Disk galaxies like our own Milky Way put the finishing touches on their stunning shapes relatively recently, a new study suggests.
Image: This spectacular image of the large spiral galaxy NGC 1232 was obtained on Sept. 21, 1998, during a period of good observing conditions. Credit: ESO
The find will likely surprise many scientists, who had thought such galaxies had been static for more than half of the universe’s 13.7-billion-year existence.
“Astronomers thought disk galaxies in the nearby universe had settled into their present form by about eight billion years ago, with little additional development since,” lead author Susan Kassin, of NASA’s Goddard Space Flight Center in Greenbelt, Md., said in a statement. “The trend we’ve observed instead shows the opposite — that galaxies were steadily changing over this time period.”
Kassin and her colleagues used NASA’s Hubble Space Telescope and the Keck Observatory in Hawaii to study 544 blue galaxies, whose color indicates that stars are forming within them.
They found that the most far-flung, ancient galaxies tend to be the most disordered, with organization steadily increasing as galaxies are observed closer and closer to the present day. Over time, the galaxies’ rotation speeds increase, and they settle into proper, well-behaved disks.
The trend holds for galaxies of all masses, but the biggest systems are always the most highly organized, researchers 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 of the University of Arizona. “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.”