Astronomers uncover a surprising trend in galaxy evolution

Four massive galaxies from the survey showing increasing organization from earlier (more distant) to more recent (closer) galaxies (top to bottom). Image credit: NASA/HST

A comprehensive study of hundreds of galaxies observed by the Keck telescopes in Hawaii and NASA's Hubble Space Telescope has revealed an unexpected pattern of change that extends back 8 billion years, or more than half the age of the universe.

"Astronomers thought disk galaxies in the nearby universe had settled into their present form by about 8 billion years ago, with little additional development since," said Susan Kassin, an astronomer at NASA's Goddard Space Flight Center and the study's lead researcher. "The trend we've observed instead shows the opposite, that galaxies were steadily changing over this time period."

Today, star-forming galaxies take the form of orderly disk-shaped systems, such as the Andromeda Galaxy or the Milky Way, where rotation dominates over other internal motions. The most distant blue galaxies in the study tend to be very different, exhibiting disorganized motions in multiple directions. There is a steady shift toward greater organization to the present time as the disorganized motions dissipate and rotation speeds increase. These galaxies are gradually settling into well-behaved disks.

"A good way to visualize what is happening is to imagine a crowd of ice skaters all getting together in a round rink at the same time," said coauthor David Koo, professor of astronomy and astrophysics at UC Santa Cruz. "At first, there would be a jumbling mix of skaters, but after awhile you are likely to see an orderly flow of skaters, all moving around smoothly in the same direction. This same evolution from early disorder to later order may well have been part of our home galaxy's life story."

The team studied a sample of 544 blue galaxies from the Deep Extragalactic Evolutionary Probe 2 (DEEP2) Redshift Survey, a project led by UC Santa Cruz astronomers Sandra Faber, Puragra Guhathakurta, and Koo, and UC Berkeley astronomer Marc Davis. Kassin worked on the survey as a postdoctoral researcher with Faber at UC Santa Cruz. Faber, now interim director of UC Observatories, led the development of the DEIMOS spectrograph at the W. M. Keck Observatory in Hawaii, which was a crucial instrument for the survey.

Located between 2 billion and 8 billion light-years away, the galaxies in the sample have stellar masses ranging from about 0.3 percent to 100 percent of the mass of our home galaxy. The Milky Way galaxy must have gone through the same rough-and-tumble evolution as the galaxies in the DEEP2 sample, and gradually settled into its present state even as the sun and solar system were being formed.

A paper describing the new findings will be published October 20 in the Astrophysical Journal. "This is the first analysis ever that shows the dynamics of disk galaxies gradually settling down from early chaotic motions to orderly circular rotation over time. It is a truly novel result," Faber said.

Blue galaxies--their color indicates stars are forming within them--show less disorganized motions and ever-faster rotation speeds the closer they are observed to the present. This trend holds true for galaxies of all masses, but the most massive systems always show the highest level of organization. Researchers say the distant blue galaxies they studied are gradually transforming into rotating disk galaxies like our own Milky Way.

"Previous studies removed galaxies that did not look like the well-ordered rotating disks now common in the universe today," said coauthor Benjamin Weiner, an astronomer at the University of Arizona in Tucson who also did postdoctoral work at UC Santa Cruz on the DEEP project. "By neglecting them, these studies examined only those rare galaxies in the distant universe that are well-behaved and concluded that galaxies didn't change."

Rather than limit their sample to certain galaxy types, the researchers instead looked at all galaxies with emission lines bright enough to be used for determining internal motions. Emission lines indicate the discrete wavelengths of radiation characteristically emitted by the gas within a the galaxy. They are revealed when a galaxy's light is separated into its component colors. These emission lines also carry information about the galaxy's internal motions and distance.

In the past 8 billion years, the number of mergers between galaxies large and small has decreased sharply. So has the overall rate of star formation and disruptions of supernova explosions associated with star formation. Scientists speculate these factors may play a role in creating the evolutionary trend they observe.

Now that astronomers see this pattern, they can adjust computer simulations of galaxy evolution until these models are able to replicate the observed trend. This will guide scientists to the physical processes most responsible for it.

The DEEP2 survey is part of the DEEP Project, jointly led by astronomers at the University of California Observatories, headquartered at UC Santa Cruz, and in the Department of Astronomy at UC Berkeley. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Md., conducts Hubble science operations. STScI is operated by the Association of Universities for Research in Astronomy, Inc. in Washington.

Images and video related to this story are available online at