above this density, the spiral fraction drops steadily, until in the dense cores of the richest clusters (with thousands of galaxies per cubic mega parsec), virtually all the galaxies are S0s, elliptical, or red spirals. Normal, star-forming, spiral galaxies inhabit low-density regions of the universe and elliptical galaxies inhabit high-density regions of the universe.

Differences in galaxy morphology are reflections of differences in galaxy histories. Many (but perhaps not all) giant elliptical galaxies form through the merging and disruption of spiral systems. The frequencies of these two types of galaxies in different environments is a reflection of how often galaxies interact. Astronomers can calculate the time it takes for galaxies to interact in different environments. An interaction is defined as an encounter or close passage where the force of gravity leaves measurable effects on one or both of the galaxies. Below a density of a few galaxies per cubic mega parsec, this time is longer than the age of the universe. Spiral galaxies in low-density regions of the universe are relatively isolated. Thus, they have weak gravitational interactions with other galaxies and rarely experience collisions. Encounters that do occur are gentle, with relative velocities of about 100 to 200 km/s — this is the typical amount of scatter in the Hubble relation. However, at the density of a rich cluster, the time scale for interaction is only a few hundred million years. (Only in astronomy would this sentence use the word only!) There has been time for dozens of interactions in the age of the universe. The strong gravity of a cluster causes the galaxies to have fast motions of 1000 to 1500 km/s. Collisions are relatively frequent and violent.

The morphology-density relation explains some of puzzles associated with the different galaxy types. Spirals like the Milky Way have too much star formation to be explained by their available reservoir of gas. In fact, spirals get injections of gas from intergalactic space, which is sparsely filled with cold gas. In low-density regions of the universe, this material can fall like a gentle rain onto the disks of spiral galaxies, replenishing the gas supply. Spirals are kept young and active by this material. Elliptical galaxies have the opposite problem, with too little star formation. Their old stars lose mass, providing an ample supply of gas. Astronomers believe that as galaxies pass through the dense core of a cluster on high-velocity orbits, they are continually swept free of gas and dust and are structurally disrupted. Additionally, the merger of two spirals that are within a factor of 3 of one another in mass will consistently become elliptical galaxies.

Presumably, the low fraction of spiral galaxies in a cluster results from collisions and mergers that strip away the gas from gas-rich galaxies. At the center of a rich cluster of galaxies, galactic cannibalism has a fascinating effect. Galaxies will gradually merge over time in the cluster core. The result of a large galaxy devouring a number of smaller galaxies could be a giant elliptical. This scenario can explain why the largest galaxies are found at the centers of rich clusters. Interactions, mergers, and collisions cannot explain all of the differences between galaxies. Only a small fraction of galaxies are the remnants of mergers between massive galaxies.

We can understand how rare a galactic merger is if we consider how long it will take for the Milky Way to collide with the Virgo cluster. The Milky Way is about 18 Mpc from the mighty Virgo cluster. We can easily work out how long it would take to fall into Virgo, traveling at 200 km/s. A distance of 18 Mpc is 1.8 × 107 × 3 × 1013 km, or 5.4 × 1020 km. Therefore, it would take 5.4 × 1020 / (200 × 3 × 107) = 9 × 1010, or 90 billion years — far longer than the age of the universe — for the Milky Way and the Virgo cluster to merge. Just as in biology, properties of galaxies are a combination of nature and nurture. Some properties result from evolution or interactions over 12-13 billion years. But other features were built in at the beginning so we must look to the early universe to explain many galaxy properties. Astronomers refer to this as the initial conditions. Galaxies evolved out of the unfamiliar conditions of the early universe when it was hot, dense, and smooth.

Astropedia Image
The Virgo cluster of galaxies. Click here for original source URL.