Galaxies Trend Towards More Ordered Disk-Like Forms Over Time, Upending Old Assumptions
New research has unveiled a pattern of continual change in galaxies gradually towards a more uniform and ordered disc shape, extending back over 8 billion years. The new research was possible by using the Keck telescopes in Hawaii and NASA’s Hubble Space Telescope.
“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. “The trend we’ve observed instead shows the opposite, that galaxies were steadily changing over this time period.”
Current star-forming galaxies have a very ordered disk-shape to them, where internal rotation is dominant over other motions. Examples are the Milky Way and the Andromeda Galaxy. “The most distant blue galaxies in the study tend to be very different, exhibiting disorganized motions in multiple directions. There is a steady shift toward greater organization to the present time as the disorganized motions dissipate and rotation speeds increase. These galaxies are gradually settling into well-behaved disks.”
Blue galaxies (blue means that stars are forming in them) exhibit more organized motions and increasing rotation speeds the closer they are to the present. This observation shows in galaxies of all masses, but the more massive the system is, the higher the level of organization is.
According to the researchers, the distant blue galaxies are essentially, over time, becoming organized rotating disk galaxies like the Milky Way. Unless of course they are in the process of crashing into other galaxies.
“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.” (an example of how assumptions in science can turn out to be drastically wrong)
Instead of limiting the sample to only certain types of galaxies, 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.”
“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 researchers think that the Milky Way galaxy must have experienced a period of the “same rough-and-tumble evolution as the galaxies in the DEEP2 sample,” before eventually self-organizing into state it’s currently in.
The number of visible galaxy mergers occurring has decreased considerably during the last 8 billion years. Along with the rate at which stars have been forming and the associated supernova explosions. The researchers think that these factors probably are at least partially responsible for creating in the evolutionary trend towards more ‘order’.
With this pattern revealed, researchers can now adjust their computer models to replicate this observation, improving their accuracy.
The new research will be published October 20th in The Astrophysical Journal.