NASA scientists have discovered an unexpected pattern of change in hundreds of galaxies observed by the Keck and Hubble space telescopes.
According to Susan Kassin, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Md, the pattern 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 Kassin. "The trend we've observed instead shows the opposite, that galaxies were steadily changing over this time period."
Indeed, 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. However, the most distant blue galaxies in the study tend to be very different, exhibiting disorganized motions in multiple directions.
As such, there is a steady shift toward greater organization to the present time as the disorganized motions dissipate and rotation speeds increase. Essentially, these galaxies are gradually settling into well-behaved disks.
The plot below 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.
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," explained 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."
To be sure, rather than limiting their sample to specific galaxy types, the researchers instead looked at all galaxies with emission lines bright enough to be used for determining internal motions. Emission lines are the discrete wavelengths of radiation characteristically emitted by the gas within a 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.
Simulations such as this will help astronomers better understand the new findings in galaxy evolution. It also tracks the development of a single disk galaxy from shortly after the Big Bang to the present day. Colors reveal old stars (red), young stars (white and bright blue) and the distribution of gas density (pale blue); the view is 300,000 light-years across.
As noted above, the team studied a sample of 544 blue galaxies from the Deep Extragalactic Evolutionary Probe 2 (DEEP2) Redshift Survey, a project that employs Hubble and the twin 10-meter telescopes at the W. M. Keck Observatory in Hawaii. Located between 2 billion and 8 billion light-years away, the galaxies 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 as the sun and solar system were being formed.
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 have positively identified the pattern, they can adjust computer simulations of galaxy evolution until these models are able to replicate the observed trend. This should help guide scientists to the physical processes most responsible for it.