Some of the first massive galaxies in the universe formed when huge gas clouds rapidly collapsed, according to Elizabeth McGrath of the University of California, Santa Cruz, Alan Stockton of the University of Hawaii, and their collaborators. This discovery, which is based on new Hubble Space Telescope images, challenges the commonly held idea that all of the earliest massive galaxies formed when smaller galaxies merged.
The standard theory of galaxy formation predicts that the most massive galaxies in the universe take a long time to grow, accumulating mass through the coalescence of many smaller galaxies in a process that continues until relatively recent times. To test this theory, McGrath, Stockton, and their collaborators searched for the oldest, most massive galaxies they could find and used clues from their shape and structure to deduce how they may have formed. High-resolution images from the Hubble Space Telescope revealed galaxies more massive than our own Milky Way that existed when the universe was very young, only one-fifth its current age. It is believed that such galaxies are the distant ancestors of the most massive galaxies in the universe today.
“We expected these galaxies to look similar to the football-shaped elliptical galaxies that we see at the centers of dense groups of galaxies today, where mergers are common. We were quite surprised to find that many of them appear instead to be flattened, rotating disks of ordered material,” says McGrath.
Disk galaxies are pancake or saucer-shaped, and their stars orbit in circles around the center of the galaxy, much like the planets in our solar system orbit around the Sun, or like a Frisbee spins as it moves through the air. This type of galaxy is more likely to have formed from a single massive cloud that collapses under its own gravity into a flattened disk rather than through violent collisions of previously formed galaxies. Computer simulations of the latter scenario predict that collisions would destroy disks and send stars from each merging galaxy into more chaotic, three-dimensional orbits, producing football-shaped, or elliptical galaxies. The most massive galaxies in the universe today all appear to be elliptical in shape, and therefore can be quite naturally explained through the merger hypothesis. The existence of massive disk galaxies in the early universe, however, challenges this perspective.
In total, McGrath and her collaborators observed seven of what are likely some of the first massive galaxies to form in the universe. Of these, four had shapes dominated by disk-like profiles. By age-dating the galaxies from studying properties of the stars within them, McGrath’s team discovered that these disk structures have remained in pristine condition for over 1 billion years. Even so, it seems inevitable that eventually these galaxies will merge with others and be reformed into the more elliptical-shaped massive, old galaxies that are familiar to us in the nearby universe.