"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 at NASA's Goddard Space Flight Center in Greenbelt, Md., said.
"The trend we've observed instead shows the opposite, that galaxies were steadily changing over this time period," she said.
Most galaxies go through a rough-and-tumble disorganized evolution before they settle into the rotating disk form seen in our Milky Way and other mature galaxies, astronomers said.
"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.
Researchers say the distant blue galaxies they studied -- blue because they are still forming stars -- are gradually transforming into rotating disk galaxies like our own Milky Way.
"By neglecting them, these [previous] studies examined only those rare galaxies in the distant universe that are well-behaved and concluded that galaxies didn't change."
"We are learning that galaxies today are like young adults," Kassin said. "Many of them had exciting youths marked by intense interactions with other galaxies, with a lot of growth spurts in mass, new stars, and heavy elements. But that chaotic growth has been slowing down as galaxies mature, and they become more organized and calmer inside."