Chapter 16: The Expanding Universe

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 redshifts 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 general properties of clusters of galaxies are:

• Anywhere from 100 to 10,000 galaxies, with hot X-ray emitting gas, and extensive dark matter.
• Total masses of 10¹⁴ to 10¹⁵ solar masses, 80-90% of which is dark matter.
• Diameters range from 1 to 5 Mpc.
• The spread of velocities of the individual galaxies is 800 to 1200 km/s.

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 ellipticals 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 extragalactic nature of those objects was understood.

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 megaparsec, 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 megaparsec), virtually all the galaxies are S0s, ellipticals, 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.

The morphology-density relation explains some of the 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 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.

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, the properties of galaxies are a combination of nature and nurture. Some properties result from evolution or interactions over 12 or 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.

Author: Chris Impey
Editor/Contributor: Pamela Gay
Editor/Contributor: Jessie Antonellis