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. It is being presented this week at a meeting of the American Astronomical Society in Austin, TX.
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 Galaxy 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,” said 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 one 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.
Other researchers who participated in this study include Gabriela Canalizo at the University of California, Riverside, Masanori Iye at the National Astronomical Observatory of Japan, and Toshinori Maihara at Kyoto University. The results are part of McGrath’s Ph.D. dissertation, which she recently completed at the University of Hawaii.
Founded in 1967, the Institute for Astronomy at the University of Hawaii at Manoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea.
IMAGE CAPTION: The Hubble Space Telescope captured this massive saucer-shaped galaxy more than 9 billion light-years distant from us. Its age implies it was formed when the Universe was very young. Meanwhile, the thin disk-like structure has remained intact since birth, apparently unharmed by destructive galaxy collisions for over one billion years. Previously, theories of galaxy formation predicted such massive galaxies could only be formed by frequent galaxy collisions that increase mass and reorganize matter into three-dimensional football-shaped objects. This galaxy and others like it may have formed instead from the collapse of a single massive gas cloud. Eventually, it will merge with neighboring galaxies to form one of the most massive galaxies in the Universe.
