A giant spiral of gas dust and stars, Messier 101 spans 170,000 light-years and contains more than a trillion stars. Astronomers have uncovered a surprising trend in galaxy evolution where galaxies like M101 and the Milky Way Galaxy continued to develop into settled disk galaxies long after previously thought. Credit: NASA/ESA Hubble
Graceful in their turnings, spiral galaxies were thought to have reached their current state billions of years ago. A study of hundreds of galaxies, however, upsets that notion revealing that spiral galaxies, like the Andromeda Galaxy and our own Milky Way, have continued to change.
“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 in Greenbelt, Md., and the study’s lead researcher in a press release. “The trend we’ve observed instead shows the opposite, that galaxies were steadily changing over this time period.”
A study of 544 star-forming galaxies observed by the Earth-based Keck and Hubble Space Telescope shows that disk galaxies like our Milky Way Galaxy unexpectedly reached their current state long after much of the universe’s star formation had ceased. Credit: NASA’s Goddard Space Flight Center
Astronomers used the twin 10-meter earth-bound W.M. Keck Observatory atop Hawaii’s Mauna Kea volcano and NASA’s Hubble Space Telescope to study 544 star-forming galaxies. Farther back in time, galaxies tend to be very different, say astronomers, with random and disorganized motions. Nearer to the present, star-forming galaxies look like well-ordered disk-shaped systems. Rotation in these galaxies trumps other internal, random motions. These galaxies are gradually settling into well-behaved disks with the most massive galaxies always showing higher organization.
This plot shows the fractions of settled disk galaxies in four time spans, each about 3 billion years long. There is a steady shift toward higher percentages of settled galaxies closer to the present time. At any given time, the most massive galaxies are the most settled. More distant and less massive galaxies on average exhibit more disorganized internal motions, with gas moving in multiple directions, and slower rotation speeds. Credit: NASA’s Goddard Space Flight Center
The sampling of galaxies studied, from the Deep Extragalactic Evolutionary Probe 2 (DEEP2) Redshift Survey, ranged between 2 billion and 8 billion light-years from Earth with masses between 0.3 percent to 100 percent that of our own Milky Way Galaxy. Researchers looked at all galaxies in this time range with emission lines bright enough to determine internal motions. Researchers focused on emission lines characteristically emitted by gas within the galaxy. The emission lines not only tell scientists about the elements that make up the galaxies but also red shifting of emission lines contains information on the internal motions and distance.
“Previous studies removed galaxies that did not look like the well-ordered rotating disks now common in the universe today,” said co-author Benjamin Weiner, an astronomer at the University of Arizona in Tucson. “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.”
In the past 8 billion years, mergers between galaxies, both large and small, has decreased. So has the overall rate of star formation and associated disruptions due to supernovae explosions. Both factors may play a role in the newly found trend, say scientists.
The Milky Way Galaxy may have gone through the same chaotic growing and changing as the galaxies in the DEEP2 sample before settling into its present state at just about the same time the Sun and Earth were forming, say team scientists. By observing the pattern, astronomers can now adjust computer simulations of galaxy evolution until they replicate the observations. Then the hunt will be on to determine the physical processes responsible for the trend.
This cosmological simulation follows the development of a single disk galaxy throughout the life of the Universe; about 13.5 billion years. Red colors show old stars, young stars show as white and bright blue while the distribution of gas shows as a pale blue. The computer-generated view spans about 300,000 light-years. The simulation, running on the Pleiades supercomputer at NASA’s Ames Research Center in Moffett Field, California, took about 1 million CPU hours to complete. Credit: F. Governato and T. Quinn (Univ. of Washington), A. Brooks (Univ. of Wisconsin, Madison), and J. Wadsley (McMaster Univ.).
A paper detailing the findings will be published in the October 20, 2012 The Astrophysical Journal.