Astronomers Reveal Pattern of Change in Evolution of Galaxies

A detailed study on the galaxies observed by the Keck telescopes in Hawaii and NASA's Hubble Space Telescope has given insights into the evolution of galaxies.

It was assumed that disk galaxies (like Milky Way) had mostly finished forming about 8 billion years ago. But the latest observations of galaxies have revealed that the long-held assumption is wrong.

The new study reveals that the galaxies have been changing over the last eight billion years. According to astronomers, the movements of gas and stars inside galaxies were actually settling down in the last eight billion years, while the galaxies were taking flat disk shapes, resembling nearby galaxies.

"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," Susan Kassin, an astronomer at NASA's Goddard Space Flight Center in Greenbelt, Md., and the study's lead researcher, said in a statement.

"The trend we've observed instead shows the opposite, that galaxies were steadily changing over this time period," she said.

Most of the star-forming galaxies today are rotational disks like our Milky Way galaxy or the Andromeda Galaxy. They are well ordered disks where rotation takes over other internal motions. The stars and gas in the disk galaxies revolve in one direction around its center, while very few galaxies have stars and gas that move in non-circular motions, according to a report from the University of California High-Performance AstroComputing Center.

The team studied a sample of 544 blue galaxies (their color indicates that stars are forming within them), that are 2 billion and 8 billion light-years away, observed from the Keck and Hubble telescopes. They revealed that the massive galaxies were mostly organized, while the distant or less massive galaxies exhibited disorganized motions.

The internal motion of the stars and gas of the less massive galaxies was not in order and they moved in a haphazard manner. 

However, those blue galaxies that were observed closer to the present show less disorganized motions and ever-faster rotation speeds. The distant blue galaxies are progressively changing into well rotating disk galaxies like our own Milky Way, said the researchers.

In the past, astronomers studied the well-ordered rotating galaxies and ignored those galaxies with disorganized motions.

"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," he said.

But, in this new study, researchers looked at galaxies with bright emission lines to determine the galaxies' internal motions. Emission lines are discrete wavelengths of radiation which are emitted by the gas within a galaxy. They shed light on information about the internal motions and distances.

These new observations suggest that our own Milky Way galaxy would have gone through similar evolutionary motions and settled into its present state with an orderly motion. The study is also believed to help scientists to adjust computer simulations and create galaxy evolution models that closely match the observed trend.

"The Epoch of Disk Settling: z ~ 1 to Now" has been published in the October 20, 2012, issue of The Astrophysical Journal.