It is my pleasure to introduce Dr. Eric McKenzie, associate director for the department of astronomy at the University of Maryland. He received his Ph.D. from the University of Florida, and his research has focused on galaxy evolution at specific wavelengths of redshifted light. He also helped me get some of my facts straight for the Tomorrow News Network story “The Orion War.”
This week, he will be answering questions about galaxy evolution and what the early universe was like.
James Pailly: Were galaxies in the early universe different than the ones we see around us today?
Dr. McKenzie: Quite a bit! They were generally smaller, for starters. Because the universe was denser, they also collided with each other more frequently. The larger galaxies that we see nowadays, including our own Milky Way, appear to have built up over time through mergers of smaller galaxies. Modern galaxies often have orderly spiral or elliptical shapes, but it’s rare to see such galaxies in the early universe.
Early galaxies formed stars more quickly than modern galaxies do. The ‘global star formation rate’ has been steadily slowing over the past 5 billion years or so as the available gas gets converted into stars. Some of the galaxy collisions caused massive waves of star formation because the galaxies’ gas got compressed by gravitational interactions; these galaxies are called ‘starbursting galaxies’.
Early galaxies also had fewer ‘metals,’ which in astronomy jargon means all the elements beyond the simplest ones, hydrogen and helium. The universe formed with essentially no heavy elements, including carbon, nitrogen, oxygen, iron, gold, uranium, and so on. These were produced later by fusion within stars or during the violent supernova explosions that occur when the largest stars die. (Pretty much all of the non-hydrogen atoms in our bodies originally came from stars. The stars cast this material into space as their lives came to an end, and the atoms came to rest in the cloud of gas that later coalesced into our solar system. I think that this is one of the most extraordinary discoveries of astronomy.)